Challenges
- The wastewater treatment plant serving Levin, a town of 20,000 inhabitants in the Horowhenua District of the North Island, was receiving unusually high flows, especially after wet periods when groundwater was suspected to be very high.
- AWT’s task was to investigate infiltration and inflow which was thought to be the cause of these high flows, locate the source, determine criticality and rehabilitation priorities.
Solutions
Our initial flow monitoring and assessment identified that one catchment representing 40% of the piped network was contributing up to 87% of the total infiltration and inflow volume. This meant we could eliminate all the other catchments from our next phase of investigations.
As we suspected, the additional flow was found to result primarily from groundwater infiltration (GWI) – which we later calculated accounted for as much as 14% of the annual total flow to the wastewater treatment plant.
Locating the GWI was the focus of the next phase – the groundwater infiltration criticality study. It was recognised that the target catchment was contributing significant volumes annually as background GWI when prolonged periods of regional rainfall caused the dry weather base flow to increase from 5.5 /s in summer to 12/s in winter.
To locate the source we undertook direct measurements of the night flow (which is mainly GWI) using portable weirs at key locations in the catchment so that we could determine the actual flow contribution in pipe lengths of between 100-600m. Our studies showed that GWI from 1.8km of trunk sewer – less than 2% of the total network – was contributing on average 382m3/day which is more than the total flow from some catchments in Levin. This volume accounted for some 55% of the GWI from the target catchment and 10% of the total annual flow to the wastewater treatment plant. The extremely high relative volume contribution in this section of pipe made it top priority for cost-effective rehabilitation. Within the remaining 38km of pipe we also identified other areas and pipe lengths needing attention, however their relative contribution made them lower priority.
Groundwater submergence analysis was carried out using a web-based tool we developed specifically for this project. We determined rehabilitation priority by analysing the groundwater level and asset data to highlight areas where groundwater submergence, pipe material, pipe age and soil type combined to increase the potential for GWI during recorded and historical extreme groundwater events. This method was verified by the night flow isolation which showed that pipes with high night flow were also submerged and generally old.
Value and benefits
- Our structured approach to this project meant that problem areas were systematically isolated to very small sections of pipe at a lower cost to our client than using traditional methods of CCTV source detection.
- Our methods also led to more certainty by using quantifiable flow data as opposed to a perceived condition – giving our client added confidence in our findings.
- Overall the key benefit for our client was cost-effective targeting of GWI and subsequent CCTV and remedial works.