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DESIGN AND CONSTRUCTION 3.3

• To what extent must the existing facilities be brought up to current design standards
and building codes as part of the project?
• How can the original design, as-built facilities, and current condition be defined, and
remaining useful life of existing facilities and structures be determined?
• How can the existing treatment facilities be kept in reliable service during the con-
struction of plant modifications and be integrated with the new facilities?
• What safety or environmental hazards (such as lead paint, asbestos insulation, and un-
derground petroleum spills) may be uncovered during the construction of the modifi-
cations?
• What provisions have been previously made in the plant to expand or upgrade the ex-
isting facilities?
• Are there any existing plant site or hydraulic constraints that must be taken into con-
sideration with the facilities improvements?

Design decisions on these issues often require reasonable compromises that must
be worked out among the design team, the owner, and regulatory agencies. The cost-
effectiveness of modifying each existing plant component versus constructing a new
component should be considered.

Custom-Built Equipment versus Preengineered versus Package
When the treatment plant under design has a capacity of less than about 10 million gal-
lons per day (mgd), it is often possible to choose between custom-designed, preengineered,
or "package" treatment units. In custom-designed units, major processes usually utilize
reinforced concrete construction. The design engineer is responsible for all dimensions of
the unit and the complete process electrical, mechanical, and structural design. Equipment
suppliers provide and are responsible for design and performance of specific equipment
such as weirs, gates, sludge collectors, and other equipment located in the process unit.
Preengineered units are generally made using steel or fiber-reinforced plastic tanks.
An equipment supplier takes responsibility for the full process, structural, electrical, and
mechanical design of the unit. The design engineer generally is responsible only for the
foundation design and items such as influent water quality, hydraulic, and mechanical
connections, electrical and instrumentation connections, and, if required, buildings or en-
closures to house the process unit. Preengineered units are strictly field-assembled.
Package units are partially or completely factory-assembled and installed on the site.
The package units are often skid-mounted, with the process components prepiped and
prewired, requiring minimal fieldwork to install. The package units are often restricted to
1 to 2 mgd in capacity, as the size of the unit is constrained by shipping limitations.
Custom-designed process units tend to have higher construction costs but lower main-
tenance needs and longer useful lives. The best decision for a specific project can be made
only after a careful analysis of the costs and benefits of each feasible alternative. In gen-
eral, as the plant capacity becomes smaller, preengineered and package units become more
cost-effective and require less time to construct.

Design Development
As part of the preliminary design phase, several key design parameters must be
developed: