Oxidation pond; flushing device role

Oxidation pond

An oxidation pond (waste stabilisation pond) is a shallow, man-made earthen basin, 1-1.5 m deep, in which sewage is treated naturally by bacteria and algae using sunlight. Detention is typically 5-30 days and the BOD removal is 70-90%.

Pollutant removal mechanism

1. Bacterial oxidation: aerobic bacteria in the upper layer oxidise the organic matter with oxygen: $\text{organics}+O_2\rightarrow CO_2+H_2O+NH_3+PO_4+\text{new cells}$.
2. Photosynthesis by algae: algae use sunlight and the CO₂, ammonia and phosphates released by the bacteria and produce oxygen: $CO_2+H_2O+\text{light}\rightarrow\text{algae}+O_2$. This oxygen, along with wind aeration at the surface, feeds the bacteria. The two groups live together in symbiosis.
3. Sedimentation and anaerobic digestion: settleable solids go to the bottom, where anaerobic bacteria convert them to CH₄, CO₂ and H₂S.
4. Pathogen removal: sunlight (UV), high pH during the day, long detention and predation kill most pathogens.

   sunlight
   \ | /
  ~~~~~~~~~~~~~~~~~~~~~~~~ surface (wind)
  Algae  --O2-->  Bacteria (aerobic)
  Algae <--CO2,NH3,PO4--  Bacteria
  - - - - - - - - - - - - - - - -
  Anaerobic sludge layer -> CH4, CO2
  ================ bottom

Role of the flushing device in sewerage

A flushing device (flushing tank or flushing manhole/hydrant) sends a large quantity of water into a sewer from time to time to clean it.

• Sewers laid at flat gradient, dead ends and house sewers at the start of a line do not get the self-cleansing velocity (0.6-0.9 m/s) because the flow is small. Solids then settle and decompose, producing blockage, hydrogen sulphide, corrosion and odour.
• The sudden discharge of 1-2 m³ of water (automatic siphonic flush tank, or hand-operated flush from a hydrant or tanker) raises the velocity and depth, scours the deposits and carries them to the larger sewers.
• Types: automatic flush tank (siphon type, filled from a water-supply connection and discharging at set intervals), non-automatic (hand-operated), and flushing by hydrants and fire-engine pumps.
• Result: sewers remain free from silting and blockage, the sewerage system functions well, and odour and corrosion are reduced.