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November 2013 MM # 1351 SANITARY SEWER FEASIBILITY STUDY & REPORT NORTH SEBASTIAN AREA COLLECTION SYSTEM ALTERNATIVES There are a number of collection system alternatives available for consideration when preparing preliminary designs. When possible, it is preferred to utilize gravity flow when compared to other systems such as vacuum or low pressure since gravity flow does not rely on mechanical elements to convey the flow. In general, the advantages and disadvantages of vacuum and low pressure alternatives to gravity sewers are as follows: ADVANTAGES • Material and trenching costs are lower because pipe size and depth requirements are reduced. • Infiltration is reduced due to sealed pipe joints DISADVANTAGES • Requires much institutional involvement because the pressure and vacuum system has many mechanical components throughout the service area. • • The operation and maintenance (O&M) cost for a pressure and vacuum system is often higher than a conventional gravity system due to the high number of mechanical elements in use. Annual preventive maintenance calls are usually scheduled for grinder pump components of pressure sewers. STEP systems also require pump -out of septic tanks at two to three year intervals. • Public education is necessary so the user knows how to deal with emergencies and how to avoid blockages or other maintenance problems associated with low pressure systems. • The number of pumps or vacuum valves that can share the same downstream force or vacuum main is limited. • Power outages can result in overflows if standby generators are not available. • Life cycle replacement costs are expected to be higher because pressure and vacuum sewers have a lower life expectancy than conventional systems. 5