WATER QUALITY CONTROL. WATER TREATMENT.

WASTE WATER TREATMENT.

Two components to water quality:

1. Safe drinking treatment of surface or subsurface water for consumption

2. Safe release treatment of municipal sewage and industrial wastewater

Water quality concern:

1. Health of people who drink the water avoidance of cholera, typhoid fever, gastroenteritis, etc.

2. Aesthetics: water color, hardness, taste, odor

3. Quality of water in the environment: dissolved oxygen, salt content, habitat

Drinking water standards: Primary and Secondary

Primary standards - health-related criteria

Secondary standards aesthetics (such as taste, odor, and color) and nonaesthetic (corrosivity and hardness)

Drinking water standards: primary standard

1) Synthetic organic chemicals (SOCs) are compounds used in the

manufacture of a wide variety of agricultural and industrial products

(insecticides, herbicides);

2) Volatile Organic Chemicals (VOCs) are synthetic chemicals that

readily vaporize at room temperature (carbon tetrachloride; 1,1,1,-

trichloroetahne (TCA); trichloroethylene (TCE) and vinyl chloride).

3) Disinfectant byproduct (DBPs) are the byproducts formed when a

disinfectant reacts with chemicals in the water to form a toxic

product. Trihalomethanes (THMs) are the byproducts of water

chlorination. The most common solution is to remove the DBP-

forming compounds from the water before the disinfection.

4) Radionuclides concentrations in drinking water are expressed in

Pico curies per liter (pCi/L). 1 pCi = 2.2 radioactive decays per

minute (1 Ci is the decay rate of 1 gram of radium). Radon occurs

naturally in some groundwater, inhaled radon gas is thought to be a

major source of lung cancer.

Water treatment (drinking water):

Disinfection of effluent to eliminate harmful pathogens. Hardness removal is optional.

Screening: removal of large floating /suspended debris, grit and sand.

Coagulation adding chemicals and agitation to promote suspended solids to form/coagulate into larger particles.

Flocculation - gentle mixing of water with chemicals to form larger flocks.

Sludge processing -mixture of solids and liquids collected from the settling tank is dewatered and disposed of.

Treatment for surface water

Settling/ clarification remove particles that settle out by gravity

Filtration removal of particles and floc by gravity settlement

Treatment for groundwater

In general, underground water is much clearer from particulate matter compared to surface water, therefore the main steps in the treatment include: 1.Aeration (removes excess and objectionable gases) 2.Flocculation (precipitation) to bind Ca and Mg ions 3.Sedimentation (gravity settling of particulate matter) 4.Recarbonation readjust pH and alkalinity 5.Filtration, disinfection, and solids processing

1. Sedimentation

During this treatment step, the particles are simply allowed to settle due to gravity effect. Particles may have very different, irregular shapes and when describing particles, an equivalent to a sphere' diameter is used. It is hydrodynamic diameter (in water) and aerodynamic diameter (in air).

The drag force is a function of the particles Reynolds number: where =density of water; vs = particle settling velocity; dp=particle hydrodynamic diameter; =absolute viscosity of water.

For the laminar flow: Re < 1 For transition flow: 1 < Re < 10 000 For turbulent flow: Re > 104

psdRe

In most environmental engineering cases, only laminar flow is considered, therefore: FD=3 vsdp The gravitational force: FG=mg=Vppg; The buoyancy force: FB=mwg=Vpg; where g=gravitational acceleration (9.8 m/s2); m=the particle mass; mw=the mass of the water displaced by the particle; Vp=particle volume=dp

3/6; p=particle density; =density of water. The balance of forces on a particle when it is at its terminal velocity is FG = FD + FB Substitution of the above equations results in the simplified form of Stokes law:

1. Sedimentation

18

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1. Sedimentation

Figure shows the trajectory of a particle that is being carried by the horizontal flow of water from one end to the other of a rectangular settling basin. The particle settles at distance, hp: where =the hydraulic detention time of the basin; Vb=the basins volume; Q=the volumetric flow rate of water through the basin.

In order to achieve higher efficiency of particles settling, the settling velocity (vs) should be equal or greater than a critical settling velocity (vo): where Ab=the surface area of the rectangular basin.

Q

Vvvh bssp

bbb

oA

Q

hA

hQ

V

hQhv

1. Sedimentation

The critical settling velocity is also known as surface loading rate or overflow rate. If we want to design a clarifier to remove all particles of a size, d, from a water stream with a flow rate, Q, the surface area, Ab, of the rectangular basin must be The same formula is applicable for circular settling basin too. However, the influent enters circular settling basins at the centre and the overflow rates are within 1.0-2.5 m3/m2 h. Also the detention time influences the efficiency of the clarifier. The hydraulic detention time in any tank is its volume divided by the influent flow rate (typically from 1 to 4 hours).

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2. Coagulation and Flocculation

Particles that are too light or small require longer time period to settle. Many of such particles are colloids (0.001 to 1 m).

These particles have overall charge and repulsion is present between the particles that prevents aggregate formation; therefore a chemical agent is required to stimulate the floc formation.

Coagulation is a chemical treatment that destabilizes particles surface making them sticky so the particles adhere to each other and consequently removed by settling and filtration.

Flocculation, sedimentation, filtration are physical treatment processes. The usual coagulant is alum Al2(SO4)3*18H2O; also FeCl3, FeSO4, and

polyelectrolytes are used. The overall reaction: Al2(SO4)3*18H2O + 6HCO3

- 2Al(OH)3 + 6CO2 + 18H2O + 3SO42-

alum aluminium hydroxide sulfates

If it is necessary to increase pH then Ca(OH)2 or Na2CO3 is added.

Coagulants are added to the raw water in rapid mix-coagulation tank; the detention time is about minute.

Flocculation is followed up and provides gentle agitation for about hour, during this period, the precipitating aluminium hydroxide forms floc.

Parameter, G, the mixing intensity is used by engineers to maximize the rate of collisions between particles, without the breaking up the flocs.

where P=the power input to the paddles; Vb=the volume of the vessel; and =the viscosity of water.

2. Coagulation and Flocculation

bV

PG

3. Filtration It is a very common procedure with a few well established techniques in

use. The rapid depth filtration: the filter consists of layers of carefully sieved

materials such as sand, anthracite coal, diatomaceous earth and a bed of graded gravel.

Processes involved: adsorption, flocculation, sedimentation. Such filters are cleaned by backwashing the medium.

The typical filtration rates (va), (loading rate or superficial velocities) are between 5 25 m3/ m2h).

The filter efficiency (production efficiency): f = (Vf Vb Vr) / Vf The effective filtration rate: ref =(Vf Vb Vr) / Af*tc where Vf=volume of water filtered per filter cycle (m

3) Vb=volume of water used to backwash the filter (m

3) Vr=volume of water used to rinse the filter after backwash (m

3) tc=duration of time for a complete filter cycle (hr) Af = filters cross-sectional area

va= Q / Af

4. Disinfection Disinfection is to kill any pathogens and prevent pathogens from growing

in the treated water.

Free chlorine is the most common disinfection procedure. It involves any of the following compounds: gas Cl2, sodium hypochlorite (NaOCl), calcium hypochlorite (Ca(OCl)2).

For Secondary disinfection, ammonia is added, which reacts with free chlorine and forms chroloamines that have longer residence time in the treated water.

Disinfectant byproducts (DBPs) include Trihalomethanes (THMs), chloroform (CHCl3), and haloacetic acids (HAAs).

Presence of organic matter promotes formation of the DBPs, therefore the disinfection is most efficient when carried out as the last treatment process step.

Alternative disinfectants are chlorine dioxide (ClO2) and ozone (O3). Both agents are effective against cysts and viruses but more costly compared to chlorination. Additionally, ozone does not leave any protective residuals in the water after the treatment.

5. Hardness and Alkanity

Hardness is defined as the concentration of Ca2+ and Mg2+ in solution.

Groundwater is especially prone to excessive hardness.

Hardness causes problem of soap curd (reaction between hardness and soap produces a sticky, gummy deposit) and

scaling (precipitation of CaCO3 and Mg(OH)2 that clogs hot

water pipes)

Alkalinity acid buffering capacity of water. In most natural water, the total amount of H+ that can be neutralized is

dominated by the alkalinity provided by the carbonate buffering

system.

5. Hardness and alkalinity

EW = AW / n e.g. EW (CaCO3) = (40+12+3x16) / 2 = 50 g/eq = 50 mg/meq for EW (Ca2+) = 40 / 2 = 20 mg/meq The general expression is meq/L (X) = [X, mg/L] / EW (X, mg/meq) Alkalinity = [HCO3

-] + 2[CO32-] + [OH] [H+]

Hardness is expressed in terms of equivalent weight, EW, which is atomic or molecular weight of a substance divided by a number, n, that relates to its valence or ionic charge.

6. Softening

The surface waters seldom have hardness exceeding 200 mg/L and softening is not regular part of the water treatment.

For waters with hardness ~ 1000 mg/L the treatment is required. It involves lime-soda ash and the ion-exchange processes.

1. Lime-soda processing, Ca(OH)2 is added to water and pH goes up to

6. Membrane Processes

All membranes can be considered semipermeable physical barrier, they allow passage of water, while severely restricting the permeation of contaminants in water (including pathogen, hardness and dissolved solids, organic and disinfectant byproduct precursors, metals, and suspended solids)

Classification of membrane: Particulate separation and solute separation

1. Particulate separation (reject particles, including pathogens) microfiltration and ultrafiltration

2. Solute separation (dissolved substance) nanofiltration and reverse osmosis

Size ranges and types of contaminants removed by membrane processes.

Wastewater treatment

Objectives of wastewater treatment:

Wastewater treatment process is divided into 3 stages:

1. Primary treatment to reduce content of solids

2. Secondary treatment to reduce BOD

3. Tertiary or advanced treatment to remove nutrients, nitrogen and phosphorus. Residues of pharmaceutical substances (e.g. antibiotics) require additional treatment.

1. Primary treatment

Screening: to remove large objects, debris

Grit chamber allows the heavy stuff to settle, a few min detention time

Primary settling tank or primary clarifier: the flow is sufficiently reduced,

detention time is ~ 1.5 3 hours during which about 50-65% of suspended solids and 25-40% of BOD are removed.

Overflow rate and detention time are the key parameters.

2. Secondary treatment, biological

Oxygen Demand Wastewater contains organic compounds such as glucose (C6H12O6).

Microorganisms in the water use these organic compounds as food while degrading these organic compounds, microorganisms consume oxygen dissolved in the water the O2 used by the organisms is replenished by mass transfer from air

I. Suspended growth treatment (microbial organisms are suspended in the water as it is treated, i.e. bioreactor):

a) Activated sludge treatment depends on two components: oxygen supply and microbial growth (sufficient microbial population).

Microbial kinetics

Microorganisms consume organic matter (substrate) that is measured in mg/L of BOD.

Mass of m/orgs fluctuates and their biomass is measured in mg/L of Volatile Suspended Solids (VSS or VS). See more details in Experiment # 2 of the lab. manual.

Activated sludge tank works as a bioreactor.

The rate of substrate entering and leaving the reactor is affected by the water rate entering and leaving, and the rate of microbial growth

is affected by changes in the mass of substrate available.

For a particular substrate and growth conditions, a proportion between substrate mass consumed and new microbial cell mass

should be achieved to maintain efficiency of the w/water treatment.

rg=microbial mass growth rate X =concentration of microorganisms (mg VSS/L) = specific biomass growth rate constant (time-1)

The Monod equation (1949)

When substrate concentration, S (mg BOD5/L), is 0, the growth rate is also 0. When substrate is present in excess, the growth rate reaches maximum rate of microbial reproduction, m.

Rate of microbial growth depends on the substrate concentration and the amount of generated biomass should be proportional to the amount of substrate consumed, i.e. certain proportion of the substrate consumed should be converted into a predictable amount of new microbial cells. Y = proportionality constant = yield coefficient , mg VSS/mg BOD5 Y relates the rate of substrate consumption, rsu, under particular conditions to the rate of microbe growth:

k relates maximum specific growth rate, m, to the yield coefficient:

No death of microbial cells is considered in this equation.

= net rate of change in microbe concentration.

Microbial growth rate is proportional to the rate of substrate consumption (rsu) minus the rate microbes die (rd).

kd=death /decay rate constant, time-1

The death rate for microbes, rd

2. Secondary treatment, biological. Suspended growth treatment: Activated sludge

Two conditions: 1. Supply of oxygen, i.e. aeration 2. Promoted growth of microbial biomass

Mixed liquor

1. Bioreactor: BOD consumption

2.

(return activated sludge)

(waste activated sludge)

Retention time:

The microbial cells mass settles in the secondary clarifier and then is returned to the activate sludge tank to maintain sufficient microbial activities.

The average cell retention time (solids retention time), SRT, c is

Activated sludge tanks are relatively inexpensive, have less problems with insects and odour, provide higher BOD removal rates.

However, activated sludge reactors require higher expertise to operate and more energy for pumps and blowers.

2. Secondary treatment, biological.

Approximate concentrations of BOD, suspended solids, total nitrogen and total phosphorus as wastewater passes through

a secondary wastewater treatment plant.

b) Membrane bioreactors (MBRs) draw water from the mixed liquor into hollow fiber membranes submerged in the activated sludge aeration tank, thus avoiding the need for a secondary clarifier.

Microfiltration fibers have a pore size is ~ 2 micrometer and effective in producing low TSS effluent.

MBRs application is very effective where wastewater reuse and reclamation is desired.

MBRs have fouling problems, are more expensive to build and operate; and their longevity is also questioned as it is a relatively new technology.

c) Aerated lagoons and oxidation ponds.

Typically, oxidation ponds are large, 1-2 m deep where sewage is decomposed by microorganisms. The decomposition near the surface is aerobic and at the ponds bottom is anaerobic; such ponds are termed facultative ponds. The ponds are easy to build and they sufficiently reduce BOD; however the effluent may contain undesirable concentrations of algae and unpleasant odour.

Attached Growth Treatment is often used as the sole secondary treatment process and as pre-treatment step before an activated sludge process.

a) Trickling filters are successfully used since 19th century. A trickling filter consists of a rotating distribution arm that sprays the feed w/water over a circular bed of plastic packing or other coarse materials.

Tall trickling filters filled with plastic media are called biotowers. The spaces between the packing allow air to circulate easily so that aerobic conditions are maintained. The media is covered by biological slime populated with microorganisms, insects, fungus, protozoa, worms, snails, etc. that are responsible for the w/water decomposition. What is the principal difference b/w suspended and attached growth treatment?

b) Rotating biological contractors (RBC) consist of a series of closely spaced, circular, plastic disks, typically 3.6 m in diameter that are attached to a rotating horizontal shaft. 40% of each disk is submerged in wastewater. Microorganisms populated on the surface of the rotating disks decompose the wastewater.

Sludge treatment: Sludge is a mixture of solids and water that remains to be disposed of. The important objective of the sludge treatment is to separate the water from the solids as much as possible. The traditional method is anaerobic digestion. It is slower than aerobic decomposition but has an advantage that a small % of the waste is converted into new bacterial cells. Most of the organics are converted into CO2 and CH4.

Many treatment plants use a two-stage digester shown below. In the 1st stage, the sludge is thoroughly mixed and heated to increase the rate of digestion (detention time 10-15 days) The 2nd stage is characterized by stratification (no mixing): liquid, solids and gas. The liquid part (supernatant) is returned for BOD removal; the sludge is dewatered and disposed of; the gas is potential fuel used for heating purpose at most of the treatment plants. The digested and dewatered sludge is potential soil conditioner but mostly it is disposed in a landfill.

Advanced treatment involves removal of nutrient elements such as nitrogen and phosphorus.

Nitrogen

Aerobic bacteria converts ammonia (NH4+) to nitrate (NO3

-) which is nitrification; then anaerobic bacteria converts nitrate to nitrogen gas (N2) which is denitrification.

The aerobic part of the reaction : NH4+ + 2O2 + bacteria NO3

- + 2H+ + H2O

For the nitrification process the detention time of at least 15 days is required.

For the anaerobic part, denitrification, the reaction:

2NO3- + organic matter + bacteria N2 + CO2 + H2O

Because the denitrification takes place after waste treatment, there may be not enough organic material for the bacteria and therefore additional nutrient is required, which is typically methanol (CH3OH).

Phosphorus

Phosphorus in w/water exists as orthophosphate (H2PO4-; HPO4

2-, and PO43-), and it is

removed by reaction with added coagulant such as alum or lime.

The reaction: Al2(SO4)3 + 2PO42- 2AlPO4 + 3SO4

2-