Welcome to the presentation onDomestic Waste Water Treatment by Pritilata Sarker Bhabi Mazumder Ranjan Kumar Das Sharmin Akhter
Objectives• Domestic waste water treatment and major ways of classifying wastewater treatment plants• How to design major unit operations and processes for treating wastewater• Microbial growth in heterogeneous cultures• Sludge volume and weight relationships• How to design sludge thickening systems• How to design aerobic and anaerobic sludge digestion systems
Introduction• Wastewater is water that has been contaminated to the degree that it is no longer beneficial, and therefore must be treated before it can be used or released back into the environment.• Four major types of wastewater are 1. Domestic/Municipal wastewater 2. Industrial 3. Urban runoff and 4. Agricultural runoff
Advanced Wastewater Treatment Plant• Remove N & P• Remove additional BOD & TSS that secondary treatment cannot remove• AWT utilizes chemical, biological & physical
N removal by AWT• Nitrification• Denitrification
Nitrification• Nitrification is an aerobic process that transforms ammonium nitrogen (reduced state) into an oxidized form (NO-3)• This process is carried out by autotropic bacteria of the genera Nitrosomonas & Nitrobacter• The overall reaction• NH+4 + 2O2 NO-3 +2H+ + H2O
Denitrification• To remove the nitrate that formed during nitrification , a second biological process known as denitrification must be used in an anoxic environment (void of dissolved oxygen)• Denitrifying , heterotrophic microorganisms reduce nitrate into nitrogen gas, which is released into the atmosphere• The equation is• 6NO-3 + 5CH3OH 3N2 + 6OH- + 5CO2 + 7H2O
P removal by AWT- EBPR• Biological phosphorus removal may be accomplished by providing alternating anaerobic/aerobic treatment of the waste water.• Such systems promote the growth of phosphorus accumulating organisms (PAOs) such as Acinetobacter.• PAOs takes up excess amount of P from wastewater during the aerobic phase of biological treatment.• These process is known as enhanced biological phosphorus removal (EBPR)
Overview of Wastewater Treatment SystemInfluent Secondary Grit Flow Aeration Chlorine Effluent clarifier meter basin contact removal basin
Preliminary Treatment• Bar racks and screens• Grit removal• Pumping• Flow measurement• Equalization• Pre-aeration• Pre-disinfection• Shredding• Flotation
Screening• Mechanically cleaned bar racks- -clear opening 1.5 -6 inch -remove large objects such as logs or other debris• Bar screen -opening 1-2 inch -remove rags, paper & other debris
Grit Removal• Grit consists of sand, silt, small gravel, cinder, coffee grounds, eggshells, and other inert materials, which typically have specific gravity around 2.65• These material are abrasive and will cause pump impellers to wear excessively, and they will accumulate in tanks, digester and pipes.• Three major types of grit removal are aerated chamber, horizontal flow through basin and vortex removal system
Primary Treatment• Primary treatment is clarification, or separation of suspended solids from the wastewater.• Primary Treatment follows preliminary treatment, settling of SS & removal of oil, grease, and scum that floats on surface of wastewater.• Since these species contain organic matter, BOD removal is accomplished.• Primary Treatment does not remove organics soluble or colloidal organic materials. Light weight organics that floats to the surface are skimmed off & pumped to the digesters for treatment
Primary Treatment (continued)• The sludge that accumulates at the bottom of a primary clarifier is normally stabilized by anaerobic digestion before disposal.• Primary sludge is is called “raw” sludge and contain pathogens and organics that produces odors
Primary Treatment (continued)• Primary clarifier: Sl Particulars Specifications 1 Size Circular/ Rectangular 2 Depth 10-16 ft 3 Length 50-300 ft 4 Width 10-80 ft 5 Dia 10-200 ft 6 Detention time 1.5-2.5 hrs 7 Avg. over flow rate 800-1200 gpd/ft2 8 Weir loading rate 10000-40000 gpd/ft2
Primary Treatment (continued)• Design of Primary clarifier: Design of Primary clarifier is based on: -Avg. over flow rate -Detention time -Weir loading rate
Design of Primary clarifier (continued)• The overflow rate (Vo) is defined as the flow rate divided by the surface area of the clarifier where Vo= over flow rate, gpd/ft2 Q= design flow rate, MGD, As= surface area of the clarifier, ft2• The surface area of the clarifier is determined by dividing the design flow rate by the overflow rate. This area calculated is then converted into either a circular or a rectangular area• Selecting detention time the total volume is calculated and clarifier depth is determined.
Design of Primary clarifier (continued)• Detention time (θ) is the average unit of time that the wastewater remains in the clarifier & is determined as follows: where V= volume of the primary clarifier, Q= Design flow rate• Weir loading rate (q) is the third parameter that must be determined when designing primary clarifiers. q= weir loading rate, gpd weir length= length of primary clarifier effluent weir
Secondary Treatment• Secondary wastewater treatment implies that a biological process is being used for treating the wastewater.• Microorganisms indigenous to the wastewater use organic carbon, along with N & P, to grow more microorganisms, primarily bacteria.• Bacteria use the organic matter as measured by BOD & COD for their energy and carbon source.• Oxidation of the organic matter produces energy that is captured in the microbe’s biochemical pathways, while a portion of a organic material is used in the synthesis of biomass.
Secondary Treatment (continued)• The following equations shows the organic materials is being oxidized for energy and organic materials is being synthesized into new microbial cells respectively- organics + O2 CO2+ H2O + energy organics + O2 + N + P C60H87O23N42P
Activated Sludge• It is an aerobic, suspended growth, biological process characterized by two major steps- 1. Substrate adsorption and utilization in the aeration basin. 2. Solids/liquid separation in the secondary clarifier
Activated Sludge• Wastewater flows into the aeration basin, where it is brought into contact with a heterogeneous culture of microbes, consisting primarily of heterotrophic bacteria. The liquid inside the aeration basin is called “mixed liquor” or “Activated sludge”. A schematic of the process is presented in the next slide
System boundary Air Alternative sludge wasting Q + Qr Secondary Effluent clarifier So Aeration basin X, V, S eQ (Q+Qr) (Q-Qr)Xi Xe, Se Xe, SeSi RAS/WAS WAS Pumping Return activated Q, X, Se station sludge, RAS Qw Xr Schematic of Activated Sludge
Design and operational parameters• The main Design and operational parameters for the activated sludge process is the mean cell residence time (MCTR).MCTR represents the average time that the microorganisms or biomass remain in the system. MCTR is also called sludge age, solid retention time (SRT) or θc. Mathematically
Design and operational parameters• Where :• X= biomass or microorganism concentration in aeration basin expressed as TSS or VSS, mg/L,• Xe= secondary effluent TSS or VSS concentration, mg/L,• Xr= TSS or VSS concentration in return activated sludge, mg/L,• V= volume of the aeration basin, mg/L, (m3)• Q= influent wastewater flow rate, MGD (m3 /d)• Qr= return activated sludge flow rate, MGD (m3 /d)• Qw= sludge wastage flow rate, MDG (m3 /d)• Qe= Q-Qw= effluent wastewater flow rate, MDG, (m3 /d)
Design and operational parameters• MCTR typically varies from 5 to 30 days and determines the overall removal efficiency of the process. Generally, the longer the MCTR the lower the effluent substrate concentration as measured by BOD or COD. MCTR is longer than the hydraulic detention time θ. The hydraulic detention time θ is defined as the volume of the reactor or the basin divided by the volumetric flow rate.
Design and operational parameters• θ= detention time based on influent wastewater flow rate, d or h and• θ’= actual detention time for recycle system, d or h.• Another design and operational parameter is the food-to-microorganism ratio which is defined as• F:M= food-to-microorganism ratio• So=influent substrate concentration to aeration basin expressed as BOD, COD or TOC, mg/L
Biochemical kinematics of activated sludge in completely mixed systemThe relationship between the MCRT and net microbialcultures growth rate isSubstrate utilization can be modeled by a Michaelis-Mintontype equation as proposed by Lawrence and McCarty (1970)where k= maximum specific substrate utilization rate, d-1.The effluent soluble substrate concentration Se
Biochemical kinematics of activated sludge in completely mixed system• The microorganism concentration (X)• where So and Se= influent and effluent soluble organic concentration, mass/volume. The volume of the aeration basin, V• The total quantity of biomass produced daily(Px)
Oxygen requirement• Oxygen serves as electron acceptor in the activated sludge process, so sufficient quantities of oxygen must be provided. The following equation is used for estimating the quantity of oxygen required to meet the total oxygen demand. The nitrogenous demand (NOD) is the amount of oxygen required for the nitrifying bacteria. where O2 is the total oxygen required to meet the carbonaceous and nitrogenous oxygen demand.
Aerator System• There are two types of Aerator System 1. Mechanical aerator 2. Diffuser• Mechanical aerator consists of mixers or brush rotors that transfer oxygen into the wastewater by spraying the wastewater into the air. Diffused aerator systems are similar to fish aquarium tanks in which oxygen diffuses through a diffuser stone or membrane, thereby transfer oxygen into the wastewater.
Trickling Filters• Alternate secondary wastewater treatment process.• Wastewater is applied to some type of filter medium.• Media:- Past -Rock or Slag.• Media:- Present- Plastic.• Biomass growing on the media uses the organic matter along with a portion of the nitrogen & phosphorus to grow new microorganisms.
SECONDARY CLARIFICATION• -It must separate the suspended & biological solids from the liquid wastewater.• - The design requires calculating the area of the clarifier based on clarification is determined by dividing the design flow rate by the overflow rate according to the following equation: Ac =
SECONDARY CLARIFICATION• Where:• Ac = the area of clarifier based on clarification,ft2 (m2)• Q = Design flow rate, excluding sludge return flow applied to the secondary clarifier, MGD (m3/d), and• Vo = Overflow rate or surface loading rate, gpd/ft2 [m3/d.m2]
SECONDARY CLARIFICATION• Based on thickening consideration, the following equation is used for determining the secondary clarifier area. AT=• Where:• AT= secondary clarifier surface area.• Q = wastewater design flow rate applied to the secondary clarifier excluding the return activated sludge flow rate, MGD (m3/d)• QR= return activated sludge flow rate, MGD (m3/d)• MLSS= suspended solids concentration in the aeration basin, and• SLR = solids loading rate design criteria, typically 25 ppd/ft2 at average daily flow.
Disinfectants used in Wastewater Treatment• -Disinfectants-Cl2, O3 & ultraviolet (UV) radiations.• -These disinfectants offer the advantage of eliminating the production of trihalmethanes (THMs), which are chlorinated organic species and suspected carcinogens that result when chlorine is added to water containing organic compounds.
Chlorination of Wastewater• -Chlorine is delivered to wastewater treatment facilities in pressurized containers, which range in size from 150-lb to 1-ton cylinders.• -Chlorine is liquefied under high pressure & withdrawn as a gas or liquid, depending on the withdrawal rate.• - At large waste water treatment plant, chlorine is withdrawn as a liquid & must pass through an evaporator to be converted to a gas.
Chlorine Contact Basin• -When chlorine is added to secondary effluent, it must have sufficient contact with the wastewater to kill pathogens.• -Long, rectangular serpentine channels simulating the plug flow regime are used in the design of chlorine contact basins.
Sludge Treatment & Disposal• Sludge or bio-solids and other residuals are generated at wastewater treatment facilities.• Screening & grit from preliminary treatment unit are normally collected in dumpsters; volume is reduced & hauled off to sanitary landfills for ultimate disposal.• Primary sludge contains organic solids & pathogens.• Secondary sludge consists of biological organisms,i.e. biomass & is stabilized using aerobic or anaerobic digestion.• Aerobic & anaerobic digestion decrease the volume & quantity of sludge
4 Steps of Sludge treatment & disposal• Sludge volume is reduced through thickening operation.• Stabilization with aerobic or anaerobic digestion.• Dewatering operations to increase the concentration of solids.• Ultimate disposal.
4 Steps of Sludge treatment & disposal• The volume (V) occupiedby wet sludge (ft3 or m3) is determined using the following equation: V=• Where:• S= Solids content,%• ϒ= Specific weight of water, 62.4 lb/ft3
Thickening Operations• Important because• It reduces the volume of the sludge to be handled, thereby reducing the size of subsequent solids handling processes such as sludge stabilization.• Sludge thickening operations & the approximate solid concentration associated with each include• a) gravity thickening (2% -10% solids)• b)gravity belt thickeners (3%-6%)• c)dissolved air flotation (3%-6%)• d)thickening centrifuges (4%-8%)
Stabilization• Anaerobic Digestion• -A biological process that takes place in an enclosed reactor with no oxygen present.• -It produces a stable sludge & a valuable by-product, methane gas,which can be combustedto provide heat for the digesters or used for generating electricity.• -It is sensitive to operate & prone to biological upsets.• -Anaerobically digested sludge is usually difficult to dewater by mechanical means.• - 3 step process- Hydrolysis Acidogenesis Methanogenesis
Anaerobic Digestion• Hydrolysis:• -Complex organic solids are hydrolyzed by bacteria.• -Carbohydrates,proteins & fats are converted to simple carbohydrates, amino acids and fatty acids.•• Acidogenesis• -Involves the conversion of soluble carbon formed during hydrolysis into organic acids & H2.•• Methanogenesis• -Bacterial conversion of fatty acids & hydrogen into CH4 & Co2 by strict anaerobes.
Anaerobic Digestion• Low or standard-rate anaerobic digester volume can be estimated as follows:• V= T1+ Q2T2• Where:• V =total digester capacity, ft3 (m3)• Q1 =volume of raw sludge fed daily, cfd (m3/d)• T1 =period required for digestion, approximately 25 d• Q2 =volume of daily digested sludge accumulation in tank, cfd (m3/d)• T2 =period of digested storage, 20 to 120 days.
Anaerobic Digestion• For determining the volume of a high-rate anaerobic digester.• V1= Q1xT• V1=digester capacity required for 1st-stage or high-rate digestion, ft3 (m3)• T=period required for digestion, d.• The volume of a 2nd-stage, VH= T1+ Q2T2
Anaerobic Digestion• Where:• VH=digester capacity required for 2nd-stage, ft3 (m3)• Q1=volume of digested sludge feed=volume of average daily raw sludge feed cfd (m3/d)• Q2=volume of daily digested sludge accumulation in tank, cfd (m3/d)• T1=period required for thickening, days• T2= period required for digested sludge storage, days
Aerobic Digestion• Similar to the activated sludge process.• -Organic sludge is aerated for extended periods to oxidized organic material.• -Works best on waste activated sludge.• -Reduces significant pathogens. Organic matter+O2 new cells+energy+CO2+H2O+end products
Dewatering• -Consists of reducing the sludge volume & increasing the solids content.• -Concentration includes-• * Centrifuges (5% to 40% solids)• * Belt filter presses (12% to 50%)• * Vacuum filtration (20% to 30%)• * Plate & frame presses (34% to 60%)• * Sand drying beds (up to 50%)
Sludge Disposal• Various options are available for proper disposal.• Incineration of sludge is an option that is becoming less attractive because of the high cost of building & operating incinerators, plus the creation of air pollution & need to landfill the ash.