Flow Patterns in the Collection System

Good collection system inspection, operation and maintenance are by far the best ways to protect the large investment the taxpayers have placed in the collection system operator’s hands. An effective inspection program will alert the operator to small problems that should be corrected before they become large problems.

Knowledge of the daily and annual variations in the collection system’s flow is useful in planning inspections and maintenance programs for the system. For example, collection system hydraulic cleaning should be scheduled during the hours of sufficient flows to accomplish the required maintenance. On the other hand cleaning with hydro-jetting should be scheduled during minimum flow periods. Upstream diversion should always be considered as a means of developing an optimum maintenance schedule.

The following chart illustrates daily fluctuations of dry weather wastewater flow from a typical residential community. The amount of flow is shown on the vertical axis of the chart, and the time of day is shown on the horizontal axis of the chart.

Diurnal Graph

Beginning at 12 p.m., we see that the flow is at 1.0 MGD and begins to drop off to a minimum of 0.5 MGD just after 4 a.m. This is typical of residential flow when people go to bed at night and are no longer using water for showers, washing clothes or doing dishes. A local industry that operates 24 hours a day and discharges process water would have an effect on the minimum flow. The residential flow increases from the minimum at just after 4 a.m. as domestic and commercial water consumption and discharges increase peak at 1.3 MGD at 10 a.m. Flow then begins to diminish. However,a second peak occurs just before 10 p.m., and then the flow drops off once again at 12 p.m.

A diurnal (day/night) graph like the one above is a good diagnostic tool that can help the operator determine the condition of the collection system and the amount of inflow or infiltration occurring. If plotted on a frequent basis and analyzed regularly the graph may alert the operator to problems such as a broken clogged or collapsed line in the system. When the operator notices a significant variations from the historical flow  data it can be assumed that some change has occurred and the cause needs to promptly investigated.

 

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