Fundamentals of Wastewater Treatment

Arya VDepartment of Civil Engineering

Indian Institute of Technology Delhi

Wastewater Characteristics

Physical

• Solids

• Color

• Temperature

• Odor

Chemical

• pH

• Alkalinity

• Organic matter

• Heavy metals

• Refractory organics

• Nutrients

Biological

• Bacteria

• Virus

• Protozoa

• Helminths

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Biochemical oxygen demand

Amount of oxygen consumed during microbial utilization of organics

▪ In a lab test, we find BOD5 by incubating the sample at 20ᵒC for 5 days after adding essential nutrients for the microbial process.

BOD = (𝐷𝑂𝐼−𝐷𝑂𝐹)/𝑃

• DOI Initial DO Conc. • DOF Final DO Conc.• P Dilution factor

BOD Kinetics

▪ Lt is the oxygen equivalent of the organics at time tand k is a reaction constant.

▪ L0 is the oxygen equivalent of the total mass of organics.

• Rate at which organics are utilized is directly proportional to the amount available.

▪ Value of reaction constant k is temperature dependent.

▪ Change in k is given by van’tHoff-Arrhenius equation.

𝐾𝑇 = 𝐾20 𝜃𝑇−20

• 𝜽 value taken is 1.047• 𝑲𝑻 represents the value

at temperature T

▪ Determine ultimate BOD and 3-day BOD at 27 C for a wastewater whose 5-day BOD at 20 C is 200 mg/L.

k=0.23 /day

Chemical oxygen demand

▪ Amount of oxygen required to oxidize the organic material in wastewater

▪ Using dichromate solution

▪ Treatability of wastewater

BOD/COD >0.5 biodegradable

Nutrients

▪ Total Nitrogen= Organic N+ Ammonia+ Nitrite+ Nitrate

▪ Total Kjeldahl Nitrogen = Organic N+ Ammonia- N

▪ Nitrification

▪ Denitrification

▪ Total phosphorus= Ortho P+ Poly P+ Org. P

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WASTEWATER TREATMENT

(i)Preliminary Treatment✓ removes large objects, girts and non-degradable materials

✓protects pumps, mechanical equipment from damage, prevent clogging of valves

✓screens and grit chamber

(ii) Primary treatment✓ remove suspended, easily settleable and floating material

✓Primary sedimentation tank (Primary clarifier)

(iii) Secondary treatment✓Remove suspended, colloidal and dissolved organic sand inorganics

matter

✓Biological treatment (ASP-Activated Sludge Process, Trickling filter, RBC-Rotating biological contactors),chemical-physical processes

(iv) Tertiary treatment. ✓Polishing the secondary treated effluent to meet the reuse / discharge

requirements

✓Absorption, Advanced oxidation, disinfection

Contaminant Process of removal

Suspended solids SedimentationScreening and CommunitionFiltrationFlotationCoagulation/SedimentationLand treatment systems

Biodegradable organics Biological treatment

Pathogens Disinfection

Nutrients Biological nutrient removalPhysico-chemical treatment

Refractory organics AdsorptionAdvanced oxidation processes

Heavy metals Chemical treatment

Dissolved inorganic compounds Ion exchangeReverse osmosis

Screens

▪ Coarse screens (6 to 150 mm)

▪ Fine screens (<6 mm)

▪ Flow velocity should not exceed1m/s, design velocity-0.3m/s

▪ Head loss less than 0.1m, Itdepends on the degree of clogging

▪ Solids removed depends on screenopening size

▪ Disposal practices: sanitary landfill, grinding and returning to the wastewater flow and incineration

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Comminutor

▪ It is a shredding device to break or cut up solids to approximately 8mm in size located across the flow path and ahead of pumping facilities

Grit chamber

▪ Removes sand, gravel and other heavy solid materials

▪ Protects mechanical equipment from abrasion and wear

▪ Reduces formation of deposits in area of low hydraulic shear in pipelines, channels and sumps

Channel-type grit chamber

▪ Grit chambers follow type-1 settling

Aerated grit chambers

▪ Turbulence created by the injection of compressed air keeps lighter organic material in suspension while the heavier grit falls to the bottom

CHANNEL-TYPE GRIT CHAMBER

AERATED GRIT CHAMBERS

Equalization tank

▪ Damping of flowrate variations to achieve constant flowrate

▪ In-line or Off-line equalization

Primary sedimentation tank

▪ Removes readily settleable solids and floating materials

▪ TSS reduction – 50 to 60%

▪ BOD reduction – 25 to 40%

▪ Organic material is slightly heavier than water and it settle slowly (1 to 2.5m/s) to the bottom of tank under gravitational force

▪ Follows Type-2 settling

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Secondary Treatment

▪ Removal of dissolved organics and remaining 40-50%

suspended solids (which are not removed in primary

treatment)

▪ Biological processes are commonly used as secondary

treatment to remove organics

▪ Biological Process is the process which decomposition of

organic matter using microorganism into acceptable end

products

Different Types of Microorganisms▪ Carbon sources: Autotrophs-CO2 from Atmosphere or

Bicarbonates

Heterotrophs- Carbon from organic matter

▪ Energy sources: Phototrophs-Sunlight

Chemotrophs-derive energy from chemical reaction

▪ Temperature: Psychrophilic -15 to 30oC

Mesophilic- 30 to 45oC

Thermophilic-45 to 70oC

▪ Oxygen Requirement: Aerobic (Aerobes)- Presence of O2

Anaerobic (Anaerobes) - Absence of O2

Facultative- work in both condition

Chemoheterotrophs microorganisms are mostly used in Biological Treatment

MICROBIAL GROWTH CURVE

▪ LAG PHASE:

The phase in which microorganism get acclimated to the surrounding environment and to the food provided

▪ LOG PHASE:

Maximum growth of microorganism occurs at a logarithmic rate

▪ STATIONARY PHASE:

Nutrients get depleted, microorganism unable to obtain food from external sources, toxic metabolite accumulation growth and death become equal

▪ ENDOGENOUS PHASE :

Cells undergo endogenous respiration, A stage in which living organisms oxidize some of their own cellular mass

MICROBIAL GROWTH IN LOG-GROWTH PHASE▪ Bacterial Cells reproduce by binary fission- divided into two new

independent cells

▪ Regeneration time

➢ time required for a cell to mature and separate

➢depends on environmental factors and food supply

➢Around 20 min

▪ Rate of reproduction is exponential

▪ Number of organisms produced (N)=2n-1

where, n= regeneration time

▪ Specific growth rate(𝝁)

𝜇 =1

𝑋×𝑑𝑋

𝑑𝑡

X = concentration of biomass, mg/L

𝑑𝑋

𝑑𝑡= growth rate of the biomass mg/L-t

µ = Specific growth rate of bacteria

MONOD EQUATION▪ It is a kinetic model which describes microbial growth as a functional

relationship between the specific growth rate and substrate concentration

𝜇 =𝜇𝑚𝑎𝑥𝑆

𝐾𝑠 + 𝑆

Where µ = Specific growth rate of bacteria

µmax = Maximum value of specific growth rate

S = Concentration of limiting substrate or nutrients

Ks = Half-Saturation constant, equal to the concentration of substrate

giving growth rate of µmax/2

▪ Yield Coefficient( Y): Amount biomass produced per unit substrate utilized

Y=dX/dS

Where, dX =change in the biomass concentration

dS=change in the substrate concentration

▪ Specific substrate utilization rate( q):

𝑞 =1

𝑋

𝑑𝑆

𝑑𝑡

▪ Decay coefficient (kd)

𝜇 =𝜇𝑚𝑎𝑥𝑆

𝐾𝑠 + 𝑆− 𝑘𝑑

𝜇 = 𝑌𝑇𝑞 − 𝑘𝑑

𝜇=𝑌𝑜𝑏𝑠𝑞

YT – True yield or Theoretical yield coefficient

Yobs – Observed yield coefficient

FACTORS AFFECTING MICROBIAL GROWTH▪ Temperature

➢Rate constants increase with increasing temperatures within the range of 0 to 55oC ,with a corresponding increase in biomass production and food utilization

➢Excessive heat denatured the enzymes and destroy the organism

▪ pH➢Microorganisms that degrade wastewater organics function best near

neutral pH

▪ Toxicants➢poison the microorganism

▪ Salt concentration ➢ interfere with internal-external pressure relationship

▪ Oxidants ➢destroy enzyme and cell materials

BASIS OF BIOLOGICAL PROCESS IN WASTE WATER

▪ Microorganism utilize the organic matter in the waste water as food

and convert it into stable products (CO2,H2O,CH4 etc) and new cells

▪ New cells know as biomass which gradually settle down as sludge in the

system

▪ Involved two distinct metabolic phases – Catabolism (respiration)

Anabolism (Synthesis)

➢Catabolism- Oxidized organic matter, nutrients etc. and produced

energy

➢Anabolism- Using that energy new cells are synthesis

DIFFERENT TYPES OF BIOLOGICAL TREATMENT PROCESS

▪ Two types- Attached growth and Suspended growth systems

▪ Suspended growth system

➢Microorganisms are maintained in suspension within the wastewater

either as single cells or as clusters of cells called flocs.

▪ Attached growth system

➢Microorganisms are attached(adhered) to some inert media such as rocks,

slog or specially designed ceramic or plastic materials.

➢During biological process wastewater contacted with this microbial films

attached to surfaces of media

Suspended growth Attached growth

Aerobic Process • Activated Sludge Process

• Aerated lagoons• SBR (Sequencing

batch reactor) • Oxidation ditch

• Trickling filters• Rotating

biological contactors

Anaerobic Process • UASB(Up flow anaerobic sludge blanket)

• Anaerobic digesters

• Anaerobic filters

ACTIVATED SLUDGE PROCESS(ASP)▪ The most conventional unit for the aerobic biological treatment of

wastewater

▪ Two Parts- Aeration tank and Secondary sedimentation tank(SST)

▪ Aeration Tank- where organic matter stabilized by the action of bacteria under aeration

▪ SST-where the sludge(biological cell mass) is separated from the effluent of aeration tank and the settle sludge (25 to 50 %) is recycled partly to the aeration tank and remaining wasted

▪ Aeration conditions are achieved by use of diffused or mechanical aerators

▪ Diffuser-Provided at the tank bottom

▪ Mechanical aerators- Provided at the surface of water(either floating or fixed)

▪ Design parameters- Aeration period, BOD loading, MLSS, Food-to-microorganism ratio(F/M), SRT etc

ACTIVATED SLUDGE PROCESS(ASP)

Qo, So, Xo

V, X, S

Qr, Xu

Qw, Xu

Qu, Xu

Qo+QrX, S

Q-Qw, Xe, S

Design assumptions

▪ The influent and effluent biomass concentrations are

negligible compared to biomass at other points in the

system

▪ The influent food concentration So is immediately diluted to

the reactor concentration S because of the complete-mix

regime

▪ All reactions occur in the aeration tank

▪ Mass balance for biomass in the system,

Biomass in + Biomass growth = Biomass out(Effluent +Wasted sludge)

𝑄0𝑋0 + 𝑉𝜇𝑚𝑎𝑥𝑆𝑋

𝐾𝑠 + 𝑆− 𝑘𝑑𝑋 = 𝑄0 − 𝑄𝑤 𝑋𝑒 + 𝑄𝑤𝑋𝑢

𝑉𝜇𝑚𝑎𝑥𝑆𝑋

𝐾𝑠+𝑆− 𝑘𝑑𝑋 =𝑄𝑤𝑋𝑢

𝜇𝑚𝑎𝑥𝑆

𝐾𝑠+𝑆=

𝑄𝑤𝑋𝑢

𝑉𝑋+ 𝑘𝑑

▪ Mass balance for food

Food in – Food consumed = Food out

𝑄0𝑆0 − 𝑉𝜇𝑚𝑎𝑥𝑆𝑋

𝑌(𝐾𝑠 + 𝑆)= 𝑄0 − 𝑄𝑤 𝑆 + 𝑄𝑤𝑆

𝑉𝜇𝑚𝑎𝑥𝑆𝑋

𝑌(𝐾𝑠+𝑆)=𝑄0(𝑆0−𝑆)

𝜇𝑚𝑎𝑥𝑆

𝐾𝑠+𝑆=

𝑄0𝑌

𝑉𝑋(𝑆0−𝑆)

Hydraulic retention time, = V/Qo

Biological Solid Retention Time, c = VX/QwXu

Diffuser Surface Aerators

SST

1) Volumetric Organic Loading Rates(OLR)-(BOD Loading)

VL = (Q*S0)/V

where, Q=Flow rate (m3/day)

S0=Influent BOD5 to aeration tank(mg/L)

V=volume of aeration tank (m3)

The mass of BOD (Kg) in the influent (without including the return sludge) per unit volume of the reactor(aeration tank) per day

2) Food-to-Mass(Microorganism) Ratio (F/M)

F/M=Q(S0-S)/VX

where, Q=Flow rate (m3/day)

S0=Influent BOD5 to aeration tank(mg/L)

S= BOD5 in the reactor (aeration tank) (mg/L)

V=volume of aeration tank (m3)

X=MLVSS (Mixed liquor (liquid) volatile suspended solids (mg/L)

DESIGN CRETERIA FOR ASP

3) Biological solid retention time (BSRT)(𝜃𝑐)

𝜃𝑐 = 𝐾𝑔 𝑜𝑓 𝑀𝐿𝑉𝑆𝑆 𝑖𝑛𝑎𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑡𝑎𝑛𝑘

𝐾𝑔 𝑜𝑓 𝑉𝑆𝑆 𝑤𝑎𝑠𝑡𝑒𝑑+𝐾𝑔 𝑜𝑓 𝑉𝑆𝑆 𝑖𝑛 𝑒𝑓𝑓𝑙𝑢𝑒𝑛𝑡

= 𝑉𝑋

𝑄𝑊𝑋𝑈

Assume kg of VSS in effluent=0

where, V=volume of aeration tank

X=Biomass concentration in reactor

𝑄𝑊= flow rate of waste sludge

𝑋𝑈 = biomass concentration in waste sludge

▪ BSRT is the duration for microorganism stays in the aeration tank

4) Sludge Volume Index (SVI)

𝑆𝑉𝐼(𝑚𝐿

𝑔) =

Settled sludge volume(mL/L)∗ 1000MLSS

▪ It the volume in mL occupied by 1g of activated sludge after settling the aerated liquor for 30min,and therefore indicates the settling characteristics of sludge in the aeration tank

▪ SVI value – Less- sludge is dense, good settling characteristics

- More-sludge is fluffy, poor settling characteristics

1 L of Wastewater

Volume of sludge settled

PONDS AND LAGOONS▪ Suspended culture biological system

▪ A large shallow earthen basin(lined), which wastewater is retained long enough for natural purification processes

▪ Ponds: some oxygen is supplied by diffusion from the air bulk of the oxygen is provided by photosynthesis

▪ Lagoons: oxygen is provided by artificial aeration (mechanical aerators)

▪ Aerobic ponds: ➢shallow ponds➢at all depth dissolved oxygen is available➢used as polishing or tertiary pond

▪ Anaerobic ponds: ➢deep ponds ➢dissolved oxygen is absent except for a relatively thin surface

layer➢used for partial treatment of strong organic wastewater (as

pretreatment)

▪ Facultative ponds: ➢both aerobic and anaerobic zones exist➢used as total treatment system for municipal wastewater

PONDS AND LAGOONS

▪ Lagoons are classified as Aerobic lagoons and Facultative lagoons (according to the degree of mechanical mixing)

▪ Aerobic lagoons:

➢Sufficient energy is supplied both to meet the oxygen requirement and to keep the entire tank content mixed and aerated

▪ Facultative lagoons:

➢Energy input is only sufficient to transfer the amount of oxygen required for biological treatment but is not sufficient to maintain the solids in suspension

➢A portion of incoming solids will settle along with a portion of biological solids produced from the conversion of the soluble organic substrate

➢In time, settled solids will undergo anaerobic decomposition

➢Eventually facultative lagoons must be dewatered and the accumulated solids are removed

FACULTATIVE LAGOONS

▪ Two zones- Aerobic, Anaerobic

▪ Upper Zone- Aerobic Zone

Aeration condition is achieved by

➢by oxygen generated by algae

➢by penetration of atmospheric oxygen (to a lesser extent)

➢Symbiotic relationship (mutually beneficial) between algae and bacteria

➢Bacteria use O2 as e- acceptor

➢Oxidize organics in wastewater to end products such as CO2,NO3

and PO4

➢Algae use these compounds with sunlight as energy source and produce oxygen as an end product. Produced O2 is used by bacteria

▪ The boundary between the aerobic and anaerobic zones

➢ Not stationary

➢aerobic zone can increases downward due to more mixing by wind and more penetration by sunlight

➢Anaerobic zone can increases upward due to clam waters and weak lighting

➢diurnal fluctuations in aerobic-anaerobic interface

▪ Lower zone- Anaerobic zone

➢Sludge along the bottom prevent oxygen transfer to that region and anaerobic conditions prevail

➢In anaerobic zone - Organic acids, Gases, Products of decomposition are foods for organisms in the aerobic zone

➢ Biological solids produced in the aerobic zone ,settle to bottom where they die and providing food for the anaerobic organisms

TRICKLING FILTERS

▪ Non-submerged fixed film biological reactor

▪ Attached culture biological system

▪ A reactor in which randomly packed solid (rock or plastic) provide surface area for biofilm growth

▪ Sorption and biological oxidation are the primary means of food removal

▪ Ideal filter packing: ➢High specific surface area per unit volume

➢Low in cost

➢High durability

➢High enough porosity

➢Chemically resistant surfaces

➢Hard

PACKING MATERIALS

ROCK PLASTIC VERTICAL FLOW

PLASTIC VERTICAL FLOW

PLASTIC CROSS FLOW

REDWOOD HORIZONTAL RANDOM PACKED

DISTRIBUTION SYSTEMS IN TRICKLING FILTERS

BIO-TOWERS

Advantages

➢Avoid plugging – due high porosity and nature of packing

➢Increase ventilation minimizes odour problem

➢Less economical

Disadvantages

➢High pumping cost

➢Head loss through the deep bed

ROTATING BIOLOGICAL CONTACTORS (RBCs)

▪ The cylindrical plastic disks are attached to a horizontal shaft and are provided at standard unit sizes of approx. 3.5 m in diameter and 7.5 m in length

▪ The RBC unit is partially submerged in a tank containing the wastewater

▪ As the RBC disks rotate out of the wastewater, aeration is accomplished by exposure to the atmosphere

▪ Separate baffled basins are needed to develop the benefits of a staged biological reactor design

NUTRIENT REMOVAL

▪ Discharging of excess nutrient content wastewater causes eutrophication in receiving water bodies

▪ Eutrophication is when a body of water becomes overly enriched with minerals and nutrients which induce excessive growth of plants and algae. This process may result in oxygen depletion of the water body

▪ Nutrients most often interest are Nitrogen and Phosphorus compounds

NUTRIENT REMOVAL OPTIONS

Nitrogen Removal

Phosphorus Removal

Nitrification and Denitrification

(Biological method)

Air stripping (physico-chemical

process)

Biological phosphorus removal

Chemical phosphorus removal

AIR STRIPPING

▪ Dissolved ammonium is converted to gaseous phase and then dispersed in air, thus allowing transfer of the ammonia from wastewater to the air.

▪ pH must be greater than 11 for complete conversion to NH3

▪ The gaseous phase NH3 and aqueous phase NH4

+ exist together in equilibrium controlled by the pH and the temperature

NH4+ + OH- = NH3 +2H20

AIR STRIPPING

NITROGEN REMOVAL

▪ nitrogen in form of organic nitrogen and ammonia

▪ two step process- nitrification and Denitrification

Nitrification

➢The conversion of ammonium to nitrate through nitrite by autotrophic bacteria such as nitrosomonas and nitrobacter in aerobic environment

Denitrification

➢The reduction of nitrate into nitrogen gas by facultative heterotrophic bacteria in anoxic environment

➢In anoxic environment, bacteria use the oxygen attached to the nitrogen in form of nitrate(electron acceptor) and where as a carbon-source(such as methanol) may be additional

Nitrification

▪ Chemoautotrophic bacteria

𝑁𝐻4+ ↔ 𝑁𝐻3 + 𝐻+

𝑁𝐻4+ + 1.5𝑂2 → 𝑁𝑂2

− + 2𝐻+ + 𝐻2𝑂 ………. Nitrosomonas

𝑁𝑂2−+0.5𝑂2 → 𝑁𝑂3

− ………………………………… Nitrobacter

Amount of oxygen consumed

Amount of alkalinity consumed

Denitrification

▪ Chemoheterotrophic bacteria under anoxic conditions

𝑁𝑂3− +

5

6𝐶𝐻3𝑂𝐻 →

1

2𝑁2 +

5

6𝐶𝑂2 +

7

6𝐻2𝑂 + 𝑂𝐻−

Alkalinity produced=

Methanol required=

Phosphorus removal

▪ Phosphorus forms include organic phosphorus, polyphosphates and orthophosphates.

▪ Principal form of phosphorus in wastewater is assumed to be orthophosphates.

▪ Orthophosphates consist of negative radicals PO43- , HPO4

2- , H2PO4-

Principal means of phosphorus removal is chemical precipitation

Al3+ + (HnPO4)(3-n)- → AlPO4 ↓ + nH+

Fe3+ + (HnPO4)(3-n)- → FePO4 ↓ + nH+

▪ Iron and aluminium salts can be added to precipitate phosphate out

▪ Phosphorus removal can be incorporated into primary or secondary treatment or may be added as a tertiary treatment.

Sludge treatment

Sludge Thickening

▪ Sludge represents concentration of solids, impurities and other objectionable materials and must be disposed of properly. Basic concept is Volume reduction.

Gravity thickeners are commonly used and they are very similar to secondary clarifiers used in suspended growth systems.• Tanks are generally deeper to provide

better thickening capacity. • Well designed gravity thickener doubles the

content of solids in the sludge (thereby reducing half the volume.

Field scale gravity thickener

Sludge Thickening

▪ Gravity

▪ Floatation

▪ Centrifugation

▪ Rotary drum

▪ Gravity Belt

Dewatering

▪ Centrifuges

▪ Vacuum filters

▪ Belt filters

▪ Filter press

Floatation system

Dissolved air floatation system for thickening activated sludge

▪ The thickened sludge is skimmed off at the top of the tank while the liquid is removed near the bottom and is returned to the aerator.

DAF system

Sludge digestion

Common means of stabilizing sludge is by biological degradation. ▪ Sludge digestion reduce the volume of the thickened sludge as well as render solids inert.

Pathway of anaerobic digestion

✓ Anaerobic decomposition produces less biomass than aerobic processes.

✓ This decomposition is carried by two groups of microbes acid formers and methane formers.

✓ Acid formers solubilize the organic solids through hydrolysis.

✓ Methane formers convert acids and alcohols along with hydrogen and carbon dioxide to methane.

Anaerobic digester

Standard Rate Anaerobic Digester

▪ Typical standard rate anaerobic digester consist of single stage operation.

▪ Sludge is fed into the digester on an intermittent basis and the supernatant is withdrawn and returned to the secondary treatment unit.

▪ Digested sludge accumulates in the bottom.

V = Volume of the digester (m3)V1 = raw sludge loading rate (m3/d)V2 = digested sludge accumulation rate (m3/d)t1 = digestion period, dt2 = digested sludge storage period, d

Standard rate anaerobic digester

High rate digester

High rate, two stage anaerobic sludge digester

▪ High rate digesters are more efficient and requires less volume than single stage digesters.

▪ Contents are mechanically mixed to ensure better contact between organics and microbes.

Sludge disposal

Wastewater disposal

Land disposal

Infiltration

▪ Percolation to groundwater▪ Recovery of water by underdrains or wells.▪ Temporary storage of treated water.▪ Easy and economical, also not constrained

by seasonal changes.