Evaluating two infiltration gallery designs for managed aquifer recharge using secondary treated wastewater
Highlights
► Two infiltration galleries for managed aquifer recharge of wastewater are compared. ► The performance of the Atlantis® system was superior to the gravel filled gallery. ► The travel time for wastewater through the vadose zone was measured with a tracer. ► The results constrain the time available for natural attenuation of contaminants.
Introduction
There is keen interest in Australia and other parts of the world to reduce the stress on urban water supplies by methods such as recharging aquifers with recycled water with later extraction for non-potable reuse. Managed Aquifer Recharge (MAR) using treated wastewater has gained wider acceptance as a method for augmenting urban water supplies for non-potable reuse (Asano and Cotruvo, 2004; Bouwer, 2002; Toze et al., 2004; Tredoux et al., 1999; Vandenbohede et al., 2009). Successful MAR with recycled water requires a balance between two inter-related issues: (i) implementing recharge on a practical scale (i.e. hydraulic performance) and (ii) achieving an adequate positive change in the quality of the groundwater in the aquifer. The latter is dependent on having adequate residence time in the subsurface for natural treatment processes to occur. There is a critical need for more information about wastewater infiltration designs that are sustainable, cost-effective and meet hydraulic and purification performance objectives. This paper offers field-based results on the hydraulic performance of two designs of infiltration gallery for infiltrating secondary treated wastewater to an unconfined aquifer.
Infiltration galleries are essentially covered percolation trenches that contain a medium or supporting structure with internal void spaces to facilitate infiltration. Different trench designs that have been tested for infiltrating wastewater include those containing different types of soil (Amoozegar et al., 2008), gravel, geosynthetic aggregate (Lowe and Siegrist, 2008) and polypropylene crates (e.g. Atlantis Leach System®). Wastewater treatment systems such as percolation trenches rely on the development of a biologically active zone (biomat) to enhance sorption, biological transformations and inactivation processes, which are further promoted as clogging of the biomat occurs (Gill et al., 2009; Lowe and Siegrist, 2008). Rapid movement of water through the soil profile may not allow sufficient time for natural attenuation, adsorption and mechanical straining processes to reduce pathogenic microorganisms, nutrients and other organic compounds. Ideally an optimal flow velocity should be achieved to allow infiltration on a practical time scale for reuse purposes and some degree of filtration, adsorption and degradation to allow water quality improvement to occur without severe impedance of flow from clogging.
The hydraulic loading rate is a key parameter that is compared to evaluate infiltrative performance. Whilst mathematical modelling of unsaturated zone flow is one approach to predicting effluent flux (Beal et al., 2006; Radcliffe and West, 2009), field experiments are also commonly used to test the hydraulic performance of different infiltration designs (Amoozegar et al., 2008; Lowe and Siegrist, 2008). An understanding of transport times in the subsurface is important for the overall performance and management of MAR systems.
The success of MAR systems relies on substantial reductions of chemical and pathogenic contaminants in the recycled water via natural processes, some of which are time-dependent (e.g. biodegradation). A better understanding of travel time through the unsaturated zone in relation to the hydraulic loading rate is needed. This paper describes the outcomes of research assessing the suitability of infiltration galleries for use in potential MAR schemes for non-potable end point uses in urban environments.
The need for cost-effective methods to augment groundwater resources has increased in developed and developing countries due to the growing scarcity of freshwater worldwide (Jha et al., 2009). As a technology for water supply, MAR has economic advantages over dams and desalination through lower infrastructure costs and minimal surface footprint (Dillon, 2005). In parts of the world with limited water resources and a high water demand compared to water availability, MAR using non-conventional water sources, such as treated wastewater, is being increasingly considered among the options to promote sustainable development (Furumai, 2008; Gikas and Angelakis, 2009; Gikas and Tchobanoglous, 2009). In urban areas, the use of reclaimed wastewater that has received adequate treatment offers a number of potential benefits in providing a stable and substantial supply of water and reducing demand for higher quality water (Furumai, 2008). This can be hampered by relatively high costs of land, public health concerns over surface storage of recycled water close to dwellings, as well as concerns over intermixed contamination with high quality aquifers.
MAR with recycled water can offer significant benefits to water stressed, urban areas if a suitable aquifer is present. One of the major issues is how to transfer the recycled water into the aquifer effectively. One option is well injection, but this can involve high infrastructure costs in establishing wells and pre-treating water to a sufficient standard to minimize clogging (Bouwer, 2002; Dillon et al., 2008; Pavelic et al., 2007). Infiltration ponds can be much less expensive to construct, but the high cost and availability of land in urban areas can make this option less appealing. Moreover, there is concern about the deterioration of wastewater quality intended for recycled water distribution during short-term storage in open surface ponds (Higgins et al., 2009), and there is potential for unwanted water loss through evaporation in arid and semi-arid regions. Exposed ponds of recycled water can also pose risks to public health and safety and there can be odour and pest (mosquito) issues. Infiltration galleries, which consist of covered trenches below ground in appropriate areas such as parks and footpaths have the potential to overcome a number of these issues.
The objective of this study was to determine if infiltration galleries are suitable for infiltrating treated wastewater to a shallow unconfined aquifer as part of a water recycling MAR scheme, while having minimal impact on the urban environment.
Section snippets
Location and gallery designs
This study used one gravel-filled infiltration gallery and one Atlantis Leach System® gallery located at the CSIRO Centre for Environment and Life Sciences in Floreat, Perth, Western Australia (Bekele et al., 2009). Each gallery had treated wastewater supplied to a central discharge chamber, which then fed the wastewater to two sections on opposite sides of the chamber (Fig. 1). Each trench was 25 m long, 1 m wide and 0.5 m deep, giving each infiltration trench a total volume of 12.5 m3. The
Hydraulic performance of the infiltration galleries
The two infiltration galleries recharged approximately 37 megalitres (ML) of treated wastewater to the shallow unconfined aquifer over a 39 month period. Due to unforeseen disruptions of inflow, it was possible to make only gross performance comparisons in interpreting the data (Fig. 3). Moreover, the west gallery which was initially constructed using gravel, clogged towards the end of the first year of operation. This gallery was then replaced with an Atlantis system similar to the design of
Discussion
One of the main obstacles experienced, while attempting to maintain the longevity of the gravel gallery receiving nutrient-rich, treated wastewater, was clogging with plant roots. Although geofabric covered the sides and top of the gallery, this measure was not sufficient to prevent the ingress of plant roots. High pressure jet cleaning, commonly used to remove sewer obstructions such as roots was not an effective long-term solution. Although the delivery pipe could be cleaned of roots, roots
Conclusions
In this pilot scale study, using 25 kL day−1 of wastewater, the infiltration capacity of the Atlantis Leach System in the east gallery was undiminished after 39 months of operation and infiltrated a total of 17 ML. The perforated PVC pipe used to promote lateral distribution of treated wastewater in the gravel gallery was longer than the Atlantis system gallery and became blocked more often, leading to less infiltration and poor hydraulic performance. These results may be practically used for
Acknowledgements
This research was funded by the Western Australian Government through the Water Foundation, the Water Corporation, WA and the CSIRO Water for a Healthy Country Flagship Program. The constructive comments by Dr Anthony J. Smith and Dr Leif Wolf at the Commonwealth Scientific and Industrial Research Organization are gratefully acknowledged.
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- 1
Present address: Environmental Ingenieur, National Park Berchtesgaden, Doktorberg 6, 83471, Germany.
- 2
Present address: Department of Water, Level 4, 168 St. Georges Terrace, Perth, Western Australia, Australia.
- 3
Present address: Water Corporation of Western Australia, PO Box 100, Leederville, Western Australia 6902, Australia.