Selection of Specific Processes

In the selection of specific processes and plant design, environmental engineers must consider the existing and potential regulatory standards; plant operators' requirements and availability; existing and projected sewage flow; flow pattern and waste characteristics; climate, topography, and availability of land; plant location within the community; and all aspects of cost. They must also assess the life-cycle cost of the plant. Lower construction costs can frequently be offset by high maintenance and increased operation costs.

The plant design cannot be optimized for all of these factors, consequently the environmental engineer must make difficult decisions in the face of uncertainty. The problem is complicated because the design life of the fa-

FIG. 7.21.1 Conventional domestic wastewater treatment process flow sheet.

cility is normally twenty-five years or more. A typical approach is for the design to minimize cost while achieving a treatment level for a set of constraints represented by the legal and physical aspects of the project.

Although many treatment plants, particularly large facilities, to be constructed include advanced forms of waste-water treatment, most plants in operation use conventional processes. For several more decades, these plants will serve a large percentage of the population. Upgrading existing facilities offers a promising alternative for improving treatment compared to completely new construction and the total loss of an existing plant's capabilities.

A principal consideration in overall plant design is to provide flexibility for expansion and upgrading of the initial plant. Flexibility for efficient plant operation allows the operator to overcome problems and provide maintenance and repair with minimum effect on plant performance. The design should also include the capability for implementing new ideas that may improve plant performance. Even minor modifications are sometimes difficult and more costly to make than the same capability built into the initial design and construction.

Anticipating plant expansion can also save money and avoid disruption. Land availability and wise plant layout are two basic considerations. Space allowances in buildings for future needs—pumps, instrumentation, and pipelines in the initial construction—are usually only a minor cost factor. For a small additional cost, environmental engineers can select chemical feeders and chlorinators for the initial project with sufficient capacity to handle anticipated increased rates. Often, the tanks designed for one purpose can be effectively used for a different process and accommodate plant expansion or upgrading.

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