Page 44 - Water and Wastewater Engineering Design Principles and Practice

THE DESIGN AND CONSTRUCTION PROCESSES 1-15

Site limitations. The location and area available for the treatment plant, availability of power,
roads, and a connection to the raw water supply or point to discharge define the facility limits. In
addition, the need for easements for the water distribution system and sewer system, and connec-
tion to the power and road grid are limitations that must be considered.

Design life. The basis for economic comparison of alternatives is the design life. Processes and
components of processes with different design lives must be brought to an equivalent life for
valid economic comparison. Standard engineering economic techniques are available to perform
this analysis. A primer on economic analysis is given at http://www.mhprofessional.com/wwe .

Cost. Cost is part of the design criteria because “(t)he ultimate selection among otherwise
acceptable unit processes or process trains is based on an economic evaluation.” (WPCF, 1977)
The degree of effort and care taken to estimate the capital investment cost and the operating and
maintenance cost depends on the stage of development of the project. In the early stages, rough
and relatively rapid estimation methods are usually the only ones justified. These are called
order-of-magnitude estimates. In the middle stages of the development of the project more
sophisticated estimates are made based on better information about the alternatives. These are
called study estimates. Authorization estimates are made to make the final choice between com-
peting alternatives to complete the project. Bid estimates are made when the decision is made
to proceed with construction of the project. To provide an accurate document against which
to control expenditures during construction, a project control estimate is made using detailed
drawings and equipment inquiries (Valle-Riestra, 1983).
Cost estimates consist of two parts: capital costs and operating costs. “The capital cost and
operating cost estimated for each alternative must be made equivalent to make an economic com-
parison.” (WPCF, 1977) Several alternative methods may be used to make equivalent economic
comparisons. These include present worth analysis, annual cash flow analysis, rate of return
analysis, benefit-cost analysis, and breakeven analysis. These are described in numerous standard
textbooks on engineering economic analysis, for example, Newnan et al. (2000) and Thuesen and
Fabrycky (2000). Consideration of both the capital cost and the operating cost on an equivalent
basis is an essential part of making the correct choice in selecting the most economical alterna-
tive, as illustrated in Table 1-5 . Using Table 1-5 , on the basis of capital cost alone, alternative
B would be selected as the more economical plant. On an equivalent basis (total annual costs),
alternative A is the more economical plant. The selection of alternative B on the basis of capital
cost alone would result in an excess expenditure of more than $1,000,000 over that of alternative
A over the 25-year life of the project.
A frequent omission failure in the examination of alternatives is the failure to consider the
null alternative. In addition, care must be taken not to include sunk costs (that is, past costs) in
the economic analysis and decision making process. The only relevant costs in an engineering
economic analysis are present and future costs (Newnan and Johnson, 1995).

Screening of alternatives. Alternative designs are examined for the feasibility of meeting
design criteria. Either experience, literature review, or rough calculations are used to determine
sizes to be used in examining feasibility. Potential sites for the project are identified based on the
rough sizes. An order-of-magnitude level of cost is made at this point.
This is a critical decision point for the project. The owner must be provided enough informa-
tion to allow a rational decision about the choices available. This information should include the