In developing countries the high cost of installation, operation and maintenance of conventional wastewater disposal works mitigates against the provision of conventional works. In particular, for smaller and more remote communities and where suitable land is cheaply available, pond-based systems have been found to be attractive in terms of both low unit costs and low operational input requirements.
Ponds, which can be simply described as relatively shallow bodies of wastewater contained in an earthen basin, are used extensively for treatment of domestic wastewater and mixtures of industrial and domestic wastewater where amenable to biological treatment by natural processes involving the use of algae and bacteria.
Pond systems have the following advantages over conventional wastewater treatment systems:
- Ponds can achieve the required degree of purification at lowest cost and with minimum maintenance by unskilled operators.
- They are well able to withstand both organic and hydraulic shock loads
- They can effectively treat a wide variety of industrial and agricultural wastes.
- They can easily be designed so that the degree of treatment is readily altered.
- The method of construction is such that, should at some future date the land be required for some other purpose, it is easily reclaimed.
- The algae produced in the pond are a potential source of high-protein food which can be conveniently exploited by fish farming.
The nomenclature used in describing pond systems can be confusing, and a common terminology is useful. The literature generally describes a pond based treatment system as a "waste-stabilization pond system". Waste-stabilization pond systems comprise a series of ponds, all of which are relatively shallow bodies of wastewater contained in an earthen basin. The primary pond is often an anaerobic pond, followed by a series of aerobic ponds. Anaerobic ponds provide a degree of pre-treatment, thereby enabling a reduction in the size requirements of the subsequent aerobic ponds. As regards aerobic ponds, these all operate in the presence of air or oxygen. There are essentially five types of aerobic ponds:
Oxidation Ponds
- Mixing is totally dependant on natural conditions, principally the wind and oxygenation is due to algal photosynthesis.
- Sludge settles at the bottom of the basin.
Mechanically Assisted Oxidation Ponds
- Mixing and oxygenation is provided totally by artificial means.
- Stirrers provide continuous mixing and suspended solids are maintained uniformly throughout the basin.
- The power level is sufficiently high to maintain the solids in suspension.
- It is normally followed by settling tanks and sludge is recycled.
Aerated Oxidation Ponds
- Air pipelines with diffusers are installed along the bottom of the pond.
- Air bubbles mix the pond but the main source of oxygen is photosynthesis.
- Sludge settles at the bottom of the basin.
Facultative Ponds
- Aerobic conditions are generally maintained in the upper layers while anaerobic conditions exist towards the bottom.
- Oxygen is supplied by artificial means e.g. floating aerators and maintained in the upper layers of the basin.
- The power level employed is only sufficient to maintain dispersion and mixing of the dissolved oxygen.
- Algal photosynthesis plays little or no part.
- Sludge forms a layer at the bottom of the basin
- Three zones exist in the basin, namely: 1) a surface zone where aerobic bacteria and algae exist in a symbiotic relationship, 2) an anaerobic bottom zone in which accumulated solids are decomposed and 3) an intermediate zone that is partly aerobic and partly anaerobic.
Suspension Mixed Aerated Lagoons
- Mechanical mixing is achieved but is not followed by settling tanks and sludge recycling.
- Solids are discharged to an oxidation or facultative pond.
- Solids settle out to form a sludge layer.