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Approaches and policy steps

Management steps to cut sewage's impact on the environment

The effects of individual sewage discharges are usually localized, but sewage is a major source of marine contamination in all regions, and is therefore a global issue.

The chief concerns are: human health impacts from exposure to pathogens, via seafood contamination or contact with contaminated water; resultant losses in fisheries and tourism revenues; and the environmental impacts of nutrients, BOD, suspended solids and other components of the sewage.

Pathogenic microorganisms in sewage-contaminated marine and estuarine waters cause a massive transmission of infectious diseases to bathers and to consumers of raw or undercooked shellfish. The global economic impact of such illness has been estimated at over US$10 billion annually.

Available measures to reduce these impacts include: appropriate siting of discharges; conventional treatment; and the development and application of alternative technologies such as composting and biogas generation.

Conventional treatment technologies are well-developed, but should not be regarded as a universal panacea. They re-quire large capital investment in collection and treatment infrastructure, often necessitate high ongoing operational costs and technical capacity, and in certain settings (e.g. low-lying areas subject to frequent flooding) have significant technical limitations.

It should not be assumed that higher levels of sewage treatment are always desirable. For some sewage-related issues (e.g. suspended solids and litter), minimal treatment is often adequate and higher levels provide only marginal improvement.

The effective removal of nutrients is expensive and only justified when sewage inputs of nutrients are of concern; in low-nutrient waters vulnerable to eutrophication (e.g. tropical lagoons), even advanced tertiary treatment may not achieve adequate reductions of nutrient discharges.

The appropriate siting of effluent discharges to enhance dilution and dispersion, minimize environmental impact, and/or reduce human exposures to sewage-borne pathogens is often less expensive, more effective, and more operationally sustainable than advanced treatment. It is, in any case, essential regardless of the level of treatment.

Priority actions:

• develop and apply more appropriate technologies and practices. In many regions the provision of conventional collection and treatment infrastructure cannot keep pace with urban growth, and may not be appropriate to local circumstances. Less centralized and less capital-intensive technologies (e.g., improved septic systems, composting toilets, small-scale biogas and treatment plants, use of artificial or natural wetlands) may offer more appropriate solutions, and improved opportunies to reuse valuable components of sewage (e.g. water and nutrients);
• immediately improve the implementation of existing measures. Relatively simple and inexpensive measures such as better design and installation of septic systems, better maintenance and operation of existing treatment plants, and improved siting of municipal sewage outfalls could yield significant gains in many, if not all, regions;
• link capital investment in infrastructure to sustainable financing for ongoing maintenance and operational costs. Large expenditures on collection and treatment works achieve little if there is not adequate financial and technical capacity for operations and maintenance. In particular, new sewerage projects in developing countries should include long-term provisions for operations and maintenance, including local capacity building; and
• prioritize issues, set objectives, and assess all alternative solutions objectively and with regard to environmental goals and the nature and uses of receiving water. Pathogens, nutrients, organic materials and other components of sewage pose different environmental risks, requiring different measures to address them that depend upon the nature and uses of the receiving waters. Siting of discharges is often more important to environmental outcomes than the level of treatment. For example, disposing raw sewage in well-flushed deep oceanic waters through a long outfall may have less environmental impact, lower costs, and less risk of failure than discharging tertiary-treated effluent into an enclosed nearshore basin.