Waste stabilization pond is one of the three methods of biological treatment of water. Others are the activated sludge process and trickling filter process. Waste stabilization pond is a biological treatment method that depends on bacteria. It has an advantage over other methods because it requires no chemical addition. In this method, three ponds are used in the process (see Figure 1):
Anaerobic pond, and
The treatment process usually goes through the three ponds. The anaerobic pond usually operates under anaerobic conditions (without oxygen and light), which is why it is usually deeper. The anaerobic conditions are made possible by the formation of its own scum/grease layer over the pond that shut out light and air. Here, microbes digest matter completely and it is normally used under high-strength waste such as industrial wastes (sewage). They are used as pre-treatment ponds to reduce BOD or COD of wastewater up to 60 – 70 %. With this reduction, the facultative pond can work under aerobic or anaerobic conditions that require only 30% of the oxygen demand of the first water to handle it.
Facultative ponds could not often act as anaerobic ponds because the surface is large and the depth is low. Working with a facultative pond requires the growth of algae. It is usually green in colour because of the algae presence and because of symbiosis between algae and bacteria (see Figure 2).
The final phase of the treatment is the maturation pond which acts as the punishing phase. Here, the pond is the shallowest and it is mostly used for the destruction of pathogens. Most people use maturation ponds as fish ponds because of their performance.
Advantages of waste stabilization ponds
1. It is cheap to construct and maintain.
2. No expertise is required in its operation.
3. It is more efficient in pathogen removal than other biological methods.
4. It is conducive in hot climates like Africa because microbes need heat. Per degree rise in temperature, microbes increase by 40%.
Marais’s theorem states that in an arrangement of ponds, maximum efficiency is achieved when the ponds are operating at the same detention time. Detention time or holding time or retention time is the time it takes for organic particles to move from entrance to exit. Detention time controls the flow rate based on the volume of the reactor. A system can operate at 5, 10, and 20 days detention time depending on the flow rate.
Design a facultative pond and maturation pond to treat 2800 m3/d of agricultural sewage which has a BOD5 of 1500 mg/l. The design temperature is 27oC and the required effluent standards are BOD5 ˂ 25 mg/l and faecal coliform (fc) ˂ 500/100 ml.
Assume that for a facultative pond, an effluent BOD value of 60 mg/l is assumed which implies that Le = 60 mg/l.
A = (Q (Li – 60))/ (18D (1.05) T-20)
D range from 1 – 1.5, D is chosen as 1.2 m; Li = 1500 mg/l
A = (28000 (1500 – 60))/ (18 x 1.2 (1.407) = 40320000/ 30.3912 = 1326700 m2
Organic surface loading, λs = 10 QLi/A = (10 x 28000 x 1500)/ 1326700 = 317 kg/ha.d
Check permissible loading, λs = 20T – 120 = 20 x 27 – 120 = 540 – 120 = 420 kg/ha.d.
Since organic surface loading ˂ permissible organic loading, the design is satisfactory. After the design, choose the pond configuration.