Page 47 - Water and Wastewater Engineering Design Principles and Practice

1-18 WATER AND WASTEWATER ENGINEERING

Extreme heat must also be considered. The following should be addressed in a detailed design:

• Operator heat exposure.
• Equipment temperature limits.
• Flammability of chemicals and fuels in storage.

• Viscosity changes in lubricants.
• Expansion of joints and piping.
• Changes in the reaction rates of biological, physical, and chemical processes.

The design is completed in sufficient detail to select standard manufacturers’ equipment, pre-
pare specifications in draft form, and firm up the site plan and layout of the facilities. The choice
of equipment is another critical decision point for the project. It is frequently to the economic
advantage of the owner to purchase the equipment directly, rather than through the construction
contract, because the contractor may have to pay taxes on the purchase and the municipality
will not. Owner-procured equipment provides the owner greater control in selection of specific
equipment. Thus, the equipment choices should be thoroughly reviewed with the owner.
As the design is finalized, well-developed drawings and specifications are prepared. Bid esti-
mates are prepared based on the detailed design. The economics of alternative means of complet-
ing the selected treatment process are considered in the bid estimate. For example, the choice of a
variable-speed drive or constant-speed drive for pumps is analyzed on a life-cycle cost basis (cap-
ital plus operating, maintenance, and replacement). Quotes are obtained to finalize cost estimates,
and an engineer’s opinion of the probable cost and cash flow projections are prepared. Because
the financial arrangements for funding the project are based on this estimate, it is imperative that
an accurate estimate of the cost be made.
The detailed design process is completed with an ongoing review of the project with the
client. Typically, these occur at 30, 60 and 90 percent completion. These are critical decision
points for the project. As before, the owner must be provided enough information to allow a ra-
tional decision about the choices available. This is not to imply that the owner needs to consent
to every nut and bolt in the design, but they should have the opportunity to review alternatives
with major economic and/or operating implications. This is also an opportunity to revisit the
assumptions made at the preceding decision points to determine if they have changed due to
circumstances or the passage of time. The null alternative should also be explicitly addressed.
At this point the project is sufficiently well understood to submit an application to the per-
mitting authority for a construction permit.
Once the final design is complete, bid documents are prepared and the bid is let. This is the
end of the formal design process and the beginning of the construction phase of the project. Un-
usual circumstances such as the unavailability of specified equipment or materials or unexpected
soil conditions may require more design work. At this point the engineer’s scope of work may
have to be renegotiated.

Incremental Design and Iteration
As noted earlier, the design process is incremental. In addition, individual steps as well as criti-
cal decision points often may require iteration. That is, a trial design may not meet performance