Mathilde Jørgensen Hedegaard

Microbial pesticide removal in rapid sand filters for drinking water treatment--potential and kinetics.

Filter sand samples, taken from aerobic rapid sand filters used for treating groundwater at three Danish waterworks, were investigated for their pesticide removal potential and to assess the kinetics of the removal process. Microcosms were set up with filter sand, treated water, and the pesticides or metabolites mecoprop (MCPP), bentazone, glyphosate and p-nitrophenol were applied in initial concentrations of 0.03-2.4 μg/L. In all the investigated waterworks the concentration of pesticides in the water decreased - MCPP decreased to 42-85%, bentazone to 15-35%, glyphosate to 7-14% and p-nitrophenol 1-3% - from the initial concentration over a period of 6-13 days. Mineralisation of three out of four investigated pesticides was observed at Sjælsø waterworks Plant II - up to 43% of the initial glyphosate was mineralised within six days. At Sjælsø waterworks Plant II the removal kinetics of bentazone revealed that less than 30 min was needed to remove 50% of the bentazone at all the tested initial concentrations (0.1-2.4 μg/L). Increased oxygen availability led to greater and faster removal of bentazone in the microcosms. After 1 h, bentazone removal (an initial bentazone concentration of 0.1 μg/L) increased from 0.21%/g filter sand to 0.75%/g filter sand, when oxygen availability was increased from 0.28 mg O2/g filter sand to 1.09 mg O2/g filter sand. Bentazone was initially cleaved in the removal process. A metabolite, which contained the carbonyl group, was removed rapidly from the water phase and slowly mineralised after 24 h, while a metabolite which contained the benzene-ring was still present in the water phase. However, the microbial removal of this metabolite was initiated over seven days.

Mecoprop (MCPP) removal in full-scale rapid sand filters at a groundwater-based waterworks

Contamination by the herbicide mecoprop (MCPP) was detected in groundwater abstraction wells at Kerteminde Waterworks in concentrations up to 0.08μg/L. MCPP was removed to below detection limit in a simple treatment line where anaerobic groundwater was aerated and subsequently filtered by primary and secondary rapid sand filters. Water quality parameters were measured throughout the waterworks, and they behaved as designed for. MCPP was removed in secondary rapid sand filters--removal was the greatest in the sand filters in the filter line with the highest contact time (63 min). In these secondary sand filters, MCPP concentration decreased from 0.037 μg/L to below the detection limit of 0.01 μg/L. MCPP was removed continuously at different filter depths (0.80 m). Additionally, biodegradation, mineralisation and adsorption were investigated in the laboratory in order to elucidate removal mechanisms in the full-scale system. Therefore, microcosms were set up with filter sand, water and (14)C-labelled MCPP at an initial concentration of 0.2 μg/L. After 24 h, 79-86% of the initial concentration of MCPP was removed. Sorption removed 11-15%, while the remaining part was removed by microbial processes, leading to a complete mineralisation of 13-18%. Microbial removal in the filter sand was similar at different depths of the rapid sand filter, while the amount of MCPP which adsorbed to the filter sand after 48 h decreased with depth from 21% of the initial MCPP in the top layer to 7% in the bottom layer. It was concluded that MCPP was removed in secondary rapid sand filters at Kerteminde Waterworks, to which both adsorption and microbial degradation contributed.

Softening of drinking water by the pellet reactor - Effects of influent water composition on calcium carbonate pellet characteristics

Pellet softening of drinking water can provide aesthetic, socioeconomic and environmental benefits in areas with hard water. Calcium carbonate pellets are the main by-product from pellet softening and their characteristics determine their reuse potentials. We characterized pellets from a pilot-scale pellet reactor treating 16 water types at 8 Danish drinking water treatment plants to investigate the variations in pellet characteristics and how they depend on the influent water composition. The pellets consisted of up to 100% calcium as calcium carbonate, but contained often also impurities such as strontium, magnesium, iron and sodium, each contributing with up to 1.3% of the pellet mass. Other elements, including heavy metals, accounted for <0.04% of the pellet mass. The quartz sand seeding material contributed with up to 15% of the pellet mass and can be a barrier for pellet reuse. Therefore, replacing this with calcium carbonate (limestone) seeding material increases the pellet purity. Modelling the chemical speciation indicated that elements not forming carbonates (e.g. potassium and magnesium), are only incorporated into pellets to a limited extent. The concentrations of strontium, magnesium, manganese, iron and nickel in the pellets had a strong positive correlation with the influent water concentration. Consequently, the pellet purity increases if the concentration of these elements is reduced in the water before softening by other treatment technologies. Potassium, arsenic and zinc showed no or only a weak correlation. The pellets precipitated as calcite, and had a reactivity of ≤25.7% and a specific surface area of ≤0.32 m2/g, which limits the potential reuse as soil amendment in agriculture. The pellet mineralogy was independent of the investigated range of influent water quality and seeding materials. Including pellet characteristics when designing the softening process can improve pellet reuse, ultimately leading to a more environmentally sustainable drinking water supply.