Sanitary sewer system failures and basement backups lead to unanticipated, costly repairs; inconvenience to the community; and intervention by regulatory agencies. Municipalities and utilities can avoid these headaches by planning and budgeting for monitoring, maintenance, and inspection, yet many utilities neglect system deficiencies until the damage is done. A cost-saving, pro-active approach is to conduct a comprehensive sanitary sewer evaluation study (SSES), or at minimum a limited version of such a study (LSSES).
An SSES is a formal program of investigation work encompassing dye water flooding, manhole and structure inspection, smoke testing, closed-circuit television (CCTV), and flow monitoring. Many municipalities already perform some of these functions, but an SSES provides a structured examination of the entire system and generates a cost-effective rehabilitation plan to reduce infiltration and inflow (I/I), satisfy regulatory requirements, and reduce the need for expensive water treatment in prioritized areas.
A comprehensive SSES is advisable if the sanitary sewer basin has never been analyzed. Depending on the basin size, an SSES will take from six months to a year to complete, as each survey step should be performed during the recommended time period for maximum effectiveness.
A follow-up LSSES will help to gauge the effectiveness of the rehabilitation work resulting from the SSES. An LSSES should also be performed when a utility notices system changes related to flow, backups, or bypasses. Usually performed within a smaller area of a sanitary sewer basin, an LSSES may be limited in the overall area analyzed or in the specific investigation tasks selected.
“An LSSES helped us by providing recommendations for doing preventive maintenance on our sewer system,” says Rick Johnson, utility manager of the New Berlin (Wis.) Water & Wastewater Division. “By rehabilitating our sanitary sewer through relining, we will save time and money in the long term. Most of all, rehabilitating allowed us to perform repairs while avoiding costly restoration charges.”
Some utilities have the ability to perform many inspection tasks with in-house resources. Others need to out-source a portion or all of these services. Once a format is developed, future investigative work can be performed in-house to reduce costs. In either case, the utility should establish a set of standards to conduct each investigation consistently so valid comparisons can be made between different sets of collected data. The data sets can be correlated by using asset-management software.
Analysis of the data gathered using the following investigation methods creates a snapshot of system performance and prioritizes problem areas.
Televising reveals and records defects that can then be rated for severity and potential to impede the hydraulic performance of a sanitary sewer system. Typical defects are pipe joint leaks, mineral deposits, roots, pipe cracks, and sags between pipe lengths. More severe problems, such as a broken or collapsed pipe, need to be addressed quickly.
Utilities that do not own CCTV equipment should consider contracting out the work, as this investigation technique provides a firsthand look at the performance of any segment of the system. CCTV inspection should be performed at least every seven to 10 years, but specific conditions may require more frequent televising.
The National Association of Sewer Service Companies (NASSCO) Pipeline Assessment and Certification Program (PACP) provides a coding standard for televised inspections. By applying consistent methods, standardized coding enables a comparative analysis of each area of the system to establish maintenance and rehabilitation priorities.
The CCTV information can be delivered in a hard-copy report format supplemented by VHS, CD, or DVD video formats. CD and DVD enables digital storage of the data using asset-management software such as GIS. Prior CCTV work performed as part of general system maintenance can also be used as part of an SSES or LSSES to assist in tracking system changes over a longer period of time.
Deteriorated manholes are a major, often overlooked source of I/I. Manhole inspection can accurately inventory these assets and confirm the structural condition of each site. Inspections are typically performed by two-worker field crews recording surveyed data on hard-copy or digital records. Digital records can be incorporated into asset-management software. Use of a standardized coding system such as NASSCO's Manhole Assessment and Certification Program (MACP) is recommended.
Manhole defects are readily apparent upon visual inspection and may include the following: holes in manhole covers, a poor fit between the manhole cover and rim, cracks and holes in the pavement around the manhole rim, cracks or misalignment between bricks in the manhole, loss or absence of mortar between the bricks or rings of the chimney, cracks in manhole walls or joints between sections, and non-watertight seals around connecting pipes. Manhole inspection should be part of a rotating maintenance program to revisit each site at least every seven to 10 years as well.
Flow monitors measure and quantify sewage flows to evaluate overall system performance. The amount of ground-water and rainwater entering the sewer system can be quantified by comparing the difference in the sewage flows during dry and wet weather conditions. Flow and rain data can be correlated to determine the percent of rainfall that entered the pipe — or rainfall-derived I/I — and the overall sewer performance.
Different technologies are available, but most flow monitors measure the velocity and depth of the flow at various time intervals using either contact or non-contact methods. Newer technologies deliver collected flow monitoring data directly to office computers, avoiding the cost of visiting the site to collect data.
Typically collected for at least six months, the data can be used to develop statistics for base flow, average day flow, maximum day flow, and maximum hour flow, indicating how the sewer performs during wet-weather events. The time period is important because monitoring the system during wet and dry months is necessary to understand the impact I/I has on the system.
Utilities that choose to perform continuous flow monitoring can purchase the equipment and train staff to install, collect, and maintain flow meters. Crew members must be certified for confined space entry. Continuous monitoring provides the flexibility to investigate different areas with very little down time, gauge the effectiveness of rehabilitation work in SSES-identified areas, and track system changes that could lead to future investigations. Utilities that do not intend to perform continuous flow monitoring outside of the SSES program can either rent the equipment or outsource the work.