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3.13 Oxidation and Stabilization Ponds

Technical Description

Many chemical compounds are biodegradable, besides the organic substances which originate from living organisms. Specialised microorganisms (e.g., bacteria and algae) are able to utilize hydrocarbons, mineral oils, phenol, heavy metals, artificial substrates, plastic, etc., as food resources. These biological processes form the basis of most new methods of wastewater treatment. Similarly, these processes often lend themselves to use in a well directed and controlled structure, for example, in activated sludge treatment systems, trickling filters, or stabilization ponds and biomechanical treatment systems.

The main biochemical processes can be separated into four groups: anaerobic, aerobic, facultative, and aerated. The appropriate planning of the pond systems make it possible to treat raw sewage, pretreat raw sewage, and final treat biologically purified sewage. Bacteria eliminate the organic substances present in the wastewater, while algae produce oxygen. Bacteria act in both an aerobic degradation mode, where the energy source is organic carbon and the oxidized products originate from CO2, SO4, and PO4; and an anaerobic-reduction mode. These latter processes include facultative processes wherein heterotrophic bacteria produce fatty acids, alcohols, and aldehydes from organic substrates; and anaerobic processes wherein methane bacteria transform carbohydrates to methane by fermentation. Likewise, algae are capable of autotrophic and heterotrophic modes of action. The photosynthetic groups are the most important because they produce oxygen for the bacteria and animals. If the water is acidic (an H-donor), then the dominant process is autotrophic photolithotrophy. If there are any organic compounds, then it is heterotrophic photolithotrophy. The main algal genera are Chlamydomonas, Chlorella, Ankistrodesmus, Scenedesmus, Oscillatoria, Anabaena, Phacus, and Euglena. Under suitable conditions, protozoans, rotifers, and nematodes feed on the algae, thereby also playing an important role in the metabolism of the pond.

Use of an oxidation pond system for the final treatment of sewage from oil refineries has been very successful. For example, after a 40 day retention period, the 500 mg/dm3 COD concentration of an oil-sewage water emulsion was reduced to between 150 and 180 mg/dm3. In another case, following the activated sludge treatment, the oily, polluted water was final treated in an aerobic oxidation pond with a 40 day retention time. This resulted in an 80% decline in oil concentration in the effluent.

Similarly, biomechanical combined oxidation system effluent treatment technologies have been developed using similar technology to purify domestic sewage from communities with a population of between 2 000 and 200 000 individuals. This technology is also particularly suitable for the treatment of strong organic wastes. The technology is similar to that used in stabilization ponds, because it is based on the natural biodegradation processes. Purification takes place in a series of treatment units. After primary settlement, the sewage flows into the first treatment unit, which is operated as a pre-treatment section and is designed to achieve a 70% to 80% BOD removal efficiency. The remaining units operate as secondary treatment systems wherein oxygen is produced by photosynthesis. This treatment efficiency can be increased by the simple application of sand filtration as a tertiary treatment stage. In this type of biomechanical system, the treatment units are arranged as rings in which oxygen is produced by photosynthetic symbiotic bacteria and algae in the secondary treatment unit which surrounds the circular pretreatment unit. Treatment efficiency is increased by recirculating the liquid between the various treatment units, generally using an automated pump system. The oxygen produced by photosynthesis is supplemented by means of a floating mechanical aerator placed in the first stage of treatment. This aerator is operated intermittently by automatic control depending on oxygen concentrations within the units.

The concentration of active biomass contained in the biomechanical units is considerably less than that required for conventional activated sludge or oxidation ditch treatment, and the aeration system has a specific power demand of approximately one tenth that used in a conventional plant. A major advantage of this system is the fact that no surplus secondary sludge is produced and; therefore, no settlement tank, sludge treatment system, or sludge return facilities are required in the secondary treatment stages.

Extent of Use

All the known biological technologies used in Hungary are used for the treatment of different sewage waters. The first stabilization ponds in Hungary were built about 60 years ago. Since the 1960s, the number of these ponds has increased, largely due to their application for the treatment of wastewater from chemical works. Stabilization ponds are also widely used in Poland. In Latvia, where there are large systems of poldered lands, nutrient-rich and pesticide-contaminated surface waters are drained water to biological stabilization ponds prior to discharge to natural surface waters.

Operation and Maintenance

Operational and running costs are 40% to 70% less than those normally associated with single stage, conventional treatment systems. Total power demand also is reduced by 50% in comparison with conventional activated sludge systems. The total treatment efficiency of the system, however, is approximately 90% of the conventional plants. Maintenance is very simple and includes the removal of excessive aquatic vegetation. This system has been tested over many years and is considered to be a proven technology in the region.

Level of Involvement

Oxidation, stabilization and biomechanical treatment technologies are generally implemented at the local administration, corporate, and household levels.

Costs

Costs depend on the scale of a project. For agricultural stabilization ponds, the ponds are sized at approximately 3 to 5 m³/ha of agricultural land served, and building costs are at the level of $10 to $20/m³ of pond volume.

Effectiveness of the Technology

The efficiency of operation of stabilization ponds depends mainly on the environmental conditions (light, temperature, etc.), and the quantity and quality of the sewage water. Sewage stabilization ponds have a great economic advantage compared to other treatment techniques, but the efficiency of treatment is lower. Biomechanical systems with primary settlement can achieve a 70% to 80% BOD removal efficiency. With secondary treatment, these units can achieve approximately 90% BOD removal efficiency. This efficiency, however, depends mainly on environmental conditions and can vary as the quantity and quality of the sewage varies.

Suitability

These technologies are suitable for use in the treatment of domestic and industrial sewage. In many cases, two or more biological methods are used in series. The number of treatment units required depends on the nature of the effluent being treated, but the construction of additional units is simple and economical. The system is particularly suited for the treatment of strongly organic wastes, especially where there are wide variations in the volumetric and BOD loadings. The system has an inherent buffering characteristic and is particularly suitable for use in situations were shock loading and pH variations are expected. It is consequently resistant to "bulking", which is often a major problem with activated sludge systems.

Advantages

This is a cost-effective technology.

Disadvantages

The disadvantage of the biomechanical systems is their dependence on climate and weather conditions. At low temperatures, as in winter, the natural biodegradation processes are very slow and the rate of algae growth is limited. In such circumstances, the efficiency of the treatment decreases. In summer, though, the systems respond with high rates of algal growth, which can result in a measurable COD concentration in the outflow. Oxidation, stabilization and biomechanical pond technologies are land-intensive technologies.

Cultural Acceptability

This technology is an efficient wastewater treatment technology, well accepted by engineers and society.

Further Development of the Technology

The technology is complete in itself.

Information Sources

Petèr Kovac and Dr Kornèlia H. Kocsis, Felsö - Tisza - Vidèki Környezetvèdelmi Felügyelösèg, 4400 Nyiregyhàza, Szèchenyi u.19, Hungary, Tel. (36-42) 310 155, fax: (36-42) 310 713.

Rolands Bebris, Ministry of Environmental Protection and Regional Development, 25 Peldu Str., 1494 Riga, Latvia, Tel. (371-7) 227145, fax: (371-7) 820442, e-mail: BEBRI@VARAM.GOV.LV.

Anna Egle, V/U "Meliorprojects", 11 Novembra Bulvaris 31, LV-1494 Riga, Latvia, Tel. (371-7) 228734.

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