Skip to main content

Point and non-point sources; waste water treatment

Short answer

Point sources discharge at a single point (e.g., factory pipe) and are treatable; non-point sources are diffuse (e.g., agricultural runoff) and hard to control. Primary treatment physically settles solids; secondary treatment uses aerobic microbes to oxidise organic matter, removing 85–95% of BOD.

What do you mean by point and non point sources of water pollutants? Give an account of primary and secondary processes of waste water treatment to control water pollution.

Point and non-point sources

  • Point sources discharge pollutants at a single, identifiable location, usually through a pipe or drain, so they can be monitored and treated. Examples: factory outlets, sewage outfalls, power plant cooling water.
  • Non-point sources are diffuse, with no single discharge point; pollution enters over a wide area, usually by runoff, and is hard to control. Examples: agricultural runoff (fertilisers, pesticides), urban storm water, atmospheric deposition, soil erosion.

Waste water treatment

Waste water → Screening → Grit → PRIMARY → SECONDARY → Disinfection → Discharge
              (preliminary)       settling   biological   (chlorine)

Primary treatment (physical)

  1. Screening and grit removal remove large objects and sand.
  2. In the sedimentation tank (clarifier) suspended solids settle as sludge. Alum or lime may be added as coagulant. This removes about 60 % of suspended solids and 30 % of BOD.

Secondary treatment (biological)

  1. The water from primary treatment is treated with aerobic microorganisms that oxidise dissolved organic matter to CO₂, water and new cells.
  2. Methods: activated sludge process (air is blown into the tank containing microbe-rich sludge) or trickling filter (waste water sprinkled over a bed of stones coated with microbial film).
  3. The secondary clarifier then settles the microbial flocs; part of the sludge is returned. BOD removal is 85 to 95 %.
  4. Sludge is digested anaerobically in a digester to give biogas (CH₄).

The effluent is then disinfected (chlorine or UV) and discharged. Tertiary treatment (nutrient removal, filtration, activated carbon) may follow when water of high quality is needed.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Engineering Chemistry chapter 3 questions →