Soil Aquifer Treatment

Many cities and agricultural areas rely on the combined use of surface water and groundwater. When demand increases, groundwater is often the most economic source of supply, but overexploitation can lead to the deterioration of water quality or a decreasing groundwater level. Eventually this leads to the depletion of groundwater reserves. Managed Aquifer Recharge (MAR) refers to different recharge techniques that release the reclaimed water from above the ground, percolating through unsaturated soil (see surface groundwater recharge), or from below the ground, by injection or recharge wells (see subsurface groundwater recharge) (CSIRO 2010). The utilisation of alternative water sources is a promising option for integrated water management (MELIN 2009). The added advantage of this method is that reclaimed water such as treated blackwater, greywater or stormwater is not just discharged into other surface waters, but reused as water for irrigation in agriculture or to intentionally recharge groundwater aquifers via MAR. Soil Aquifer Treatment (SAT) is one of many MAR methods, which is receiving growing attention because it features advantages such as inherent natural treatment, inbuilt storage capacity to buffer seasonal variations of supply and demand as well as mixing with natural water bodies, which promotes the acceptance of further uses, particularly indirect potable use (MELIN 2009).

SAT is one method used to recharge groundwater aquifers from the surface or from below the surface. Typically, SAT is used to enter either stormwater (SHUTES et al. 2010) or pre-treated wastewater (MELIN 2009) through a recharge basin (also: infiltration basin) or an injection well (direct injection, see also surface groundwater recharge) e.g. when space is not available for a recharge basin (see also subsurface groundwater recharge).

As the effluent moves through the soil and the aquifer, it can undergo significant quality improvements through physical, chemical and biological processes. The water is stored in the underlying unconfined aquifer generally for subsequent reuse, such as irrigation or even for drinking water purposes (generally after a water purification step). In short, SAT has benefits both in treatment in the dominated unsaturated zone, which acts like a natural filter and storage within the saturated zone (groundwater aquifer) (MIOTLINSKI et al. 2010).

Depending on the wastewater quality, land availability and intended water supply usage, SAT can be complemented by various pre-treatment technologies such as horizontal, vertical and free-surface constructed wetlands, waste stabilisation ponds, UASB reactor or advanced treatments such as activated sludge, membrane filtration, etc.

The quality of water extracted from a recharge aquifer depends on the quality of the reclaimed water introduced (after pre-treatment), the method of recharge, the characteristics of the aquifer, the residence time, the amount of blending with other waters and the history of the system (TILLEY et al. 2008). Generally it poses challenges with regard to public health (AERTGEERTS and ANGELAKIS 2003; ASANO and COTRUVO 2004). Especially groundwater recharge with recycled wastewater presents a wide range of technical and health challenges (ASANO and COTRUVO 2004). Although the unsaturated part of the soil is known to act as a filter for a variety of contaminants, groundwater recharge should not be viewed as a treatment method (TILLEY et al. 2008).

The SAT method can have the following objectives:

Safer Water Storage

Underground water storage can reduce the evaporation rate (especially in arid and semi-arid regions) (MIOTLINSKI et al. 2010), the potential breeding places for insect-vector diseases (especially stormwater drainage) (EAWAG and SANDEC 2008), and risk of contamination and pollution (e.g. algae) compared to water stored on the surface. Taste and odour are also better (ASANO and COTRUVO 2004). Aquifer recharge leads to storage of surplus water for later use. The increased storage capacity buffers seasonal and weather pattern variations of water availability and demand. It is thus a safe and reliable climate change adaptation method to cope with water shortages (PITTOCK et al. 2009).

Mixing Water Flows

SAT enables the mixing of reclaimed and treated water with natural water bodies. This promotes the acceptance of further uses, particularly indirect potable use (MELIN 2009). Because the water is abstracted from wells user acceptance is generally high (ASANO and COTRUVO 2004).

Quality Improvement

SAT is most commonly used to remove residual organic material, nitrogen and pathogenic microorganisms before storing reclaimed water in the aquifer (NCSWS 2001). Using SAT, biological oxygen demand (BOD) can be removed efficiently and ammonia can be effectively nitrified under most conditions. Furthermore, a SAT system effectively removes bacteria and viruses (JIMENEZ 2008). However, denitrification does not readily occur.

Mitigate Saltwater or Contaminants Intrusion

If the groundwater is too salty for use, recharge with fresher water e.g. stormwater will displace the saltier water, help to mitigate or control saltwater intrusion into coastal aquifers, or generally prevent intrusion of contaminants into the aquifer (BIXIO et al. 2006; PITTOCK et al. 2009).

Example: the Shafdan Treatment Plant, Israel

Since the 1970’s, the Shafdan treatment plant (in the El Dan region, central Israel) uses the SAT method. Today it is one of the biggest reclamation sites applying SAT, where the method has ever since adapted to changing conditions (CIKUREL 2006).

The plant infiltrates 130 x 106 m3/year of a biological secondary effluent with nitrogen elimination through a series of open ponds. The cycle lasts four days and the recharge rate is 70–210 m/year. This way, 90% of BOD, Total Suspended Solids (TSS), ammonia, nitrogen, and phosphorus are removed. However there is an increase in salinity, due to the proximity to the coast, and in boron content, due to the type of soil (ORON 2001; JIMENEZ 2008).

Characteristics of Soil Aquifer Treatment in the Shafdan treatment plant (El Dan, central Israel). Source: ORON (2001)

Today, the Shafdan plant applies a short SAT (to prevent production of proteins which lead to clogging of nanofiltration membranes) combined with nanofiltration. This combination of technologies was found to be superior to the conventional SAT technology in terms of land use, time parameters, and water quality. The method enabled quick and efficient removal of microorganisms and micropollutants, resulting in the production of water near drinkable quality and suitable for irrigation use. The process was also found to be superior to the combination of SAT and ultrafiltration, which, although it occupies a smaller land area, is relatively more energy intensive, does not remove all micropollutants, and only produces water quality for irrigation use (LOFTUS 2011).