Cultus Lake Park Sanitary Sewer Infrastructure Assessment

Fraser Valley Regional DistrictCultus Lake Sanitary Sewer Infrastructure Assessment – Final Report4.3 Preliminary TreatmentPrior to introducing raw sewage to the wastewater treatment plant, the raw sewage hasto be screened to protect pumps, valves, pipelines, and other equipment from damage orclogging by rags and large objects. In Cultus Lake, the raw sewage is pumped from SL 1to an existing distribution chamber which distributes the flow equally to four septic tanks.It is proposed to intercept the raw sewage prior to existing septic tanks and route the rawsewage to the treatment plant where it can be screened. Flow from others future areasand the west field can be routed directly to the treatment plant using pump stations. Referto Appendix C, Drawings which shows the process flow diagrams and preliminary layoutfor Option 2 and 3 respectively4.3.1 Grinding and ScreeningThe sewage will be pumped by the influent pumps to the fine screens through thegrinders. The grinders will shred the large objects and rags, and the fine screen removesthe shredded screenings from influent water. The fine screen is provided with a press thatcompacts, and dewaters the captured screenings for final disposal.The conceptual design was carried based on providing, two influent pumps(duty/standby), two grinders (duty/standby) and two fine screens (duty/standby) completewith the associated pipes, valves and flowmeters.4.3.2 EqualizationThe peak wet weather flow to the plant is 42 L/s, and the maximum day design flow is1893 m3/day (22 L/s). An equalization tank should be provided to store the excesswastewater during wet weather flow events and to minimize the hydraulic shock loads onthe treatment plant. MSR require treatment plants to be designed based on two trains asa minimum, with the remaining capacity of each train must be at least 75% of the designmaximum flow. The downstream trains will have capacity of 150% of the designmaximum flow (i.e. 22 L/s x 1.5 = 33 L/s) which is less than the peak wet weather flow(42 L/s). Equalization will be required to store excess wastewater above the designmaximum flow.Currently, the East field has four septic tanks with a total capacity of 368 m 3 . These tankscan be converted and utilized as an equalization tank. The capacity of the tanks willprovide approximately 11.3 hrs. of retention time based on a peak wet weather flow andmaximum downstream treatment capacity of 42 L/s and 33 L/s respectively.The flow from the fine screens will be discharged by gravity to the equalization tanks andthree equalization tank pumps (2 duty/1 standby) will pump the screened wastewater tothe secondary treatment.4.3.3 Preliminary Treatment SizingTable 8 summarizes the preliminary treatment equipment Sizing.Project No. 30825 – September, 2012 Page 11