Stabilisation pond treatment

A common practice for improving the effluent treated in the aerated lagoon is the use of a stabilization/polishing pond system (Costa-Pierce, 1998). This system depends on the action of aerobic bacteria on the soluble organics contained in the waste streams. The organic carbon is converted to carbon dioxide and bacterial cells. Algal growth is stimulated by incident sunlight which penetrates to a depth of 1-1.5 m. Photosynthesis results in the production of excess oxygen which is available to the aerobic bacteria; additional oxygen is provided by mass transfer at the air/water interface. The ponds are designed to provide a residence time of 2-20 days, with surface loadings of 5.5-22 g BOD5/day per square metre.

The ponds are usually multiple cell units operated in series to eliminate the possibility of short circuiting and to permit sedimentation of dead algae and bacterial cells. The ponds are constructed with inlet and outlet structures located in positions to minimise short circuiting due to wind-induced currents; the dimensions and geometry are designed to maximise mixing. These systems can achieve 80-95% removal of BOD5 and approximately 80% removal of suspended solids, with most of the solids discharged as algae cells. During winter periods the degree of treatment decreases markedly as temperatures decrease and ice cover eliminates algal growth.

Aerobic stabilization ponds are utilised where land is readily available. In regions where soils are permeable, it is often necessary to use plastic, asphaltic or clay liners to prevent contamination of adjacent groundwater. Although obsolete and not really 'process controlled' these systems are still in use because of their low cost and efficiency when low-standard effluents are required.

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