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"Improving the Efficiency of Urban Stormwater Harvesting and Reuse - an Operational Perspective"

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"Improving the efficiency of urban stormwater harvesting and reuse - an operational perspective" B.F. Naumann*, R.A. Allison**, and R.N. Williams** * Salisbury Water, City of Salisbury, SA, Australia ([email protected]) ** DesignFlow, Stirling SA, Australia ([email protected], [email protected]) Extended Abstract The Unity Park Biofiltration project is an important part of the City of Salisbury's ongoing commitment to stormwater re-use within the community. The project features a small footprint treatment technology (bio-filtration) for cleansing urban stormwater run-off. The objective is to demonstrate the applicability of the technology for stormwater reuse applications and to facilitate widespread application in other urban areas across Australia, which may not have the open space for traditional wetland treatment and reuse of urban stormwater. The overall process involves harvesting stormwater, cleansing the water in a combination of constructed wetlands and/or biofilters, and storing the water for later reuse in natural underground aquifers. Extensive supporting infrastructure has been constructed. This includes a harvesting weir on the adjacent ‘Dry Creek’; a harvest main; nine aquifer storage and recovery (ASR) wells in a Road Reserve; three distribution pump stations and an additional 20km of reticulation in Council's citywide ‘purple pipe' network to supply the recycled stormwater to customers. The initial project involved the construction of six 200 m2 bio-filtration cells and two conventional treatment wetlands, to facilitate research and development trials. The ultimate objective is ‘to improve the efficiency of future urban stormwater harvesting systems and encourage greater uptake across Australia’. Wetlands have been used successfully for some time to treat stormwater flows prior to aquifer injection and subsequent reuse. A major limitation, however, is the ‘land take’ required for wetlands. This is especially the case in highly urbanised areas where large areas of suitable flat undeveloped land are rare. The required footprint for treatment is often a limitation for harvest quantities and reduces the economic viability of a project. The performance of biofiltration for stormwater treatment is well established, but the application to large scale stormwater harvesting schemes is yet to be demonstrated. Biofiltration potentially can treat similar amounts of water as wetlands but occupy less than 10% of the footprint area. Costs are also potentially in the order of 3-5 times less than wetlands. This has significant implications for urban stormwater harvesting schemes, particularly in built up areas, where available space for wetlands is limited. The biofilters at Unity Park are designed to harvest up to 600 ML each year with treated water being pumped to a nearby ASR well field for storage. Each of the cells is being monitored to determine an optimal configuration and hydrologic regime. Construction was closely monitored with attention to detail to ensure each cell was constructed to facilitate research. The primary research question is: ‘What is the optimal configuration of biofiltration to treat the maximum volume of stormwater to a suitable standard for reuse?’ This question has directed much of the design for the experimental project. The current research program explores the treatment implications of different flow distribution methods, the benefit of a submerged zone (and the impact of providing a carbon source), two different filter media types, different pallets of plants and varying resting times between loading. The system operates by stormwater being pumped from nearby Dry Creek (at up to 100 L/s) into an inlet pond (6ML) for primary sediment removal and to enable flows to be regulated. From the inlet pond, flows are distributed under gravity to the biofiltration cells and two conventional wetlands (a further set of five biofilters cells are to be commissioned in the near future) ensuring that the inflows to each cell receive water of the same quality. Valves are used to control flows into each cell. A typical layout of a Biofiltration Cell As water soaks into the soil nutrients and The plants also use some of the water and the filtered contaminants are extracted by plants. nutrients to grow. The roots support many microscopic organisms which assist in the filtration process, and maintain the porosity of the soil. During heavy rainfall events water ponds on the surface Filtered water is collected and distributed by a for a short time providing a greater capacity for water perforated pipe at the base of the system. treatment. As a safety mechanism, all biofiltration cells have an overflow pipe to divert excess flows away from the system during large storms events. This protects the surrounding area from flooding. Figure 1. Typical cell cross-section (adapted from waterbydesign.com.au) Figure 2. One of the trial biofilters cells at Unity Park Water level variations within each cell (including the saturated zones) are monitored. Regular samples from the inlet pond provide data on the quality of inflows. Composite samples from the cells are tested for suspended solids, metals, and nutrient concentrations. A snapshot of early results are provided in Tables 1 to 4 below. These early results show that the quality of the inflows is relatively good. It should be noted that the samples were collected from only one event towards the end of a normal harvest period. Water of this quality is quite normal in a mature urban catchment with little or no land development taking place. The composite samples from the cells were consolidated from grab samples that were obtained after the biofiltration system had been running for approximately three hours. Despite the early stage of development of the cells, the results show the low levels of pollutant concentrations that can be achieved. TSS on 10-8-12 14.0 Dry Creek 12.0 Inlet 10.0 Cell 1 8.0 Cell 2 6.0 Cell 3 4.0 Cell 4 2.0 Cell 5 Cell 6 0.0 10-Aug-12 wetland Table 1. Total Suspended Solids (mg/L) Turbidity on 10-8-12 20.0 Dry Creek 18.0 16.0 Inlet 14.0 Cell 1 12.0 Cell 2 10.0 8.0 Cell 3 6.0 Cell 4 4.0 Cell 5 2.0 Cell 6 0.0 10-Aug-12 wetland Table 2. Turbidity (NTU) TP on 10-8-12 0.180 Dry Creek 0.160 0.140 Inlet 0.120 Cell 1 0.100 Cell 2 0.080 Cell 3 0.060 Cell 4 0.040 Cell 5 0.020 Cell 6 0.000 10-Aug-12 wetland Table 3. Total Phosphorous (mg/L) (Note: Soil sample Cell 6, Total P = 42.5mg/L, This was approximately double the value in other cells) TN on 10-8-12 0.700 Dry Creek 0.600 Inlet 0.500 Cell 1 0.400 Cell 2 0.300 Cell 3 0.200 Cell 4 0.100 Cell 5 Cell 6 0.000 10-Aug-12 wetland Table 4. Total Nitrogen (mg/L) Ongoing Research issues: • Loading duration and resting • Surface flow distribution (cost vs evenness) • Media infiltration rates (150 & 300 mm/hr) • Plant types, diversity, coverage • Saturated zone • Carbon source in saturated zone • Pollutant accumulation in media Water quality testing will reveal influence of: • Flow distribution on plant health • Influence of saturated zone for plant health/water quality • Impact of carbon source • Help to determine simple, effective biofiltration configuration for harvesting Lessons learnt – Construction • Importance of collaboration between designer and construction contractors and early involvement of operations personnel • Attention to detail Lesson learnt – Operations More time and effort is required to ensure plant health is maintained in extended dry periods Weed removal is a far greater issue than conventional wetlands Too early to assess ongoing operational and maintenance costs and how they compare with conventional wetlands Conclusions • Clean Catchment Clean Water From an operational perspective, maintaining a clean catchment is the most effective way of improving the efficiency of urban stormwater harvesting and reuse Bio Filters offer another treatment option that is shaping up to be both cost effective and space efficient. .
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