Most of the organic matter present in waste waters is decomposed during secondary treatment. Waste waters now contain plenty of nitrates, phosphates and ammonium salts, which are the products of bio-degradation. In addition, these waters contain large population of microbes – bacteria, algae, fungi, protozoans etc. Phosphates, nitrates and ammonium salts are essential plant nutrients.
If released into natural waters, such waters cause the problem of eutrophication or excessive plant growth. The microbial population present in these waters has to be eliminated before these waters are discharged in a river or a lake as many of the microbes are dangerous disease-causing organisms.
After the secondary treatment, waste waters are subjected to tertiary treatment. It is during this phase of the treatment that much of the plant nutrients and microbes are eliminated. This is done in biological ponds in which waste waters are allowed to stay for some time. Biological ponds are a series of simple shallow tanks about 1 metre in depth which are interconnected with each other.
Rich population of algae develops in these ponds as plant nutrients are absorbed. Primary producers (algae) support a series of consumers. The mineral content of the water passes on to higher trophic levels and is eliminated. Profuse algal growth generates enough oxygen in the system to support a diverse community of aerobes which consume much of the harmful pathogenic forms.
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After Iagooning in biological ponds the waste waters are subjected to the process of disinfection which further eliminates the pathogenic organisms still present. This may be achieved by chlorination, treatment with potassium permanganate, ultra-violet irradiation or any such physical or chemical means. After the tertiary treatment the waste waters are finally fit for being discharged in a natural water body such as a lake or a river.