Oxidation pond: theory and commissioning

An oxidation pond (waste stabilisation pond) is a shallow, man-made earthen basin, usually 1-1.5 m deep, in which raw or settled sewage is treated naturally by the combined action of bacteria and algae, using sunlight. It is cheap and suits small towns with plenty of land.

Theory and pollutant removal mechanism

The removal depends on a symbiotic relationship between bacteria and algae:

1. Aerobic bacteria oxidise the organic matter: $\text{organics} + O_2 \rightarrow CO_2 + H_2O + \text{new cells} + NH_3 + PO_4$
2. Algae use sunlight, the CO₂, ammonia and phosphates released by bacteria and produce oxygen by photosynthesis: $CO_2 + H_2O + \text{sunlight} \rightarrow \text{algae} + O_2$
3. This oxygen is used by the bacteria, so no mechanical aeration is needed. Wind action also aerates the surface.
4. Settleable solids go to the bottom, where anaerobic bacteria digest them to CH₄, CO₂ and H₂S. The sludge layer is thin and stabilised.
5. Pathogens die by sunlight (UV), high pH, long detention and predation.

   sunlight
     \ | /
  ---------------------------- water surface
  Algae: CO2 + sunlight -> O2   ^ O2 (and wind)
          |   O2 down   CO2 up  |
  Bacteria (aerobic zone): organics + O2 -> CO2
  - - - - - - - - - - - - - - - - - - - - - -
  Anaerobic zone: sludge -> CH4, CO2, H2S
  ============ bottom (sludge) ==============

Typical data: detention 5-30 days, organic loading 200-400 kg BOD/ha/day, BOD removal 70-90%.

Commissioning (starting up the pond)

Commissioning means bringing a newly built pond into stable operation.

1. Inspect the embankment, inlet, outlet and lining; remove vegetation and make the pond leak-free.
2. Fill with clean water first (river or tube-well water), to the operating depth, to prevent weed growth, embankment erosion and bad odours; filling with raw sewage directly gives odour and anaerobic conditions.
3. Seed the pond with active sludge or contents of a working pond (about 5-10% of volume) to start the culture.
4. Start loading gradually: begin with a low flow (about 1/3 of design) or low BOD, then raise it over several weeks as algae develop (the water turns green).
5. In cold seasons commission in spring/summer when sun and temperature help algae growth.
6. Check DO, pH (above 7.5 in the day) and colour regularly; if the pond turns grey/black (anaerobic), reduce loading or recirculate effluent.
7. Do not discharge effluent until the pond reaches steady performance.