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8.3 Economic factors

Sound practices require that costs are optimised. An indication of relative costs of technologies described in this Source Book is provided in table 2.5 and in Appendix 2. Optimising the cost of technology for wastewater management needs to consider (1) availability of land, (2) labour costs, (3) land uses and (4) economy of scale. Land is required for wastewater and stormwater management either underground to lay pipes or on the ground for a treatment plant or for land-based disposal. If low cost land is available a lower cost technology utilising more land can be chosen rather than a higher cost technology using less land area. Lagoons, for example, can be installed rather than an Activated Sludge Treatment Plant, because both can achieve the same standard for final BOD and SS. Labour cost for construction and maintenance is an important consideration. On-site treatment systems are generally more conducive to the use of manual labour for construction and maintenance, whereas off-site treatment systems generally require specialised equipment and skilled labour.

Availability of land, when an on-site system is used, enables reuse of treated wastewater at the site. Similarly when an off-site system is used, nearby agriculture, horticulture, forestry or industrial activities can present an opportunity for reusing the wastewater.

Economy of scale may be taken advantage when total cost of treatment is considered. Individual on-site systems do not present an opportunity for economy of scale for cost reduction, unless they are constructed in standard sizes and prefabricated components are manufactured in large quantities. Off-site treatment of wastewater from many households provides an opportunity for cost-saving in treatment. The cost of treatment per unit volume of wastewater will decrease with an increase in population served. The cost of collection will, however, increase, because larger diameter pipes and additional pumps and pumping stations are required. This will counter the cost saving in centralised treatment. In centralised collection and treatment systems with deep sewerage, the cost of pipes and pumps is generally a substantial proportion of the capital cost (up to 85%). There will be an optimum size of population served by an off-site treatment system when the combined cost of collection and treatment are considered. When opportunities for water reuse are also considered (piping of reuse water, availability of land or opportunities for reuse) there seems to be an optimum to the size of population served (Figure 2.46).

The economics of wastewater management needs to consider the benefits of improvement to public health and long-term affordability of sanitation services to the community. The benefits of improved public health to the economy of a country is difficult to quantify, although estimates have been made on the cost to the economy as a result of people suffering from illnesses from waterborne diseases (Appendix 1). Similarly the economic benefits of the protection of the environment from improper disposal of wastewater and stormwater is difficult to estimate. A case for subsidy to communities to install wastewater treatment facilities has been put forward (The all beneficiaries contribute (abc) principle).

Figure 2.46: Cost of treatment as a function of population served

From a community’s point of view the affordability of a wastewater collection and treatment system is an important factor. A percentage of the average person's income in a community, or of the average value of housing appears to be a figure that can be used as a measure of what a community can afford. What the percentage figure should be is determined by the importance given by community members to having the wastewater system in their community. The priority given to wastewater management in turn is dependent on the community having the information that will help them decide on its importance relative to other household and community needs. Hygiene promotion and education is needed to provide this information. An example of an excellent hygiene promotion is a publication by WHO (WSSCC Working Group on Promotion of Sanitation, 1998).

Selection of technology

Procedures to consider economic and environmental factors in a systematic way have been developed. These range from a single decision-making flowsheet to a computer software package.

Figure 2.47 and Figure 2.48 illustrate flowsheets that have been developed for selection of wastewater technology in developing countries in both urban and rural communities. Computerised decision making software is based on the same methodology as illustrated in the flowsheets. Two programs (SANEX and WAWTTAR) are briefly described in the accompanying boxed sections to illustrate their advantages and limitations.

Figure 2.47: Simple decision making flowsheet for choosing wastewater treatment systems (Pickford, 1995)

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