Interrelationships between biological, chemical, and physical processes as an analog to clogging in aquifer storage and recovery (ASR) wells

Abstract

Laboratory columns were used to understand and predict bore clogging issues at a South Australian recycled water aquifer storage and recovery site, before field trials proceeded. The columns were used to study biogeochemical processes resulting from the continuous injection of recycled water into an aquifer matrix. The results showed that despite levels of suspended solids (SS) between 3–4 mg/l in the influent, flow rates (hydraulic conductivities) were maintained at 20–50% of initial flow through three identical columns for a period of 22 days. An initial decline in the hydraulic conductivity (K) through the columns was evident early in the experiment, and decreased from 0.78 m/day to 0.062 m/day in the first 7 days of the experiment. This was thought to be due to physical clogging by suspended solids and then biological clogging by biomass accumulation and polysaccharide production at the inflow end of the columns. The physical bioclogging was relieved midway through the experiment due to calcite dissolution mainly at the inlet end of the columns. Calcite reprecipitation at the outflow end of the columns was evident from SEM micrographs.

Introduction

In arid parts of the world, as the conventional use of water resources approaches the limits of sustainability, the need to address water reclamation and appropriate reuse is becoming critical.

Aquifer Storage and Recovery (ASR) is one of the ways excess water can be stored when available and reused subsequently when needed. Artificial recharge through well injection has been practised widely in many parts of the world such as in Israel (Harpaz, 1971), The Netherlands (Peters, 1989), the USA (Pyne, 1995) and Australia (Pavelic and Dillon, 1997).

With borehole recharge the negative impact often encountered is clogging. Injection well clogging can occur within minutes to years after the start of an ASR project. Although most of the well clogging can be alleviated by regeneration methods such as backwashing, clogging remains the main factor affecting the feasibility of new ASR projects. Clogging during recharge results in an increase in pressure in the injection well and reduced injection rate. Previous researchers (Osei-Bonsu, 1996, Shaw et al., 1985, Taylor and Jaffe, 1990; Vandevivere and Baveye, 1992b) have documented a number of causes leading to clogging. These include bacterial production of polysaccharides and biomass, entrapment of suspended solids, entrained air (gas binding), and dispersion of clay within the formation. Some water quality parameters of concern when injecting wastewater include suspended solids (SS), sodium adsorption ratio (SAR), total dissolved solids (TDS), and the concentration of nutrients that could lead to biological growth and thereby clogging in the vicinity of the injection well or further within the aquifer system.

Clogging of injection wells due to the introduction of recharge water containing suspended solids remains a key determinant in injection well performance despite advances in water treatment and injection well technology. Numerous studies indicate that clogging of the injection well tends to be primarily due to suspended solids (SS), namely physical clogging (Harpaz, 1971, Hauser and Lotspeich, 1967, Vecchioli, 1972, Olsthoorn, 1982). Previous experimental results have shown that recharge water used for ASR should have levels of SS<2 mg/l to avoid physical clogging problems (Okubo and Matsumoto, 1983). Although previous research in the same aquifer system has shown that SS levels in excess of 25 mg/l have not caused clogging in a calcareous aquifer at a stormwater ASR site in South Australia (Pavelic et al., 1998).

Biological clogging is the second most important factor causing injection well clogging (Oberdorfer and Peterson, 1985). Unless sterilised, the pretreated water when injected will always contain microorganisms, which in a high nutrient environment will exhibit a high regrowth potential.

Filtration and regrowth of microorganisms in the vicinity of the ASR well, can be of concern. Rebhun and Schwarz (1968), and Vecchioli (1970), found high counts of non-fecal coliforms in recovered water when there was a pause between injection and extraction, even though the injected water was prechlorinated at the time of injection.

Considering the lack of research in the field of bioclogging in ASR wells and the actual relevance of this type of clogging in ASR systems, it is evident that more research into the prediction and minimisation of bioclogging is warranted. The focus of the work reported is on the simulation of ASR operations in the laboratory and development of simple tests/procedures that help predict bioclogging, before a full scale ASR project is implemented.

Several methods have been used to predict the potential clogging rates for the recharge water used, these include the modified fouling index (MFI) (Hutchinson, 1997), the assimilable and biodegradable organic carbon content (AOC, bDOC) of the recharged water (Schippers et al., 1995), and mathematical empirical and analytical models (Okubo and Matsumoto, 1979, Taylor et al., 1990, Vandevivere et al., 1995). The disadvantage is that these predictive clogging tools focus on a single clogging process by assessing either biological, physical, chemical or mechanical clogging. However, all these processes are interrelated and the current work aims at developing an understanding of these interrelationships. Column infiltration studies using treated wastewater have previously been used to study biological clogging (Ehrlich et al., 1973, Okubo and Matsumoto, 1983, Rebhun and Schwarz, 1968, Vecchioli, 1970). Column studies involving the continuous injection of treated wastewater rather than artificial wastewater and using aquifer material from the target aquifer to study clogging processes have not been performed widely and no conclusive results have been obtained to date.

The main objective of this study was to inject recycled water continuously through laboratory columns packed with aquifer material collected at an ASR site, and to evaluate clogging and precipitation/dissolution processes taking place within the columns. The columns were used as a tool to investigate the combined effect of physical, biological, chemical and mechanical factors on injection well clogging in the field situation. The water injected through the columns was secondary treated wastewater which was further post treated by dissolved air flotation/filtration (DAF/F) to simulate the type of water to be injected at an ASR recharge operation that will take place in South Australia.