Soil Aquifer Treatment

Design of the functioning of soil aquifer treatment (SAT)

During wet cycles, infiltration basins (also “recharge basins”) (for unconfined aquifers) or injection wells (for confined and unconfined aquifers) are loaded with treated effluent or stormwater (MELIN 2009; SAKTHIVADIVEL 2007), followed by dry cycles for percolation. Involving both aerobic and anaerobic milieus, SAT facilitates to polish water for indirect potable or irrigation uses, but also offers natural storage and buffering capacity. Partial removal of organic and inorganic nitrogen, organic carbon as well as significant reduction effects in terms of phosphorus, some non-aromatic organics (including polysaccharides and proteins), trace metals or pathogens have been observed (MIOTLINSKI et al. 2010; NRMMC 2010; JIMENEZ 2008). Relying on sub-surface transport and storage, this method is specifically valuable for areas with high evaporation rates (MIOTLINSKI et al. 2010). Design and performance of SAT systems strongly depend on influent quality, geo-hydrological characteristics of soil and aquifer and operational schedule of infiltration components (hydraulic loading, drying intervals) as well as intended reuse purpose (NRMMC 2010; NCSWS 2001), whereas the following general set-up can be proposed: (1) Capture Zone (2) Pre-treatment (e.g. horizontal, vertical and free-surface CWs, waste stabilisation ponds, USAB reactors or advanced treatment such as ASP or membrane filtration systems) (3) Recharge unit (4) Subsurface storage (in the aquifer) (5) Recovery well or GW-recharge (6) Post-treatment (and Disinfection) (7) End use: drinking water supply, irrigation/industry, discharge to ecosystems. Conventional systems are designed for retention times in the aquifer of up to 12 months, “Short SAT” systems rely on retention times of 30-60 days. Land use requirements vary with the infiltration method used (LOFTUS 2011; CIKUREL 2006).

O&M requirements strongly depend on design and complexity of the infiltration system, the aquifer, the characteristics of the treated effluent as well as on the extraction method. For simple systems, e.g. where roof run-off from single houses is infiltrated on a small-scale and extracted for non-potable uses, only limited expertise might be required (infiltration is management on behalf of the householder themselves). However, in these cases, a local authority should provide design requirements for the householders and also monitor regional effects on the aquifer. For more complex, large-scale systems, operation and regulation requires more expertise to guarantee for ecologically sustainable operation. This specifically refers to the issue of risk management, which requires a deep understanding of the system (NRMMC 2010). A common maintenance issue is the development of a clogging layer on the infiltration basin’s surface (or around the injection well) due to accumulation of biofilm, algae, suspended solids and chemical precipitates, resulting in the decline of the infiltration rate.

Following PITTOCK et al. (2009), the economically most favourable use of SAT output water is for substitution of potable water for uses such as irrigation, environmental restoration, cleaning, sanitation or industrial uses. Capital costs and running costs generally depend on the number of injection wells or recovery wells, or the area of infiltration ponds/galleries required to recharge/recover water at the required rate. In general, the higher the total costs per unit volume of recovered water, the lower the yields of the extraction well. Treatment processes required to avert clogging can cause a major part of the costs. Space can especially become very cost-relevant in urban settings where land prices are high (MOITLINSKI et al. 2010). Moreover, for large-scale systems, also investigation costs for understanding the soil profile and the aquifer dynamics can be substantial (NRMMC 2010).

In Basel (Switzerland) water from the river Rhine is used to provide around 60% of the total drinking water demand (25 Mm³/y) applying SAT in the “Langen Erlen” – a former floodplain landscape in the city. Following rapid sand filtration, river water is applied on eleven forested recharge basins, whereas three sections (0.5 ha each) are consecutively flooded for ten days, followed by a drying period of 20 days. The vegetation on the recharge basins consists of typical floodplain plants (such as ash tree, alders, willows, bird cherry, reed canary grass and sedges) building up a floodplain forest ecosystem whose plant roots and soil fauna keep the soil permeability continuously high. The shade offered by the plants prevents strong warming of the upper soil layers and hereby also algae growth. The infiltration capacity (1-2 m³/m²/d) of the recharge basins has been constant for decades. With a surface area demand of around 10 m²/inh. the system is primarily suitable for areas with low land prices and/or large forests (RUETSCHI 2004). Another prominent example for large-scale SAT, is the Shafdan treatment in the Dan Region (Central Israel), where parts of the wastewater from seven cities is treated with SAT prior to re-use for irrigation in the South of the country. Infiltrating around 130-140 Mm³/y on a total area of 80 ha, this is one of the biggest reclamation sites applying SAT (CIKUREL 2006). The effluent percolates through a deep vadose zone (15-30 m) and is horizontally spread through the saturated zone. 1-2 days surface spreading is followed by 2-6 days drying period and a retention time in the aquifer of 6-12 months resulting in “accidental drinking water quality”. Facing increasing urbanisation pressure, a short SAT was introduced in combination with nano-filtration being superior to the conventional SAT technology in terms of land use, time parameters, and water quality (efficient removal of microorganisms and micro-pollutants) (LOFTUS 2011; CIKUREL 2006).

Investigations on potentials and challenges of (pilot-scale) SAT applications have been undertaken in e.g. Ahmedabad (NEMA et al. 2001), Delhi (JAMWAL & MITTAL 2010); Chennai City (DEEPA & KRISHNAVENI 2012). In Ahmedabad a pilot project was jointly conducted by Physical Research Laboratory, National Environmental Engineering Research Institute (NEERI) and Ahmedabad Municipal Corporation. SAT was applied for purifying the municipal secondary treated water and augmentation of groundwater. During an experiment period of 138 days in the post-monsoon period of 1996, an average of 1,650 m³/d of primary settled sewage entered the pilot system, and a recovery rate of about 60% could be achieved. Analysis of the system performance proved a reduction in the cost of centralised recharge collection, treatment and disposal; rejuvenation and restoration of groundwater for agricultural use. SAKTHIVADIVEL (2007), who investigated potentials of various groundwater recharge approaches in India, proposes that aquifers best suited for artificial recharge are those, which can absorb and retain large quantities of water. For surface spreading schemes in the arid zone, recent river alluvium (where water table is subject to pronounced natural fluctuations) but also coastal dunes and deltas were identified as favourable sites. Another aspect that needs to be addressed is the public perception of SAT (or wastewater reuse in general). NIJHAWAN et al. (2013) investigated the public perception of wastewater reuse through artificial groundwater recharge in India based on public consultation through questionnaires. The idea of using wastewater for artificial groundwater recharge was supported by a large number of respondents. However, there was significant concern over the quality of treated municipal wastewater and the general feasibility of using this water for groundwater recharge. The authors emphasised on the need for extensive awareness raising and strict process monitoring to sufficiently protect groundwater bodies from pollution.

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