Page 40 - Water and Wastewater Engineering Design Principles and Practice

THE DESIGN AND CONSTRUCTION PROCESSES 1-11

Establishment of Design Criteria. Design criteria are the boundary conditions that establish
the functional performance of the facility. Two general types of criteria are used: performance
and prescriptive. Performance criteria define the desired objective, but not the means of achiev-
ing it. Prescriptive criteria define the explicit details of how the facility will be built. The design
criteria are frequently a combination of the two types of criteria.
Water and wastewater treatment systems will be used for illustration in the following para-
graphs. Some of the factors to be considered will differ for water supply and sewer systems.
Six factors are normally considered in establishing the design criteria for water and wastewater
treatment systems:
• Raw water or wastewater characteristics.

• Environmental and regulatory standards.
• System reliability.
• Facility limits.

• Design life.
• Cost.

Raw water or wastewater characteristics. Water characteristics include the demand for water
and the composition of the untreated ( raw ) water. Wastewater characteristics include the flow
rate of the wastewater and its composition.

Sound design practice must anticipate the range of conditions that the facility or process can reasonably be
expected to encounter during the design period. The range of conditions for a plant typically varies from
a reasonably certain minimum in its first year of operation to the maximum anticipated in the last year of
the design service period in a service area with growth of customers. . . . Often the minimum is overlooked
and the maximum is overstated. (WEF, 1991)
Consideration of the flowrates during the early years of operation is often overlooked, and over sizing
of equipment and inefficient operations can result. (Metcalf & Eddy, Inc., 2003).
The water characteristics include:
• Raw water composition.
• Hourly, daily, weekly, monthly, and seasonal variations in raw water composition and
availability.
• Variations in demand from domestic, industrial, commercial, and institutional activities.
The wastewater characteristics include:
• Composition and strength of the wastewater.

• Hourly, daily, weekly, monthly, and seasonal variations in flow and strength of the waste-
water.
• Contributions from industrial and commercial activities.

• Rainfall/runoff intrusion.