Pinpointing blockages to stop sanitary sewer overflows before they happen
Managing sanitary sewer overflows (SSOs) is an ongoing concern for many sewer utilities, and while most have established plans to deal with them when and where they occur, preventing them from happening is the preferred solution. Blockages are among the primary causes of SSOs, so if you know when and where blockages are developing, you can mitigate the problem well before an SSO occurs. Let us examine how.
1. Determine the root causes of blockages. At one major utility we examined in the Southeastern U.S., 65% of the reported SSOs resulted from blockages. A closer look revealed that 59% of the blockages were caused by grease, 24% were caused by roots, 16% were caused by debris, and 1% were caused by vandalism. All of these are preventable.
2. Determine what can be done about blockages. Blockages represent an operation and maintenance (O&M) problem and, if left unresolved, have a high probability of causing SSOs. Therefore, once a blockage occurs, response time is critical. If identified early enough, the O&M response operates in a rescue mode where the blockage is removed, flow conditions are restored to normal, and an SSO is averted. If identified too late, an SSO has already occurred, and the O&M response operates in a recovery mode where the blockage is removed, flow conditions are restored to normal, and cleanup and reporting activities are performed. However, both rescue and recovery imply an emergency response. Therefore, a preventive approach is often preferred.
3. Determine if a preventive cleaning approach is appropriate. Sewer utilities generally have adopted preventive approaches in the form of preventive sewer cleaning. The approach is simple: clean every gravity sewer over a predetermined period of time, often once every three to 10 years. While this approach is straightforward and makes a positive impact, gravity sewers often are cleaned when they do not need to be, and gravity sewers that need to be cleaned may not be. As a result, some blockages continue to go undetected and cause SSOs. Therefore, sewer utilities often will adapt this approach and implement higher-frequency preventive cleaning for specific gravity sewers where the probability or consequence of SSOs is greater. While incremental improvements are observed at these hot spot locations, the same weakness is still noted. Gravity sewers that do not need to be cleaned often are redundantly cleaned, and gravity sewers that need to be cleaned may not be.
4. Investigate emerging level monitoring technologies. To optimize the use of sewer cleaning personnel and equipment, level monitoring technologies have emerged in recent years to detect blockages at the earliest stages and allow sewer utilities to respond in a proactive mode rather than a rescue or recovery mode—applying the right resources in the right place at the right time.
Monitoring In Action
To observe flow conditions before, during and after blockages, a level monitor was installed just upstream of an 18-in.-diameter gravity sewer with known, recurring grease problems in the middle Tennessee area. Previously, this location was cleaned on a high frequency basis—once every two weeks—to reduce the probability of an SSO.
A level monitor that uses a non-contact, ultrasonic level sensor was installed in the manhole located just upstream of the gravity sewer with the recurring blockage. After installation, regularly-scheduled preventive sewer cleaning was suspended, leaving the level monitor to watch and wait. During an 18-month period from July 1, 2016 through Dec. 31, 2017, three developing blockages were observed.
All three developing blockages were characterized by a gradual increase in the maximum daily flow depth. However, the first developing blockage differed from the second and third blockages in one important way: the minimum flow depth showed a gradual increase in the first developing blockage, but not in the second and third blockages. The gradual increase in maximum daily flow depth indicated the presence of a developing blockage, while the increase or lack of increase in the minimum daily flow depth provided information about the type of developing blockage. In this case, the first developing blockage was caused by debris accumulation in the invert of the gravity sewer, while the second and third blockages were caused by grease accumulation on the pipe wall near the wastewater flow surface.
The transition from high-frequency cleaning to proactive, on-demand cleaning reduced the number of mobilizations to this location from 36 to three—a 92% reduction. The payback period on the level monitoring equipment installation and service was less than six months, with an estimated savings of 77% in preventive cleaning over a 60-month period. And no SSOs occurred.
Break Through the Blockade
Sewer level monitoring technology has demonstrated the ability to detect developing blockages in sufficient time to provide a planned, proactive response rather than a reactive rescue or recovery response, allowing a more cost-effective use of preventive cleaning personnel and equipment while still eliminating SSOs. Early detection of developing blockages is easily obtained anywhere from several days up to one week before urgent attention is required, and early detection with up to two weeks of lead time may be possible in
Data science and machine learning algorithms are now being employed to identify the telltale signs of developing blockages, alerting sewer utilities so they can respond appropriately and stop SSOs before