Leverage assessment and predictive modeling to proactively mitigate inflow and infiltration

Inflow and infiltration (I&I) - extraneous rainwater, groundwater, and flow from domestic or commercial sources - can lead to capacity overload in wastewater collection systems. I&I is often under-accounted for during detailed design and can add pressure to municipalities’ wastewater collection systems, requiring upsizing or even replacement of their wastewater conveyance and treatment facilities to accommodate excessive flow.

While complete I&I prevention is impractical, municipalities do have options to consider besides infrastructure upsizing or replacement: a proactive I&I mitigation program leveraging predictive modeling and modern assessment methodologies.

A targeted assessment plan that returns a five-to-ten-year outlook on maintenance requirements may at first seem like a superfluous expense - the assessment isn’t fixing the infrastructure, after all. However, the results of our work developing and implementing a proactive program with the Woodlands Water Agency (WWA) prove that targeted assessment with predictive modeling can help municipalities efficiently maintain wastewater collection infrastructure and avoid a costly replacement project.

Program overview

WWA manages the “retail” wastewater collection system, including about 311 miles of gravity sewer pipelines measuring 15 inches or smaller, in The Woodlands, Texas. The retail system discharges into the “wholesale” wastewater collection system made up of roughly 51 miles of larger-diameter gravity mains and managed by San Jacinto River Authority (SJRA).

It’s an extensive collection system, which is why the project team chose a multi-year approach, with each subsequent year increasing in sophistication. The project team studied the system holistically and deployed human resources only where I&I was expected using an iterative approach.

Over the course of three phases, the team conducted a modified sanitary sewer evaluation survey (SSES). They followed the industry standard approach, using pre-inspection flow monitoring, basin prioritization, and field investigation including smoke testing, manhole inspection, and closed conduit television (CCTV). But the SSES didn’t conclude with reporting and recommendations for rehabilitation alone; instead, its conclusion included reporting, recommendations, and a preliminary predictive model, a tested and effective tool that WWA could continue to use and refine to deploy operational staff efficiently.

Discussion of phased approach and assessment highlights

Phase 1 focused on extensive data collection and analysis. The project team leveraged existing flow monitoring data from a previous project to establish a hierarchy, based on responses to rainfall events, of sub-basins. They then incorporated that data into an existing hydraulic model of the entire wastewater collection system to calculate the rainfall-derived inflow and infiltration (RDII), which was determined by comparing the collection system’s responses to wet and dry weather conditions.

Results of RDII analysis revealed excessive RDII in the eastern and southeastern parts of the system.

While high volume RDII sub-basins usually have high peak flow RDII, the relationship is not linear. There are varying degrees of I&I. The study results in figure 1 highlight the I&I hotspots in the system as well as the area to inspect in more detail.

Within the initial batch of priority sub-basins, the project team conducted large scale sanitary sewer smoke testing as a cost-effective way to identify areas where extraneous flow may be entering the retail wastewater collection system.

The project team then used the smoke testing results to choose locations for night flow isolations, manhole assessments, and gravity main inspections.

Phase 1 results, which informed Phase 2, indicated that priority should be given to assets near bodies of water that are susceptible to ponding. During Phase 2, the project team replaced smoke testing with night flow isolations, to study how the system responded to rain events without the interference of typical daytime patterns. Results of the night flow isolation were then used to identify target areas for further manhole and gravity main inspections.

Using flow monitoring, RDII, and all other data collected during Phase 1 and 2 field investigations, a preliminary predictive model was created and then deployed in Phase 3 to act as a guide, pointing to other areas of the collection system that needed investigation. The statistical model used information about factors correlated to poor condition to identify those areas.

The project team then used the predictive model to target pipelines and manhole covers most likely to be in poor condition, such as the gravity mains pictured below.

Findings

Over the course of the phased program with WWA, the assessment process was continually refined. Adding predictive modeling to the standard SSES approach enables increasingly accurate identification of collection system infrastructure requiring rehabilitation, as demonstrated in the following tables featuring summaries of manholes and gravity mains recommended for renewal during the program's three phases.

Table 1: Summary of manholes recommended for renewal

*Year 3 included targeted infrastructure assessment selections and was not considered a spike in assets requiring some level of rehabilitation leftover from findings in earlier years.

Table 2: Summary of gravity mains recommended for renewal

*Year 3 included targeted infrastructure assessment selections and was not considered a spike in assets requiring some level of rehabilitation leftover from findings in earlier years.

As the tables show, by phase 3 WWA needed less overall assessment. And by phase 3, WWA’s detection of assets requiring rehabilitation had increased. The phase 3 predictive model increased the manhole detection rate by about 20% and the gravity main detection rate by 14%. So, assessment costs and the amount of effort needed to identify degraded infrastructure went down as the detection rate of degraded infrastructure went up.

For an owner relying on third-party operators or limited operational staff this program can significantly reduce inspection costs. In addition, effective I&I mitigation safeguards the integrity of the entire wastewater collection system, protecting public health and safety.

For a comprehensive discussion of our methods, please refer to "Incorporating Modern Techniques to Build a Framework for Inflow and Infiltration Assessment in Wastewater Systems," published in the third issue of this year's Texas WET, the official magazine of the Water Environment Association of Texas (WEAT).