Environmental Pollution and Control III: Water Pollution — Types, Sources and Impacts, Sampling and Analysis of Water Quality Parameters, Microbiological Analysis by MPN, the Indian Drinking Water Standard IS 10500:2012, Drinking Water Treatment, Primary, Secondary and Advanced Wastewater Treatment, and Common Effluent Treatment Plants

A grab sample represents one place and moment; a composite sample combines portions taken over time, often in proportion to flow, to represent a day's effluent. Samples must be preserved because many parameters change in storage: dissolved oxygen is fixed on site with the Winkler reagents; samples for metals are acidified with nitric acid; samples for COD and nutrients are acidified with sulphuric acid and cooled to about 4 °C; and BOD samples are cooled and analysed within hours. pH, temperature, conductivity and DO are best measured in the field.

Hardness is caused by divalent cations, chiefly calcium and magnesium. Temporary (carbonate) hardness is due to their bicarbonates and is removed by boiling, which precipitates calcium carbonate; permanent (non-carbonate) hardness is due to their sulphates and chlorides and is not. Hardness is expressed as mg/L of CaCO₃ by multiplying each ion's concentration by the ratio of the equivalent weight of CaCO₃ (50) to its own (20 for Ca²⁺ and about 12.15 for Mg²⁺): hardness = 2.5 × Ca + 4.12 × Mg. Water with 60 mg/L of calcium and 24.3 mg/L of magnesium has a hardness of 150 + 100 = 250 mg/L as CaCO₃, which is hard.

Microbiological quality is judged by indicator organisms. In the multiple-tube fermentation method, portions of the sample, usually five tubes at each of three dilutions, are incubated in lactose broth at 35 °C for 24 to 48 hours; gas production is a positive presumptive test, confirmed in brilliant green lactose bile broth and completed by isolating the organism. The pattern of positive tubes is converted, using tables derived from probability theory, into the most probable number (MPN) of coliforms per 100 mL, a statistical estimate rather than a count. Thermotolerant (faecal) coliforms and E. coli are identified by incubation at 44.5 °C. The membrane filtration method, which filters a measured volume and counts the colonies grown on the membrane, gives a direct count per 100 mL.

IS 10500:2012, issued by the Bureau of Indian Standards, sets an acceptable limit and, for some parameters, a higher permissible limit that applies only where no alternative source is available. For toxic substances and for pH, nitrate and bacteriological quality there is no relaxation. Arsenic, as the standard now stands after amendment, is held at 0.01 mg/L without relaxation, the WHO guideline value.

A conventional surface-water treatment plant follows screening and aeration (which releases gases and oxidises iron and manganese) with coagulation, flocculation, sedimentation, filtration and disinfection. Coagulants such as alum are dispersed by rapid mixing and the flocs are grown by slow mixing, with the dose set by jar test. In the clarifier, particles settle if their settling velocity exceeds the surface overflow rate, Q ÷ A, the flow divided by the plan area, typically about 20 to 40 m³/m²/day for flocculated water; a plant treating 10,000 m³/day at 25 m³/m²/day needs 400 m² of settling area, and the tank's depth sets its detention time, V ÷ Q, commonly two to four hours. Slow sand filters run at about 0.1 to 0.2 m/h and purify biologically in the schmutzdecke, needing no coagulation but large areas; rapid sand filters run some thirty times faster, at about 5 m/h, after coagulation, and are cleaned by backwashing.

Disinfection is most often by chlorine. In water chlorine forms hypochlorous acid, HOCl, which dissociates to hypochlorite, OCl⁻, with a pKa of about 7.5; HOCl is roughly 80 to 100 times more effective as a disinfectant, so chlorination works better at lower pH. Chlorine first reacts with reducing substances and with ammonia to form chloramines; beyond the breakpoint, further chlorine appears as free residual chlorine, and breakpoint chlorination ensures a free residual that protects the distribution system, at least 0.2 mg/L at the consumer's tap. Chlorine reacting with natural organic matter forms disinfection by-products such as trihalomethanes, some of them suspected carcinogens; ozone and ultraviolet light are alternatives that leave no residual. Softening removes hardness: in lime-soda softening, lime removes carbonate hardness and magnesium, and soda ash is needed to remove non-carbonate calcium hardness; ion exchange with sodium-form resins or zeolites exchanges calcium and magnesium for sodium; and reverse osmosis removes most dissolved salts.

In the activated sludge process, settled sewage is aerated with a dense suspension of micro-organisms (the mixed liquor), which oxidise the organic matter and flocculate; the mixed liquor then settles in a secondary clarifier, most of the sludge is returned to the aeration tank and the excess is wasted. Two design parameters dominate. The food-to-microorganism ratio, F/M = QS₀ ÷ VX, where Q is the flow, S₀ the influent BOD, V the aeration tank volume and X the mixed liquor suspended solids, is typically 0.2 to 0.4 per day for conventional plants: for Q = 10,000 m³/day, S₀ = 200 mg/L, V = 5,000 m³ and X = 2,500 mg/L, F/M = (10,000 × 200) ÷ (5,000 × 2,500) = 0.16 per day, a lightly loaded plant, with a hydraulic retention time V ÷ Q of 0.5 day, or 12 hours. The sludge volume index, the volume in mL occupied by 1 g of sludge after 30 minutes of settling, SVI = settled volume (mL/L) × 1000 ÷ MLSS (mg/L), measures settleability: 250 mL/L at 2,500 mg/L gives 100 mL/g, good settling, while values above about 150 mL/g indicate bulking, often caused by filamentous bacteria. In India UASB reactors, which treat sewage anaerobically with little energy and produce biogas, and waste stabilisation ponds are widely used where land is available. The sludge from all these processes is thickened, digested, dewatered and disposed of or used on land.

Small and medium industries in industrial estates cannot each afford a treatment plant, so their effluents are collected and treated jointly in a common effluent treatment plant (CETP), supported in India by a central scheme with financial assistance since the early 1990s and run by a special-purpose company of the member units. Each unit must pre-treat its effluent to the CETP's inlet standards so that toxic or incompatible wastes do not upset the biological treatment; the CETP must then meet the discharge standards. The general standards for discharge of effluents into inland surface water under the Environment (Protection) Rules include a BOD of 30 mg/L, a COD of 250 mg/L and suspended solids of 100 mg/L. Where water is scarce or rivers are sensitive, zero liquid discharge, recovering water by evaporation and crystallisation, has been required of some industries.