Two Sides of the World One Surprising Collection System Truth

INTRODUCTION The Albuquerque Bernalillo County Water Utility Authority (ABCWUA) in Albuquerque, New Mexico, has vast experience designing, installing, operating, and maintaining small-diameter gravity sewers. To create an extensive study on how the design criterion of slope affects Sanitary Sewer failure performance, the ABCWUA collaborated with the Yarra Valley Water (YVW) Utility in Melbourne, Victoria. Collectively, this research compiled 30-years of SSO data relative to each system's slope, age, and piping material. Shown in Table 1 is the systematic comparison and contrast between ABCWUA and YVW. The purpose of this research is to analyze the performance of small-diameter gravity sewers in Collection Systems. The current criterion for Sanitary Sewer design is steep slopes (>1%) are required for gravity sewers to be adequately functional in self-cleansing. However, this design criterion applied worldwide yield deeper and steeper sewers. When designing for situations where this steep slope criterion is not met, pumping systems are a more expensive alternative but often provide unsatisfactory performance. Sanitary Sewer failure performance is measured by Sanitary Sewer Overflows (SSOs). This research supports the following, steep sewers do not provide compensatory benefits in terms of odors, asset management of existing assets is suboptimal, and tractive force should be the method used for small-diameter pipes. METHODOLOGY The SSO Rates, denoted per equation 1, measure the relative Sanitary Sewer performance. SSO Rate = (Number of SSOs)/(([(Sewer Length (Kilometers))/100])/Years) (1) The number of SSOs is the respective count of failures for a given year. The sewer length is the total linear sewer distance in km, and the years are the total number of time periods in the study. Table 2 presents the selected ABCWUA and the YVW Sanitary Sewer system piping diameters, describing the pipe material and slopes used for the analysis. Table 3 explains the definitions for the flat, middle, and steep slopes. Inspections of ABCWUA pipes validated and complemented the theoretical results with observations from the field. Traditionally, low velocities, resulting from flat slopes and low flows, are understood to result in solids deposition and odors. The presence of solid sediments at the bottom of a pipe is attributed to a situation where the line can not self-cleanse. A set of field observations were performed to monitor actual lines. This study selected four flat and steep line pairings with the same construction age and material type (e.g., 1965, vitrified clay pipe (VCP)). In Table 6, two lines were studied for each selected flat or steep location in the ABCWUA Collection System, one with a segment loading number (SLN) of 1 (i.e., dead-end) and the other SLN of 7. A previous study by Holstad (2019) showed SSO frequency increases on piping segments with larger SLN values. RESULTS See Table 5 for the summary of the two systems rolled up over the decades built. Sewers from 1960 to 1989 were selected because older sewers did not have design slopes available, and 1989 was old enough to experience some deterioration. Both systems showed lower SSO Rates in flat slopes than steep. The difference in the magnitudes of the reported SSO Rates is partially due to ABCWUA having a proactive cleaning program, whereas YVW has a reactive response to spills. The primary exposure of interest in this study was slope, the secondary exposures were material and age, and the outcome was SSO Rate. The Bayesian network analyses confirmed slope, material, and age impact SSO Rate. Figure 1 provides the identical directed acyclic graph (DAG) representation of both systems. The DAGs were identical but not the calculated strengths for the directed edges representing the individual dependencies between the variable nodes in each network. Per Table 6, for ABCWUA, material primarily drove SSO Rates (i.e., VCP lines had higher SSO Rates than polyvinyl chloride (PVC)). While for YVW, the decade was the main variable impacting SSO Rates (i.e., older lines built in the 1960s had higher SSO Rates than the 1970s and 1980s). However, for both utilities, the main exposure of interest, slope, was second in strength percentage (i.e., steep lines had higher SSO Rates than flat). Lastly, the directed edge from decade to material explained the transition from VCP to PVC over time. ABCWUA closed-circuit television (CCTV) inspections revealed flat lines had more debris depositions than steep. However, none of these steep lines had deposits significant enough to cause an SSO. Figure 2 shows the top segment in a typical flat line has approximately 1-inch debris, and the downstream segment has a channel cut in the debris layer. Figure 3 reinforces this finding with YVW fault collection data showcasing that most blockages were attributed to tree roots and fat build-up. The hydrogen sulfide (H2S) data is provided in Table 7, the results found flat lines did not experience significant odors. While we placed OdaLogs on flat lines, the customers, who talked with us, were adamant there existed no odor issues in their neighborhood.   DISCUSSION How can flat lines have lower SSO Rates? The answer may be in what an SSO is, a blockage where the sewage upstream is greater than the available storage volume. The storage volume is simply the space occupied by the pipes and manholes upstream of the blockage and below the spilling manhole. As shown in Figures 4 and 5, the flat (0.3%) and steep (3.0%) systems have approximately 1200 cubic feet (cf) and 300 cf of storage for the same pipe and manhole configurations. For a typical flow rate of 10,500 gallons per day (gpd), and recognizing that blockages are seldom complete, a steep line will overflow in approximately 7 hours while a flat line will attenuate storage and not overflow. The CCTV observations match well with previous computational and model analyses (Littlewood et al. 2003), which found that under intermittent flow, solids moved differently but adequately to avoid SSO-causing deposits. The low H2S is due to the availability of oxygen to exposed deposits (ASCE 1989). CONCLUSION The above findings challenge a fundamental understanding of Collection Systems, that flat sewers provide inferior service and should be avoided (EPA 1974, ASCE 2007). Instead, flat sewers should be preferred over alternatives such as lift stations, low-pressure grinder pump systems, etc. These alternative systems are more expensive, are not SSO-free, and are prone to odors. Asset management is based on risk assessment. Once flat sewers are recognized as less likely to spill, resources can be focused on the steeper lines more likely to spill. For instance, YVW is trialing a proactive cleaning program and developing a manhole-level monitoring program to prevent SSOs and will focus on the lines more likely to spill. See Figure 6 for their modified asset management approach.