Waste Water Treatment Notes

8/6/2019 Waste Water Treatment Notes

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BIOLOGICAL

K.GOPALAKRISHNA

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MUNICIPAL WASTE WATER COMPOSITION

Domestic

Commercial

Permitted Industries (Garages etc)

Services – Hotels, Resorts, Residential Schools, Hospitals etc

Storm Water – 15% from Overflow Weirs

Total Solids – 2%

(90 % are inert; 10 % active bio or chem.)

Physical

T.S.S. Colloidal D.S. Floating (E.S.S.)(60%) (30%) (8%)

(2%)

TSS – Settles in 2 hr D.T

Colloidal - < 0.002 mm diameter

D.S. – Ionized form

F.S. – Includes grease and oil (ESS)

Tests

Total Solids

30 mins settling

Dry in oven at 103°c for 20 mins

Cool in desiccators

Difference in Weight (to fourth decimal)

(All units as mg /l or g / m3)

Colloidal Solids.

Inorganic solids with a diameter less than 0.002 mm and Sp Gr more than 1.

Filter through What man filter – 10

Dry filter paper

2



Difference in weight

Volatile Suspended Solids

Represents the volatile organic and inorganic components.

Ignite in muffle furnace at 550°c (Loss of weights represents organics)

Fixed Solids

Balance Solids

represents Minerals like Ca++, Mg++, Fe++

Ether Soluble Solids

Sox let apparatus

represents grease

Quantification

Domestic – q – specific waste water production in lpcd.

This is about 60% to 90% of water supplied

QD = N.q x 10-3 m3 /d.

Industrial wastes – Depends upon Industry and number of working hours.

It may be noted that only permitted industries which form the infrastructure of the

municipal sector are allowed to form the municipal waste waters. These include

laundries, motor garages, small scale sector units etc. They are expected to pretreat the

waste water to the required norms before being let off to the municipal sewers.

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Storm Flow

Rainfall intensity data

Error: Reference source not found

ψ – Runoff Coefficient

A – Cumulative area to be drained in hectares

γτe – Return Storm water (te - 15 mins)

QSW = (A .ψ .γτe x 10-3) m3 / day

Infiltration: Expressed as\1. Rate per unit surface area – 5 to 50 m3/ ha / day (avg 20)

2. Rate per unit length of sewer – 10 to 20 m3/ km / day

3. Rate per unit length and diameter

Peak Flow

Maximum daily flow ≈ 1.8 to 2 average daily flow

Maximum hourly flow ≈ (average daily flow in m3/ day / Y) m3/ hr

Y – 10 to 18

4



Design Flow

Design Flow (in m3/ hr): Daily flow (m3/ day) + Daily flow (m3/ day) + Infiltration

(m3/day) Y = 10 to 18 working hours 24

Characteristics of Sewage

Waste water contains more than 99 percent water coming as return water and less

than 1 percent solids which are added in the form of either organic or inorganic solids.

99.8% Water

0.2 % Solids (both organic and inorganic)

Classification of Suspended Particles

CHARECTERISTICS OF MUNICIPAL WASTE WATER

Material Diameter (mm)

Coarse Gravel > 2Fine Gravel 2 – 1Coarse Sand 1 – 0.5

Medium Sand 0.5 – 0.25Fine Sand 0.25 – 0.1Very Fine Sand 0.1 – 0.05

Silt 0.05 – 0.01Fine Silt 0.01 – 0.005

Clay 0.01 – 0.001Fine Clay 0.001 – 0.0001

Colloidal Clay < 0.0001

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PHYSICAL CHARECTERISTICS

The significant physical characteristics of waste water is its total solid content

which is composed of floating, settleable, colloidal and dissolved contents. Other parameters include particle size distribution, turbidity, color, temperature, conductivity,density, specific gravity and specific weight.

Solids : Waste water contains a variety of solid materials varying from coarse tocolloidal. Coarse materials are usually removed before the sample is analyzed for solids. The various solid classification is identified in the following table.

TEST DESCRIPTION

Total Solids (TS)

Total Volatile Solids (TVS)

Total Fixed Solids (TFS)

Total Suspended Solids (TSS)

Volatile Suspended Solids (VSS)

Fixed Suspended Solids (FSS)

Total Dissolved Solids (TDS)( TS-TSS)

Total Volatile Dissolved Solids (TVDS)

Fixed Dissolved Solids (FDS)

Residue remaining at 103 to 105 C

Solids that are volatilized at 500 C

Residue left after igniting at 500 C

Portion of TS retained on filter paper with pore size 1.5 micrometers

Solids Volatilized when TSS is ignited at

500 CResidue remaining after TSS is ignited at500 C

These are typically colloidal solids rangingfrom 0.001 to 1 micrometers

Solids Volatized when TDS is ignited at500 C

Residue after TDS is ignited at 500 C

The standard test for settleable solids consist of placing a waste water sample on a one liter Imhoff Cone and noting volume of settled solids after onehour. Typically about 60% of suspended solids in a municipal waste water issettleable.

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Turbidity : Turbidity is a measure of the light transmitting property of thewater, is another test used to indicate the quality of waste water. The results of turbidity are reported in Nephelometric turbidity units (NTU). It should be notedthat the presence of air bubbles in the fluid will cause erroneous readings.

There is reasonable relationship between turbidity and total suspendedsolids (TSS) for the settled and filtered secondary effluents from activated sludge process. The general relationship is as follows:

TSS (mg/l) = (TSS f ) (T)TSS – Total suspended solids in mg/lTSS f - Conversion factor T – Turbidity in NTUTSS f – 2.3 to 2.4 for secondary settled effluent

1.3 to 1.6 for secondary filtered effluent

COLOUR : Condition refers to the age of the waste water, which is determinedqualitatively by its color. Fresh waste water is usually light brownish grey. Assepticity increases it tends to dark grey and finally to black. In most cases thecolor is due to metallic sulphides reacting with metals, which is formed by thesulphides reacting with metals under anaerobic conditions. Some industrialeffluents can also add color to domestic waste water.

CHLORIDES : Chloride is a constituent of concern in waste water as it canimpact the final reuse application of treated waste water. Chloride in naturalwater result from leaching from rocks and soils, salt water intrusion. In additionagricultural, industrial and domestic waste water discharges act as a source of chlorides. Human excreta contain about 6 grams of chloride per person.Conventional biological treatments do not remove chloride as they are non biodegradable.

NITROGEN: Total nitrogen comprises of organic nitrogen, ammonia. Nitriteand nitrate nitrogen. The organic fraction consists of a complex mixture of compounds including amino acid, amino sugars and proteins. These could besoluble or particulate. Organic nitrogen is determined analytically using theKjeldhal method. The aqueous sample is first boiled to drive off the ammonia andthen it is digested. During digestion organic nitrogen is converted to ammoniumthrough the action of heat and acid. Total Kjeldhal nitrogen (TKN) is determinedin the same manner as organic nitrogen, except that the ammonia is not driven off before digestion step. TKN is therefore sum of total organic nitrogen andammonia nitrogen. Nitrate nitrogen determined calorimetrically is relativelyunstable and is easily oxidized to nitrate form. Although present in lowconcentrations, nitrate can be very important as it is extremely toxic to fish andother aquatic species. Nitrites present in treated effluents are oxidized by chlorineand this increases the cost of disinfection. Nitrate nitrogen is the end product of

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nitrification and is typically found in treated effluents from 15 to 20 mg/l. Nitratenitrogen is determined by colorimetric tests or with specific-ion electrodes.

PHOSPHOROUS : Municipal waste waters may contain 4 to 16 mg/l P as total phosphorous which may comprise of ortho-phosphate, poly-phosphate and

organic phosphate. The ortho-phosphates are available for biological metabolismwithout further breakdown. The poly-phosphates include molecules with two or more phosphorous atoms, oxygen atoms and in some cases hydrogen atomscombined in a complex molecule. Poly-phosphates undergo hydrolysis inaqueous solutions and revert to ortho-phosphate from. Ortho-phosphate can bedetermined by adding ammonium molybdate which will form a color complex.The poly-phosphates and organic phosphates must be converted to ortho- phosphates using an acid digestion step before they can be determined in a similar manner.

SULFUR : Sulphates are reduced biologically to sulphide under anaerobic

conditions which in turn can combine with hydrogen to form hydrogen sulphide.The accumulated hydrogen sulphide can then be oxidized biologically to sulfuricacid which is corrosive. Sulphates are reduced to sulphides in sludge digestersand may upset the biological process if sulphide concentration exceeds 200 mg/l.

GASES : The gases commonly found in untreated waste water are compoundsof nitrogen, carbon di oxide, hydrogen sulphide, ammonia and methane. Thelatter three are derived from the decomposition of organic matter and are of concern to health and safety. The actual quantity of these gases that can be present depend upon (a) the solubility of gas defined by Henry’s law (b) the partial pressure (c) the temperature (d) the concentration.

Hydrogen sulphide is formed from the anaerobic decomposition of organicmatter containing sulfur or from the reduction of mineral sulphites or Sulphates.The blackening of waste water and sludge usually results from the formation of hydrogen sulphide that has combined with the iron present to form ferroussulphide (FeS). Various other metallic sulphides are also formed. Althoughhydrogen sulphide is the most important gas formed from the odor point, other volatile compounds such as indol, skatol, mercaptans may cause odor moreoffensive than hydrogen sulphide.

METALIC CONSTITUENTS : Trace quantities of metals such as cadmium.Chromium. Copper, zinc, iron, lead, manganese, nickel are present generally inwaste water. Most of these of these metals are necessary for the growth of biological life and the absence of sufficient quantity of these could limit themicrobial and algal growth. However metals in excessive quantities will interferewith the beneficial uses because of their toxicity. Sources of these metals includeresidential sectors, ground water infiltration, commercial and industrialdischarges.

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Metals are determined typically by flame atomic adsorption, electrothermal adsorption, inductively coupled plasma or IPC/mass spectrometry.Metals can be classified as (a) dissolved metals present in unacidified samples (b)suspended metals present in unacidified samples that are retained on a membranefilter (c) total metals – a sum of the above two determined after digestion (d) acid

extractable metals after present unfiltered sample is treated with a hot dilutemineral acid.

ORGANIC CONSTITUENTS : Organic constituents are normally composedof a combination of carbon, hydrogen and oxygen together with nitrogen in somecases. In waste water it typically consists of proteins (40 to 60%), carbohydrates(25 to 50%) and oils and fats (8 to 12%). Urea the major constituent in urine tsalso present in fresh waste water. However it quickly decomposes due to ureahydrolysis leading to ammonia.

Laboratory investigations for organics include (1) Biochemical Oxygen

Demand (BOD) (2) Chemical Oxygen Demand (COD) (3) Total OrganicCarbon (TOC). The TOC test, done instrumentally is used to determine the totalorganic carbon in an aqueous samples. The test methods for TOC utilize heat andoxygen, ultra violet radiation, chemical oxidants or some combination of thesemethods to convert organic carbon to carbon di oxide which is then measuredwith an infrared analyzer or by other means. The TOC tests are gaining favor, because it takes only 5 to 10 minutes to complete.

INDIVIUAL ORGANIC COMPONDS : Individual organic compounds aredetermined to assess the presence of priority pollutants identified by USEPA.Priority pollutants both organic and inorganic are selected on the basis of their carcinogenicity, mutagenecity, tetrogenecity or high acute toxicity. The analyticalmethods used to determine these require the use of sophisticated instrumentscapable of measuring trace concentration in the range of 10 -12 to 10 -13 mg/l. Gaschromatograph (GC) and high performance liquid chromatograph (HPLC)methods are most commonly used to detect these compounds. Typical detectorsused in conjunction with gas chromatography include electrolytic conductivity,electron capture (ECD), flame ionization (FID), photo ionization (PID) and massspectrometer (GCMS). Typical detectors for high performance liquidchromatography include photo iodide array (PDAD) and post column reactor (PCR). Over 180 individual organic compounds can be determined by using theabove one of two methods.

VOLATILE ORGANIC COMPOUNDS (VOC) : Organic compounds thathave a boiling point less than or equal to 100 C and / or a vapor pressure morethan 1mm Hg at 25 C are generally considered to be volatile organic compounds(VOC). Vinyl Chloride which has a boiling point of minus 13 C and a vapor pressure of 2548 mm of Hg at 20 C is an example of a highly volatile organiccompound. These are of common concern because (1) Once such compounds are

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in Vapor state, they are much more mobile and therefore more likely to bereleased to the atmosphere(2) the presence of some of these compounds in the atmosphere may pose a

significant health risk (3) they contribute to the general increase in thereactive hydrocarbons in the atmosphere, which can lead to the formation of

photochemical oxidants. The release of these compounds in sewers andtreatment plants especially at head works is of concern with respect of healthof personnel involved.

DISINFECTION BYPRODUCTS : It has been found that when chlorine isadded to waste water, variety of organic compounds containing chlorine areformed. Collectively these compounds are known as disinfection byproducts(DBP). Although present in low concentrations, they are of concern because of suspected carcinogenicity. Typical classes of these compounds includetrihalomethane (THM), halo acetic acid (HAA), trichlorophenol and aldehydes.Because of the concerns, considerable attention has been focused over the past

few years on the use of ultra violet (UV) disinfection as a replacement tochlorination. In addition considerable attention has been focused on themodifications to the conventional treatment processes to improve the treatment of these compounds and to advanced processes for the removal of these.

PESTICIDES AND AGRICULTURAL CHEMICALS : Pesticides,herbicides and other agricultural chemicals are toxic to many organisms andtherefore can be significant contaminants to surface of waters. These chemicalsare not common constituents of domestic waste water, but result primarily fromsurface run off from agricultural, vacant and park lands. Concentration of thesemay retard the treatment process, result in fish kills or may result in toxic foodchain to humans.

BIOLOGICAL CHARECTERISTICS : The biological characteristics of waste water are of fundamental importance in the control of diseases caused by pathogenic organisms of human origin, and because of their fundamental role played in the stabilization and decomposition of organic matter. The se include bacteria, fungi, algae, protozoa and viruses.

Bacteria are single celled prokaryotic organisms. The interior of the cellcontains a colloidal suspension of proteins, carbohydrates etc called cytoplasm.This contains ribonucleic acid (RNA) which plays a major role in synthesis of proteins. It also contains deoxyribonucleic acid (DNA) which contains all thenecessary information for reproduction.

Protozoa are motile microscopic eukaryotes that are usually single celled.The majority of protozoa are aerobic heterotrophes and often consume bacteria asan energy source.

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Viruses are composed of a nucleic acid core ( either DNA or RNA)surrounded by an outer shell of proteins called a capsid. Viruses are intracellular parasites that multiply only within a host cell. Bacteriophages are viruses thatinfect a bacteria as a host cell. They have not been implicated in humaninfections.

Algae are unicellular or multicellular autotrophic photosyntheticeukaryotes. The blue green algae cyanobacter is a prokaryotic organism.

Fungi are multicellular non photosynthetic, heterotrophic eukaryotes.Most fungi are strict or facultative aerobes which reproduce sexually or asexually, by binary fission, budding or spore formation. Molds produce microscopic unitswhich collectively form a filamentous mass called mycelium. Yeasts are fungithat cannot form mycelium and are therefore unicellular.

Many types of harmless bacteria colonize in the human intestinal tract and

are routinely shed in feces. They contain a large number of bacillus collectivelyknown as coli form bacteria. Each person discharges from 100 to 400 billion coliform bacteria per day which are taken as indicator microorganisms for the presence of pathogens.

Typical Sewage Analysis

Total Solids

Total Suspended Solids

Total Volatile Solids

Total Fixed Solids

Dissolved Oxygen

Biological Oxygen Demand

Chemical Oxygen Demand

Total Nitrogen

Ammonia Nitrogen

Organic Nitrogen

Nitrite Nitrogen

Nitrate Nitrogen

Sulphates or Sulphides

Chlorides


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Living Organisms

Macro – Worms

Micro – Bacteria, Virus, Protozoa, Algae and Fungus

Dissolved Oxygen

Fresh Water will have DO depending upon water temperature and ambient

pressure

At 10° C and 1 atm – 11.3 mg/l

At 30° C and 1 atm – 7.8 mg/l

(atm – atmosphere)

DO is determined by Winkler’s modification method or probes.

Winkler’s Azide modification Method ( D.O)

Sample (BOD bottle) + 2ml MnSO4 + 2ml Alkali Iodide azide

Brown precipitate – Presence of DO

White precipitate – Absence of DO

+ 2ml H2SO4 (fixing)

Starch Indicator – Blue Color

Titrate 0.02 Sodium thiosulphate – disappearance of blue color

Mn++ + 2OH- MnOH (white precipitate)

Mn++ + 2OH- + O MnO2 + H2O (brown precipitate)

MnO2 + 2I- + 4H+ Mn++ + I2 + 2H2O

2Na2 S2O3 5H2O + I 2 Na 2 S4 O6 + 2 NaI + 10 H2O

or 2S2O3- + I2 S4O6

2- + 2I-

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MnSO4 + Sodium azide (Na N3) --------- 4 ml

Volume for Titration = 203300

3002004

=−

xml

1 ml of 0.025 N Na2 S2O3 ------- 0.2 mg DO

For 203 ml ---------- 1 mg / l DO

Kinetics of BOD

(Biochemical Oxygen Demand)

It is assumed that for major portion it occupies a first order curve

The rate of Biochemical oxidation of organic matter is directly proportional to

remaining concentration of unoxidized matter.

dL / dt = -KLOr dL / L = -Kdt

Integrating:

Log e Lt = -Kt + C

at t = 0, Lt =L and C = Log eL

Log e Lt - Log eL = - Kt

Log e Lt / L = -KtLog10 Lt / L = - 0.434 Kt = -K 1t

Lt / L = 10-K 1t

K 1 is deoxygenation constant (per day)

Lt / L is the remaining fraction of oxidizable matter

1 – Lt / L fraction oxidized in t days

Xt amount oxidized in t days

Xt = L (1- Lt /L)= L (1- 10-K

1t)

L = Xt / (1- 10-K 1t)

K 1 (T) = K 1 20 (1.047) T-20

t could be 3 or 5 days; referred as BOD327 or BOD5

20

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C. O. D

(Chemical Oxygen Demand)

Organic matter + Cr 2 O72- + H+ 2 Cr 3+ + CO2 + H2O

Digestion for 3 hours

Titration with Ferrous Ammonium Sulphate; Indicator – Ferroin

BOD / COD – Biodegradability of waste

BOD / COD > 0.7 B.T

BOD / COD 0.7 to 0.4 B.T with acclimatization

BOD / COD < 0.4 PCT preferred

BOD per capita 54 gm / cap / day

Nitrogen

The nitrogen in waste water could be present as organic nitrogen, nitrite or nitrate

nitrogen.

Total organic Nitrogen – 20% of BOD ( For Indian municipal waste waters)

Chlorides

Chlorides could be from human wastes or from Industries.

Each normal human being ejects about 6 gm / cap / day both in urine and feces.

Chlorides are non biodegradable either by aerobic or anaerobic system.

Normal sewage 1000 mg / l

Excess chlorides interferes with biological treatment systems.


Measured using Sox let apparatus indicates grease, oils & fats

Normal Sewage 100 to 150 mg / l

Excess leads to incrustation of pipes

Should be reduced to 10 mg / l before B.T

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pH value

Normal 6.5 to 7.5

Highly acidic or alkaline

Condition indicates industrial wastes presence

Normal Municipal Waste water

BOD327 ≈ BOD5

20 – 350 mg / l

COD – 400 to 450 mg / l

Organic Nitrogen – 70 to 80 mg / lChlorides – 600 to 800 mg / l

Sulphates – 40 to 60 mg / l

ESS – 100 to 150 mg / l

MPN - 105 to 106 per 100 ml

Treatment – Unit Operations

I. Primary Treatment

a) Screens Coarse screens

Fine screens

b) Grit chamber

c) Skimming tank

d) Primary sedimentation tank

II. Secondary Treatment

a) Biological Treatment

b) Final sedimentation tank

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III. Tertiary Treatment

a) Nitrogen removal Ammonia stripping

Denitrification

b) Phosphate removal

c) Chlorides removal by RO

d) Activated Carbon Adsorption for dissolved gasses and phenols

e) Sand filters

f) Disinfection

IV. Sludge Treatment and Disposal

a) Sludge thicknessb) Sludge Digesters

c) Sludge drying beds

d) Bag filters

e) Vacuum filters

f) Incineration

Biological Treatments

Aerobic

a) Activated sludge process

b) Biofilters (Trickling filters)

c) Aerated Lagoons

d) Fluidized bed reactors

e) Biodiscs (Rotating Biological Contactors)

f) Stabilization ponds

g) Oxidation ditch (Pasveer ditch)

Anaerobic

a) Anaerobic Contact filters

b) Up flow anaerobic sludge blanket (UASB)

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c) Anaerobic Lagoons

d) Septic tanks

e) Imhoff tanks

In the aerobic system the organic matter is bio oxidized in the presence of dissolved

oxygen. In the anaerobic system the organic matter is bio reduced in the absence of

dissolved oxygen. The amount energy evolved in an aerobic oxidation for microbial

sustain is 16 to 20 times the amount of energy evolved in an anaerobic reduction. Hence

aerobic degradation is 16 to 20 times faster than an anaerobic degradation making aerobic

treatment more compact.


Flow Sheet – Municipal Wastewater Treatment System

Theory of Settling

A particle in a still fluid of lesser density will move to settle. It will accelerate

initially until the frictional resistance of the fluid equals the impelling force and

there on it reaches a constant settling velocity (HSV).

Discrete Settling No change in shape or size of particle and HSV remains constant. Depth is no

criteria for efficiency. It is applicable for plain sedimentation.

Hindered Settling

Shape and size increases with settling. There is acceleration with depth. It is

applicable for flocculent settling.

Frictional Resistance or Drag

FD = CD*A*ρ*(υ2/2)…………. (Kg.m/Sec2)

Where,

CD = Coefficient of drag. (Relative to viscosity)

A = Projected surface area (πd2/4)

υ = HSV m/sec

ρ = Mass density of fluid

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Sludge

Digester



Impelling Force,

Fi = (ρ1-ρ)*g*V

Where,

ρ1 = Mass density of particle

ρ = Mass density of fluid

V = Volume of particle = (Π*d3/6)

At steady state condition, Fi = Fd

CD*(Πd2/4)*ρ (υ2/2) = (ρ1-ρ)*g*(Π*d3/6)

υ = ρ

ρ ρ

**3

*)(**4 1

DC

d g −

…………………..eqn1

This is referred as hydraulic settling velocity. But, Reynolds’s number which

relates inertia to viscous force

Re =*d υ

ν

ν = Kinematic viscosity of fluid (m2/sec)

By Stoke’s law for the drag of small particles (spheres) in a viscous neglecting

inertial forces

FD = 3*π*ν*υ*d*ρ

Assumptions: Particles are sphericalParticles are inert

3*π*ν*υ*d*ρ = CD*(Πd2/4)* ρ(υ2/2)

or CD =

24*

* d

ν

υ =

24

Re

Substituting in equation 1

υ = ( )11

218

g d

ρ ρ

ν ρ

−

As per Stoke’s law this is valid for Laminar (Re = 1) or nearly laminar flow.

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Let L and B be the effective length and breadth and H be HMD of tank.

Design Flow (Q) = BHV1

Where, V1 = Average flow through velocity

V1/υ = L/H or V1= L*υ/H

Substituting ………. υ = (Q/ B*L)

Design settling velocity in (m/h) is same as SL= Hydraulic surface loading.

Hence, we can get the surface dimension of tank.HMD is fixed (2.5 to 3 m) and checked for the following conditions.

1) Re = (V1*R / ν)

R = Hydraulic mean depth

For rectangular ……………. (BH/ (B+2H))

ν = Kinematic viscosity in m2/sec (1.42 * 10-6 m2/sec at 350c)

2) Stability Conditions

Fr = V12/ (g*R) > 10-5

g = 9.81 m/sec2

2) Check for bottom scour L < 2H

This is possible by construction of battles.

To calculate Design velocity (υ)

LV

1

υ H

Sludge Zone

Inlet Outlet

Free Board

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Example: Total wt of suspended particle = 200 gms/m3

% Net Cum% withwith velocity<= υ (p)

90 gms/m3 with υ = 2.5 m/h 45 10060 gms/m3 with υ = 1.2 m/h 30 5530 gms/m3 with υ = 0.6 m/h 15 2520 gms/m3 with υ = 0.2 m/h 10 10

Removal ratio (r) =Ci

CeCi −

Ci and Ce in mg/l or gm/m3

Based on this υ is determined. Re for Laminar Re<= 1

Transient Re= 1-2000

DT υ ρ1

Grit Chamber 3 to 5 minutes 36m/h 2650Aerated Grit Chamber 6 minutes 25m/h 2650PST 2.5 hrs 1.5m/h 1650SST 3.5hrs 0.6m/h 1350

r

10

0

80

40

20 1-r

υ

υ m/day or 10-3 m/sec

P

20



21



Activated Sludge Proces s

The biological systems are classified as either suspended growth system where

the bioflocs are kept in suspension either in aerobic or anaerobic condition. Activated

sludge process, aerated lagoons, anaerobic lagoons etc fall under this category. In an

attached growth system the bioflocs are adsorbed over an inert media, examples are

biofilters, RBC, Anaerobic contact filter etc.

In an activated sludge process, the wastewater (refilled as mixed liquor) is kept

agitated leading to bioflocs. The bioflocs which are in an activated state are settled in the

final sedimentation tank. Parts of the bioflocs are returned as return sludge to increase the

acclimatized micro-organisms in the mixed liquor.

S PST S2* ∞ FST Se

QD M QD

Activities

1. Agitation leads to better contact between M.O. and colloidal BOD

2. Agitation sets M.O. in virulent action

3. Agitation leads to adsorption forming bioflocs

4. Agitation of DO in MLSS

Design features

1. Sludge loading or F/M ratio2. Volume of reactor

3. Excess sludge produced

4. Recirculation

5. Aeration required

6. Design of aerators

22

QR



Oxidation of organic matter takes place in three stages

1. Production of cell material + substrate respiration

8 CH2O + NH3 + 3O2 C5H7 NO2 + 3CO2 + 6H2O + S

2. Auto oxidation (Endogenous respiration)

C5H7 NO2 + 5O2 5CO2 + NH3 + 2H2O + S

(Cells)

3. Nitrification

NH3 + O2 NO2- (Nitrosomonas)

NO2- + O2 NO3

- (Nirtobacter)

Design of activated Sludge Process

I. Sludge Load or F/M ratio (LS)

L = QD X Si* X 10-3 (kg / day)

V volume in m3

M MLSS in kg / m3

LS = L / MV


Nitrification (depends upon Temp)

Full Nitrification occurs at 17ºc for Ls – 0.3



(No Nitrification below 7ºc)

High Rate ASP will have Ls – 3 to 2

Conventional ASP will have Ls – 0.5 to 0.2

Extended aeration will have Ls – 0.05 to 0.01

Methods of aeration

Completely Mixed Flow (using mechanical aeration)

Plug Flow (using compressed air aeration)

CMF

23

0.33



1. Mechanical aeration by rotors

2. Instant dispersion, Less concentration of reactants, longer aeration period

required

3. Larger Tank volume

4. Less efficiency (actual DT – 0.7 Theoretical DT)

5. Better agitation, hence suited for industrial wastes.

PF

1. Compressed air aeration

2. Flow is laminar, better efficiency, hence lesser tank volume (actual DT =

Theoretical DT)

3. No mixing, hence suited for uniform (municipal wastes)

Compressed air aeration

1. Fine bubble aeration: Bubble size is 2mm to 8mm diameter, through

porous material. Ridge & Furrow systems, Porous Plate, aeration dome etc.

2. Course bubble aeration: Bubble size greater than 8mm, Better agitation,

but poorer oxygenation effect through perforated pipes.

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OC ( Oxygenation Capacity)

Oxygenation capacity of a given system is defined as the rate of increase of

oxygen concentration with time at STP [1atm & 10ºc] in kg O2 / hour

OR ( Oxygenation Rate )

Oxygenation rate of a given system is defined as the rate of increase of oxygen

concentration under design Temp and pressure in kg / hour

OC = K 2 Cs*

OR = K T (CST – CL)

K2 Reaeration constant at STP

K T Reaeration constant at DTP

CL Minimum O2 Concentration in tank [≈ 2 mg / l]

Design details:

Data: QD , QH , Min working temperature (Design)Si , Se ,M, Ls and SVI

L = Si* QD X 10-3 (Kgs/day)

Se = 20mg/l

E = 100*

* X

S i

SeS i −

I Aeration Tank Volume

Choose Ls, MLSS as per requirements

Take density of sludge (SVI) as 100 ml/gm

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Total dry weight of ASP – Ws

Ws - s L

L(kgs)

VAT -M W s (m3) or M L

L s

(m3)

Check for tAT =

D

AT

Q

V

Recirculation:

QD Se

QR Ps


SVI =a

v

X

P 1000

)/(

)/(

l mg

l ml

Pv – Volume concentration (MLSS) in ml/l

Xa – weight concentration (MLSS) in mg/l

SVI = ml/gm

Pv

Xa= MR (gm/l) =

SVI

1000

M – MLSS concentration in AT in gms/l or Kg.m3

MR - MLSS concentration in return sludge

MR =SVI

1000

R % = 100 X Q

Q

D

R

QR MR = (Q+QR ) M = QM+QR M

26

Ps

FSTAT



QR (MR -M) = QM

Q

Q R =M M

M

R −

R% = 100 X M M

M

R − = M SVI

M

−1000

100

= 3

3

10.1

10..100−

−

− SVI M

SVI M

M – Kg/m3 , SVI –

ml/gm

Aeration Requirements:

Kr s – O2 uptake rate for substrate respiration. (Kg O2 / Kg BOD)

Kr e – O2 uptake for endogenous respiration. ( Kg O2/Kg MLSS day)

Krn - O2 uptake for Nitrification. ( Kg O2/Kg day) ( All are temp. dependent)

O2 consumption / day – R od

R od = Kr s 100

E L + Kr e Ws + Kr n L NOx

(Maximum hourly BOD factor – 15)

Roh = 15

1

(---,,---) + 24

1

(--- ,, ---) + 24

1

(---,,---)

OC = K 2 Cs*

OR = K T ( CST – CL )

OR

OC

= )(

*

2

LST T

S

C C K

C K

− T K

K 2

= T D

D10

D – Diffusion co efficient for O2 at a given temperature ( cm2/sec)

OC = OR T D

D10 )(

*

LST

S

C C

C

−

OR =α

oh R

α - Gas transfer efficiency (0.6 to 0.8)

27



OC (Kgs/hr) = α

oh R

T D

D10)(

*

LST

S

C C

C

−

HP =2.1

oh R (hp)…………… empirical

For Municipal wastes:

Kr s –0.1 to 0.5

Kr e – 0.15 to 0.2

Krn – 3.5 to 4.5

Excess sludge production:

Ps (Excess sludge) Kg MLSS/day = [1.2 Ls0.23 100

E

L] Kgs/ day

[1.2 takes care of the unit per day]

Sludge age or Mean cell residence time = s

s

P

W

days

For conventional ASP – 3 – 4 days

For Extended Aeration – 20 – 25 days

Contact stabilization process:

Error: Reference source not foundAeration in return sludge could be 30 mts

This will enrich the biomass and hence the efficiency of the system.

28

PS

T

FS

T ∞

∞



Operational problems:

(i) Rising of sludge:

This is due to excessive nitrification in aeration tank and denitrification in AT or

FST. If the sludge in FST is not removed in six hours, it may lead to sludge overshooting.

This could also happen due to dead pockets in the aeration tank due to improper working

of aerators or voltage fluctuation.

(ii) Bulking of sludge:

This is due to growth of filamentous protozoa in activated sludge resulting inincrease of SVI (> 300 ml/gm). This is due to toxic shock loads from industries (mainly

heavy metals) or substrate shock loads. This can be controlled temporarily by

chlorinating the return sludge with 5 to 10 mg/l chlorine. On long term the shock loads

should be controlled.

(iii) Foaming:

Foams occur in the aeration tank due to excess grease (>10 mg/l). The foam may

create in aerosols and carried for long distances with pathogens. On short term this could

be controlled by spraying defoamers ( like Alkyl benzynesulphonate) and by proper

grease removal.

Monitoring of ASP:

1. BOD

2. MLSS

3. SVI

29



TRICKLING FILTER ( BIO FILTER )

Attached growth system with the following

1. centrifugal action2. Adsorption and forming zoogleal film with micro organisms.

3. aerobic system with 1/3rd void space(media 4 “ or 10 cms)

Advantages:

No artificial aeration is needed, hence lot of saving on energy. Little excess

sludge is produced and hence FST could be smaller.

Maintenance is easier and cheaper. But first cost is very high and requires more

land space. There may also be problems of psychoda flies in a conventional TF. A

conventional TF with no recirculation works on a low F/M ratio.

Designs:

BOD volume loading - Lv

Lv = TF V

L

0.1 to 0.15 daym

kgBOD3

Depth of filter – D eff - 2 – 2.5 m

Media 4 “or 10 cms

30

PST

TF FST



Vitrified clay rings of 10 cms have also been used. Hydraulic head need for

distribution is 0.6 m above TF surface. These may have occasional psychoda flushing.

Ponding is due to clogging of the filters. Ponding leads to anaerobic conditions in the

filters, odour problems and reduction in efficiencies. If the influent BOD (pre settled)

exceeds 400 mg/l, it may lead to clogging needing dilution by recirculation.

High rate TF with recirculation:


In a high rate trickling filter presettlement is necessary to prevent clogging.

a. surface loading (hydraulic)

SL = 1.2 to 1.6 m/h

)/7.0( hm ATF

QP SL >==

b. BOD volume loading

LV =TF V

L

d m

KgBOD3

c. Recirculation ratio

R% = 100×Q

QR

I Design of HRTF

E reqd = 10*

*×

−

i

ei

S

S S

E = y La+1

100or LV =

2)1100(

−

a

E

For a given E get LV

31

Q

Si



a Si (mg/L)0.4

4

0.4

0

0.3

7

<= 300

~ 350

- 350-400

Given L, calculate VTF

Choosing DH varying from 2.5 to 4m

Get ATF - check

LATF daym

KgBOD2

4≤

SL hm /7.0≥ < 1.6 m/h

Operation problems:

i. Bulking – spray chlorine in return flow, control shock loads.

ii. Psychoda flies – check recirculation and anaerobic condition due

to clogging.

iii. Maintenance of arms and nozzles.iv. Cleaning media once in six months and replacement of media once

in five years.

The TF works similar to extended aeration system with sludge age as 30 to 45 days.

32



STABILIZATION PONDS (OXIDATION PONDS )

Types of ponds:

i. Fully aerobic (shallow ponds) meant for algal cultivation – 30 cm.ii. Facultative – depth 1.2 m to 1.5 m with 30 cm to 40 cm anaerobic.

–waste stabilization.

iii. Maturation ponds – depth 1m used as polishing pond.

lato Visible radiation (langlays)Max min

50

4030

20

10

26

66126

182

225

07

2470

120

162


33

Performance of Stabilization Pond

Influent

Waste water

Inorganic

+organic

AerobicAnaerobic

Bacteria Algae

NO3

, PO4

, CO2 NH

4, CH

4, CO

2



Algae – C106H180O45 N16P1

(Euglema, spirogyra and blue green algae)

By dry weight – 52.5% C, 9.2% N, 1.3% P.

So the waste should have this minimum CNP.

106 CO2 + 90 H2O + 16 NO3 + PO4 C106H180O45 N16P +154O2

So theoretically for every gm of algae 2 Gms of oxygen is produced.

In practice for every gm of algae 1.7 Gms of oxygen is produced.

Average radiation = min + [(max – min) x SCF]

(Cals/cm2/day)

Conversion efficiency – 4 to 6% of visible light energy.

Energy in algae – 6000 cal/gm.

Designs:

Si – INF BOD gm/m3

Se - eff BOD gm/m3 [60 mg/L]

BOD (ult) removed – (Si – Se) gm/m3

O2 required (Si – Se) QD gm/day.

QD - average design flow.

P1 = wt of oxygen released

Wt of associated algae

Total weight of algae required = (Si – Se) QD = (al) gms/day

P1

Energy for algal formation – 6 K Cals/gm

Total energy required = [(al) 6000] cals/day

= [E] cals/day

Average radiation received in Dec-Jan – {min + (max-min) SCF} cals /cm2/day

(This is in the northern hemisphere and it should be taken as may-June if it is

northern hemisphere).

34



SCF – sky clearance factor in Dec-Jan

= [x] x 10 cals/hac/day

Consider energy conversion 6 to 8%

Net effective radiation – 0.06 [x] 108 cals/hac/day.

Area of pond required =

810[x]0.06

/)( daycals E hectares.

Volume of pond =

× 75.1

[x]0.06

E m3

DT (Days) =Volume of pond

DQ

Min DT = 1/0.11 say 8 to 10 days(for avoiding flushing of cells.)

CONSTRUCTIONAL FEATURES OF STABILIZATION POND

USING PUDDLE CLAY BED

35

Puddle Clay : Clay + Coarse aggregate

Puddle Clay Bed (70 cm)

1.75 m

F.B - 0.25 m

1 m

1 : 2.5 1 : 1.5

Slope

Sand Cushion (30 cm)

Grass

Turfin

g

Pre Cast Slabs



CONSTUCTIONAL FEATURES OF STABILIZATION POND

USING LDPE ( LOW DENSITY POLYETHYLENE) SHEETS

OPERATIONAL PROBLEMS

Sulphides in ponds:

Due to anaerobic condition at the bottom of the pond, Sulphates are reduced to sulphides

by desulpho vibrio. By this algal inhibition takes place if S2- > 4 mg/L. Further S2- will

consume DO in the pond with excess of S2- algae will disappear and pink sulphur bacteria

(Beggiatora) will appear which give no oxygen.

Average S2- in mg/l at25oc = [0.000158(Kg BOD 5 / ha day)- 0.001655(t days) =0.0553] x

S2- mg/L

(Ref: JWPCF- Feb 79)

Sludge accumulation:

Wet sludge(m3/cap/Yr)

The sludge must be removed once in two years to

avoid raising of sludge.

Algal harvest:

1. Rice husk filters.

36

(Using LDPE sheets for Bed)

1.75 m

1 m

Sand Cushion (20 cm)

GrassTurfing

Pre Cast Slabs

0.25 m



2. Coagulation

3. Centrifugation

4. Coagulation using natural coagulants likes nirmali, red sorella seeds.

Other Operation problems in stabilization ponds:

. Heavy metals.

2. Sulphides in ponds.

. Raising of sludge.

4. Grease and oil

Maturation ponds

Polishing ponds with one meter depth where water hyacinth (eichornea crassipes)

is cultivated. This is a water weed with 95% water by volume. Hence it may lead to loss

of water. Under favourable conditions, each hectare of hyacinth can remove 30 to 40 Kgs

of nitrogen and potassium, 10 to15 Kgs of phosphorous, and 3 to 4 Kgs of magnesium

from effluents. In Indian conditions 250 to 400 tons (dry weight) per hectare of hyacinth

could be cultivated. Hyacinth could be harvested (mechanically or labor intensive

means), comminuted (chopped or crushed) and bio digested with nitrogen supplement for

60 to 80% CH4 which has a calorific value of 5300 Kcals/m3 and this could be upgraded

to 7900 Kcals/m3 by lime scrubbing.

One hectare under ideal conditions can yield 900 – 1800 Kg/day which can be a calorific

equivalent to 400 to 500 liters of petrol. The sludge is a good fertilizer.

37



Land disposal and Irrigation

1. Irrigational use2. Infiltration to ground water 3. Storing in basins for long term evaporation

Irrigational use is recommended in semi arid where rainfall< 500 mm/yr. In India there are 150 sewage farms with 15,000 hectares.

Kinetics of land application

Land application apart from supplying water will be conditioning soilwith nutrients.

38



Disadvantages:

1. Health hazard both direct (farmers) and indirect (food chain).2. If waste water does not match with soil condition, may result in

damage to soil ( N2 robbing due to excess carbon, soil build up).3. Heavy metal etc are phyto- toxic.4. Excessive land requirements.5. Odour and fly problem.6. The water demand may be seasonal.

Salt build up:

I – effective field capacity in mmCi – average salt concentrationin effluent

Cs – concentration in leachate and soilC p – concentration in ppt (negligible)

P – ppt , L – leachate

39



By water and salt balance

[Cs / Ci ] = [ 1 / (L / I+P) ]

Cs / Ci = salt build up ratio

L / I+P = leaching ratio (fraction)

Typical value of leaching ratio

0.1 to 0.2 in clay0.2 to 0.3 in normal soil

0.3 to 0.4 in sandy soil

if leaching ratio is 0.2 Cs = 5Ci

TDS or Ec( Electrical conductivity)

TDS mg/ l 0.64 Ec µmhos / cmIn general water with Ec < 750 µmhos is good for application.

Crop Ec permissibleµmhos / cm

Grass 1800Wheat 1200Veg. 800fruits 400

Sodium content:

This affects soil by cation exchange (with Ca++ and Mg++) resulting in break down of soil. The water become less porous and will have lesswater holding capacity (field capacity) and aeration (soil sickness).Hence lime is added when excess of Na+ (salt) content.

% Na+ = 100 (Na+ ) / Na+ + Ca++ + Mg++ + K +

40



R.S.C = Residential sodium carbonate

(defined alkalinity)

RSC = (CO3

--

+ HCO3) – (Ca++

+ Mg++

) all in mg/l

RSC Condition

< 1.25 Safe1.25-2.5 Marginal> 2.5 unsuitable

Sodium Absorption Ratio (SAR)

SAR = Na+ / √(Ca++ + Mg++) /2 (all in meq.)

SAR (meq./l) Hazard to soil0-10 Low

10-18 Medium18-26 High> 26 Very high

Thus we can find suitability of application of these wastes. For correctionwe can add lime gypsum.

% NaCondition

20 Very good20-40 Good40-60 Medium> 60 unsuitable

41



Anaerobic Lagoons

Anaerobic lagoons generally provided prior to aerobic unit in case

of high BOD loadings. They help reducing BOD by 40 – 60%. Anaerobicactivity also helps in nurturing the nature of solids by liquid fraction.

Disadvantages:

Odour, high start-up period, highly sensitive to inhibitors likeheavy metals. A buffer zone of 1000 m is recommended for habitation.

Stages:

1. liquefaction and acid formation by acedogens.Acetic, butyric, propionic and volaric acids.

2. Methane formation by methanogens.

These are obligate anaerobes. Any trace of pH is inhibitive. Highlysensitive for temp. they are thermophillic bacteria and no methane takes

place below 150 C.

K s at T 0C = 0.002 (1.035) T-20

K s - system rate coefficient

Methanogens are highly sensitive to pH.

Below pH 7.5 no methane formation occurs.

(Ks (per day) at 30 0C = 0.145 / day)

DT =(loge (Si /Se)) / Ks

Ex. Si – 1000 mg / l, Se – 500 mg / l, (50% removal)

DT = (loge (1000/500)) / 0.145= 4.5 day

For 40% removal

42



DT = (loge (1000/600)) / 0.145=3.5 days

DT beyond 5 day are not recommended as the pond tend to turnfacultative reducing methane formation.

Pond temp 0C DT (days) ף (BOD)

15 - 20 0C 5 days 30%

20 – 25 0C 3 – 4 days 40 – 50%

25 – 35 0C 3 days 50 – 60%

Anaerobic lagoons of 2.5 m to 4 m are constructed. However 3 m arevery common.

Example:

Data:

Population = 5000Q = 120 lpcdDesign temp. = 25 0CBODinf = 1200 mg / l

Facultative pond loading – 150 kg / hec / day

Ks = 0.145

Estimate land saving by having an anaerobic lagoon prior to facultative pond. The anaerobic lagoon is designed for 50% - 40%

Case 1:

43



BODinf = 1200 mg / lBODeff = 600 mg / l

DT = (loge (1200/500)) / 0.145=5 days

Volume of lagoon = 5000 x 120 x 10-3 x 5= 3000 m2

Depth = 3mArea = 1000 m2 = 0.1 hectares

Area of stabilization pond alone = ( 5000 x 120 x 10-3

x 1200 x 10-3

) / 150= 4.8 hectares

Stab. pond + anae. lagoon = ( 5000 x 120 x 10-3 x 1200 x 10-3 x 0.5) / 150

= 2.4 hectares

Total = 2.4 + 0.1 + 2.5 hectares

Land savings = 48%

Xxx

Aerated Lagoons

44



Mechanically aerated lagoons are earthen basins 2.5 to 3 m deep, provided withsurface aerators installed on floats (Poly urathene foam). Raw sewage is feddirectly after screening. The D.T. could be 1 to 10 days, and hence smaller tanksare needed.

There are essentially three types.1. Facultative type2. Aerobic flow through type3. Extended aeration type

1.Facultative type

These are akin to the algal ponds except that oxygen is now derived from

mechanical aeration instead of algal photosynthesis.

FB

Baffle

The power input would be sufficient for diffusing enough oxygen into the liquid, but not sufficient for maintaining solids in suspension. These settle in the pondand undergo anaerobic digestion.

2.Aerobic flow through type lagoon:

Baffle

45

Anaerobic

Aerobic

Eff Inf

Aerobic

Eff Inf



Here the power input is high enough to keep all the solids in suspension as in anactivated sludge process, but no attempts are made to hold back the solids andthey flow through the effluent. Efficiency is not very high unless attempts are

made to hold back solids in SST.

3.Extended aeration type

Similar to above but solids are retained and a part of solids in recirculated.

Fac Flow – through Ext – aerationSolids build up 30-150 mg/l 30 to 300 4000 – 5000

Power Low More MoreSludge Accumulates in pond

Solids in effluent Stabilized sludgewith drawn

DT 4-10 days 2-6 days 1-2 daysDepth 3-5 m 3-5 m 3-5 mη 75-85% 70-80% 95-98% Nitrification -No- -No- GoodMisc Simple to operate

More landEff / good qualitySuitable for Sewage

farming

Simple to operateMore power billsEff / suitable for specific purpose

only

More trained personnel neededMore power billLeast land suitable

for Ind & municipalwaste

Design of facultative aerobic lagoon

46

Recirculation Excess Sludge



Population = 70,000Si = 300 mg/lSe > 70 mg/lQD = 12,400 m3/d

Waste water Temp. (min.) - 10o

CColiforms - 106 / 100 mlK L (20oC) - 0.6 per. day

(K L is BOD removal rate in the lagoon)

Aerators – 2 kg O2/KWH at STP (OE)field Capacity - 0.75 (STP)

DT - 8 daysLagoon volume – 12,400 x 8 = 99,200 m3

62

400

Depth – 4m

Dispersion No

D = 30.8 W

_D_ = _Dt_ = (30.8 x 31) (8 x 24)UL L2 (400 x 2)2

= 0.3

BOD removal rate at 10oC

K L (10o) = 0.6 (1.035)10-20 = 0.42 / day

K Lt = 0.42 / day x 8 = 3.36

From Chart S = 0.23So

BOD η = 77%

47



Actual BOD in eff = 0.23 x 300= 69 mg/l

Power required in winter:

O2 required per day= 0.77 [1.4 (300) x 12,400 x 10-3]

= 4010 Kg / day

= 334 Kg / hr

Power required = _______334________ = 220 KW0.75 (2 Kg O2/KWH)

= 165 HPIn summer power required will be more due to more BOD removal. This has to

be calculated winter criteria is important for Lagoon size. Summer criteria isimportant for power calculations.

Sludge accumulation:

Cleaning Period - 5 yearsSludge Vol - 0.05 m3 / cap / yr x 5 x 70,000

= 17,500 m3

Provide additional 1m for sludge.

Land required

Net area = 400 x 62 = 24,800 m 2

Gross area will be 30% more.= 31,000 m2 (3.1 hectares)

= 31,000 = 0.478 m2/cap70,000

(Stab. pond – 3.7 m2 / cap. (ASP – 0.2 m2 / cap)

Coliform removal

K b = 1.2 at 20oC or 0.21 at 10oCK bt = 0.21 x 8 = 1.68from chart Removal = 70%Hence for MPN (103) chlorination is required.

48



Design of aerobic flow through lagoon:

Q, So Solids – X mg/l Q, S, XVol - V

θc = t = V (mean cell residence time)Q

At steady state:

Y (So-S) Q - K d (XV) = XQ (Kg/hr) Net solids produced Solids leaving

Y - Yield coefficientK d - BOD degradation rate (Per day)

K’ = deoxygenation constant (per day)

Data

Population - 70,000Si - 300 mg/l

Se > 70 mg/l

QD - 12,400 m3/dWaste temp (Min) - 15oCK 1 - 0.015 per day at 20oCY - 0.5

49

Y (So-S)X = 1 + K dt

1 + K dtS = YK’t



K d - 0.07 per day

Assume DT - 3 daysLagoon Size - 12, 400 x 3 = 37,200 m3

Depth - 4 m

Lagoon area = 37,200 = 9300 m2

4= 100m x 93m

K’15oC = K’20

oC (1.035)T-20

= 0.015 (1.035)15-20 = 0.0126/day

BODeff in winter = 1 + K dt = 1 + (0.07) (3)YK 1t 0.5 (0.0126)3

= 64 mg/l (Soluble)

Solids in lagoon

X = Y(So-S) = 0.5 (300 – 64) = 97 mg/l1+K dt 1 + (0.07)3

Final BOD eff = 64 + 97 = 161 mg/l.

Efficiency with FST = 300 – 64 = 79%300

Efficiency without FST = 300 – 142 = 52%300

Hence FST is required.

O2 required per day = 0.52 { 1.4 (300) x 12,400 x 10-3 }

= 3000 kg / day = 130 Kg/hr

Power required = ___130 Kg/hr__ = 87 KW0.75(2 Kg O2/kwh)

= 120 HP

50



ROTATING BIOLOGICAL CONTACTORS (BIODISCS)

These are attached growth system where in pretreated wastewater for grit and

grease removal is treated in a facultative tank with 3 to 4 hours hydraulic retention

period. Large size discs made of inert material rotate at a slow speed of 3 to 4 rpm

wherein adsorption of the colloidal matter taken place forming zoogleal media. The

adsorbed media slough off as bioflocs which are sedimented either in the same tank or in

a final sedimentation. The discs are kept 2/3 submerged so that air is drawn is to keep the

system aerobic

There are over 2000 installations in Europe. Could be plain surfaced, corrugated

or slotted. Relatively low power and land requirement. But has more mechanical parts

and so operation and maintenance is difficult. More suited as a package unit for

recirculation with tube settlers.

51



Zoogleal film layer develops on the disc and sloughens as sludge. The speed

could be 3 to 6 r.p.m. Diameter of the discs 3 to 4 m. Plastic, asbestos, PVC or

polystyrene discs are used. Shaft diameter – 6 yo 8 cm. Spacing 2 to 5 cm (clear)

Loading g BOD/m2 d BOD removal (ή)

6 to 10 90 %

20 to 25 80 %

Minimum DO desirable – 2 mg/L

Effluent BOD (S) mg/L

Dr. Poepel’s data

Submergence – 2/3 d

Q (So –S) = Ka (S)1/2 A

A – Cumulative area of discs m2

So – Influent concentration mg/L

S – Effluent concentration mg/L

Dr. Poepel’s dataDr. Poepel’s data

Slope – 0.53 ± 0.03

1 10 100 1000 1500

50

100BODremovalrate (ra)(gms BOD/

m2

day)

52



Q (So – S) in gms/day

g Ka

daym=

3

gmsS

m=

If we know value of Ka

22

3 2 2 o

mg gms KaA KaAS or S

L m Q Q

= − ± +

Design a bio disc for 1000 persons to remove 80% BOD using 3m dia. discs

(plain surfaced) spaced 5 cm c/c.

Temperature expected – 20OC

Organic loading – 20 gms/m3 day

BOD5 – 54 gms/cap/day

Flow – 200 lpcd

QD = 200 m3 / day

54 1000270 /

200

xSo mg L= =

1. Disc area required = 254 10002700

20

xm=

2. Area per disc =2

22 144

d x sides m

π =

3. No. of discs =2700

19514

=

53



4. with spacing as 5 cm, tank length is 195 x 5 = 9.75 m say 10 m.

Tank width with clearance – 3.2 m

Tank depth – 2 m (2/3 is submerged)

Tank volume – 62.4 m3

5. Detention time = 3

62.47.5

200 /

V hours

Q m d = =

6. Hydraulic loading on disc =3 3

2

2

200 / 1074 /

2700

m dxl m day

m= −

7. Assume power required at 10 kWH / cap / year =

10kWH/cap-year x 1000 persons=1.5HP

24 hr x 365days

So install a 2 HP motor for disc rotation. It has been claimed that there is a

30% power saving compared to ASP.

8. Check if ή assumed - 80 % is okay.

22

2 2 o

mg KaA KaAS S

L Q Q

= − ± +

22

(2.3)2700 2.3 2700270 49 /

2 200 2 200

xmg L

x x

= − ± + =

ή =270 49

81%

270

−= Hence OK.

9. Excess sludge ( ; 0.6 kg / kg BOD removed)

= 0.6 [200 (m3/d) (270-49) x 10-3] = 26.5 kg/d

At 1% solids concentration = 2.7 m3/d

54



SLUDGE TREATMENT AND DISPOSAL

The cost of sludge disposal is about 30 to 40% of total cost of treatment. It involves involume reduction by dewatering, stabilization and destruction of pathogens.

Origins of sludge are (1) primary sludge (2) secondary sludge from biological or chemical treatment

The quantity varies as per 1. domestic and social habits2. dietary habits – vegetarian system gives for more sludge than protein based3. Sewerage system: combined system produces more sludge than separate.4. Industrial wastes.

Quantity of secondary sludge depends on the process

Conventional ASP per kg BOD: 0.8-0.9 kg DSConventional TF per kg BOD: 0.3 – 0.5 kg DSHigh rate TF per kg BOD: 0.8 – 0.95 kg DSEAP per kg BOD: 0.3 – 0.5 kg DS

Sludge Analysis:

1. Dry residue or total dry solids.2. volatile solids3. percentage ash (metals)

55



4. Settling rate: in a graduated cylinder of 6 cms diameter, the volumetric measuresof settled sludge per liter after 6 hrs.

Characterization of sludge:

1. pH should be about 7 or slightly alkaline.2. volatile acids calorimetric tests3. Alkyl benzene sulphonate test: to measure detergents. More than 1.5 % ABS will

impair digester performance.4. Heavy metals such as Cd, Ni, Cr, Cu, Zn, Hg, Pb can interface with performance.5. NPK and Ca values for fertilizer value6. pathogen including worms7. calorific value – important if to be incinerated8. centrifugability for dewatering

Disposal of sludge:

1. Agricultural use: by spraying digested wet sludge or flesh activated sludge.Alternatively sludge is dried and mixed with soil as cakes. Domestic sludge may be more suitable. Industrial sludge should be checked for toxicity in particular heavy metals.

2. Land Filling: sludge should be fairly dry or mixed with refuse. Care should be

taken about leaching and general ground water pollution. It can also be mixedwith fly ash from thermal power station.3. sludge can be incinerated, but air pollution should be controlled.4. Marine discharge; widely practiced in many countries including UK. But if toxic

chemicals like Hg are discharged, it may accumulate in fish etc and end up infood chain. Further, the reduction in DO level in water due to discharge should bewatched.

Anaerobic Sludge Digesters: Acid formers-acidogensMethane formers - strict anaerobes.Reproduction rate of acid formers is very fast (one to two hours) compared to methaneformers (two to three hours)

Cysteine is the first stage of acid production

4C3H7O2 NS + 8H2O 4CH3COOH + 4CO2 + 4NH3 + 4H2S + 8H(cysteine)

56



CH3COOH 5CH4 + CO2 + H2O +8H

Overall reaction4C3H7O2 NS + 6H2O 5CH4 + 7H2O + 4NH3 + 4H2S

70C to 200C – psychrophilic – 12 to 25 weeks200C to 400C – mesophilic – 20 to 30 days400C to 650C – thermophilic – 3 to 5 days

There are four important parameters in digestion1. temperature2. time of digestion3. correct feeding (no inhibitors)4. proper seeding and mixing

Digesters are designed for Kg VSS/ m3 of tank. (in the mesophilic range – 2.5 KgVSS/m3 ). If the total contribution of sludge is 80 gm/cap/day with 65% VSS and 95%m/c (5% DS), we will have 1.5 l/cap/day and a detention time of 30 days.

Heating and mixing if necessary:

Steam injection: efficient but expensive internal heating with hot water coils. Mixingwill be convection (not an efficient method)

Stirrer with heating coils: efficient, but we have to depend upon many mechanical parts.

Recirculation of gas: we can have an external heating by heat exchangers outside.We can combine external heating with gas circulation for mixing. Advantage is easier mixing and maintenance.

Sludge thickeners: dewatering is generally done by agitation and sedimentation.

Gravity thickening:

1. Fill and draw: three to four tanks in series, we can hopper buttons and desludgewith hydrostatic pressure.

2. Continue thickeners: provided with a stirrer and a scrapper (one revolution per hour) detention time is about 15 hours.

3. Flotation method; used for secondary sludge like ASP. Fine bubble aeration liftssludge to surface. Final m/c – 5% with detention time (DT) 30 mts to 60 mts.

4. Dewatering on sludge drying beds: more suitable for tropical countries and whereland is not expensive. Coarse gravel 5 to 10 cms, coarse sand – 2 to 5 cms and

57



sludge depth – 15 cms. Free board – 30 cms surface scrapping of dried sludge can be done manually or mechanically.

% water Characteristics88 Fluid limit for pumping

70 Plastic60 Rigid50 Stable lumps40 Appears dry25 Dry cakes

Sludge conditioning:Chemicals like lime, ferrous, Sulphates or polyelectrolyte could be used for conditioningfor mechanical dewatering. The presence of ammonium bicarbonate in sludge increasesthe demand for these chemicals. This is reduced by elutriation which is washing the

sludge with water twice the volume of sludge. This reduces the chemical demand by 70%and increases specific resistance. This is done after thickening and before mechanicaldewatering.

Mechanical dewatering:

Has no effect on pathogens. Done either by vacuum filtration or pressure application.

(a) Vacuum filtration

Works at 0.8 to 0.9 atmospheres. The vacuum is applied inside, which sucks the liquidleaving the solids on the strainer made of polyethylene or polythene. The solids are

scrapped.

(b) Rotary concentrators:

Consists of long cylinders with strainer sludge is introduced at one end and as the unit isrotated the water is lost through the strainer.

(c) Pressure application in bag filters:

Preconditioning of sludge is essential. Sludge is pumped through strainer bags and theseare pressed to squeeze the water out. The pressure applied is 60 to 100 psi (4 to 6kg/cm2). It is a batch process and it gives a solid of 455 DS.

(d) Centrifuge:Sludge conditioning by polyelectrolyte is essential. Centrifuge speed should be optimumto avoid shearing.

G = 5.5 N2 d107

G = gravitational force

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N= rpmD= av Φ in mm

G should be less than 10,000 to avoid shearing.

Sludge digesters:

The organic solids are liquefied and gasified by acid formers and methane formers. Thedigested sludge with about 40% m/c is further dried.

The optimum temperature for digestion (mesophilic) is 30OC to 35OC and it takes 30 to50 days for digestion. In thermophilic range it takes less than 10 days.

Mixing is done thoroughly in digesters to distribute the incoming sludge, to reduce scum,to maintain uniform temperature. Power drivers mechanical mixing devices are used.

General circular tanks of 6 to 12 m depth with hopper bottom are designed. In India, theaverage sludge production is taken as 35 to 60 l/c. it could be fixed or floating dome type.

Loading Factors:

Based on1. kg volatile solids added per day per m3 digester capacity or 2. kg volatile solids added per day per kg volatile solids in the digester.

For conventional digesters 0.5 to 1.6 kg/m3 d of volatile solids could be used.For high rate digesters it could be 1.6 to 6.4 kg/m3 d.

Population basis:

Type of plant Dry solids(gm/cap/d)

Volume(m3/1000cap/d)

Volume required35-45d (m3/d)

primary 72 1.44 50-65Primary + TF 108 2.70 95-122

Primary + ASP 114 3.8 133

59



OPERATION AND MAINTENANCE OF BIOLOGICAL

TREATMENT PLANTS

INTRODUCTION

Studies of waste treatment facilities have shown that any inadequacy in the

design, staff organization, or operation and maintenance (O and M) has invariably led to

a waste of capital, man-power and energy. Wise use of personnel according to well

organized O and M program can conserve treatment efficiency at a minimum total cost.

(1) The basic requirements of successful operation and maintenance of wastewater

treatment plants are:

(i) a thorough knowledge of the processes and equipments,

(ii) proper and adequate tools,

(iii) assignment of specific maintenance responsibilities to operating staff,

(iv) training of all operating staff in proper operating procedures and

maintenance practices,(v) efficient quality control through laboratory tests,

(vi) maintenance of records on operating efficiency of different units.

(2) The importance of plant operation comes into focus when we realize that it is the

culmination of all preceding efforts aimed at for control of water pollution. A

60



plant which may be over loaded or has other short comings can produce its best

results if competent and careful operating techniques are applied. The word

“Operator” is applied to anyone charged with the responsibility of the operation

of the treat5ment plant. Operation of wastewater treatment plants is greatly

affected by the motivation and training of the individual operator.

This training must include basic knowledge of unit processes, the plant

equipments, how to recognize potential trouble, now to diagnose, and to make

temporary repairs, and finally to know as to where to obtain additional assistance.

“Voluntary Certification” and “Mandatory Certification” is required for rating the

operators ability in operation of plants and at the same time classifying plants as

to the grade of operator ability required.

(3) The certificate is indicative of the knowledge, experience, and competency of theoperator. Certification programs give the operator improved status, greater

flexibility in changing jobs, and opportunity for higher wages. Certification

usually provides benefits to the municipal or industrial employer by increased

efficiency in plant operators and maintenance, more reliable reports and record,

and more confidence in the operator’s recommendations for repairs and

improvements.

To prevent our wastewater treatment system from being perpetuated as

“Monuments to inefficiency” the key of operation and maintenance gap must be

closed.

(4) Availability of operation and maintenance manuals for each facility enables the

plant operators to check for specific problems and apply corrective measures.

In this article the problems associated with the operation and maintenance of

different wastewater treatment units are described.

(a) Activated Sludge Process

“Activated sludge” describes a continuous flow, biological treatment systems

characterized by a suspension of aerobic micro-organisms (MLSS) maintained in a

relatively homogenous state y the mixing and turbulence induced in conjunction with the

aeration process. Basically, the activated sludge process (ASP) used micro-organisms in

61



suspension to oxidize soluble and colloidal organics in the presence of molecular oxygen.

During oxidation process, a portion of the organic materials is synthesized into new cells.

A part of the synthesized cells then undergo auto oxidation (self-oxidation or endogenous

respiration) in the aeration tank. Oxygen is required to support the synthesis and auto-

oxidation reaction. To operate the process in a continuous basis, the solids generated

must be separated in a clarifier, the major portion is recycled to the aeration tank an

excess sludge is withdrawn, from the clarifier under flow for additional handli8ng an

disposal, and there are different modifications in the ASP system viz, conventional, high-

rate, extended serration and contact stabilization process. The operational variables in an

ASP include, rate of water flow, air supply, MLSS, aeration period, DO in aeration tank

and clarifiers, rate of sludge return and condition of sludge.

The most important control parameters in activated sludge system are the DO inthe aeration tank and the MLSS. It is desirable to maintain a minimum aeration tank DO

of about 1 to 2 mg/L. If the DO levels are not controlled, localized regions may become

saturated with oxygen or super saturated with nitrogen or carbon dioxide. These

conditions lead to adsorption of fine bubbles on the flock, causing poor setting and

possible flotation. Activated sludge can have poor settling characteristics because of (1)

poor bio-flocculation, (2) excessive bound water, (3) small gas bubble entrainments in the

floc, (4) growth of type of bacteria or fungi (filamentous organisms) that have a large

surface area compared to their mass, (5) excessive amounts of hexane soluble oils and

grease. To maintain the required MLSS in the aeration tank the sludge settled in the

clarifier is returned to the aeration tank through sludge return pumps. The return flow is

to be adjusted such that it is approximately equal to the percentage ratio of the volume

occupied by the settleable solids from the aeration tanks effluent to the volume of the

clarified liquid (supernatant) after settling for 30 min. in a 1000 ml graduated cylinder.

This ratio should not be less than 15 percent at any time. For example, if after 30 min of

settling, the settleable solids occupied a volume of 150 ml, the percentage would be equal

to 17.7 percent [(150ml/850 ml) x 100]. Another method often used to control the rate of

return sludge pumping as well as plant operation I based on an empirical measurement

known as the sludge volume index (SVI). This index is defined as the volume in

62



milliliters occupied by one gram of activated sludge (MLSS), dry weight, after settling

for 30 min in a 1000 ml graduated cylinder.

MLSSof ml/l

1000xsludgesettledof ml.SVI =

Sludge under 100 SVI will settle well. Sludge which does not settle well or settles andcompacts poorly, leaving a small amount of clear supernatant is called bulking sludge. (6)

Bulking sludge is an operational problem that occurs in higher loaded plants with

insufficient aeration, presence of toxic substances in the influent, frequent organic shock

loads containing exceptionally high amount of carbohydrates. Bulking results from

increased growth of filamentous bacteria and with resultant poor settling of the MLSS in

the final clarifier. To correct this condition rapidly, the filamentous organisms, because of

their large surface area to volume ratio, can be selectively destroyed by large doses of chlorine or hydrogen peroxide. The latter is more effective because it has less deleterious

effect on the desirable organisms. Sludge bulking can be controlled by cutting off

aeration tank in-flow and re-aeration the sludge for at least 6 to 8 hours before re-

commencing the plant flow.

(b) Trickling filter

Biological process used for the treatment of wastewater can be classified as

suspended growth system of fixed film systems. Fixed film systems provide surface area

for the growth of a zooglean slime. This slime or film contains the major portion of

micro-organisms that provide treatment. The fixed film systems with stationary media are

known as tricking filters, trickling filters contain a stationary medium providing surface

are and void space allows air and wastewater to pass through the medium and co me in

contact with the micro-organisms in the film. The organisms utilize the oxygen and

material in the wastewater for their metabolism. Developments in the design and

operation of trickling filters, historically, have been popular because of their ability torecover from shock loads and perform well with a minimum of skilled technical

supervision. The problems associated with trickling filters are mainly distribution,

clogging, ponding under drains, odour and filter files.

63



All clogged spray nozzles or orifices in the revolving distributors should be

cleaned as soon as clogging is noticed. Dosing tanks should be kept free from

accumulation of deposits. All parts of the filter bed should receive equal loading.

Periodical tests should be carried out using water tight pans of standard size 90 cm X 120

cm set flush with the top of filter media and end to end along the radius. The media

surface shall be dived into two concentric circles with the area of the inner being 10

percent of the total area covered by the distributor. The sewage collection in the pan for

10 revolutions of the distributor when the air is still is measured*. The rate of distribution

should not vary ±5 percent from the mean rate of distribution in the inner 10 percent area.

Pools or ponds some times form on the surface of the filter. This is due to organic growth

or retained organic matter from poorly settled waste. Some times, this is due to careless

dumping of fine materials in one place at the time of placing filter media. In many cases,forking or raking the media to a depth of 20 to 30 cm will effectively remove ponding.

Washing the filter media with a jet of water or giving rest to the filter for 2 or 3 days may

also be effective. Pre-chlorination of the waste or application of caustic soda up 10 mg/L

has also been tried with success to eliminate clogging and ponding problems. When using

chemicals treatment may be given for 8 hours period on alternate days.

Filters under drains should be inspected frequently for clogging. If clogging

evidenced by reduced flow from any drain, this should be flushed and cleaned with sewer

rods.

Psychoda filter flies sometimes infest the filter and cause not only nuisance to the

workers but also clog the beds.* Application of chlorine at a rate of 3 to 5 mg/L or

gammexane at a rate of 180 gm/ha or DDT at a rate of 3 to 10 kg/ha of wall surface once

in a week are the methods available for the flushing of the larvae. Adult flies are

controlled by pyrethrum spray.

(c) Sludge Digestion Tanks

For startup, the digester tanks with fixed covers should be filled initially with

water, sludge or sewage to expel air. In tanks with floating cover, the cover should be

brought down to the lowest point before filling of the tank is commenced. In order to

64



reduce initial lag period, raw sludge mixed with digested sludge in the ratio of 2:1 to 4:1

may be pumped into the digester so that alkaline digestion starts within few days after

loading. The addition of fresh sludge should commence only after this stage. If digested

sludge is not available, raw sludge along should be pumped and kept for 2 to 3 weeks

before the digester can be loaded. Open digester can be charged directly.

The raw sludge feeding rate should be such that the volatile solids in the digester

should not exceed 3 to 5 percent so that digestion is not inhibited. Generally a loading

rate of 1 to 2 kg of fresh solids to every 40 to 50 kg of digesting volatile solids should be

the ratio to maintain a uniform digestion rate.

Where the digesters are equipped with mixing devices they should be operated in

accordance with the manufacturer’s instructions, where facilities for recirculation by

pumping exists, they should be used for mixing digester contents breaking down scum,mixing lime with sludge for pH adjustment etc., where there is no mixing and re-

circulation facility, the operator has to rely upon natural mixing of raw and digested

sludge in the digestion tank.

Digestion is generally carried out in the mesophilic range and the temperature of

the sludge generally varies from 25OC to 35OC. Thermometers, to record temperature,

should be dept in order and reading noted twice or thrice a day.

Sludge should be withdrawn from the digesters only when it is fully digested,

judged by the dark grayish brown color without visible raw sewage sludge solids. Sludge

should be sampled and tested to find out the condition before withdrawing. Generally not

more than 10 percent of the capacity of digester should be drawn at a time, sludge

withdrawn being limited by the capacity of the sludge drying beds.

Frequent pH test of the sludge should be made and this should be correlated with

the alkalinity of the supernatant of the sludge which may range from 1500 to 3000 kg/L.

This affords an excellent check on operation. Digestion proceeds most favorably at pH

values of 7.0, it is usually desirable to raise the pH be adding lime to the sludge as it is

enters the digester. The alkalinity of the supernatant is a useful guide to control the

dosage. A start may be made using 20 to 40 kg of lime per m 3 of sludge, with more added

if the pH value or alkalinity does not rise appreciable in a few days.

65



Difficulties in the digestion tanks such as foaming due to overloading or

accumulation of acid sludge or excessive formation of H2S have to be corrected by

neutralization an adjustment of pH. H2S in moist gas leads to corrosion of meters piping

and flame trap through which the digester gas is drawn. This can be overcome by the

removal of the H2S by passing the gas through iron oxide or other scrubbers or by heating

the gas to a high temperature to eliminate moisture in it

Gas pipes should be kept free from sediments, gas meters being periodically

lubricated and fusible plugs in the flame traps frequently checked.

(d) Anaerobic lagoons

Anaerobic digestion in lagoons is a mixed culture bacteria process consisting of

two phases (a) oxidation of organic matter to organic acids like acetic, butryc and proponic acid and (b) fermentation of these organic acids into methane and other gases.

The former process is accomplished in the digester by facultative anaerobic bacteria

which are abundant in nature. The second phase, methane formation is brought about by

methane bacteria, which are strict anaerobes. Anaerobic lagooning, as applied in the

treatment of wastewater represents the controlling application of a process which occurs

under natural conditions when organic matter decays in the presence of water but absence

of air. If the object is to operate the lagoons at the maximum efficiency, it is necessary tokeep the environment ideal for the bacteria to grow and oxidize the organic matter.

Operation and Maintenance – Anaerobic lagoons

1. The anaerobic lagoons should be filled with fresh water and fresh cow dung

should be added and allowed to digest for a period of 30 days. The quantity of

cow dung (4 percent suspension) required is about 10 percent by volume of the

lagoon. Slowly waste may be fed and pH maintained between 6.8 and 7.8. In

about a months time lagoon will be ready for regular operation. This

commissioning of the anaerobic lagoon should be started one month before

starting of the prior units.

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(e) Extended Aeration (ASP) plants

1. The first step in order to put the aeration tank into regular operation is to build up

enough microbial solids. Mixed liquor suspended solids (MLSS) concentration

desired to be maintained in the tank will be between 3000 to 5000 mg/L with an

average of 4000 mg/L. The steps to be followed for developing the solids are

given below.

2. The tank may be filled with wastewater up to the operation level. Once the tank is

filled up to that level the aerators will be switched on to aerate the tank contents

for 24 hours. After aeration for this period, the aerators will be cut off and the

tank contents will be allowed to settle in the tank for about 2 hours. The

supernatant, in the tank, will then be drawn off. Therefore, raw wastewater will be

admitted to the tank to the operational level and the process described above will be repeated every day till such time that the MLSS concentration in the tank is

attained to a level of about 4000 mg/L. This process of building up the solids in

the tank will take 2 to 4 weeks.

3. Once the solids level is reached to about 4000 mg/L, the tank is then ready for

continuous inflow of wastewater enters, there will be overflow from the tank

entering into the settling tank. Depending upon the design of the settling tank, it

may take abut 2 to 3 hours to fill the tank. The sludge in the settling tank will betaken to the return sludge pump through a valve. Opening of the valve should be

controlled to effect an average flow of the incoming of wastewater. The return

sludge is pumped continuously and put back into the tank. While this process is

on, the liquid level in the settling tank will rise up slowly as the wastewater

continuously flows into the aeration tank. The overflow from the settling tank will

constitute the treated effluent. As stated earlier aerators and the return sludge

pumps in the treatment plant should be operated continuously even when there is

an intermittent flow of wastewater into the aeration tank.

4. With continued operation of the aeration tank and the return sludge, the MLSS

concentration will increased until the capacity of the system to settle and retain

the solids is exceeded. For this reason, it is necessary to withdraw a small fraction

of the solids in the mixed liquor and dispose of it separately. This excess sludge is

68



withdrawn from the settled sludge which is normally returned to the aeration tank.

The volume of the excess sludge to be wasted per day is usually between 0.26 and

0.43 percent of the wastewater inflow. Instead of wasting sludge everyday. It may

be accumulated for 3 to 4 days and then wasted in one lot.

5. The excess sludge should be spread to a depth of 8 to 12 inches and allowed to

day for sufficient time. Generally about 10 days drying period is required before it

can be spaded out. If experience indicates more time for drying, then additional

drying beds will have to be provided. The sludge, after it is dried sufficiently, will

be collected and used as manure. Just prior to rainy season, some extra sludge

may be wasted from the system so as to provide a buffer capacity for storage of

excess sludge during monsoon season when the sludge on the drying beds may

not loose moisture readily.6. The day to day operational requirements may range from regular lubrication of

bearings and greasing of reduction gear units of aerators. Also the recording of

such parameters as flow measurements, suspended solids and other characteristic

may be carried out regularly.

7. Occasional samples may be withdrawn and got analyzed at a convenient

laboratory to help to maintain records of performance of the installation.

Representative samples should be taken, preserved properly during transport and

tested.

8. Tests for BOD on the influent and effluent, and for suspended solids on the

influent and effluent, mixed liquor suspended solids and return sludge should be

carried out regularly once in a week. Frequency of sampling can be increased if

desired. Occasional samples may be collected and analyzed for the nutrients

(Phosphates, ammonia, nitrates, nitrite etc.) in the influent and effluent.

9. It is always necessary to maintain proper records. This would help to evaluate

performance of the aeration tank and help to furnish useful date when further

expansion of the plant is contemplated.

10. W well maintained plant with properly cleared surroundings and a small patch of

garden with appropriate landscaping can convert a waste disposal plant into a

pleasant spot, which would be a credit to the authorities. Plants like coconut, palm

69



and eucalyptus may also be planted so that it can be natural screen or filter for any

unpleasant odors.

(f) Oxidation ponds or Waste stabilization ponds

Oxidation ponds or waste stabilization ponds are no longer a novelty.

Considerable experience has accumulated over the last three decades in the design,

operation and maintenance of these ponds and the Public Health Engineers are now able

to use them with confidence as a simple and reliable means of treatment of sewage and

certain industrial wastes at a fraction of cost of conventional waste treatment plants

hereto used.

As simple as a septic tank, and yet as effective as a complete sewage treatment

plant it is relatively a new tool in the hand of Public Health Engineers awaiting wider application of these ponds, particularly in the tropics where sunshine is plentiful and

money is scarce, for the treatment of industrial waste also. The term “Waste Stabilization

Ponds” has been more widely adopted as it is more descriptive of the real function, and

includes aerobic as well as anaerobic modes of stabilization. The word “waste” includes

both sewage as well as industrial wastes. Neither the term “Oxidation Pond” nor the term

“sewage lagoons” include all these aspects.

This process involves two steps in the decomposition of the organic matter present in the wastewater. The carbonaceous matter in the effluents is first broken down

by the aerobic organisms with the formation of carbon dioxide. The carbon dioxide so

formed is utilized by algae during photosynthesis which liberate oxygen. This oxygen

dissolved in water is used by the aerobic bacteria for further oxidation of organic matter.

Operation and Maintenance

After the stabilization pond is construction is completed and the pond bottom is

cleared of all loose debris and vegetation, raw waste may be gradually allowed to enter

and fill the pond upto a depth of 15 to 30 cm (6 to 12 inches) only. Each day thereafter

only a small quantity of raw wastewater may be admitted to maintain the above level, till

such time as algal growth establishes itself naturally. Soils generally harbor the spores of

various algae and spontaneous growth of algae is likely t take place within a week or two

70



showing visible green to dark green growth. After the first algal bloom has established

itself further raw waste may be admitted gradually till the entire pond is filled up. The

pond may be then being allowed to rest for 2 or 3 days to ensure that algal growth has

firmly established itself. The pond is then ready for continuous inflow of wastewater.

Where algal cultures are readily available or considered desirable to use for

certain reasons, artificial addition of small quantities (about a bucketful) of algal

suspension obtained from any nearby village tank or well may be spread over the pond

surface to hasten the establishment of a healthy algal bloom.

Where artificial addition of nutrients like nitrogen or phosphorous is required for

treating an industrial waste deficient in the same, additional facilities for dosing of

chemicals have to be provided. The nutrient addition should be started as soon as the raw

waste is first admitted to the pond and continued throughout its operation.Operation and maintenance of oxidation ponds is a matter of “good house

keeping”. Odour and color of the lagoons should be noted daily. Accumulation of

floating scum, dead algae in the corners of the pond as well as the grass and weeds

growing at the water marine should be cleaned properly and frequently. Waste

stabilization pond condition and the preventive steps to be taken, if they are faulty, are

given below.

Overloading of Ponds

Stabilization ponds can withstand to a certain extent fluctuations on the waste

load. But as far as possible sudden or extreme variation in the characteristics of the waste

entering the pond should be avoided. In case where anaerobic conditions are found to be

developing due to overloading, measures like pond surface agitation (by pump

recirculation or motor boat operation) and addition of chemicals like sodium nitrate may

be adopted. However overloading should be avoided by studying the causes for the same

or by expanding the size of the pond to take care of the increased load.

Pond conditions and preventive steps.

S.No. Observation Pond condition Preventive steps1. Bright green color to some

depth below surfaceVery good

Only remove marginal andfloating scum accumulation

71



2. Blue green in color Good

Algamats and scumaccumulation to be removed

3. Green or blue color onsurface to small

Tending to beover loaded

If overloading is temporary, by pass the flow to reduce load on pond remove floating matter

4. Surface pink, red, or greycolor Tending to beanaerobic

May be due to the presence of sulfur bacteria. By pass part of the influent until normalcy isestablished.

5. Grey or dark brown oilyappearance of the surfacewith dark grey floatingsolids

Overloaded andanaerobic

If temporary by pass part of theinfluent permanent, remediessuch as cleaning and recommissioning the pond are to be taken.

Corrosion control in wastewater treatment plants

The problem of corrosion is more severe in case of waste treatment plants. This is

due to the fact that the nature of the liquid to be handled in this case is more corrosive

(low resistivity and chemically more active). It contains solids which are more likely to

cause abrasion in pump components thus removing the protective coating and

accelerating the corrosion process. The waste with low pH and containing sulphides from

the reduction of Sulphates will promote corrosion. Since there are many mechanical and

electrical components or equipments in case of wastewater treatment plat than in the caseof water treatment plants the aspect of corrosion in more serious. The cost of mechanical

and electrical equipment in a wastewater treatment plant can be anything between 30

percent and 45 percent of the total cost.

A wastewater treatment plant generally consists of screen and grit removal,

primary and settling ranks, sludge digesters, biological treatment processes like trickling

filters, activated sludge plants or its modifications and the carious pumping units and

piping etc. In case of certain industrial wastes neutralization tanks for acid or alkaliwastes, equalization tanks with pre-aeration facilities or mixing facilities are susceptible

to corrosion.

Neutralization and Equalization Tanks

72



Where there are batch process discharging acidic or alkaline wastes are required

to be equalilsed to balance out fluctuations in quality and quantity. These equalized

wastes have to be mixed continuously throughout the period will the effluents are to next

process of neutralization. In such cases the equalization as well as neutralization tanks

may have to be provided with acid resistant lining of tiles or bricks. The chemicals used

for neutralizing should be stored in acid or alkali resistant containers and fed to the tanks

through PVC piping.

Sedimentation tanks

The primary sedimentation tanks handle raw waste which is allowed to settle at

the bottom. The settling tanks are generally provided with mechanical scrapers to divert

the sludge to a zone to facilitate continuously or periodical withdrawal of sludge. The

scraper arms are constantly immersed in wastewater and are subjected to corrosion.

Since, sewage and most of the industrial wastes have much lower resistivity that water,

the parts are likely to corrode much faster than in the case of water. The specifications for the steel used for the under-water mechanisms should be carefully drawn to ensure

maximum protection from corrosion. It is normally specified that all the steel below the

liquid level shall be at least 1/4” thick. It is a good practice to keep all chains, bearings or

brackets above the liquid surface. All castings in the driving mechanism should be high

grade cast iron.

It is possible to give cathodic protection to the scraper mechanism of the clarifier

either by sacrificial anode or by impressed current. The choice of either of the method or

cathodic protection will depend upon the comparative costs. In any case, the cost of such

protective measures will not be higher than the cost of good quality acid resistant paint.

Sludge digestion tank

73



In the sludge digestion tanks, digestion is carried under anaerobic conditions. The

wastes containing appreciable contents of Sulphates, under anaerobic conditions will be

reduced to sulphides forming hydrogen sulphide. The corrosion due to hydrogen sulphide

is in fact due to sulphuric acid formed in presence of moisture. This will attack digester

walls, digester dome and mechanical equipment to such an extent that breakdown may

occur, ultimately. It is recommended that the cement resistant to attack by H2S, such as

blast furnace slag cement, should be used in the construction of digesters.

It is observed that the draft tubes inside the digesters are sometimes provided of

mild steel. This is not a good practice since the life of such metallic tubes is limited in

highly corrosive interior. Hume or concrete pipes of thicker cross section are re

commended for use as draft tubes. Use of guy rods inside the digester should be

discouraged.

Activated sludge plants

In the activated sludge plant oxygen is provided to the waste either by compressed

air system or by surface aeration system. In the compressed aeration system the clogging

of porous filter material is or frequent occurrence. Clogging can be on either side of the

filter. Air side clogging may be due to corrosion inside the compressed air supply line or

dust drawn by the compressors. Dust in air can be eliminated by provision of proper air filters. Clogging due to corrosion can be minimized only by the use of air supply

pipelines of non-corrosive material.

In the surface aeration system, the conditions in the aeration tank are more

conductive to corrosion since in addition to the corrosiveness of the liquid, oxygen I

present to aggravate the situation. Proper material selection and coating are therefore

necessary for protection of the exposed parts of the rotor. It may be mentioned here that

the protective coating has to be applied at regular intervals since it is found such coatings

have very short life. If floating aerators are provided, it is desirable to have corrosion

resistant lining, such as fiberglass, for the floats.

Trickling filters

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In trickling filters the mechanical components include the header, the distribution

arm and distribution nozzles, the header and distribution arms are normally of mild steel

and should be protected from corrosion by proper painting etc. The corrosion and

resulting blockage of distribution nozzles are of common occurrences. This can be voided

by selection of proper corrosion resistant material such as brass or PVC for nozzles.

Sewage and wastewater pumps

For pumps and pumping equipment, painting is the usual protective measure.

Both the interior and exterior surface or pumps should be painted after rust scale and

deposits are removed by and blasting, wire brushing or rubbing with sand paper.

Preventive maintenance

It will be seen from the above discussion that anticorrosive paints, coatings

linings have to be used in various equipments to prevent corrosion. The paints, coatings

and linings require periodical renewal. Proper maintenance demands that a scheduling be

drawn up in each paint so that the operator may abide by it and undertake repainting or cleaning at appropriate intervals without waiting for corrosion to become obvious. The

accent should be on preventive maintenance rather than just maintenance in the form of

repairs, and replacements of broken down parts.

No doubt, proper design and specifications at the tendering stage would go a long

way in ensuring long life. However, once the plant is built it is entirely in the hands of the

operator or the supervisor to ensure proper preventive maintenance and carry out

judicious replacement of spare parts piping etc. bearing in mind the requirements

Documents

Waste Water Treatment Notes