The incoming flows to a Water Pollution Control Plant fluctuate throughout any given day. Higher "base" flows occur in the morning, noon and night when domestic water usage is high, while lower base flows occur in between these peak periods. Groundwater infiltration and direct inflows (I/I) into the collection system (i.e. extraneous flows) also contribute to the overall sewage flow received by a treatment plant.
The average day flow represents the total quantity of sewage received by the Water Pollution Control Plant over the course of a year divided by the number of days in a year. The peak day flow represents the highest volume of sewage received over a given 24 hour day period.
The Petawawa WPCP has been designed to accommodate an average day flow of 8,730 cubic metres per day (2:1 peaking factor). These flows are based on a 20 year design period which takes into account population growth within the community as well as CFB Petawawa requirements.
Sewage flows also tend to fluctuate over the course a year (i.e. seasonal variations). For example, flows during the spring thaw are usually much higher than during other time periods of the year due to higher infiltration and direct inflows into the collection system. The magnitude of these flows largely depend on the condition of the collection system, surrounding soil, groundwater table and direct connections ( e.g. roof drains, weeping tiles, etc.).
A maximum day design flow usually accounts for very high 1/1 flows which typically occur during the spring thaw. The maximum day flow allows for very high I/I rates occurring concurrent with the maximum 24 hour domestic flows. This flow is intended to represent "worst case" conditions and the design of hydraulic devices and controls in a plant is usually based on maximum day flows. However, the treatment process is not normally designed to handle such high flows and must be by-passed.
Based on the Operating Certificate of Approval (C of A), the Petawawa WPCP is "approved" to treat sewage at an average daily flow of 8,730 cubic metres per day and a peak daily flow of 17,460 cubic metres per day. The plant is not permitted to accept flows in excess of 17,460 cubic metres per day, and any flows larger should be by-passed.