Raw Water Quality and Pretreatment in Managed Aquifer Recharge for Drinking Water Production in Finland

Abstract

Abstract:The main objective of managed aquifer recharge (MAR) in Finland is the removal of natural organic matter (NOM) from surface waters. A typical MAR procedure consists of the infiltration of surface water into a Quaternary glaciofluvial esker with subsequent withdrawal of the MAR treated water from wells a few hundred meters downstream. The infiltrated water should have a residence time of at least approximately one month before withdrawal to provide sufficient time for the subsurface processes needed to break down or remove humic substances. Most of the Finnish MAR plants do not have pretreatment and raw water is infiltrated directly into the soil. The objectives of this paper are to present MAR experiences and to discuss the need for and choice of pretreatment. Data from basin, sprinkling, and well infiltration processes are presented. Total organic carbon (TOC) concentrations of the raw waters presented here varied from 6.5 to 11 mg/L and after MAR the TOC concentrations of the abstracted waters were approximately 2 mg/L. The overall reduction of organic matter in the treatment (with or without pretreatment) was 70%-85%. Mechanical pretreatment can be used for clogging prevention. Turbidity of the Finnish lakes used as raw water does not necessitate pretreatment in basin and sprinkling infiltration, however, pretreatment in well infiltration needs to be judged separately. River waters may have high turbidity requiring pretreatment. Biodegradation of NOM in the saturated groundwater zone consumes dissolved oxygen. Thus, a high NOM concentration may create conditions for dissolution of iron and manganese from the soil. These conditions may be avoided by the addition of chemical pretreatment. Raw waters with TOC content up to at least approximately 8 mg/L were infiltrated without any considerations of chemical pretreatment, which should be evaluated based on local conditions.

Combining Chemical Flocculation and Disc Filtration with Managed Aquifer Recharge

“…The technique utilizes services provided by nature to reduce natural organic matter (NOM), bacteria, and viruses [3]. Artificial recharge plants can, depending on the study, remove 50-85% of NOM from surface waters [3][4][5][6]. However, problems arise when WTPs are pushed to the limit due to increasing drinking water demands.…”

Water quality changes and organic matter removal using natural bank infiltration at a boreal lake in Finland

“…The retention time from the lake to the first observation well 30 was 1-2 weeks, and to the most distant observation sites, well 2 and spring 1, around 1-1.5 years. The total retention time exceeded the retention time of a typical MAR system, which may vary from 5 to 100 days (Jokela et al 2017); Kuehn and Mueller 2000. The estimated surface-water inflow rate of 870-1,265 m 3 /day corresponds to a small MAR site.…”

“…In Scandinavia, TOC concentrations between <0.2 to 30 mg/L are measured in lakes (Skjelkvåle et al 2001). At the largest MAR plant in Finland, water is pre-treated to a TOC concentration of approximately 3 mg/L before infiltration (Niinikoski et al 2016), but at several sites water with a TOC concentration of 6.5-11 mg/L is infiltrated in MAR (Jokela et al 2017). In the Lake Iso Tiilijärvi water, 87% of organic carbon was in the dissolved form, which is a typical share in lake waters (Thurman 1985).…”

Water Quality Considerations on the Rise as the Use of Managed Aquifer Recharge Systems Widens

“…Along with the variation of different source waters, the type and degree to which pretreatment is required to allow infiltration [22,23] are diverging as well. Pretreatment, e.g., by coagulation, sand filtration or soil aquifer treatment (SAT), is needed to remove suspended fines from the source water, in order to prevent infiltration well clogging (e.g., [24]) and to maximize the recharge capacity during surface infiltration [21,24,25].…”

“…Pretreatment, e.g., by coagulation, sand filtration or soil aquifer treatment (SAT), is needed to remove suspended fines from the source water, in order to prevent infiltration well clogging (e.g., [24]) and to maximize the recharge capacity during surface infiltration [21,24,25]. In addition, to limit the clogging risk by microbial growth, disinfection of the source water, e.g., using UV [21] or chemical pretreatment, e.g., to remove high organic matter contents [23], can be considered. In addition to measures to minimize clogging risks, proper monitoring methods and interpretation tools are required to assess the impact on the development of MAR operations, e.g., during riverbed infiltration [26].…”