Water Disinfection, DO, BOD, COD and Sewage Treatment
Disinfection
Killing pathogens — distinct from sterilisation, which destroys all organisms. Water treatment aims at the former.
Chlorination
Chlorine gas or a hypochlorite hydrolyses in water:
Hypochlorous acid HOCl is the active agent — uncharged, so it penetrates the cell wall, where it oxidises enzymes. The hypochlorite ion OCl⁻ is charged and far less effective, perhaps eighty times weaker.
Hence the pH dependence, which is the practical point. The equilibrium lies towards HOCl in acid and towards OCl⁻ in alkali. Chlorination is therefore much more effective at pH 6–7 than at pH 9, and a swimming pool that has drifted alkaline disinfects poorly however much chlorine is added.
Advantages: cheap, effective, and — uniquely — provides a residual that continues protecting the water through the distribution network. Limitations: forms trihalomethanes with natural organic matter, which are suspected carcinogens; gives taste and odour; and is ineffective against Cryptosporidium cysts.
Break point chlorination
Add chlorine progressively to water containing ammonia and organics, and plot the residual against the dose. The curve has three regions and a minimum:
- Chlorine destroyed. The first chlorine is consumed by reducing agents — Fe²⁺, Mn²⁺, H₂S, organics. Residual stays near zero.
- Combined residual rises. Chlorine reacts with ammonia to form chloramines: These disinfect weakly and cause the characteristic "chlorine" smell of a poorly managed pool — a smell that indicates too little free chlorine, not too much.
- Break point. Further chlorine oxidises the chloramines away: The residual falls to a minimum — the break point.
- Beyond it, added chlorine appears as free residual chlorine, which is the effective disinfectant.
Why dose past the break point. Below it, the chlorine present is combined and weak. Beyond it, free chlorine is available, ammonia and organics have been destroyed, and taste and odour actually improve with further dosing. Dosing to just short of the break point is the worst of both.
Ozone and UV
Ozonation. O₃ generated on site by electrical discharge, since it cannot be stored. A far more powerful oxidant than chlorine, acting faster, destroying viruses and Cryptosporidium, and improving taste, colour and odour. It forms no trihalomethanes. But it leaves no residual, is energy-intensive, and equipment costs more — so it is often used with a small chlorine dose afterwards purely to maintain a residual.
UV irradiation. 254 nm light damages microbial DNA, forming thymine dimers that prevent replication. No chemicals, no by-products, no taste, very fast, and effective against Cryptosporidium. But again no residual, and it requires clear water — turbidity shields organisms from the light. Lamps also foul and age.
The recurring trade-off is residual protection. Only chlorine persists through the distribution network. Ozone and UV are superior at the point of treatment and offer nothing downstream of it, which is why large systems combine them.
DO, BOD and COD
Dissolved oxygen (DO) — oxygen dissolved in water, mg L⁻¹. Saturation is only about 9 mg L⁻¹ at 20 °C — a very small reserve. Aquatic life generally needs above 4–5 mg L⁻¹.
DO falls as temperature rises, which is why thermal discharge from power stations harms rivers, and why an organic load is far more damaging in summer.
Biochemical oxygen demand (BOD) — the oxygen consumed by microorganisms in oxidising biodegradable organic matter, measured over 5 days at 20 °C (BOD₅). It measures the biodegradable load, which is what will actually deplete a river's oxygen.
Chemical oxygen demand (COD) — the oxygen equivalent of organic matter oxidisable by a strong chemical oxidant, usually acidified potassium dichromate, in about 3 hours.
Always COD > BOD, because chemical oxidation attacks non-biodegradable material as well.
The ratio is the diagnostic:
- BOD/COD > 0.6 — largely biodegradable, so biological treatment will work well. Typical of domestic sewage.
- BOD/COD < 0.3 — largely non-biodegradable, often toxic industrial effluent. Biological treatment will fail, and physical or chemical treatment is needed.
Why both are measured. COD is fast and reproducible but says nothing about biodegradability. BOD reflects the real environmental impact but takes five days. Together they say how much load there is and whether bacteria can deal with it.
Typical values: clean river BOD below 1 mg L⁻¹; raw domestic sewage 200–400; treated effluent should be below 30.
Sewage treatment
Primary — physical. Screening removes rags and grit; sedimentation in settling tanks removes suspended solids as primary sludge; skimming removes grease. Achieves about 30% BOD removal and 60% of suspended solids. No biology involved.
Secondary — biological. Microorganisms consume the dissolved and colloidal organic matter that settling cannot remove. Achieves 85–95% BOD removal, and is where most of the work is done. Two classical aerobic processes:
Activated sludge process — sewage is aerated in a tank with a suspended culture of microorganisms, then settled; part of the settled sludge is returned to maintain the population. Compact and efficient, but needs continuous aeration, which dominates the energy cost of a treatment works, and skilled operation.
Trickling filter — sewage is sprayed by a rotating distributor over a bed of stones or plastic media, 2–3 m deep. A biofilm grows on the media; as sewage trickles over it, the organisms absorb and oxidise the organic matter, with air passing naturally upward through the voids. Excess biofilm sloughs off and is removed by a secondary settling tank.
Advantages: simple, robust, low energy since it needs no forced aeration, and tolerant of shock loads. Limitations: larger land area, lower efficiency than activated sludge, odour and fly nuisance, and it can clog.
Tertiary — polishing. Filtration, activated carbon, nutrient removal of nitrogen and phosphorus, and disinfection. Required where the effluent goes to a sensitive water body or is to be reused.
The UASB process
Upflow Anaerobic Sludge Blanket — an anaerobic alternative, widely used in India including for domestic sewage.
Working. Sewage is fed in at the bottom and flows upward through a blanket of dense granular anaerobic sludge. The granules — a few millimetres across, self-forming — carry a consortium of bacteria that convert organic matter to biogas, roughly 70% methane. A three-phase separator at the top separates gas, settled solids and treated liquid; the solids fall back, retaining the biomass.
The elegance of it: the granules settle well, so the biomass stays in the reactor while the liquid passes through quickly. Solids retention is decoupled from liquid retention, which is what allows a compact reactor.
Advantages, and why it suits Indian conditions:
- Produces energy as biogas rather than consuming it. Activated sludge spends most of its energy on aeration; UASB needs none.
- Very low sludge production — anaerobic organisms grow slowly — so sludge disposal costs fall sharply.
- Compact, low operating cost, works well in warm climates.
Limitations: slow start-up while granules develop, of the order of months; sensitive to temperature, so it performs poorly in cold climates; effluent usually needs post-treatment, since BOD removal is 70–85% rather than the 95% of activated sludge; and it produces H₂S, which smells and is corrosive.