ENVIRONMENTAL ENGINEERING

343

Lecture 10:

Wastewater treatment 2- primary treatment

Philadelphia UniversityFaculty of Engineering

Department of Civil EngineeringFirst Semester, 2013/2014

TYPICAL COMPOSITION OF UNTREATED DOMESTIC WASTEWATER [METCALF AND EDDY, 1991].

Concentration (mg/lt.)

Contaminants Weak Medium Strong

Total Solids (TS) 350 720 1200

Total Dissolved Solid (TDS) 250 500 850

Suspended Solid (SS) 100 220 350

Settleable Solid 5 10 20

Biological Oxygen Demand (BOD5) 110 220 400

Total Organic Carbon (TOC) 80 160 290

Chemical Oxygen Demand (COD) 250 500 1000

Nitrogen (total as N) 20 40 85

Organic 8 15 35

Free ammonia 12 25 50

Nitrites 0 0 0

Nitrates 0 0 0

Phosphorous (total as P) 4 8 15

Chlorides 30 50 100

Sulfate 20 30 50

Alkalinity (as CaCO3) 50 100 200

Grease 50 100 150

Total coliform (no/100 ml) 106-107 107-108 107-109

COMPOSITION OF WW RECALL

99.9 % Water

0.1% Solids

Solids are divided into Organic and inorganic

solids

Inorganic(30%)

Organic (70%)

Suspended Organics

(Colloidal –protein)

Dissolved Organics

(Carbohydrates)

Dissolved Inorganics(metals, salts,P,N)

Suspended Inorganics

(Grit)

Secondary Treatment

Dissolved Organics and Suspended

Organics

Primary Treatment

60% SS; 30% BOD and Grit

Tertiary Treatme

nt

RECALL Composition of WW

Suspended Solids

Biodegradable Organics

Pathogens

Body waste, food waste rags, paper, biological cells

Soluble organics

Bacteria, virus etc.

Protein (40-60%) – amino acids

Carbohydrates 25-50% - sugars starch, cellulose

Lipids (10%)- fats, oils and grease

Contain Carbon – exert an oxygen demand

Recall - Wastewater Treatment Plants

Municipal treatment is divided into: Primary, Secondary and Tertiary

Primary Treatment – removes solid materials from stream-Large debris may be removed by screens or reduced in size by grinding device.

Inorganic solids are removed by the grit chamber

Much of the organic suspended solids are removed by sedimentation

Primary treatment removes 50% SS and 30% BOD http://www.thewatertreatments.com/waste-water-treatment/primary-treatment-sewage-

treatment-waste

SCREENING - CHARACTERISTICS

Screening is the first operation performed on wastewater.

The purpose of screening is to remove coarse solids e.g. sticks, rags, boards, napkins etc. which can clog valves and pipes and damage pumps.

Screens- Placed in rectangular channel- Properly ventilated to prevent

accumulation of gases- A straight channel ahead of the

screens to provide distribution of flow

- Flow velocity 0.3m/s and not greater than 1.0m/s

Coarse-Vertical bars >1cm- Inclines away from flow- Cleaned manually by

raking for small plants- Mechanically cleaned in

large plants

Fine- Woven wire cloth- Or perforated plates

mounted on a rotating disk or drum or on a travelling belt

- Mechanically cleaned on a continuous basis

Screening

Screened solids are caught with raw sewage:- Promptly disposed in sanitary landfill- Grind and returned to wastewater flow- Incineration

Screening

Screening

http://www.chishun.com.tw/ebarscreen.htm

-Located across the flow path and intercepts the coarse solids and shred them to approximately 8mm in size.

-Comprise of a screen and cutting teeth-High maintenance equipment and provisions should be made

to by-pass flow during repairs. Small plants use screens as by-pass while large plants install comminutors in parallel.

Comminutor

http://water.me.vccs.edu/courses/ENV110/lesson13b.htmhttp://nsm1.nsm.iup.edu/tsimmons/Environmental%20Health%20Photolibrary/ENVH_Photolibrary_Wastewater_Images.shtm

Grit Chamber

The objective of the Grit Chamber is to remove inorganics, and some larger organics e.g. pebbles, sand, silt, egg shells, glass, metal fragments, bone chips, seeds, coffee and tea grounds.

Why we remove Grits?- Grits are abrasive in nature and will cause wear on pumps- Grit deposits in pipes, sumps and clarifiers can absorb grease and solidify.- They are non-biodegradable and occupy valuable space in the digester

What are the sources of Grits- - The major contributor is infiltration and depends on the type, age and

condition of sewerage systems- Industrial waste- Domestic garbage grinders

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Aerated Grit Chamber

How does it work? Used in large treatment plants. Turbulence is created by the injection

of compressed air which keeps lighter organics in suspension while heavier grit falls to the bottom.

Advantages- Separation of non-target organics is done via roll velocity rather

than horizontal velocity. Therefore manual control of horizontal velocity is not necessary. Adjustment of air quantity is used for settling control.

- Aeration removes anoxic gases which are present when wastewaters enters in an anaerobic state.

Flow measurement does not remove contaminants, however, knowledge of hydraulic loading rates is critical to the operations of the reactors.

Hydraulic loading influences operation parameters as air flow, recirculation rates, infiltration rates, inflow quantities

Flow Measurement

Methods of Flow Measurementsa. Digital Flow metersb. Parshall FlumeThe Parshall flume consists of a

metal or concrete channel structure with three main sections: (1) a converging section at the upstream end, leading to (2) a constricted or throat section and (3) a diverging section at the downstream end

http://www.fao.org/docrep/R4082E/r4082e06.htm

How does the system work?• Primary sedimentation is a unit operation designed to concentrate

and remove suspended organic solids from wastewater. • Most of the SS in wastewater are “sticky” in nature and flocculate

naturally. Primary settling proceeds as type-2 settling without the addition of chemical coagulation, mechanical mixing and flocculation operation.

• Organic matter is slightly heavier than water and settles slowly 1.0 to 2.5m/h. Oil and grease float to the surface and must be skimmed off.

• Scum is skimmed off and sent to the scum tank and disposed with screen material, grit or digested sludge

• Sludge should be removed from the primary sedimentary tank before anaerobic condition develop. Gas bubbles would adhere to the solid particles and lift them towards the surface. Therefore it should be removed 30 min to 60 min after settling

• Hydraulic detention time in sedimentation basin range from 1.5-2.5 hrs under average flow conditions . A 2.0 hr detection time is typical

Primary Sedimentation Tank

Primary Sedimentation Tank

Design Equations:1. Overflow rate, q =

Q/A(surface_area)

2. Detention time, td = Vvolume of tank /Qflow rate

Homework Design the a primary sedimentation tank for San

Fernando WWTP if the average flow 2.0 million gallons per day and 4.5 Million gallons per day at peak flow. Assume a tank depth of 3.0 m and assume the use of one tank and assume the use of two tanks. Comment of the results.

http://apesnature.homestead.com/chapter17.html

http://www.thewatertreatments.com/waste-water-treatment/primary-treatment-sewage-treatment-waste

What are the Objectives?Effluent from primary treatment still contains 40-50% original

suspended solids and virtually ALL dissolved organics and inorganics

The removal of organics both suspended (colloidal) and dissolved is called secondary treatment.

How are these objectives achieved?The use of biological processes is the most cost effective method

used to treat municipal wastewaters.

Microorganisms use organics in wastewater as a food source and convert them to biological cells (Biomass)

Because wastewater contains a wide variety of organics, a variety

of organisms or mixed culture is required for complete treatment.

Secondary Treatment Processes

Efficient treatment then depends on understanding the requirements for optimal growth as well as recognizing unfavorable conditions.

Typically, a domestic wastewater prior to entering the treatment plant will contain from 100,000 to 1,000,000 microorganisms per milliliter. 

While all microorganisms found in wastewater treatment plants have some role in the decomposition of wastes, probably the three most significant microbial groups are the bacteria, fungi, and protozoa. 

Bacteria have the primary role of decomposing wastewater compounds, forming settleable solids, and at times are the source of operational problems. 

The general group called fungi are significant since many operational problems are caused by members of this group. 

Protozoa are microorganisms that play a key role as predators and help control the bacterial populations. 

Microorganism require Energy source (biosynthesis, maintenance) & Carbon source (Growth of microorganisms)

Microorganisms in Wastewater

General information on the classification of Microorganisms

• Nutritional Requirements: On the basis of chemical form of carbon required, microorganisms are classified as – Autotrophic: organisms that use CO2 or HCO3

- (bicarbonate ion) as their sole source of carbon.

– Heterotrophic: organisms that use carbon from organic compounds.

• Energy Requirements: On the basis of energy source required, microorganisms are classified as – Phototrophs: organisms that use light as their energy source. – Chemotrophs: organisms that employ oxidation-reduction

reactions to provide energy. They are further classified on the basis of chemical compounds oxidized (i.e., electron donor)

• Chemoorganotrophs: Organisms that use complex organic molecules as their electron donor.

• Chemoautotrophs: Organisms that use simple inorganic molecules such as hydrogen sulfide or ammonia as their electron donor.

Microorganisms

• Temperature Range: On the basis of temperature range within which they can proliferate, microorganisms are classified as – Psychrophilic: organisms whose growth is optimum blow 20°C. – Mesophilic: organisms whose growth is optimum within 25 to

40°C. – Thermophilic: organisms whose growth is optimum within 45

to 70°C. • Oxygen Requirements: On the basis of oxygen

requirement microorganisms are classified as – Aerobes: organisms that use molecular oxygen as electron

acceptor. – Anaerobes: organisms that use some molecule other than

molecular oxygen as electron acceptor. – Facultative organisms : organisms that can use either

molecular oxygen or some other chemical compound as electron acceptor.

Microorganisms

Growth Pattern of Micro organisms • When a small number of viable bacterial cells are placed

in a close vessel containing excessive food supply in a suitable environment, conditions are established in which unrestricted growth takes place.

• However, growth of an organism do not go on indefinitely, and after a characteristic size is reached, the cell divides due to hereditary and internal limitations.

Growth and food utilization may follow a pattern similar to as shown in figure:

Microorganisms

Stationary phase: Maximum growth cannot occur indefinitely. Food becomes limiting, overcrowding, waste products build up. Due to limiting food, cells begin endogenous catabolism while other cells die. Eventually rate of growth = rate of death

Endogenous phase: Endogenous catabolism continues but death of cells dominate and biomass slowly decreases.

Microorganisms

Lag Phase: is a period of adaptation of the microorganisms to their new environment and food- null growth rate.

Log-Growth Phase: Once the microorganism becomes accustom the growth phase begins and proceeds exponentially. Biological cells grow and reproduce. Regeneration can occur in as little as 20 minutes.

What factors affect the rate of biomass production?

Rate of biomass production and food utilization is a function of pH, Temperature and toxins

- Reaction rates increase within the range of 0 to 55 degrees, and can double with every 10 degree increase in temperature. Excess heat can destroy organisms.

- Microorganisms functions best in neutral pH conditions, but their tolerance ranges from pH 6 to 9

- Factors such as toxins (poison MO), salt concentration (affect internal/external cell pressure) and oxidants (destroy enzymes) influence growth.

- They are capable of adjusting to a wide range of environmental factors provided it occurs gradually.

Microorganisms

Reactors Types of Biological treatment Systems

Continuous-flow Reactors

Suspended Growth

Ponds and Lagoons

Completely Mixed without Sludge

Recycle

Plug Flow with sludge recycle

(Activated Sludge System)

Completely Mixed with Sludge

Recycle

Activated Sludge System)

Attached Growth

Trickling Filter

Batch reactors

Activated Sludge System

A biological treatment process in which a mixture of sewage and activated sludge is completely mixed and aerated (sufficiently). In this process the microorganisms are mixed thoroughly with the organic compounds under conditions that stimulate their growth through us of organic compounds as foods. As the microorganisms grow and are mixed by the agitation of the air, the individual organisms clumps together (flocculate) to form an active mass of microbes (biological flocs) called activated sludge Air is continuously injected to mix the activated sludge to supply the oxygen needed for the M.O to break down the organic matter.The mixer of Activated sludge and the wastewater in the aeration tank is called Mixed Liquor. The mixed liquor flows from the Aeration Tank to a secondary clarifier where the activated sludge is settled out

• The activated sludge (Biological Solids) is subsequently separated from the treated sewage by settlement and 22 to 30 % is re-used (called return sludge) to maintain high population of microbes to permit breakdown of organic matter.

• The remaining sludge is discarded (called waste activated sludge)

• The activated sludge is controlled by balancing between waste activated sludge and return activated sludge.• If too low, less effective• If too high, overflow the secondary tank and flow into the

receiving streams• Sludge age/solids retention time (SRT) the average amount of

time that microorganisms are kept in the system.• 8 m3 required for each m3 wastewater

In conventional activated sludge systems, the wastewater is aerated for 6-8 hrs ; in long, rectangular aeration tanks

Activated Sludge System –

Activated Sludge System – Completely Mixed with Sludge Recycle

Two mass balances are required to define the design the reactor (biomass and food)1: Biomass influent + net biomass growth= Biomass effluent + biomass waste

2:Food influent –food consumed = Food effluent +Food waste

Trickling Filter consist of a bed of coarse material, such as (stone or plastic materials) over which wastewater is applied

Stones are used to increase the surface area for biofilm growth (microorganisms) that biodegrade the waste.

Under high organic load, the void space between the rocks plug causing flooding, and failure of the system restriction of the amount of oxygen available for microbes Oder and filter flies

Part of the liquid effluent may be recycled to the trickling filter for additional treatment or to maintain hydraulic flow rates

WASTEWATER TREATMENT PLANTS

TRICKLING FILTER

CLARIFIER

WASTEWATER TREATMENT PLANTS

Sludge Digestion Sludge from primary treatment as well as the

secondary clarifier are sent to the sludge digestion unit where it is undergoes anaerobic degradation.

The results are: • Methane – Used for power generation• CO2 • Liquid – contains high organic – recycled through

the plant• Inert solids – contains high minerals – soil

conditioner/ fertilizer on agricultural lands

SLUDGE DIGESTER

Oxidation ponds / Waste stabilization pond / sewage logons Self purification

Aerobic Facultative Anaerobic Aerated

Disinfection: Mainly by chlorine

WASTEWATER TREATMENT PLANTS