Waste Stabilization Ponds or Basins (Oxidation Ponds/Lagoons)

WASTE STABILIZATION PONDS OR BASINS (OXIDATION PONDS/ LAGOONS)

• Stabilisation ponds are open, flow through earthern basins, designed and constructed to treat domestic sewage and biodegradable industrial waste waters.

• WSBS have long detention periods, extending to several days.

• Low cost system and is used in rural areas.

• These ponds are completely mixed biological reactors without solids retur

Stabilisation ponds may be classified according to nature of biological activity: 1. Aerobic, 2. Anaerobic, 3. Facultative (aerobic - anaerobic).

Construction:

• These are earthern pits made of impervious soil (clay).

• Lining is provided to prevent seepage of ground water.

• The depth varies from 1 m to 1.8 m.

• The walls of the pond are sloped and the topwidth varies from 2.5 to 3 m.

• Minimum free board of 0.6 m. is provided.

• Influent enters at the centre of the pond and the effluent overflows in a corner.

• The detention time is usually 2-6 weeks.

• In the stabilisation ponds, microbial plants like algae, protozoans, bacteria and rotifers grow and metabolize the organic wastes in sewage.

• The sewage contains organic matter that provides food to the microbial Obra population. The microbes stabilize the organic matter (oxidation) into CO2, помо nitrates and nutrients. The nutrients released are taken by the algal population Isidoroim and in return it releases oxygen by photosynthesis.

• The algae (microscopic plants) which grows in the presence of sunlight, produce oxygen by the action of photosynthesis and this oxygen is utilised by the bacteria for oxidising the waste organic matter. This process is known as "bacterial-algal symbiosis" and results in complete stabilization of organic wastes.

• When tank depth is greater, lack of mixing and aeration occurs, which results in anaerobic digestion of the settled solids in the tank bottom.

• The dissolved oxygen in pond water is high during afternoon due to the release of oxygen by algae in the presence of sunlight.

• BOD reduction is above 95% (in summer) but as algae contribute to suspended solids/BOD, the effluent standard BOD < 30 mg/l cannot be met.

• Algae contribute about 50-70 mg/l of SS/BOD.

• Algae is a serious problem in lagoon treatment.

• Hence algae should be removed before discharge by gravity settling, floatation or filtration.

Advantages of Stabilisation Ponds

(i) Low initial cost

(ii) Low operating cost.

(iii) Regulation of effluent discharge is possible. 

Disadvantages

(i) Requires extensive land required laertnyeol esgl Used in rural areas.

(ii) Odour problems.

(iii) Effluent quality standard of 30 mg/l for suspended solids is not met.

 

(1) Aerobic ponds (Algae Ponds)

• In aerobic stabilization ponds, the oxygen is supplied by natural surface aeration and by algal photosynthesis.

• Aerobic ponds are shallow (0.5 to 1.2 m depth) and functions aerobically throughout the depth.

• L/B ratio is 3:1 to present short circuiting.

• Detention period = 7 days.

• Contents of the pond are stirred to maintain aerobic conditions.

 

The daily sewage flow containing organic material provides food to the aerobic microbial population which stabilises/oxidises the organic matter into nitrates and CO2. The CO2 and nitrates so formed are used by algal population for growth and production of new algal cells, thereby releasing oxygen. This oxygen in turn is used by microbial population for oxidation of organic waste. This is called bacterial-algal symbiosis.

Oxygen transfer depends on:

(i) Lagoon surface area to volume.

Larger the ratio, better will be the O₂ diffusion into the lagoon. ailuast doidwa

(ii) Temperature of lagoon.

Greater solubility of O, in water and hence greater diffusion rate at lower temperature.

(iii) Turbulence.

Generally provided by the wave action.

(iv) Bacterial oxygen uptake rate.

The faster the microbes consume D.O, the greater will be the rate at which oxygen is replenished.

 

(2) Anaerobic Ponds:

The entire depth of pond is in anaerobic condition. The anaerobic decomposition takes place in two steps.

Step 1: Decomposition of dissolved organic waste by acid-producing bacteria into organic acids (such as acetic, butyric acid) and new anaerobic microbial cells.

Step 2: Decomposition of these acids into end products methane CH₄, carbon dioxide CO₂ and water by methane producing bacteria.

→ The black colour of lagoon indicates that the lagoon is functioning properly.

→ Process releases H2S septic odours and the effluent is only partially purified.

The abnoxious odours are due to the reduction of sulphate compounds into hydrogen sulphide (H2S) by acid producing bacteria. The solution is to limit the concentration of sulphate in the influent.

• Anaerobic ponds require less surface area than aerobic ponds (1/30^-th)

• Pond depth is 2.5 m to 5 m for improved heat retention.

• L/B = 2:1; Detention is 2 to 5 days.

• BOD reduction is 65 to 85% during summer and 45% to 65% during winter.

• Sludge is accumulated for 10 years.

• Anaerobic ponds are used for the treatment of strong industrial wastes and has limited use in sewage treatment.

 

(3) Facultative Ponds (Oxidation Ponds)

• Combination of aerobic and anaerobic ponds.

• Constructed of Intermediate depth = 1 to 1.5 m.

Facultative ponds consists of three zones:

(i) Aerobic zone at the top.

(ii) Anaerobic zone at the bottom.

(iii) Facultative zone between aerobic zone and anaerobic zone.

• The aerobic zone is similar to aerobic ponds (Bacterial Algal Symbosis). To no In the top layers, Algae grows and utilize CO, during day light for photosynthesis and liberate O2 and maintains the aerobic conditions.

• The aerobic conditions promotes the oxidation of organic waste matter by aerobic bacteria.

• In facultative zone, the facultative bacteria oxidize the incoming organic waste as well as products of anaerobic decomposition.

• Greater depths prevent mixing and settled solids remain in the bottom.

• When sewage enters the pond, the suspended and colloidal matter settles at the bottom and undergoes Anaerobic Fermentation (Methane Fermentation) and liberates Methane -0.25 g of methane is liberated for 1 g of BOD_u

Aerated Lagoons:

• Used when wind mixing and oxygenation by algae is not sufficient and when stabilisation ponds are overloaded with industrial waste at reduced temperatures.

• Aerated lagoons are intermediate between Oxidation Ponds and ASP.

• The oxygenation is provided by mechanical surface aerators installed on rafts or floats.

• Greater Depth of about 2.5 m to 4 m is provided.

• Land required is less.

• Smaller Detention Time

Types → (1) Complete Mixed

 (2) Partially Mixed

Type → Aerobic Aerated Lagoons

Facultative Aerated Lagoon.

(1) Complete Mix Aerated Lagoon.

• Greater aeration is provided to keep solids in suspension to maintain aerobic conditions.

 • Similar to ASP but without recirculation.

• First unit provided with surface aerators to maintain the solids in suspension.

• Second unit is used as a settling tank which removes the suspended solids.

SS concentration - 60 to 300 mg/l.

Detention - 2 to 10 days.

O₂required -  0.7 to 1.3 kg/kg BOD5

BOD removal - 75 to 85%.

 

(2) Partially Mixed System.

•  These are operated at low rate of aeration which is not suffiicient to keep all the solids in suspension. Therefore, the solids tend to settle down.

• Only the top layers remain aerobic.

•  Large portion of incoming solids in sewage settle at the tank bottom which boobs undergoes anaerobic decomposition.

•  Effluent produced is more stable.

Settled Sludge (SS) Concentration - 30 to 150 mg/l.

Detention Time -  3 to 5 days. 

O₂ required for oxygenation - 0.8 kg/kg BOD, removed.

BOD removal - 75 to 90%.

• The settled sludge needs to be removed after few years.

Design Criteria for Oxidation Ponds:

Depth  - 1 m to 1.8 m

Detention - 2 to 6 weeks - depends on sunlight and temperature.

- 7 to 42 days.

Surface area - Based on organic loading - 300 to 150 kg/ha/d (India).

Area = 0.5 to 1 hectare

Advantages of Oxidation Ponds

• Cheap and low cost process.

• Suitable for small towns and cities.

• Suitable for hot dry country (India).

•  Low maintenance.

•  No skilled supervision required.

•  Flexible for fluctuations in flow and organic loading.

Disadvantages of Oxidation Ponds

•  Mosquito Breeding.

•  Algal growth.

•  Bad Odours.