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2.4 Sustainable versus unsustainable wastewater and stormwater management

The natural purification processes and biogeochemical cycles described in section 2 (2.2) provide a basis for determining what is environmentally sustainable management practices for wastewater and stormwater. Discharge of wastewater and stormwater into an environment exceeding the natural purification capacity of that environment will result in the accumulation of organic materials (carbon), nitrogen, phosphorus or other pollutants that cannot be absorbed by the ecosystem constituting the receiving environment. Accumulation of organic materials will result in a high oxygen demand that cannot be met by oxygen transfer from the atmosphere. Undesirable anaerobic conditions are a consequence (See section 1 (1) on discharge of wastewater with a high BOD to a river). Figure 2.8 illustrates an unsustainable practice where the natural purification capacity of a river into which wastewater is discharged is exceeded, and where in addition the local biogeochemical cycles are not closed.

Figure 2.8: Unsustainable wastewater management practice from not closing the local biogeochemical cycles resulting in the natural purification capacity of the receiving environment to be exceeded (Lange and Otterpohl, 1997).

Nitrogen and phosphorus in wastewater are discharged to a river resulting in their accumulation in the river. Eutrophication of the river is an outcome. The nitrogen and phosporus in the wastewater come from food consumed by people. To grow this food fertilisers containing nitrogen and phosphorus are required. These are manufactured chemically from atmospheric nitrogen and from phosphate rock. The flow of materials (N & P) is one way from the atmosphere for N and from the phosphate rock mine for P into the river. There is depletion of a resource (mined phosphate rock) and accumulation and pollution in the river. This practice is unlikely to be sustainable in the long term, because phosphate rock deposits will be exhausted and pollution of the river by N and P needs further treatment of the wastewater.

One way of managing the wastewater sustainably is by closing the material cycles locally (Figure 2.9).

Nutrients in the wastewater are reused to grow food. In this way there is not the need to to use as much chemical fertilisers and at the same time there much less discharge of nutrients to the river. The problem of resource depletion and pollution of the river is overcome by closing the material cycles. Figure 2.9 also emphasises the need to treat industrial wastewaters containing toxic substances separately, and not to mix industrial wastewaters with domestic wastewater. In addition stormwater should be separately collected and treated and infiltrated locally.

Figure 2.9: Sustainable wastewater management practice by closing the local biogeochemical cycles (Lange and Otterpohl, 1997).

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