Wastewater Treatment

INTRODUCTIONWe will start with an overview of treatment

processes

1) Why do we treat water and wastewater?The main objectives of the conventional

wastewater treatment processes are the reduction in biochemical oxygen demand, suspended solids and pathogenic organisms.

Also necessary to remove nutrients such as N and P, toxic components, non-biologically degradable compounds and dissolved solids.

Removal of these materials are necessary for the simple reason that discharge to the environment will result in “damage” of some sort.

2) What are the materials in water and wastewater that we must remove?There are a wide range of these pollutants (contaminants) ranging from municipal sewage to highly specific industrial wastes. The usual approach in discussing treatment schemes is to categorize pollutants into general classes so that a general class of treatment methods can be applied.

Note that many pollutants fall into several categories.

For example, some biodegradable organic matter (one category) is in the form of suspended solids (another category).

So removal of SS sometimes results in the removal of organic matter.

3) To what level do we need to remove contaminants?

The degree to which drinking water must be

treated depends on the raw water quality and the desired quality of the finished water.

Similarly the degree of treatment of a wastewater depends on the quality of the raw waste and the required effluent quality.

BOD5 = 30 mg/L monthly averageSuspended Solids = 30 mg/L monthly averagepH (if there is industrial input) = 6 – 9

continuous

For drinking water treatment the requirements are, of course, much more stringent with many more categories and lower contaminant limits.

Turbidity (a measure of suspended solids): less than 0.5 NTU in at least 95% of samples taken each month.

Lead: 0.005 mg/L

Copper: 1.3 mg/L

Total Coliform: no coliform detection in more than 5% of samples collected each month.

4) How are these contaminants removed from water and wastewater?

Contaminant removal is accomplished by a series of unit processes or unit operations.

Unit operation is a physical ,chemical or biological treatment process.

The system of integrated unit processes or unit processes used to treat a water or wastewater is called a treatment train.

Clarifier

General overview of plant components

Secondary Sludge

Primary Sludge

Clarifier

Raw Wastewater Influent

PRIMARY

DISINFECTION

BiologicalTreatment

System

SECONDARY

Clean Wastewater EffluentDischarge to Receiving Waters

Preliminary Residuals(i.e., grit, rags, etc.)A

B

C

WastewaterTreatmentResiduals

BiosolidsProcessing

and Disposal

Clarifier

PRELIMINARY

Usually to Landfill

AdvanceTreatment

System

TERTIARY

ChemicalTreatment

Conventional Wastewater Treatment Conventional Wastewater Treatment ProcessProcess

PRELIMINARYBar Screening Grit RemovalComminutorsFlow equalization

PRIMARYFlocculationSedimentation Clarifiers

SECONDARYAerobic Processes

Anoxic ProcessesAnaerobic ProcessesCombined Aerobic-AnoxicAnaerobic ProcessesPond ProcessesChlorine compounds

Bromine ChlorideOzoneUV Radiation

grinding, degritting, blending, thickening, stabilization, conditioning, disinfection, dewatering, heat drying, thermal reduction, ultimate disposal

Chemical precipitation Adsorption

DISINFECTION

TERTIARY

Reverse osmosisIon ExchangeMembrane filtrationSolvent extractionAdvanced oxidation process

Sludge Treatment and

Disposal

Preliminary treatment

Mechanical Processes 1 Screening ● The first unit operation encountered in wastewater-

treatment plants is screening. A screen is a device with openings, generally of uniform size that is used to retain the coarse solids found in wastewater.

● According to the method of cleaning, screens are designated as hand cleaned or mechanically cleaned.

● According to the size of openings, screens are designated as coarse or fine. Coarse screens have openings of ¼ inch or more, and fine screens have openings of less than ¼ inch.

Mechanical Screen

Preliminary Treatment

SCREENING

Figure Definition sketch for types of screens used in wastewater treatment

Grit chamberGrit chamberAerated grit chamber : diffused air keeps organicsolids in suspension as grit settles

Vortex - Type Grit ChambersVortex is created-Grit move to the outside of the unit and gets collected

ComminutorComminutor •In this device all of the wastewater flow passes through the grinder assembly

•The grinder consists of a screen or slotted basket, a rotating or oscillating cutter and a stationary cutter•Solids pass through the screen and are chopped or shredded between the two cutters

3 Flow Equalization ● Flow equalization is used to overcome the operational

problems caused by flow variations, to improve the performance of the downstream processes, and is also used as an emergency tank to equalize wastewater effluent in case of any process failure in the treatment process.

● The design must provide for sufficient mixing to prevent solids deposition and concentration variations and also to provide aeration to prevent odor problems.

● The best location for equalization facilities to be at existing and proposed treatment plant sites. In some cases, equalization after primary treatment and before biological treatment may be appropriate.

Location

(a) Locate in-line

(b) off-line

Primary treatment

Primary treatmentPrimary treatmentThe objective of primary treatment is the

removal of settle-able organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming

Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD5), 50 to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment

Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation

In many industrialized countries, primary treatment is the minimum level of pre-application treatment required for wastewater irrigation

Contd…Contd…Primary sedimentation tanks or clarifiers may be

round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours

Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units

Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units

Primary Treatment

Flocculation

Physical Treatment 1 Sedimentation ● Sedimentation is the separation from water, by

gravitational settling, of suspended particles that are heavier than water.

● Sedimentation is used for separation of grit and particulate matter in the primary settling basin, separation of biological-floc in the activated-sludge settling basin, and separation of chemical-floc when the chemical coagulation process is used. It is also used for solids concentration in sludge thickeners.

● Sedimentation basins are constructed in a variety of shapes and sizes, circular tanks or rectangular tanks.

● The basin is comprised of four zones according to function:

1-The inlet zone. 2-The settling zone. 3-The sludge zone. 4-The outlet zone.

● The inlet zone is a region where the incoming suspension is distributed uniformly over the cross-section of the tank.

● In the settling zone, the particles settle at the same rate as they would in a quiescent.

● In the outlet zone, the clarified liquid is collected uniformly over the cross-section of the basin.

● The solids collect in a sludge zone at the bottom of the tank.

Rectangular basin Circular basin

Rectangular basin Circular basin

Chemical Treatment

Chemical unit processes-precipitationWidely used, technology for the removal of metals and

other inorganics, suspended solids, fats, oils, greases, and some other organic substances from wastewater.

Precipitation is a method of causing contaminants that are either dissolved or suspended in solution to settle out of solution as a solid precipitate, which can then be filtered, centrifuged, or otherwise separated from the liquid portion.

Precipitation is assisted through the use of a coagulant, an agent which causes smaller particles suspended in solution to gather into larger aggregates.

When colloidal matter such as emulsified oil or metal bearing particles are treated with metal salts and lime or NaOH, the metal salts act as primary coagulants.

The positively charged metal ions combine with the negative colloid particles and neutralize their charge.

The particles then repel each other less strongly and tend to coagulate or collect into larger particles.  

Chemicals for precipitationLime – Calcium Oxide, CaOFerrous Sulfate – Fe(SO4)3

Alum– Al2(SO4)3.14H2OFerric Chloride – FeCl3Polymer

AdvantagesChemical precipitation is a well-established

technology with ready availability of equipment and many chemicals.

Some treatment chemicals, especially lime, are very inexpensive.

Completely enclosed systems are often conveniently self-operating and low maintenance.

DisadvantagesCompeting reactions, varying levels of

alkalinity and other factors typically make calculation of proper chemical dosages impossible.

Chemical precipitation may require working with corrosive chemicals, increasing operator safety concerns.

The addition of treatment chemicals, especially lime, may increase the volume of waste sludge up to 50 percent.

Large amounts of chemicals may need to be transported to the treatment location.

Polymers can be expensive.

APPLICATION OF DIFF. CHEMICALSLime – Calcium Oxide, CaOProduces calcium carbonate in

wastewater which acts as a coagulant for hardness and particulate matter. Often used in conjunction with other coagulants, since: (1) by itself, large quantities of lime are required for effectiveness, and (2) lime typically generates more sludge than other coagulants.

Ferrous Sulphate – Fe(SO4)3 Typically used with lime to soften water.

The chemical combination forms calcium sulfate and ferric hydroxide. Wastewater must contain dissolved oxygen for reaction to proceed successfully.

Ferric Chloride – FeCl3 Reacts with alkalinity or phosphates to

form insoluble iron salts.

Alum or Filter Alum – Al2(SO4)3.14H2OUsed for water softening and phosphate removal.

Reacts with available alkalinity (carbonate, bicarbonate and hydroxide) or phosphate to form insoluble aluminium salts.

PolymerHigh molecular weight compounds (usually

synthetic) which can be anionic, cationic, or non-ionic.

When added to wastewater, can be used for charge neutralization for emulsion-breaking, or as bridge-making coagulants, or both. Can also be used as filter aids and sludge conditioners.

Secondary treatment

Secondary treatmentSecondary treatmentThe objective of secondary treatment is the further

treatment of the effluent from primary treatment to remove the residual organics and suspended solids

Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products (principally CO2, NH3, and H2O, CH4)

Several aerobic biological processes are used for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter

Contd…Contd…High-rate biological processes are characterized by

relatively small reactor volumes and high concentrations of microorganisms compared with low rate processes

Consequently, the growth rate of new organisms is much greater in high-rate systems because of the well controlled environment

The microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary effluent

The sedimentation tanks used in secondary treatment, often referred to as secondary clarifiers, operate in the same basic manner as the primary clarifiers described previously

The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing

Common high-rate processes include the activated sludge processes, trickling filters or biofilters, oxidation ditches and rotating biological contactors (RBC)

A combination of two of these processes in series (e.g. biofilter followed by activated sludge) is sometimes used to treat municipal wastewater containing a high concentration of organic material from industrial sources

Biological unit processes In the case of domestic wastewater treatment,

the objective of biological treatment is:– To stabilize the organic content– To remove nutrients such as nitrogen and phosphorus

Types:Aerobic ProcessesAnoxic ProcessesAnaerobic ProcessesCombined Aerobic-Anoxic-Anaerobic Processes

Pond Processes

Attached GrowthSuspended Growth Combined Systems

Aerobic MaturationFacultativeAnaerobic

Attached Growth ProcessWhat can this process do?

1. Remove Nutrient2. Remove dissolved organic solids3. Remove suspended organic solids4. Remove suspended solids

Cross-section of an attached growth biomass film

Wastewater

Oxygen (the natural or forced draft)

Organic/ nutrient

filter media

Biomass : viscous, jelly-like substance containing bacteria

Major Aerobic Biological Processes

ACTIVATED SLUDGE PROCESSES…

The most common suspended growth process used for municipal wastewater treatment is the activated sludge process.

Activated sludge plant involves:

1.wastewater aeration in the presence of a microbial suspension,

2.solid-liquid separation following aeration,

3.discharge of clarified effluent,

4.wasting of excess biomass, and

5.return of remaining biomass to the aeration tank.

Process The process involves air or oxygen being introduced

into a mixture of primary treated or screened sewage or industrial wastewater combined with organisms to develop a biological floc which reduces the organic content of the sewage.

The combination of wastewater and biological mass is commonly known as mixed liquor.

In all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated  supernatant is run off to undergo further treatment before discharge. 

Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new wastewater entering the tank.

This fraction of the floc is called return activated sludge (R.A.S.). Excess sludge is called surplus activated sludge(S.A.S.) or waste activated sludge(W.A.S).

S.A.S is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance.

S.A.S is stored in sludge tanks and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal.

Advantages Diverse; can be used for one household up a

huge plant Removes organics Oxidation and Nitrification achieved Biological nitrification without adding chemicals Biological Phosphorus removal Solids/ Liquids separation Stabilization of sludge Capable of removing ~ 97% of suspended solids The most widely used wastewater treatment

process

Disadvantages Does not remove color from industrial wastes and

may increase the color through formation of highly colored intermediates through oxidation

Does not remove nutrients, tertiary treatment is necessary

Problem of getting well settled sludge

Recycle biomass keeps high biomass concentration in aeration tanks

Types of Activated Sludge ProcessesPlug Flow  wastewater is routed through a series of

channels constructed in the aeration basin. Wastewater Flows to tank & is treated as it

winds its way through the tank. As the wastewater goes through the system,

BOD and organics concentration are greatly reduced.

Variations to this method include:

adding return sludge and/or in decreasing amounts at various locations along length of the tank;

wastewater BOD is reduced as it passes through tank,

air requirements and number of bacteria required also decrease accordingly.

Complete Mix  wastewater may be immediately mixed

throughout the entire contents of the aeration basin (mixed with oxygen and bacteria).

This is the most common method used today. Since the wastewater is completely mixed

with bacteria and oxygen, the volatile suspended solids concentration and oxygen demand are the same throughout the tank.

Contact Stabilization Microorganisms consume organics in the

contact tank. Raw wastewater flows into the contact tank

where it is aerated and mixed with bacteria. Soluble materials pass through bacterial cell

walls, while insoluble materials stick to the outside.

Solids settle out later and are wasted from the system or returned to a stabilization tank.

Microbes digest organics in the stabilization tank, and are then recycled back to the contact tank, because they need more food.

Detention time is minimized, so the size of the contact tank can be smaller.

Volume requirements for the stabilization tank are also smaller because the basin receives only concentrated return sludge, there is no incoming raw wastewater.

Often no primary clarifier before the contact tank due to the rapid uptake of soluble and insoluble food.

Extended Aeration 

Used to treat industrial wastewater containing soluble organics that need longer detention times.

This is the same as complete mix, with just a longer aeration.

Advantage - long detention time in the aeration tank; provides equalization to absorb sudden/temporary shock loads.

Less sludge is generally produced because some of the bacteria are digested in the aeration tank.

One of the simpler modifications to operate.

Design ConsiderationThe quality or characteristics of raw waste

water to be treated.

The desired quality or characteristics of effluent or treated waste water.

The type of reactor that will be used.

Volumetric and organic loading that will be applied to the reactor.

Amount of O2 required and the aeration system will provide to supply O2 and to support mixing.

The quantity of sludge that will be generated and wasted for its further management.

Besides these nutrient requirements of microbes, environmental conditions under which plant operated.

Design stepsThe design computations require the

determination of:

Volume or dimensions of the aeration tankAmount of O2 required and power needed

for aerationQuantity of sludge that will produced for

particular waste and treatment conditions Volume and dimensions of sec. settling

tank

Design criteria No of aeration tanks, N= min. 2 (small plants) = 4 or more (large

plants) Depth of waste water in tank= 3-4.5 m (usually) = 4.5-7.5 m (diffuse

aeration) = 1-6 m (surface

aeration)

Air requirement:I. 20-55 m3 of air/Kg of BOD removed for

diffuse aeration when F/M => 0.3II.70-115 m3 air/Kg of BOD removed for

diffuse aeration when F/M <= 0.3Power required for complete mixing : 10-

14 kW/1000 m3 of tank volume for surface aeration system

Activated Sludge ProcessTypes of AS Systems: Conventional, Complete-Mix, Sequencing Batch Reactor Extended Aeration Deep Tank Deep Shaft

Advantages/DisadvantagesAdvantages

Flexible, can adapt to minor pH, organic and temperature changes

Small area required Degree of nitrification

is controllable Relatively minor odor

problems

Disadvantages High operating costs

(skilled labor, electricity, etc.)

Generates solids requiring sludge disposal

Some process alternatives are sensitive to shock loads and metallic or other poisons

Requires continuous air supply

Activated Sludge Principles • Wastewater is aerated in a tank • Bacteria are encouraged to grow by providing

• Oxygen • Food (BOD)• Nutrients • Correct temperature • Time

• As bacteria consume BOD, they grow and multiply • Treated wastewater flows into secondary clarifier • Bacterial cells settle, removed from clarifier as sludge • Part of sludge is recycled back to activated sludge tank,

to maintain bacteria population • Remainder of sludge is wasted

Kinetics of Microbial Growth • Biochemical reaction• • Biomass concentration. • The concentration of biomass, X (mg/L),

increases as a function of time due to conversion of food to biomass:

• Where is the specific growth rate constant (d-1).

This represents the mass of cells produced/mass of cells per unit of time.

Effect of substrate concentration on growth rate constant

Monod Kinetics

• Growth rate • Growth rate constant, , is a function of the

substrate concentration, S. • Two constants are used to describe the growth

rate m (mg/L) is the maximum growth rate constant (the rate

at which the susbtrate concentration is not limiting) – Ks is the half-saturation constant (mg/L) (i.e.,

concentration of S when = m/2

•

• Biomass production

• Where kd represents the endogenous decay rate (d-1) (i.e., microorganism death rate).

– Substituting the growth rate constant:

• Substrate utilization

Where Y is the yield factor (mg of biomass produced/mg of food consumed)

• Y range:– Aerobic: 0.4 - 0.8 mg/mg

• Food to microorganism ratio (F/M) • Represents the daily mass of food supplied to the

microbial biomass, X, in the mixed liquor suspended solids, MLSS

• Units are Kg BOD5/Kg MLSS/day

• Since the hydraulic retention time, = V/Qo, then

Typical range of F/M ratio in activated sludge units

Treatment Process F/MKg BOD5/Kg MLSS/day

Extended aeration 0.03 - 0.8

Conventional 0.8 - 2.0

High rate > 2.0

Design parameters for activated sludge processesProcess d h F/M Qr/Q X (mg/L)

Conventional 5-15 4-8 0.2-0.4 0.25-5 1,500-3,000

Complete-mix 5-15 3-5 0.2-0.6 0.25-1 3,000-6,000

Step-aeration 5-15 3-5 0.2-0.4 0.25-0.75 2,000-3,500

Modified-aeration

0.2-0.5 1.5-3 1.5-5.0 0.05-0.15 200 – 500

Contact-stabilization

5-15 0.5-13-6

0.2-0.6 0.25-1 1,000-3,0004,000-10,000

Extended-aeration

20-30 18-36 0.05-0.15 0.75-1.5 3,000-6,000

High-rate aeration

5-10 0.5-2 0.4-1.5 1-5 4,000-10,000

Pure-oxygen 8-20 1-3 0.25-1.0 0.25-0.5 6,000-8,000

Operational characteristics of activated sludge processes

Process Flow model Aeration system BOD5 removal efficiency (%)

Conventional Plug-flow Diffused air, mechanical aerators

85-95

Complete-mix Complete-mix Diffused air, mechanical aerators

85-95

Step-aeration Plug-flow Diffused air 85-95

Modified-aeration Plug-flow Diffused air 60-75

Contact-stabilization

Plug-flow Diffused air, mechanical aerators

80-90

Extended-aeration Complete-mix Diffused air, mechanical aerators

75-95

High-rate aeration Complete-mix Diffused air, mechanical aerators

75-90

Pure-oxygen Complete-mix Mechanical aerators 85-95

Schematic of activated sludge unit

Activated Sludge Design Equations

• Influent biomass + biomass production = effluent biomass + sludge wasted

•

• Substitute biomass production equation

•

• Assume that influent and effluent biomass concentrations are negligible and solve

•

Mass balance of biomass production

Mass balance of food substrate • Influent substrate + substrate consumed = effluent

susbtrate + sludge wasted substrate•

• Substitute substrate removal equation•

• Assume that no biochemical action takes place in clarifier. Therefore the substrate concentration in the aeration basin is equal to the substrate concentrations in the effluent and the waste activated sludge. Solve:

•

Overall equations – Combine the mass balance equations for food and biomass:

• • The cell residence time is:•

• and the hydraulic retention time is,= V/Qo » Substitute and rearrange:

• • • Compute the F/M ratio

1.3 Trickling Filter “ Biological Air Filters” ● The trickling filter consists of a bed of a highly permeable

medium to which microorganisms are attached and through which wastewater is percolated or trickled.

● The filter media usually consist of either rock (slag is also used) or a variety of plastic packing materials.

● Rock filter beds are usually circular and the liquid wastewater is distributed over the top of the bed by a rotary distributor, and the wastewater was allowed to contact the media for a short time.

● The collected liquid is passed to a settling tank where the solids are separated from the treated wastewater. In practice, portion of the liquid collected in the under-drain system or the settled effluent is recycled, usually to dilute the strength of the incoming wastewater and to maintain the biological slime layer in a moist condition.

● The limitations of the trickling filter included a relatively high incidence of clogging, the long rest period required, and the relatively low loading that could be used.

Trickling Filter

Trickling FiltersTrickling FiltersA trickling filter or biofilter consists of a basin or

tower filled with support media such as stones, plastic shapes, or wooden slats

Wastewater is applied intermittently, or sometimes continuously, over the media

Microorganisms become attached to the media and form a biological layer or fixed film

Organic matter in the wastewater diffuses into the film, where it is metabolized

Oxygen is normally supplied to the film by the natural flow of air either up or down through the media, depending on the relative temperatures of the wastewater and ambient air

The thickness of the biofilm increases as new organisms grow

Periodically, portions of the film slough off the media

The sloughed material is separated from the liquid in a secondary clarifier and discharged to sludge processing

Clarified liquid from the secondary clarifier is the secondary effluent and a portion is often recycled to the biofilter to improve hydraulic distribution of the wastewater over the filter

Trickling Filters

Advantages/Disadvantages Advantages

Good quality (80-90% BOD5 removal) for 2-stage efficiency could reach 95%

Moderate operating costs (lower than activated sludge)

Withstands shock loads better than other biological processes

Disadvantages High capital costs Clogging of

distributors or beds

TRICKLING FILTER PROCESSES…

Trickling filter is an attached growth process i.e. process in which microorganisms responsible for treatment are attached to an inert packing material. Packing material used in attached growth processes include rock, gravel, slag, sand, redwood, and a wide range of plastic and other synthetic materials.

Process Description The wastewater in trickling filter is distributed

over the top area of a vessel containing non-submerged packing material.

Air circulation in the void space, by either natural draft or blowers, provides

oxygen for the microorganisms growing as an attached biofilm.

The organic material present in the wastewater metabolised by the biomass attached to the medium.

The biological slime grows in thickness as the organic matter abstracted from the flowing wastewater is synthesized into new cellular material.

Flow Diagram for Trickling Filters

Recycle

Primaryclarifier Trickling

filter

Finalclarifier

Wastesludge

FinaleffluentInfluent

Q

Or

Recirculation= A portion of the TF effluent recycled through the filterRecirculation ratio (R) = returned flow (Or)/ influent flow (Q)

Advantages

simplicity of operation

resistance to shock loads

low sludge yield

low power requirements

Disadvantages

relatively low BOD removal (85%)

high suspended solids in the effluent (20 -30 mg/L)

little operational control

Types of FiltersTrickling filters are classified as high rate or low rate, based on the organic and hydraulic loading applied to the unit.

Hydraulic loading rate is the total flow including recirculation applied on unit area of the filter in a day.

Organic loading rate is the 5 day 20°C BOD, excluding the BOD of the recirculant, applied per unit volume in a day.

Recirculation is generally not adopted in low rate filters.

A well operated low rate trickling filter in combination with secondary settling tank may remove 75 to 90% BOD and suitable for treatment of low to medium strength domestic wastewaters.

The high rate trickling filter, single stage or two stage are recommended for medium to relatively high strength domestic and industrial wastewater.

The BOD removal efficiency is around 75 to 90%.

Single stage unit consists of a primary settling tank, filter, secondary settling tank and facilities for recirculation of the effluent.

Two stage filters consist of two filters in series with a primary settling tank, an intermediate settling tank which may be omitted in certain cases and a final settling tank.

Process Design Generally trickling filter design is based on

empirical relationships to find the required filter volume for a designed degree of wastewater treatment.

NRC equations commonly used. NRC (National Research Council of USA)

equations give satisfactory values when there is no re-circulation, the seasonal variations in temperature are not large and fluctuations with high organic loading.

NRC equations: These equations are applicable to both low rate and high rate filters. The efficiency of single stage or first stage of two stage filters, E2 is given by

For the second stage filter, the efficiency E3 is given by

E2=              100                      1+0.44(F1.BOD/V1.Rf1)1/2

E3=                      100                             [(1+0.44)/(1- E2)](F2.BOD/V2.Rf2)1/2

where E2= % efficiency in BOD removal of single stage or first stage of two-stage filter

E3=% efficiency of second stage filterF1.BOD= BOD loading of settled raw sewage in single

stage of the two-stage filter in kg/dF2.BOD= F1.BOD(1- E2)= BOD loading on second-stage filter

in kg/dV1= volume of first stage filter, m3

V2= volume of second stage filter, m3

Rf1= Recirculation factor for first stage, R1= Recirculation ratio for first stage filterRf2= Recirculation factor for second stage, R2= Recirculation ratio for second stage filter.

Rf1= 1+R (1+R/10)2

R=recycle ratio F=recirculation factor

Q. Problem: Design a low rate filter to treat 6.0 Mld of sewage of BOD of 210 mg/l. The final effluent should be 30 mg/l and organic loading rate is 320 g/m3/d.

Solution: Assume 30% of BOD load removed in primary sedimentation i.e., = 210 x 0.30 = 63 mg/l. Remaining BOD = 210 - 63 = 147 mg/l.Percent of BOD removal required = (147-30) x 100/147 = 80%

BOD load applied to the filter = flow x conc. of sewage (kg/d) = 6 x 106 x 147/106 = 882 kg/d

To find out filter volume, using NRC equation E2=              100                         

       1+0.44(F1.BOD/V1.Rf1)1/2

80 =               100              Rf1= 1, (no recirculation)         1+0.44(882/V1)1/2

V1= 2704 m3

Depth of filter = 1.5 m, Filter area = 2704/1.5 = 1802.66 m2, and Diameter = 48 m

Hydraulic loading rate = 6 x 106/103 x 1/1802.66 = 3.33m3/d/m2 < 4 hence o.k.

Organic loading rate = 882 x 1000 / 2704 = 326.18 g/d/m3 which is approx. equal to 320

Rotating Biological ContactorsRotating Biological ContactorsRotating biological contactors (RBCs) are fixed-

film reactors similar to biofilters in that organisms are attached to support media

In the case of the RBC, the support media are slowly rotating discs that are partially submerged in flowing wastewater in the reactor

Oxygen is supplied to the attached biofilm from the air when the film is out of the water and from the liquid when submerged, since oxygen is transferred to the wastewater by surface turbulence created by the discs' rotation

Sloughed pieces of biofilm are removed in the same manner described for biofilters

Contd…Contd…High-rate biological treatment processes, in

combination with primary sedimentation, typically remove 85 % of the BOD5 and SS originally present in the raw wastewater and some of the heavy metals

Activated sludge generally produces an effluent of slightly higher quality, in terms of these constituents, than biofilters or RBCs

When coupled with a disinfection step, these processes can provide substantial but not complete removal of bacteria and virus

However, they remove very little phosphorus, nitrogen, non-biodegradable organics, or dissolved minerals.

Rotating Biological Contactors It consists of a series of circular disks of polystyrene or

polyvinyl chloride that are submerged in wastewater and rotated slowly through it

The disk rotation alternately contacts the biomass with the organic material and then with atmosphere for adsorption of oxygen

Excess solids are removed by shearing forces created by the rotation mechanism

Advantages/DisadvantagesAdvantages

Short contact periods Handles a wide range

of flows Easily separates

biomass from waste stream

Low operating costs Short retention time Low sludge production Excellent process

control

Disadvantages Need for covering

units installed in cold climate to protect against freezing

Shaft bearings and mechanical drive units require frequent maintenance

Major Anaerobic Biological Processes

Anaerobic Contact ProcessUntreated wastewater is mixed with

recycled sludge solids and then digested in a sealed reactor

The mixture is separated in a clarifier The supernatant is discharged as

effluent, and settled sludge is recycled

Advantages/DisadvantagesAdvantages

Methane recoverySmall area

requiredVolatile solids

destruction

DisadvantagesHeat requiredEffluent in reduced

chemical form requires further treatment

Requires skilled operation

Sludge to be disposed off is minimal

Upflow Anaerobic Sludge Blanket Wastewater flows

upward through a sludge blanket composed of biological granules that decompose organic matter

Some of the generated gas attaches to granules that rise and strike degassing baffles releasing the gas

Free gas is collected by special domes

The effluent passes into a settling chamber

Tertiary / advanced treatment

Tertiary and/or advanced Tertiary and/or advanced treatmenttreatment

Tertiary and/or advanced wastewater treatment is employed when specific wastewater constituents which cannot be removed by secondary treatment must be removed

Because advanced treatment usually follows high-rate secondary treatment, it is sometimes referred to as tertiary treatment

However, advanced treatment processes are sometimes combined with primary or secondary treatment (e.g., chemical addition to primary clarifiers or aeration basins to remove phosphorus) or used in place of secondary treatment (e.g., overland flow treatment of primary effluent)

An adaptation of the activated sludge process is often used to remove nitrogen and phosphorus

Effluent from primary clarifiers flows to the biological reactor, which is physically divided into five zones by baffles and weirs

In sequence these zones are: (i) anaerobic fermentation zone (characterized by very low dissolved oxygen levels and the absence of nitrates); (ii) anoxic zone (low dissolved oxygen levels but nitrates present); (iii) aerobic zone (aerated); (iv) secondary anoxic zone; and

(v) final aeration zone

The function of the first zone is to condition the group of bacteria responsible for phosphorus removal by stressing them under low oxidation-reduction conditions, which results in a release of phosphorus equilibrium in the cells of the bacteria

On subsequent exposure to an adequate supply of oxygen and phosphorus in the aerated zones, these cells rapidly accumulate phosphorus considerably in excess of their normal metabolic requirements

Phosphorus is removed from the system with the waste activated sludge

Most of the nitrogen in the influent is in the ammonia form, and this passes through the first two zones virtually unaltered

In the third aerobic zone, the sludge age is such that almost complete nitrification takes place, and the ammonia nitrogen is converted to nitrites and then to nitrates

The nitrate-rich mixed liquor is then recycled from the aerobic zone back to the first anoxic zone

Here de-nitrification occurs, where the recycled nitrates, in the absence of dissolved oxygen, are reduced by facultative bacteria to nitrogen gas, using the influent organic carbon compounds as hydrogen donors

The nitrogen gas merely escapes to atmosphere. In the second anoxic zone, those nitrates which were not recycled are reduced by the endogenous respiration of bacteria

In the final re-aeration zone, dissolved oxygen levels are again raised to prevent further de-nitrification, which would impair settling in the secondary clarifiers to which the mixed liquor then flows

DisinfectionDisinfection Selective destruction of disease-Selective destruction of disease-

causing organismscausing organisms

1.1. ChlorineChlorine2.2. UV LightUV Light3.3. Ozone (gas)Ozone (gas)

Chlorine DisinfectionChlorine Disinfection

1.1. Liquid chlorine Liquid chlorine 2.2. Sodium hypochlorite (tablets) Sodium hypochlorite (tablets)

Note: not allowed to discharge chlorineNote: not allowed to discharge chlorine(it must be removed after disinfection)(it must be removed after disinfection)

* * chlorine removed with either…chlorine removed with either… a. sulfur dioxidea. sulfur dioxide b. sodium bisulfiteb. sodium bisulfite

Tablet ChlorinatorTablet Chlorinator(Calcium Hypochlorite)(Calcium Hypochlorite)

Chlorine DisinfectionChlorine Disinfection To be effective…To be effective…

• Chlorine concentrationChlorine concentration• Contact timeContact time• Proper mixingProper mixing• TemperatureTemperature• Number and type of organismsNumber and type of organisms

Chlorine DoseChlorine Dose

UV DisinfectionUV Disinfection UV LightUV Light

• Specific wavelengths have biocidal Specific wavelengths have biocidal properties (~254 nm)properties (~254 nm)

• Quartz, mercury-vapor lampsQuartz, mercury-vapor lamps• Cleaning required Cleaning required • No residualNo residual

UV DisinfectionUV Disinfectiona specific wavelength of lighta specific wavelength of light

UV DisinfectionUV Disinfection

Ozone DisinfectionOzone Disinfection OO33 a gas, must be generated on-site a gas, must be generated on-site

Bubbled into a basin Bubbled into a basin (or pipeline)(or pipeline) with treated with treated effluenteffluent

Great disinfectant!Great disinfectant!

No residual…ozone degrades to oxygen, ONo residual…ozone degrades to oxygen, O22

Costs More, Need equipment and electricityCosts More, Need equipment and electricity

Ozone DisinfectionOzone Disinfection

Flow DiagramOzone Generation

Unit operations in advanced Unit operations in advanced treatmenttreatment

1. Removal of suspended solids: Removal of suspended solids in advanced

treatment implies the removal of those materials that have been carried over from a secondary settler.

Microstraining Rotating drum-type filter to screen suspended solids Filtering media consists of finely woven stainless steel

fabric with mesh size of 23-35 microns Fabric is mounted on periphery and water is allowed to

pass from inside to the outside

Coagulation and filtration A method in which certain chemicals are rapidly

dispersed in wastewater to change the characteristics of the suspended particles so that they coalasce and sink rapidly

In industrial wastewater treatment, coagulation is frequently used for oily emulsions and finely divided and non-settlable solids such as pigments, paper-fibre, meat and tannery effluents

Most widely used coagulants are aluminium sulphate, ferric sulphate and ferric chloride

Typical reactions are: Al2(SO4)3 + 6H2O 2Al(OH)3 + 3H2SO4

3H2SO4 + 3Ca(HCO3)2 3CaSO4 + 6H2CO3

6H2CO3 6CO2 + 6H2O Overall reaction is represented as: Al2(SO4)3 + 3Ca(HCO3)2 2Al(OH)3 + 3CaSO4

ii. Removal of dissolved solids

Adsorption on activated carbon

Solvent extraction

Ion exchange

Reverse osmosis

Electrodialysis

In many situations, where the risk of public exposure to the reclaimed water or residual constituents is high, the intent of the treatment is to minimize the probability of human exposure to enteric viruses and other pathogens

Effective disinfection of viruses is believed to be inhibited by suspended and colloidal solids in the water, therefore these solids must be removed by advanced treatment before the disinfection step

The sequence of treatment often specified in the United States is: secondary treatment followed by chemical coagulation, sedimentation, filtration, and disinfection

This level of treatment is assumed to produce an effluent free from detectable viruses

Municipal Sewage TreatmentMunicipal Sewage Treatmento Primary treatmentPrimary treatment

• Removing suspended and floating particles by Removing suspended and floating particles by mechanical processes mechanical processes

o Secondary treatmentSecondary treatment• Treating wastewater biologically to decompose Treating wastewater biologically to decompose

suspended organic material; reduces BOD suspended organic material; reduces BOD

Municipal Sewage TreatmentMunicipal Sewage Treatment

o Sewage SludgeSewage Sludge• Solids remaining after primary and Solids remaining after primary and

secondary sewage treatment has been secondary sewage treatment has been completed completed

o Tertiary treatmentTertiary treatment• Advanced wastewater treatment methods Advanced wastewater treatment methods

that are sometimes employed after primary that are sometimes employed after primary and secondary treatments and secondary treatments

• Reduce phosphorus and nitrogenReduce phosphorus and nitrogen

Municipal Sewage TreatmentMunicipal Sewage Treatment

Individual Septic System-Individual Septic System-Septic TankSeptic Tank