In designing an on-site wastewater treatment plant, the primary criterion for selection of one design over another is protection of the public health and health of the workers while preventing environmental degradation. Secondary criteria are cost and ease of operating and maintaining the system. The fate of any residuals resulting from the treatment and disposal system must be considered in the selection process. There are a variety of wastewater treatment options available for a specific wastewater problem; it is rather difficult to make a decision on the selection of the design option that prevents public health hazards and maintains environmental quality at the least cost.

The first step in the design of an on-site system is the selection of the most appropriate components to make up the system. Because the site characteristics constrain the method of disposal more than other components, the disposal component must be selected first. Selection of waste-water modification and treatment components follow. To select the disposal method properly, a detai1ed site evaluation is required. However, the site characteristics that must be evaluated may vary with the disposal method. Because it is neither economical nor practical to evaluate a site for every conceivable system design, the purpose of this first step is to eliminate the disposal options with the least potential so that the detailed site evaluation can concentrate on the most promising options. To effectively screen the disposal options, the wastewater stream to be treated and disposed of must be characterized as detailed as possible, and an initial site investigation made.

The estimated daily wastewater volume and any short- or long-term variations in flow affect the size of many of the system components. In addition, the concentrations of various constituents can affect the treatment and disposal options chosen.

Once a process designer or engineer has completed a survey of characteristics of the wastewater and an on-site evaluation of perspective treatment facilities, he or she needs to choose a set of wastewater treatment processes that are able to achieve the objective of the design at a cost that is acceptable. Among all factors that are relevant to the selection of the treatment processes, economical consideration is the most important factor in deciding which process(es) to be included in the final selection.

In order to estimate the costs of the processes in consideration, the data from the wastewater characterization should be available along with the design parameters for the processes and the empirical cost correlations for these processes. The costs related to alternative processes and information on the quality of effluent should also be collected prior to the development of cost estimation in compliance with the regulations regarding waste-water discharge.

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