An important item to be investigated when designing a water or wastewater treatment plant is the design of the collection troughs required by the many different basins in the treatment facility. These collection troughs have a direct bearing on both the process and the economic aspects of the facility.
Sizing to Handle Maximum Flow Rate
The process aspect of collection troughs involves the requirement to handle a maximum specific flow rate. All basin collection troughs have one thing in common: they must be sized properly so the hydraulics of the troughs will not adversely affect the intended unit process function and design. The trough must be designed properly so that flooding of the weirs does not occur. If flooding of any portion of the weirs did occur, the basin hydraulics would lose their continuity; thus the individual unit process would suffer. Each basin requires a different arrangement for the collection troughs, depending upon its designated application.
Proper Economic Sizing of the Trough
Another important aspect of the design of the collection trough would be that of economics. A trough should not be over-sized because this would increase the cost of the total project. On the other hand, a trough should not be undersized, because that would adversely affect the process aspect of the project and the headloss required for it. A proper economic sizing of the trough should be a compromise between an economic determination of the cost of the trough versus the cost of handling additional flow and headloss.
Planning for Extensions of Existing Installations
Many of the treatment facilities now being planned are extensions of existing installations. Most existing treatment facilities are located on or near a receiving body of water and therefore the outfalls are set in accordance with the existing water level of the body of water. This outfall condition would affect all the unit process basins upstream of the outfall structure and set and/or limit the head available to them.
If head is no problem in the treatment scheme, a collection trough that discharges into a collection/outlet channel will be at a free discharge condition. The water level, as it flows into the outlet channel, will be flowing at approximately critical depth. To compute the upstream water depth (Ho), 1.73 x the critical depth (Hc) can be used.
Hc = [Q^2/gb^2]^1/3 Q in cfs; b in ft.; g=32.2
The use of free discharge arrangement would give the most economical design, because it yields the smallest basin trough cross section possible. However this design utilizes the maximum head.