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How can the inflow and infiltration problem be solved?
Information Provided by Global Water Instrumentation.
The reduction and control of inflow and infiltration in sanitary sewer systems should be considered with regard to a disciplined, long-term monitoring and maintenance program. The first step to resolving any inflow and infiltration problems is determining how significant the problem is. Typically a sanitary sewer system evaluation is performed to assess the system. An evaluation of the sewer system will determine the quantity of inflow and infiltration, determine their sources and provide guidance to determine a cost effective corrective action plan.
As with most situations you can't manage what you can't measure, and the first step to managing the inflow and infiltration issue is to measure the extent of problem. To quantify the inflow and infiltration into a sanitary sewer system means a significant attempt to locate and record information that relating to a variety of issues including, but not limited to, observed overflows, measured or observed surcharges, reported bypasses, customer backup complaints, and chronic maintenance activities. The information should be obtained from different places including maintenance records, sewer maps, complaint records, assorted department files, work orders, past studies, engineering reports, and interviews with personnel who are responsible for maintenance and management of the sanitary sewer system. A large amount of information can be found using these sources as well as others. Once the data has been found it must be recorded and displayed in a way that will show possible relations between overflows, bypasses and other related factors such as capacity models, rainfall records, maintenance activities, and reported backups. If electronic maps of the sanitary sewer system are available, they should also be used to confirm the result of the data findings.
Once the data has been researched and correlations found, the city or agency must establish sewer flow monitoring points at various locations within the system. Typically, sanitary sewer systems can be broken down into associated watersheds. Then those watersheds can be separated into basins and if necessary sub-basins.
Flow monitoring instrumentation must be placed in sanitary sewer systems at locations appropriate to obtain the data desired. To measure wastewater flows through the sanitary sewer system it is important to select the appropriate flow meter. Many types of flow monitoring instrumentation are available and pricing varies accordingly. Simple instruments like a flow probe measure water velocity and depth but do not record data. This type of instrument is good for spot flow checks or random checks of permanently installed flow meters. Often long term flow measurements can be made using simple water level recorders. In this case only water level is recorded then the data is exported into a spreadsheet and the data can be processed through an equation or lookup table that cross references water level to flow for that particular site. The advantage of water level recorders is that they are relatively inexpensive and multiple units can be purchased with a moderate investment to monitor the water level (flow) throughout the sanitary sewer system. Alternatively more sophisticated flow meters can output, display, and record flow information directly. Often these instruments also have output that can trigger wastewater samplers or other devices. These instruments are typically a larger investment, but have greater monitoring abilities.
The following "rules-of-thumb" may be used to determine a monitoring and evaluation strategy to adequately measure amount of inflow and infiltration in a sanitary sewer system. These parameters vary depending on the overall city or agency goals.
One flow meter for every 30,000 - 50,000 feet of sanitary sewer pipe
The flow meter recording should be set at 15-minute intervals
Flow meter capable of measuring surcharges
One rain gauge for every 2-4 flow meters
Minimum monitoring period - 45 days with 60 days being optimal
Measurement of between 6-8 separate rainfall events
The system should be monitored during a period of high seasonal groundwater
Once the flow monitoring data has been collected it should be carefully evaluated. Adjustments to account for periodic flow profiling at the monitoring site, errors associated with grease or deposits on the sensors, drift of the depth recordings, and downtimes related to flow meter malfunction. The corrected data should be tabulated and analyzed to make comparisons between the measured inflow and infiltration and the corresponding rainfall intensity. Data under surcharge conditions should be avoided for analysis purposes. The analysis will provide two essential parameters that are used to quantify the inflow and infiltration problem. The first parameter is a comparison between different basins so that basins can be prioritized for future studies and potential inflow and infiltration reduction. The second parameter is information that will be useful if subsequent relief or replacement sewer systems are necessary to reduce or eliminate overflow or bypass conditions.
Basins can be ranked in a range of ways. Rankings might include unit inflow or infiltration rates such as gallons/day/foot, mgd/1,000', gpd/inch-mile of pipe, mgd/acre, etc. By changing the raw flow data into a measured unit rate, comparisons may be made between basins as well as comparisons relating factors such as general age of the sanitary sewer system, frequency of reported overflows, etc.
In addition to flow monitoring there are other tests that a city or agency can use to identify sources of inflow and infiltration. These tests include dye and smoke testing and visual inspection. Smoke and dye testing work by introducing either dye or smoke into the sanitary sewer system and determining where it comes out. Visual inspection can be done with remote television monitoring devices and used to look for cracks or other damage in a sewer pipe.
Once a source of inflow and infiltration has been discovered the city or agency can take appropriate action to resolve the problem, including fixing or replacing damaged or leaky sewer pipes and notifying property owners of improper connections. Periodically the city or agency must monitor and measure their sanitary sewer system to maintain the integrity of the system and determine new sources of inflow and infiltration. Continuous monitoring is also beneficial to the cities and agencies so appropriate cost increases can be applied to communities/basins that are heavy contributors to inflow and infiltration into the sanitary sewer system.
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