Short-Term Flow Monitoring Skewing Rehabilitation Efforts

Sewer flow monitoring is widely used to locate inflow and infiltration (I/I) problems in wastewater collection systems. Short-term flow monitoring studies, studies lasting three months or less, distinguish inflow from infiltration based on the time it takes for sewer flows to return to dry weather levels following a rain event. A delayed return to a dry weather diurnal flow pattern may also result from different times of concentration for various branches in the upstream collection system. These times of concentration are unique to every flow meter location and are dependent on the size of the overall drainage area and the drainage area for each upstream branch of the system. Short-term studies may also attempt to quantify infiltration by monitoring night flows; however, night flows may also include actual wastewater contributions, especially in areas with 24-hour industrial operations and large drainage areas with extended flow travel times. Since inflow and infiltration volumes are difficult to differentiate, Concord chose to categorize excess flow in another way. Excess sewer flows that can be attributed to a response from a rainfall event are referred to as rainwater intrusion. Seasonal changes to the dry weather flow average that can be attributed to changes in the groundwater table are referred to as groundwater intrusion. Rainwater intrusion includes inflow and infiltration; however, it is an event-based deviation from the average dry weather diurnal curve. The average dry weather diurnal curve fluctuates over the course of the year. Groundwater intrusion is the volume increase in the baseline flow represented by the shift of the average dry weather diurnal curve. A long-term flow monitoring study that utilizes this method for categorizing excess sewer flow is a rainwater intrusion and groundwater intrusion (R/G) study.In the process of evaluating the effectiveness of Concord's flow monitoring program, results revealed that short-term studies may inadvertently skew rehabilitation efforts toward rainwater intrusion issues. Rehabilitation activities that target rainwater intrusion may not adequately address groundwater intrusion problems. For that reason, it is important to understand how to locate and quantify excess flow entering the wastewater collection system and determine if this excess flow is a result of rainwater, groundwater, or both. Operators with strained utility budgets cannot afford to transport and treat flow that is not contributed by paying customers, and they cannot afford to correct every defect found in the system. A reliable process of identifying and prioritizing the most significant problems must be used to select system improvements with the greatest benefits.Concord's program successfully reduced the wastewater collection system's response to rainfall; however increases in excess sewer flow during the spring season have continued to persist. This seasonal excess has been linked to the months of the year when the groundwater table is highest. Following this revelation, the City began trying to quantify groundwater intrusion problems and locate areas within the collection system most significantly impacted by seasonal dry weather flow changes. Even though rainwater intrusion was reduced in Concord's wastewater collection system, the overall excess sewer flow volumes for the year were not significantly reduced. If groundwater intrusion problems are not located, associated defects will worsen over time as the infrastructure ages and the surrounding soil settles and shifts. Therefore, long-term studies to locate and quantify these problems may be necessary to ensure that rehabilitation projects are properly prioritized.