4/26/2011 SEWER COMMISSION Minutes

Home Browse Search

Page 28 filter and controls into a single package. In reality, many of the existing STEP systems fall somewhere in- between. Historically, the former has a poorer performance record. Each ma-te-ialahould-a-e-vatuatedlelative�o -g�vity�castancLreliability—For-example,.STEP—pumps-can-- — range from a $150 cast iron effluent pump that will provide 5 years of service up to a $500 multi-stage high head effluent pumps that should last more than 20 years. Effluent filters can range from less than 0.5 square feet of flow through area of 1/4" mesh up to filters with more than 20 square feet at 1/8"filtering size. Tanks can range from structurally unsound tanks that crack or collapse during installation and leak immediately to water-tight tanks with water-tight connections that can provide up to 50 years of anticipated life with proper installation. When substandard materials are utilized, either in new construction, or for replacement parts, an increase in 0&M costs is a certainty. A key component during construction is the inspection and/or replacement of the existing building sewer, the source of most of the inflow that can plague system performance. 0&M of on-site STEP systems is typically divided into two activities, reactive maintenance (RM)and preventative maintenance (PM). The protocol for on-site system maintenance varies greatly, even when comparing similar systems. Most systems have been operated with varied degrees of emphasis placed on PM activities,the frequency of which is dictated by the most sensitive components. Extremely aggressive PM programs have been able to reduce RM service calls to a point that they become relatively insignificant in the overall 0&M costs. Unfortunately an overly aggressive PM program can also result in higher overall 0&M costs when PM activities unnecessarily target components that have a significant level of reliability with less PM. A less known RM protocol is a run-to-fail approach,whereby maintenance is only performed when equipment fails. Essentially, the operator waits for an alarm. While this approach is fairly uncommon, it has some important implications. While daily 0&M costs (at least in the early years) may be extremely low, increasing frequency of major repairs and replacement activities will escalate as the system suffers from neglect.Additionally, public perception of a run-to-fail approach is generally poor because this approach may place the customer in a more responsible position of identifying system alarms and reporting them to the management entity. Most studies reveal that customers want as little responsibility as possible related to their systems. In practicality, the most cost-efficient STEP management approaches balance PM and RM to achieve the lowest overall cost for 0&M. The STEP systems that typically achieve the best overall 0&M cost tend to base them on the required PM cycles for each component. For example, most field experience indicates that tank accumulations measured every 3 years will allow the operators to determine the frequency of pumping required. Tank effluent screens are generally reported to need inspection and, if necessary, annual cleaning. The minimum frequency for pump and controls inspection is also about one year. Thus, a program can be designed to do the annually required tasks, and during those visits do the other less frequently required tasks at their required frequency. Thus, the least frequent operator visits would be annually, and would consist of inspecting the screens and the pump and controls. Other tasks would be added as necessary(e.g., tank accumulation checks every 3 years).As the records of high-quality STEP components are verified by unbiased experience, these minimum frequencies may increase.. One of most important components of STEP 0&M cost is tank pump-out intervals. Extended tank pump intervals can be achieved through the use of larger tanks and through the measurement of sludge and scum accumulation. In a managed STEP system,where tank inspections are executed as part of a PM program, pump-out intervals can be extended until they are functionally required. When a management program is in place, as is the case with centrally managed STEP system, pump-outs still provide earlier pump-out intervals for