Page 46 - Water and Wastewater Engineering Design Principles and Practice

THE DESIGN AND CONSTRUCTION PROCESSES 1-17

In conjunction with the client, the engineer establishes the level of sophistication of the
facility. The following are examples of the items to be established:
1. Degree of automation.

2. Nature of maintenance history.
3. Number of people to operate the facility.
4. Qualifications of personnel required to operate and maintain the facility.

The availability and responsibility for providing connection to the electric grid, road access,
fuel requirements, and sludge disposal alternatives are also established at this stage. In addition,
facility aesthetics (architecture) and construction impacts on the local community are discussed.
The engineer completes a design of the major processes. This design includes sufficient
calculations to firm up the estimated land requirements, directs the location of soil borings, and
establishes horizontal and vertical control surveying.
An authorization estimate is made to provide a basis for making the final choice of the
treatment processes and to allow the client’s budget planning to proceed. Sufficient informa-
tion is available at this stage to allow a rigorous cost estimate comparison, such as that shown in
Table 1-5 , to be completed.
This is another critical decision point for the project. As noted previously, the owner must
be provided enough information to allow a rational decision about the choices available. This
information should include the alternatives, the consequences of deciding one alternative over
another, and the data and/or logic the engineer used to arrive at the consequences. This is also
an opportunity to revisit the assumptions made in screening the alternatives to determine if they
have changed due to circumstances or the passage of time. The null alternative should also be
explicitly addressed.

Final Design
At this point the project alternative has been selected. Detailed calculations and justifications
are prepared. In these calculations, a range of conditions are examined. For example, minimum
values for hydraulics, reactor oxygen, mixing, biological nutrient control, alkalinity, seasonal
nitrification temperature, and unit equipment sizing and maximum values for waste solids, reactor
sizing, oxygen demand, and return sludge are recommended (WEF, 1991). In addition, in cold
weather regions, the following should be addressed in the detailed design (WEF, 1991):
• The potential for ice formation on equipment.

• Freezing of the process equipment.
• Freezing of chemicals in storage.
• Freezing of pipes not located below the frost line.
• Viscosity changes in lubricants.

• Snow and ice accumulations on structures, equipment, and roads.
• Changes in the reaction rates of biological, physical and chemical processes.