Sustainability & Environment (Agri-Food Systems) · Pollution Control & Environmental Engineering · also Water Supply and Wastewater Engineering, Environmental Engineering I & II, Effluent Treatment
Making water safe to drink and wastewater safe to release or reuse: unit operations, biological treatment, reactor kinetics, sludge, industrial effluents and zero liquid discharge. Farm-scale lagoons and manure systems are in the Agricultural Engineering subject waste-management-environmental-engg; this subject covers municipal and agro-industrial treatment to GATE ES depth.
In the exams: GATE ES Section 5 (whole section, numerical); ICAR AICE subject 71 Unit 4; UGC NET Unit 6; NCEES FE/PE Environmental
Water and wastewater treatment protect public health and the environment by making water safe to drink and wastewater safe to release or reuse. Treated drinking water and sewerage are among the greatest public-health advances in history, yet billions of people still lack safely managed drinking water or sanitation, and a large share of the world's wastewater, especially in low- and middle-income countries, is discharged with little or no treatment. In India, the Central Pollution Control Board's assessments have repeatedly found that installed sewage treatment capacity covers well under the volume of sewage generated in urban areas, while agro-based industries -- dairies, sugar mills, distilleries, food processors -- produce some of the strongest effluents of any sector.
This subject covers municipal and agro-industrial treatment to the depth examined in GATE Environmental Science and Engineering Section 5, which is largely numerical. It begins with water and wastewater quality parameters and BOD kinetics, population forecasting and design flows, then follows water through treatment (screening, sedimentation, coagulation and flocculation, filtration, desalination and disinfection) and distribution, and wastewater through sewers and primary, secondary and tertiary treatment, with activated sludge, trickling filters, anaerobic treatment and UASB reactors, reactor kinetics and sludge treatment. It ends with agro-industrial effluents and with common effluent treatment plants, recycling and zero liquid discharge.
Farm-scale lagoons, manure systems and on-farm waste management are in the Agricultural Engineering subject Waste Management and Environmental Engineering; the effects of pollutants on receiving waters, including the oxygen sag curve and eutrophication, are in Water Quality and Agricultural Non-point Source Pollution; pipe and open-channel hydraulics are in Hydrology and Water Resources; and reuse of treated wastewater in agriculture is in Agricultural Water Conservation, Water Footprint and Governance. This manual focuses on the treatment processes themselves.
Indian practice follows the Central Public Health and Environmental Engineering Organisation (CPHEEO) manuals and CPCB discharge standards; other countries use their own codes. Design values quoted (loading rates, detention times, coefficients) are typical textbook ranges for illustration; actual designs use the applicable national manual and site data. Every worked example has been checked by calculation.
Physical parameters include turbidity (NTU), colour, temperature, and solids. Total solids (TS) are the residue after evaporation at 103-105 degrees C; they divide into total suspended solids (TSS, retained on a filter) and total dissolved solids (TDS, passing it); each divides further into volatile solids (lost on ignition at 550 degrees C, a rough measure of organic matter) and fixed solids. Chemical parameters include pH, alkalinity, hardness, dissolved oxygen, nutrients, metals and organic content. Hardness is expressed as mg/L of calcium carbonate: hardness = Ca x (50/20.04) + Mg x (50/12.15), i.e. about 2.5 x Ca + 4.1 x Mg (both in mg/L); water with Ca 40 mg/L and Mg 12 mg/L has about 149 mg/L hardness as CaCO3. Nitrogen forms include organic N, ammonia, nitrite and nitrate; total Kjeldahl nitrogen (TKN) is organic N plus ammonia N. Biological parameters include coliforms and E. coli as faecal indicators.
Organic content is measured several ways. Biochemical oxygen demand (BOD) is the oxygen consumed by microbes oxidising biodegradable organic matter, usually over 5 days at 20 degrees C (BOD5). Chemical oxygen demand (COD) uses strong chemical oxidation by potassium dichromate in acid and measures nearly all organic matter, biodegradable or not; it is always at least as large as BOD and takes about two to three hours. Total organic carbon (TOC) is measured by combustion. The BOD5/COD ratio indicates biodegradability: above about 0.5 for readily biodegradable wastes such as domestic sewage and many food-industry effluents, much lower for wastes with recalcitrant organics. Typical untreated domestic sewage has BOD5 roughly 150-350 mg/L and TSS roughly 150-400 mg/L, varying with water use.
BOD exertion is modelled as first order: the BOD remaining at time t is L_t = L0 e^(-kt), and the BOD exerted is BOD_t = L0 (1 - e^(-kt)), where L0 is the ultimate carbonaceous BOD and k the rate constant (base e, per day). With k = 0.23 per day, BOD5 = L0 (1 - e^(-1.15)) = 0.68 L0. When constants are given in base 10 (K), k = 2.303 K. The rate constant depends on temperature: k_T = k_20 theta^(T - 20), with theta commonly taken as 1.047 for BOD; at 30 degrees C, k = 0.23 x 1.047^10 = 0.36 per day, so BOD is exerted faster in warm climates. Nitrogenous BOD, from oxidation of ammonia to nitrate by nitrifying bacteria, typically begins after several days and is suppressed with an inhibitor when only carbonaceous BOD is wanted. The theoretical oxygen demand of ammonia nitrification is 4.57 g O2 per g NH4-N.
BOD is measured by incubating diluted samples, seeded if necessary, in sealed bottles and measuring DO before and after: BOD = (D1 - D2) / P for unseeded samples, where P is the decimal fraction of sample in the bottle. Constants k and L0 can be estimated from a BOD time series by methods such as the Thomas graphical method or least squares. BOD is the basis of treatment plant design, effluent standards and river pollution models.
Water supply and sewerage systems are designed for the population expected at the end of a design period, commonly about 30 years for major components in India (shorter for mechanical equipment, longer for some intakes and conduits). Standard forecasting methods use past census data. The arithmetic increase method assumes a constant increase per decade: Pn = P0 + n x, where x is the average decadal increase -- suitable for large, established cities growing slowly. The geometric increase method assumes a constant percentage growth rate: Pn = P0 (1 + r)^n, where r is the average growth rate per decade (or per year with n in years) -- suitable for young, rapidly growing towns, but liable to overestimate for long periods. The incremental increase method adds the average increment of increase: Pn = P0 + n x + n(n + 1) y / 2, where y is the average change in decadal increase. Graphical, comparative, logistic-curve and master-plan methods are also used. Example: a town of 100,000 growing at 2% a year (geometric) reaches 100,000 x 1.02^30 = about 181,000 in 30 years.
Per capita water demand depends on city size, climate, habits and supply. CPHEEO guidelines in India recommend 135 litres per capita per day (lpcd) for cities with existing or planned sewerage, 70 lpcd for towns without sewerage, and 150 lpcd for metropolitan and mega cities, with additional allowances for institutional, commercial and fire demand and for unaccounted water. Demand fluctuates: the maximum daily demand is often taken as about 1.8 times the average, and the peak hourly demand as about 1.5 times the maximum daily (about 2.7 times average). Fire demand is estimated by formulas based on population.
Sewage flow is commonly taken as about 80% of water supplied, the rest being lost or used outside the sewer system; infiltration of groundwater into sewers adds to it. Peak sewage flows are estimated with peak factors that decrease as population increases (Babbitt's and Harmon's formulas, or CPHEEO's tabulated factors of about 2 to 3). Example: for 150,000 people at 135 lpcd, average water demand is 20.25 million litres per day (MLD); sewage is about 16.2 MLD; with a peak factor of 2.25, the design peak sewage flow is about 36.5 MLD. Storm-water flows for combined or storm sewers are estimated with the rational method (see Hydrology and Water Resources).
Screens remove large floating and suspended objects: coarse screens (bar racks with openings of tens of millimetres) protect pumps and downstream units; fine screens remove smaller material. Grit chambers settle sand and grit (specific gravity about 2.65) while keeping organic solids in suspension, typically by controlling horizontal velocity near 0.3 m/s. Sedimentation removes settleable solids by gravity. Four types are recognised: Type I (discrete particles settling independently, as in grit chambers, described by Stokes' law), Type II (flocculent particles that coalesce and accelerate, as in primary clarifiers and chemically coagulated water), Type III (zone or hindered settling, where high concentrations settle as a mass, as in secondary clarifiers) and Type IV (compression at the bottom of thickeners).
In an ideal settling basin, a particle is completely removed if its settling velocity vs is at least the overflow rate (surface loading rate) vo = Q / A, which is independent of depth; particles with vs < vo are removed in the fraction vs / vo. Detention time t = V / Q. Example: a clarifier treating 10,000 m3/d at an overflow rate of 30 m3/m2/d needs 333 m2 of surface; at 3 m depth its volume is 1,000 m3 and detention time 2.4 hours; a particle settling at 0.2 mm/s (17.3 m/d) is removed about 58%. Primary clarifiers typically remove about 50-70% of suspended solids and 25-40% of BOD from sewage.
Colloidal particles (clay, colour, microorganisms) are too small to settle and carry negative surface charges that keep them apart (described by the electrical double layer and zeta potential). Coagulation destabilises them by adding coagulants -- aluminium sulphate (alum), ferric chloride or sulphate, polyaluminium chloride -- through mechanisms of double-layer compression, charge neutralisation, sweep flocculation (enmeshment in precipitating metal hydroxide) and inter-particle bridging by polymers. Alum reacts with alkalinity, so low-alkalinity waters may need lime or soda ash; the optimum dose and pH are found by jar tests. Rapid mixing disperses the coagulant in seconds to a minute.
Flocculation then gently mixes the water for perhaps 20-40 minutes so that destabilised particles collide and grow into settleable flocs. Mixing intensity is expressed by the velocity gradient G = (P / (mu V))^(1/2), where P is power input, mu dynamic viscosity and V volume; rapid mixing uses G of several hundred to around 1,000 per second, flocculation about 20-80 per second, and the product G t (dimensionless) of the order of 10^4 to 10^5. Example: 500 W in a 10 m3 tank of water (mu = 0.001 Pa s) gives G = (500 / 0.01)^(1/2) = 224 per second, appropriate for rapid mixing. Coagulation, flocculation and sedimentation together remove most turbidity and many pathogens before filtration.
Filtration removes remaining particles, flocs and pathogens by passing water through granular media. Slow sand filters operate at low rates (of the order of 0.1-0.4 m/h) and rely on a biologically active surface layer, the schmutzdecke, for treatment; they are simple and effective but need large areas and periodic scraping. Rapid sand filters operate at about 5-15 m/h after coagulation and settling; particles are captured within the bed, and filters are cleaned by backwashing with water (and often air) when head loss or turbidity breakthrough limits are reached. Dual- and multimedia filters (anthracite over sand) use the bed depth more effectively. Example: treating 10,000 m3/d (417 m3/h) at 5 m/h requires about 83 m2 of filter area, divided into several units so that one can be backwashed.
Membrane processes include microfiltration and ultrafiltration (particles and pathogens), nanofiltration (hardness, some organics) and reverse osmosis (RO), which removes dissolved salts by applying pressure greater than the osmotic pressure. Osmotic pressure is approximately pi = i C R T (van 't Hoff): seawater with about 35 g/L of salt has an osmotic pressure of roughly 25-30 bar, so seawater RO operates at about 55-80 bar. Thermal desalination (multi-stage flash, multi-effect distillation) is used where energy is cheap. Desalination produces concentrated brine requiring careful disposal, and energy use is its main cost and environmental burden.
Disinfection inactivates pathogens. Chlorine is the most widely used disinfectant: in water it forms hypochlorous acid (HOCl), a strong disinfectant that dissociates to the weaker hypochlorite ion (OCl-) with a pKa of about 7.5, so chlorination is more effective at lower pH. Chlorine reacts first with reducing substances and ammonia (forming chloramines, weaker but longer-lasting combined chlorine); breakpoint chlorination adds enough chlorine to oxidise ammonia and leave a free residual. Chlorine demand = dose - residual. A residual of about 0.2-0.5 mg/L at the consumer's tap protects against recontamination. Example: a plant treating 10,000 m3/d at a dose of 2 mg/L uses 20 kg of chlorine a day.
Disinfection kinetics follow the Chick-Watson model, ln(N/N0) = -k C^n t, which underlies the CT concept (residual concentration times contact time) used in regulations: for n = 1, a 3-log (99.9%) inactivation with k = 0.1 L/(mg min) at C = 1 mg/L needs t = ln(1,000)/0.1 = 69 minutes. Cryptosporidium oocysts resist chlorine, so filtration, ultraviolet irradiation or ozone is needed. Ultraviolet light inactivates microorganisms by damaging nucleic acids, without residual; ozone is a powerful oxidant and disinfectant, also without lasting residual. Chlorination of water containing natural organic matter forms disinfection by-products such as trihalomethanes and haloacetic acids, which are regulated; removing organic precursors before chlorination reduces them.
Distribution systems deliver treated water at adequate pressure and quantity. Systems may be gravity-fed (from elevated sources), pumped, or combined (pumping to elevated service reservoirs that supply by gravity). Network layouts include dead-end (tree) systems, simple but with stagnant ends and interruptions during repairs; grid-iron (looped) systems, which supply each point from more than one direction; ring systems; and radial systems with zonal service reservoirs. Pipes are sized for peak demand plus fire flow while keeping residual pressures above minimum values; looped networks are analysed by the Hardy Cross method or by computer models (see Hydrology and Water Resources for pipe-flow equations).
Service reservoirs (elevated tanks and ground-level reservoirs) balance the difference between a steady supply from the treatment plant and the varying hourly demand, provide emergency and fire storage, and maintain pressure. Balancing storage is found from a mass curve of cumulative supply and demand: the required storage is the sum of the maximum surplus and maximum deficit. Intermittent supply, common in Indian cities, causes contamination through negative pressures and unequal distribution; moving to continuous pressurised supply, reducing non-revenue water through leak detection and metering, and maintaining chlorine residuals are key reforms.
Sewerage systems may be separate (sanitary sewers for wastewater, storm drains for rainwater) or combined (both in one pipe, with combined sewer overflows discharging untreated mixtures during heavy rain). Sewers are mostly gravity pipes designed with Manning's equation to flow partly full at design peak (commonly not more than about 0.8 of diameter) so that gases can vent. They must achieve a self-cleansing velocity -- about 0.6 m/s for sanitary sewers in CPHEEO practice, and higher for storm sewers carrying grit -- to prevent deposition, while staying below velocities (roughly 3 m/s) that cause erosion of pipe material. Example: a 300 mm sewer with n = 0.013 laid at a slope of 0.005 flowing full has hydraulic radius D/4 = 0.075 m and velocity (1/0.013) x 0.075^(2/3) x 0.005^(1/2) = 0.97 m/s, carrying about 0.068 m3/s.
Appurtenances include manholes for access and inspection at changes of direction, gradient or size and at regular intervals; drop manholes where a branch sewer enters at a much higher level; inverted siphons (depressed sewers) to pass under obstacles such as rivers; lamp holes and cleanouts; grease and oil traps for kitchens and food industries; catch basins and street inlets for storm water; and sewage pumping stations where gravity flow is not possible. Hydrogen sulphide generated in sewers under anaerobic conditions causes odour, toxicity to workers and crown corrosion of concrete pipes; ventilation, adequate velocities and corrosion-resistant materials address it. Manual cleaning of sewers is dangerous and, in India, manual scavenging is prohibited by law (2013 Act).
Wastewater treatment is organised in stages. Preliminary treatment removes large objects, grit and grease (screens, grit chambers, skimming). Primary treatment removes settleable solids by sedimentation, typically reducing suspended solids by about half to two-thirds and BOD by a quarter to two-fifths; the settled primary sludge goes to sludge treatment. Secondary treatment uses microorganisms to remove dissolved and colloidal biodegradable organic matter, converting it to carbon dioxide, water and new biomass that is then settled out. Processes include suspended-growth systems (activated sludge and its variants, aerated lagoons, waste stabilisation ponds) and attached-growth systems (trickling filters, rotating biological contactors, moving bed biofilm reactors), and anaerobic processes such as UASB reactors. Well-operated secondary treatment produces effluents with BOD and suspended solids of the order of 10-30 mg/L.
Tertiary (advanced) treatment further improves effluent quality for sensitive receiving waters or reuse: nutrient removal (biological nitrification-denitrification, biological or chemical phosphorus removal), filtration, membrane processes, adsorption on activated carbon and disinfection. India's discharge standards for sewage treatment plants have been tightened to require lower BOD, suspended solids and nutrient levels in many areas, which has driven adoption of advanced processes.
Low-cost natural systems are important for small towns and rural areas. Waste stabilisation ponds -- anaerobic, facultative and maturation ponds in series -- rely on sunlight, algae and bacteria and are effective at pathogen removal, but need large areas. Constructed wetlands (surface-flow and subsurface-flow) treat domestic and some agro-industrial wastewaters. Duckweed and other aquatic plant systems have been used in some places.
Biological nitrogen removal combines nitrification (aerobic oxidation of ammonia to nitrate by autotrophic bacteria, which requires long sludge ages and consumes about 4.57 g O2 and 7.14 g alkalinity as CaCO3 per g of ammonia-N oxidised) with denitrification (anoxic reduction of nitrate to nitrogen gas by heterotrophs using organic carbon). Configurations such as the Modified Ludzack-Ettinger process place an anoxic zone before the aerobic zone with recycle of nitrate-rich mixed liquor. Enhanced biological phosphorus removal uses an anaerobic zone so that phosphate-accumulating organisms take up excess phosphorus in the following aerobic zone; chemical removal precipitates phosphorus with alum, ferric salts or lime. Nutrient removal prevents eutrophication of receiving waters (see Water Quality and Agricultural Non-point Source Pollution).
In the activated sludge process, wastewater is aerated in a tank with a suspended mass of microorganisms (mixed liquor); the mixture flows to a secondary clarifier, where the biological floc settles; most settled sludge is returned to the aeration tank and the excess is wasted. Key design and operating parameters are: mixed liquor suspended solids (MLSS, roughly 1,500-3,500 mg/L in conventional plants; mixed liquor volatile suspended solids, MLVSS, is the biological fraction); hydraulic retention time (HRT = V/Q, about 4-8 hours conventionally); food-to-microorganism ratio, F/M = Q S0 / (V X), typically about 0.2-0.5 per day for conventional plants and lower for extended aeration; solids retention time or mean cell residence time, SRT = V X / (Qw Xr + (Q - Qw) Xe), about 5-15 days conventionally and longer for nitrification; and return sludge ratio R = Qr / Q.
Example: a plant treating 10,000 m3/d of settled sewage with BOD 200 mg/L in a 3,000 m3 aeration tank at MLSS 2,500 mg/L has HRT 7.2 h and F/M = 10,000 x 200 / (3,000 x 2,500) = 0.27 per day. If 60 m3/d of return sludge at 10,000 mg/L is wasted and effluent solids are negligible, SRT = 3,000 x 2,500 / (60 x 10,000) = 12.5 days. Settleability is measured by the sludge volume index, SVI = (settled volume after 30 minutes, mL/L) x 1,000 / MLSS (mg/L); a sludge settling to 250 mL/L at MLSS 2,500 mg/L has SVI 100 mL/g, indicating good settling, while SVI above about 150-200 indicates bulking, often caused by filamentous organisms.
Microbial growth is described by the Monod equation, mu = mu_max S / (Ks + S), where S is substrate concentration and Ks the half-saturation constant. For a completely mixed activated sludge process at steady state, the effluent substrate concentration depends only on SRT: S = Ks (1 + kd theta_c) / [theta_c (Y k - kd) - 1], where Y is the yield coefficient, k the maximum specific substrate utilisation rate (mu_max = Y k) and kd the endogenous decay coefficient. With Ks = 60 mg/L, Y = 0.6, k = 5 per day, kd = 0.06 per day and theta_c = 10 days, S = 60 x 1.6 / (10 x 2.94 - 1) = 3.4 mg/L, showing why SRT is the key control parameter. Variants include extended aeration (long SRT, low sludge production), sequencing batch reactors, oxidation ditches, membrane bioreactors and moving bed biofilm reactors.
Trickling filters pass wastewater over a bed of stone or plastic media covered with biofilm; organic matter is removed as it flows past, and sloughed biofilm is settled in a secondary clarifier. Low-rate filters use no recirculation and low hydraulic loading; high-rate filters recirculate effluent. The NRC (US National Research Council) formula estimates BOD removal efficiency for a single-stage stone filter as E = 100 / [1 + 0.4432 (W / (V F))^(1/2)], with W the BOD load (kg/d), V the media volume (m3) and F the recirculation factor (1 + R) / (1 + 0.1 R)^2. Example: W = 500 kg/d, V = 1,000 m3 and R = 1 give F = 1.65 and E = 80%. Trickling filters use less energy than activated sludge but achieve somewhat lower removal and are sensitive to temperature and odour.
Anaerobic treatment converts organic matter to biogas (methane and carbon dioxide) in the absence of oxygen through four stages carried out by different microbial groups: hydrolysis of complex polymers (carbohydrates, proteins, lipids) to soluble monomers; acidogenesis (fermentation) to volatile fatty acids, alcohols, hydrogen and CO2; acetogenesis to acetate, hydrogen and CO2; and methanogenesis, in which methanogenic archaea produce methane from acetate (acetoclastic, the main route) and from hydrogen and CO2 (hydrogenotrophic). Methanogens grow slowly and are sensitive to pH (optimum about 6.8-7.4), temperature, volatile fatty acid accumulation, ammonia and toxic substances, so stable operation requires balanced loading and adequate alkalinity. Mesophilic digesters operate around 30-38 degrees C and thermophilic ones around 50-57 degrees C.
Anaerobic treatment suits high-strength wastewaters -- distillery spentwash, sugar mill, dairy, starch, slaughterhouse and food-processing effluents -- and sludge. Compared with aerobic treatment it needs no aeration energy, produces far less sludge, and yields energy: theoretically 0.35 m3 of methane (at standard temperature and pressure) per kg of COD removed. Example: removing 5,000 kg COD/d yields up to about 1,750 m3 of methane per day, with an energy content of roughly 60 GJ (methane is about 35.8 MJ/m3). Disadvantages include slow start-up, sensitivity to shocks, odour, and effluent that usually needs aerobic post-treatment to meet discharge standards.
The upflow anaerobic sludge blanket (UASB) reactor, developed by Gatze Lettinga and colleagues in the Netherlands in the 1970s, passes wastewater upward through a dense blanket of granular sludge; a gas-liquid-solid separator at the top collects biogas and returns settling solids. Granules with good settling properties allow high biomass concentrations and short hydraulic retention times -- of the order of 6-10 hours for sewage in warm climates and longer, with high organic loading rates, for industrial wastewaters. India adopted UASB technology widely for sewage treatment, notably under the Ganga Action Plan, although post-treatment of UASB effluent (for example in polishing ponds) has often been inadequate to meet standards for BOD, solids and coliforms, leading to later combinations with aerobic processes.
Other high-rate anaerobic reactors include anaerobic filters, expanded granular sludge bed (EGSB) and internal circulation reactors, and anaerobic lagoons for strong effluents in warm regions. Covered lagoons and digesters capture methane that would otherwise escape; uncaptured methane from anaerobic treatment is a significant greenhouse gas source, so biogas capture and use (or flaring) matter for both energy and climate (see Agricultural GHG Emissions and Mitigation).
Treatment units are modelled as ideal reactors. In a batch reactor, there is no flow during reaction; for a first-order reaction, C = C0 e^(-kt). In a continuously stirred tank reactor (CSTR, completely mixed), the contents are uniform and equal to the effluent; for first-order kinetics at steady state, C / C0 = 1 / (1 + k theta), where theta = V/Q is the hydraulic retention time. In a plug-flow reactor (PFR), fluid elements pass through in order without mixing along the flow path, behaving like a batch reactor with reaction time theta: C / C0 = e^(-k theta). For zero-order reactions, C = C0 - k theta in both CSTR and PFR; for second-order reactions, the PFR gives 1/C = 1/C0 + k theta and the CSTR requires solving k theta C^2 + C - C0 = 0.
For reactions of order greater than zero, a PFR needs a smaller volume than a CSTR to achieve the same removal, because the CSTR operates entirely at the low effluent concentration. Example: with k = 0.5 per day and theta = 4 days, a CSTR achieves C/C0 = 1/(1 + 2) = 0.33 (67% removal), while a PFR achieves e^(-2) = 0.135 (86.5% removal). Splitting the CSTR into n equal tanks in series improves performance towards plug flow: C/C0 = 1 / (1 + k theta / n)^n; two tanks of 2 days each give 1/(1 + 1)^2 = 0.25 (75% removal).
Real reactors lie between the ideal extremes because of dispersion, short-circuiting and dead zones. Tracer studies -- adding a pulse or step of a conservative tracer and measuring its outlet concentration over time -- give the residence time distribution, from which the dispersion number or equivalent number of tanks in series is estimated. The CSTR is valued for its resistance to shock loads and toxic spikes, because incoming flow is immediately diluted; plug-flow configurations give higher removal for a given volume and favour good settling sludge in activated sludge systems. Chlorine contact tanks are designed as near-plug-flow serpentine channels so that all water receives the required contact time.
Reactor kinetics are used throughout treatment design: BOD removal in lagoons and ponds (often modelled as CSTRs in series), pathogen die-off in maturation ponds, disinfection contact tanks, activated sludge (CSTR kinetics with recycle), and chemical oxidation. Temperature corrections of the Arrhenius form k_T = k_20 theta^(T - 20) are applied to rate constants. The same principles appear in models of rivers and lakes (see Water Quality and Agricultural Non-point Source Pollution).
Treatment concentrates pollutants into sludge: primary sludge from primary clarifiers and secondary (waste activated or humus) sludge from biological processes. Sludge is mostly water: primary sludge may contain a few per cent solids and waste activated sludge often less than 1-2%. Because volume falls sharply as water is removed, sludge treatment aims to reduce water content, stabilise organic matter, kill pathogens and enable safe use or disposal. For a given mass of dry solids, the volume of sludge is approximately inversely proportional to its solids concentration: V2 / V1 = P_s1 / P_s2 = (100 - p1) / (100 - p2), where P_s are solids percentages and p moisture percentages. Example: 1,000 kg of dry solids at 2% solids occupies about 50 m3; thickened to 5% solids, it occupies about 20 m3 -- a 60% reduction in volume from a small change in solids content. Typical production is of the order of tens of grams of dry solids per person per day for a conventional plant, so a city of a million people produces tens of tonnes of dry solids daily, which must be transported, treated and disposed of safely.
Thickening (gravity thickeners, dissolved air flotation, gravity belts, centrifuges) raises solids to a few per cent. Stabilisation reduces odour, pathogens and volatile solids: anaerobic digestion (mesophilic or thermophilic, with SRT commonly 15-30 days, producing biogas), aerobic digestion, lime stabilisation and composting. Conditioning with polymers or chemicals improves dewatering. Dewatering (belt filter presses, centrifuges, filter presses, sludge drying beds -- the last common in warm climates) raises solids to roughly 15-35% or more, producing a cake that can be handled as a solid. Thermal drying, incineration, pyrolysis and other thermal processes further reduce volume.
Disposal and use routes include land application as biosolids (supplying nutrients and organic matter but requiring control of pathogens, heavy metals, and emerging contaminants such as PFAS and pharmaceuticals), composting with other organic wastes, landfilling, incineration with ash disposal or phosphorus recovery, and use in construction materials. Regulations set pathogen and metal limits for land application in many countries; some restrict it. Phosphorus recovery from sludge and ash is increasingly promoted (see Nutrient Stewardship). Sludge management often accounts for a large share of a treatment plant's operating cost.
Agro-industries produce effluents rich in biodegradable organic matter, often far stronger than sewage. Dairy effluents contain milk solids, fats, lactose and cleaning chemicals; BOD commonly ranges from several hundred to a few thousand mg/L, with large fluctuations between product changeovers and cleaning cycles, and fats and oils cause operational problems. Sugar mill effluents come mainly from washing, process water and leakages, with BOD of the order of 1,000-2,000 mg/L and seasonal operation during the crushing season. Molasses-based distillery spentwash is among the most polluting industrial wastes: it is dark brown (from melanoidins and other colourants), acidic (pH about 4-5), very hot when discharged, and extremely strong, with BOD of the order of 45,000-60,000 mg/L and COD of the order of 80,000-120,000 mg/L, produced at roughly 8-15 litres per litre of alcohol; it is also rich in potassium. Food-processing effluents (fruit and vegetable processing, starch, edible oil, meat and poultry, slaughterhouses) vary widely; slaughterhouse effluents carry blood, fat and pathogens. Example: a distillery producing 100 kL of spentwash a day at COD 100,000 mg/L generates 10,000 kg of COD a day -- equivalent in COD to the sewage of several hundred thousand people.
Treatment combines source reduction and recovery with treatment. Dairies reduce losses through good housekeeping, clean-in-place optimisation and recovery of whey (once a waste, now a product); effluents are treated by fat removal (oil and grease traps, dissolved air flotation), equalisation, and anaerobic (UASB or anaerobic filter) plus aerobic treatment. Sugar mills use equalisation, aerobic treatment (activated sludge, aerated lagoons) and reuse of water within the mill; press mud is composted. Distillery spentwash is treated by anaerobic digestion (biomethanation) with energy recovery, followed by further treatment; because the residual colour and COD are very difficult to remove, Indian regulations have required distilleries to achieve zero liquid discharge through concentration by multiple-effect evaporation and incineration of the concentrate in boilers (with potash-rich ash recovery), bio-composting with press mud, or other approved routes. Food-processing effluents are commonly treated anaerobically then aerobically, with screening and fat removal first.
Irrigation with treated agro-industrial effluents (fertigation) is permitted in some cases under controls, because these effluents contain nutrients; but over-application causes soil salinity, sodicity, organic overloading and groundwater pollution. CPCB and state boards set industry-specific discharge standards in India.
Small and medium enterprises clustered in industrial estates often cannot afford or operate their own treatment plants. Common effluent treatment plants (CETPs) collect effluents from many units and treat them centrally, sharing costs and expertise. India promoted CETPs from the early 1990s with financial assistance from the central and state governments, for clusters of tanneries, textile dyeing units, electroplaters, chemical units and food processors. Each member industry is usually required to provide primary treatment and to meet inlet standards set by the CETP so that the combined wastewater is treatable. CPCB assessments have found many CETPs failing to meet discharge standards because of overloading, variable and toxic inflows, poor operation and maintenance, inadequate funding and weak monitoring; continuous online monitoring of effluent quality is now required for many CETPs and large industries. Successful CETPs combine clear cost-sharing based on pollution load, enforcement of inlet standards on members, professional operators, and regulators with the capacity to inspect -- institutional conditions that matter as much as the treatment technology.
Water recycling within industries -- reuse of treated effluent for cooling, washing, boiler feed after treatment, or process water -- reduces fresh water demand and effluent volume. Zero liquid discharge (ZLD) goes further: no liquid effluent leaves the facility, all water being recovered for reuse and dissolved solids recovered as solid salts or sludge. A typical ZLD train comprises pretreatment, biological treatment where needed, ultrafiltration, reverse osmosis (recovering most water as permeate), and treatment of RO reject brine by multiple-effect evaporators and crystallisers or agitated thin-film dryers, producing salts for disposal or recovery. In India, a Madras High Court order (2011) required textile dyeing and bleaching units in Tiruppur to achieve ZLD after pollution of the Noyyal river, and CPCB has required ZLD for specified sectors such as distilleries and some textile and tannery clusters.
ZLD protects water bodies and recovers water, but it is expensive in capital and especially energy (evaporation), generates mixed salts that are difficult to sell or dispose of safely, and has high operating demands; failures lead to illegal discharges or storage of brine. Policy therefore increasingly favours a hierarchy: minimise waste at source and improve process efficiency first, recycle water within the facility, then apply ZLD where receiving waters are critically polluted or water is scarce. Treated municipal wastewater is also increasingly supplied to industry and thermal power plants -- India's tariff policy requires thermal power plants within a set distance of sewage treatment plants to use treated sewage -- which reduces freshwater use.
Treat this subject as a set of design equations to master with units: BOD_t, k_T, hardness, population methods, demand and peak factors, vo = Q/A, G, filtration rate, chlorine demand and Chick-Watson, Manning for sewers, F/M, SRT, SVI, Monod effluent, NRC efficiency, CSTR, PFR and series reactors, sludge volume and methane yield. Know the qualitative facts too: settling types, coagulation mechanisms, HOCl and OCl-, UASB origins, distillery spentwash characteristics and ZLD trains. Practise full GATE-style problems with unit conversions, because the numerical section rewards speed and accuracy.
As a treatment plant engineer, consultant or pollution control board officer you size and troubleshoot units, interpret operating data (MLSS, SVI, DO, sludge age), respond to bulking, foaming and shock loads, check compliance with discharge standards, inspect industries and CETPs, and evaluate proposals for upgrades, reuse and ZLD. You balance treatment performance, energy, sludge costs and operator skills, and you keep the records and laboratory quality control that make compliance data credible to regulators, industry clients, courts and the public.
As a utility head, regulator or industry environmental manager you plan sewerage and treatment capacity for growing cities, set and enforce standards, choose technologies suited to local capacity, finance operation and maintenance, decide when to require ZLD or allow reuse, and manage public health risks. You must ensure that plants built actually run, which is often harder than building them, and you must secure tariffs, budgets and trained staff for decades of operation as well as capital for construction, and explain these needs to city councils.
As a researcher you work on biological process kinetics and microbiology, nutrient removal and recovery, anaerobic technologies, membranes and desalination energy, emerging contaminants and antimicrobial resistance in treatment, sludge valorisation, low-cost decentralised systems, and process modelling and control. You advise standard-setting bodies, evaluate new technologies at pilot and full scale, publish the design parameters that practitioners rely on, and train the environmental engineers and operators who will design and run future treatment plants in cities, towns and industries across the world.
Design values are typical textbook ranges for illustration; the manual does not reproduce CPHEEO design tables, national discharge standards in detail (which change), or detailed process design of nutrient removal configurations. Industrial effluent characteristics vary widely by plant and are given as orders of magnitude. Membrane design, advanced oxidation, detailed sewer network design, water treatment for specific contaminants (arsenic, fluoride, iron), and small-scale sanitation and faecal sludge management are covered only briefly or not at all. Policy facts (ZLD requirements, standards, tariff policy) should be checked against current regulations.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Every numerical answer was computed by script; Design values and coefficients are stated in each stem because they differ between codes and sites; every numerical answer was computed by script.
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