Stabilization ponds are usually classified according to the biological activity that takes place in the ponds as: 1. Aerobic Ponds 2. Anaerobic Ponds 3. Facultative Ponds.
1. Aerobic Ponds (Algae Ponds):
The aerobic ponds (also known as algae ponds) are designed to maintain completely aerobic conditions. In these ponds oxygen is supplied by natural surface aeration and by algal photosynthesis. The ponds are kept shallow with depth less than 0.5 m to 1.2 m. Shallower depths will encourage growth of rooted aquatic plants while greater depth may interfere with mixing and oxygen transfer from the surface.
Very shallow depth of aerobic pond (depth 0.15 to 0.45 m) is used for the treatment of irrigation return water or any other industrial sewage where the aim is the removal of nitrogen by algal growth. However, for the treatment of domestic sewage the depth is kept between 1 to 1.2 m. The length to width ratio of the pond depends on the geometry of the land but should not exceed 3:1.
This tends to prevent short circuiting. The influent and effluent structures are so located that entire pond volume is utilized. The contents of the pond are stirred occasionally to prevent anaerobic conditions in the settled sludge. Except for the algal population, the microbiological population present in these ponds are similar to that in activated sludge system.
The daily flow of sewage containing organic material provides necessary food to the aerobic population which stabilizes the organic matter by oxidizing it. During the process of oxidation, ammonia, carbon dioxide and other substances are liberated which are used by the algal population for their growth to produce more algal cells.
The algal populations in the presence of sunlight liberate free oxygen which is again used by the aerobic population to decompose the organic matter present in the sewage. The action taking place in these ponds is known as bacterial-algal-symbiosis, and it is the symbiotic relation between bacteria and algae leads to the stabilization of the organic matter present in the sewage. The algal-bacterial interaction in an aerobic pond is shown in Fig. 15.4.
Oxygen transfer in an aerobic pond depends on the following factors:
(i) Ratio of Pond Surface Area to Volume – Larger the ratio better will be the oxygen diffusion into the pond.
(ii) Turbulence – Generally provided by the wave action.
(iii) Temperature of Pond – Greater solubility of oxygen in water and hence greater diffusion rate at lower temperature.
(iv) Bacterial Oxygen Uptake Rate – The faster the micro-population consumes the dissolved oxygen, the greater will be the rate at which oxygen is replenished.
The composition of biological cell mass is approximated by the formula C120H180O45N15KP. According to this empirical formula, the basic requirement of bacterial cell growth is nitrogen (N), potassium (K) and phosphorous (P), and hence sufficient quantities of these should be available in the sewage, and if not, they should be added to the sewage to assure biological oxidation. A good rule of thumb to follow is to maintain a BOD/N/K/P ratio in the pond influent of 100/5/1/1.
In addition to the nutrient levels required, temperature and pH also influence the successful operation of aerobic ponds. Since the depth of aerobic ponds is less and its surface area is quite large, liquid temperature approximates the ambient temperature. Hence the expected BOD removal efficiency, which is 80 to 95 per cent under normal conditions, decrease during winter months. Further if pH is a problem, neutralization and equalization may be necessary prior to aerobic ponding.
For aerobic ponds the organic loading may be taken as 150 to 200 kg BOD5 per hectare per day during cold weather, which may be increased considerably during summer. Further the detention period should be about 7 days for proper development of algae.
It may, however, be mentioned that aerobic ponds develop intense algal growth and have been used on experimental basis only.
In anaerobic ponds the entire depth is in anaerobic condition except an extremely thin top layer. The anaerobic micro-organisms do not require the presence of dissolved oxygen in the water in order to function. Their requirement is met from the oxygen chemically contained in the organic materials.
The anaerobic decomposition takes place in two separate but interrelated steps:
Decomposition of dissolved organic waste, by acid-producing bacteria, to organic acids such as acetic, propionic and butyric acids, and
Further decomposition of these acids to the end products viz., methane, carbon dioxide and water, by methane-producing bacteria.
This is depicted in Fig. 15.5-
Effective operation of anaerobic ponds requires a balance between step 1 and step 2, because the methane producing bacteria are sensitive to the concentration of volatile acids. A portion of the waste material is used by the anaerobic biosystem as source of energy and in the synthesis of new bacterial cells. Sludge or solids buildup is therefore much less in the anaerobic system. The contents of the anaerobic pond are black in colour which is an effective indication that the pond is functioning properly.
The anaerobic process is usually accompanied by obnoxious odours and the effluent is only partially purified. The obnoxious odours are as a result of the reduction of sulphate compounds to hydrogen sulphide (H2S) gas by the acid producing bacteria. At high concentration H2S attacks painted surface and is deleterious if inhaled for an extended period. The long term solution of this problem is to limit the concentration of sulphate in the influent.
Anaerobic ponds require much less surface area than the aerobic ponds. These ponds are constructed with a relatively greater depth, ranging from 2.5 to 5 m, to conserve heat and minimize land area requirement. The efficient length to width ratio for these ponds is 2:1.
The levees of anaerobic ponds are similar to those of aerobic/facultative ponds, both in geometry as well as in construction. For these ponds the organic loading in summer may be taken as 1000 to 3000 kg BOD5 per hectare per day with BOD removal of 65 to 85%, and in winter it may be taken as 400 to 1000 kg BOD5 per hectare per day with BOD
removal of 50 to 65%.
The detention period for these ponds may range from 5 to 50 days. For anaerobic ponds, less surface area is required because the organic loading is about 10 times more than for aerobic ponds. Further the depth of the pond being more the surface area required is reduced.
Deposition and digestion of sewage solids is the main function of anaerobic ponds. With suspended solids loading of about 1000 kg per hectare per day, sludge may be allowed to accumulate for 10 years or more before removal is necessary. The ideal pH range is 6.6 to 7.6. The anaerobic process functions optimally over two temperature ranges- the mesophilic range of 29° to 38°C, and the thermophilic range of 49° to 57°C.
The greater depth provided in anaerobic ponds thus helps in maximizing heat retention. The anaerobic ponds are used mainly for pretreatment of strong industrial wastes and sometimes for the treatment of municipal sewage. These ponds may also be used in series with facultative ponds for complete treatment of sewage. The anaerobic ponds usually have an odour problem and these ponds are not commonly used in sewage treatment.
In facultative ponds the stabilization of sewage is brought about by a combination of aerobic, anaerobic and facultative bacteria. The facultative ponds function aerobically at the surface while anaerobic conditions prevail at the bottom. Hence these ponds combine the features of both aerobic and anaerobic ponds.
The depth of facultative ponds ranges from 1 to 1.5 m. As shown in Fig. 15.6 three zones exist in a facultative pond:
(1) An aerobic zone at the top where aerobic bacteria and algae exist in a symbiotic relationship, as previously discussed;
(2) An anaerobic zone at the bottom in which accumulated solids are actively decomposed by anaerobic bacteria; and
(3) A facultative zone in between the aerobic and anaerobic zones, that is partly aerobic and partly anaerobic, in which decomposition of organic matter is carried out by facultative bacteria.
The action in the aerobic zone is similar to the one found in the aerobic ponds giving rise to bacterial-algal-symbiosis. Further the top aerobic layer acts as a good check against odour evolution from the pond. The pond depth inhibits mixing, hence organic solids which settle remain on the bottom and are subjected to anaerobic decomposition.
The treatment effected by this type of ponds is comparable to that of conventional secondary treatment processes such as trickling filters, activated sludge process, etc., both in regard to BOD and bacterial removal. Hence facultative ponds are best suited and most commonly used for treatment of sewage. Most of the stabilization ponds adopted in India are of facultative type.
The effluent from a facultative pond usually contains plenty of algae which has considerable BOD and also results in a high concentration of suspended solids (SS). The effluent may have BOD5 and SS value in the range of 50 to 100 mg/l. However, the effluent, even with such a high BOD5 and SS concentration, will not cause nuisance when disposed of on land or discharged into receiving waters because the algal cells do not readily decompose or exert oxygen demand under natural conditions. In fact, the algae increase the oxygen levels in the receiving water by continued photosynthesis.
The quality of facultative pond effluents is usually based on the BOD5 of the filtered effluent, the assumption being that the suspended solids in the effluent are all algae. The filtration procedure adopted for the test is the same as for the suspended solids test.
Well-designed facultative ponds give about 80 to 90% BOD reduction based on BOD5 of the filtered effluent.
Facultative ponds also effect high bacterial reduction, the efficiency being particularly high in multiple ponds operated in series. Coliform and faecal streptococci removals are as high as 99.99%. Intestinal pathogens are also completely removed in these ponds. Further Cysts of Entamoeba hystolytica and helminthic larvae are also eliminated.
The facultative pond is simple and cheap to construct. It does not require skilled operation and is easy to maintain. Properly designed, the pond gives consistently good performance. The facultative pond has therefore become very popular for municipal, and institutional sewage treatment.
The method is suited wherever land is cheap and readily available and may be used for treating sewage either for discharge into streams or lakes or for use on land. The effluent from stabilization pond may be used for pisiculture (fish cultivation).
The method is particularly useful for interim sewage treatment when due to lack of funds or due to meagre flow in the initial stages, it is considered inexpedient to construct initially the treatment plant envisaged ultimately.
The facultative pond is also suited for the treatment of industrial wastes which are biodegradable provided the wastes are not coloured and do not contain substances toxic to algae.
Because of high level of performance in terms of pathogen removal and reliability, effluents from ponds having a minimum detention time of 4 to 6 days can be safely used for irrigation for crops which are not to be eaten raw. For unrestricted irrigation, series of pond systems may be designed to meet the microbial quality criteria for irrigation water.