V&A Newsletter_Case History_August

Wastewater managers in Cotati, a city of about 6,000 in California’s Sonoma County, decided to take a proactive approach to heading off a problem that affects many other small cities across the country: excess stormwater flow into the wastewater treatment system. The California firm of Winzler and Kelly Consulting Engineers was retained, with V&A as subconsultant, to carry out an Infiltration/Inflow Study and Reduction Program to locate sources of excessive flow (“infiltration/inflow” or “I/I”) into the City’s sanitary sewer collection system. V&A’s services included flow monitoring and rainfall monitoring services; I/I analysis; collection basin reconnaissance and investigations; CCTV (closed-circuit television) investigations; and smoke testing.

The Stormwater Problem
Most sanitary sewer collection systems have increased flow volumes during the rainy season. The additional stormwater flow that enters the collection system during wet weather, generally classed as infiltration/inflow (I/I), is relatively clean water that requires little or no treatment once the initial surge (carrying street debris, oil, etc.) has passed. I/I volumes can be very substantial, to a point where treatment facilities may not have the capacity to handle the increased flows. In addition to c

apacity issues, reducing I/I within a collection system will reduce the treatment costs at the treatment facility.

During a storm event, additional flow over the expected baseline flow (i.e., the dry-weather flow) is classified as I/I and is calculated by subtracting the baseline flow from the real-time monitored flow.

I/I sources are further categorized by the type of source: groundwater infiltration (GWI), rainfall-dependent infiltration (RDI), and stormwater inflow (SWI).

The most common sources of both GWI and RDI are defects in deteriorated sewer pipes, such as cracks, offset joints, root intrusion points, and broken pipes. Typically, groundwater or rainwater enters the pipelines through these defects. RDI is more significantly influenced by the size and duration of the storm event. SWI locations are usually easier to find than GWI and RDI sources, and also usually less expensive to correct. These sources include direct and indirect cross connections with storm drainage systems, roof downspouts, and various types of surface drains.

Project Approach
V&A partitioned the collection system into ten basins and conducted dry-weather and wet-weather flow monitoring. The flow monitoring data was analyzed to determine the basins that contribute the most I/I into the collection system. These basins would then be investigated via a variety of reconnaissance techniques to try to determine direct point sources of inflow, and/or the areas responsible for high levels of infiltration contribution. Direct and quick identification of these sources could provide the City with a way to reduce the sanitary sewer treatment costs.

Results
Flow monitoring, rainfall monitoring, and I/I analysis determined that Basin 9 in particular had a significantly large source of SWI during the larger storm events.

To investigate the source of the inflow, V&A utilized a variety of investigative techniques, including smoke testing, low-flow reconnaissance investigations, interviews, and CCTV pipe inspections. The investigation required more than two months, but it succeeded in pinpointing the the source of the offending inflow.

A small business had three small (4-inch) outside floor drains situated adjacent to the business property, which were directly connected to the sanitary sewer system. As it happened, this property was also located at the low point within a fairly extensive watershed basin. During the previous winter, the property had flooded multiple times, sometimes resulting in more than 24 inches of standing water. As a result, the floor drains were conducting flood water into the system at a rate of more than 500,000 gallons a day.

The connection of the floor drains to the sanitary sewer was later confirmed using a tracing dye test, and the characteristics of a flood event were consistent with the flow monitoring data.

Modification of the floor drains to not allow storm water into the sanitary sewer system is a relatively inexpensive fix and can be accomplished before the start of the next wet-weather season.

Correcting this single defect will also eliminate a half-million-gallon (0.5 MGD) spike of flow at the downstream treatment facility.
__________________________________________________________________________


Vol 1. No 3, August 3002 *Case History* City of Cotati, Ca Sewer System Master Plan		for previous articles, visit the Infrastructure Preservation News archives and V&A's web site at www.vaengr.com
Inflow/Infiltration Investigation Helps California City Control Stormwater Flow, Save WW Treatment $$ Wastewater managers in Cotati, a city of about 6,000 in California’s Sonoma County, decided to take a proactive approach to heading off a problem that affects many other small cities across the country: excess stormwater flow into the wastewater treatment system. The California firm of Winzler and Kelly Consulting Engineers was retained, with V&A as subconsultant, to carry out an Infiltration/Inflow Study and Reduction Program to locate sources of excessive flow (“infiltration/inflow” or “I/I”) into the City’s sanitary sewer collection system. V&A’s services included flow monitoring and rainfall monitoring services; I/I analysis; collection basin reconnaissance and investigations; CCTV (closed-circuit television) investigations; and smoke testing. The Stormwater Problem Most sanitary sewer collection systems have increased flow volumes during the rainy season. The additional stormwater flow that enters the collection system during wet weather, generally classed as infiltration/inflow (I/I), is relatively clean water that requires little or no treatment once the initial surge (carrying street debris, oil, etc.) has passed. I/I volumes can be very substantial, to a point where treatment facilities may not have the capacity to handle the increased flows. In addition to capacity issues, reducing I/I within a collection system will reduce the treatment costs at the treatment facility. During a storm event, additional flow over the expected baseline flow (i.e., the dry-weather flow) is classified as I/I and is calculated by subtracting the baseline flow from the real-time monitored flow. I/I sources are further categorized by the type of source: groundwater infiltration (GWI), rainfall-dependent infiltration (RDI), and stormwater inflow (SWI). The most common sources of both GWI and RDI are defects in deteriorated sewer pipes, such as cracks, offset joints, root intrusion points, and broken pipes. Typically, groundwater or rainwater enters the pipelines through these defects. RDI is more significantly influenced by the size and duration of the storm event. SWI locations are usually easier to find than GWI and RDI sources, and also usually less expensive to correct. These sources include direct and indirect cross connections with storm drainage systems, roof downspouts, and various types of surface drains. Project Approach V&A partitioned the collection system into ten basins and conducted dry-weather and wet-weather flow monitoring. The flow monitoring data was analyzed to determine the basins that contribute the most I/I into the collection system. These basins would then be investigated via a variety of reconnaissance techniques to try to determine direct point sources of inflow, and/or the areas responsible for high levels of infiltration contribution. Direct and quick identification of these sources could provide the City with a way to reduce the sanitary sewer treatment costs. Results Flow monitoring, rainfall monitoring, and I/I analysis determined that Basin 9 in particular had a significantly large source of SWI during the larger storm events. To investigate the source of the inflow, V&A utilized a variety of investigative techniques, including smoke testing, low-flow reconnaissance investigations, interviews, and CCTV pipe inspections. The investigation required more than two months, but it succeeded in pinpointing the the source of the offending inflow. A small business had three small (4-inch) outside floor drains situated adjacent to the business property, which were directly connected to the sanitary sewer system. As it happened, this property was also located at the low point within a fairly extensive watershed basin. During the previous winter, the property had flooded multiple times, sometimes resulting in more than 24 inches of standing water. As a result, the floor drains were conducting flood water into the system at a rate of more than 500,000 gallons a day. The connection of the floor drains to the sanitary sewer was later confirmed using a tracing dye test, and the characteristics of a flood event were consistent with the flow monitoring data. Modification of the floor drains to not allow storm water into the sanitary sewer system is a relatively inexpensive fix and can be accomplished before the start of the next wet-weather season. Correcting this single defect will also eliminate a half-million-gallon (0.5 MGD) spike of flow at the downstream treatment facility. __________________________________________________________________________ V&A Consulting Engineers 1999 Harrison Street, Suite 975, Oakland, CA 94612 phone 510.903.6600 fax 510.903.6601 info@vaengr.com copyright 2003