Biodegradation of Aqueous Organic Matter over Seasonal Changes: Bioreactor Experiments with Indigenous Lake Water Bacteria

Artificial groundwater recharge for drinking water production involves infiltration of surface water through sandy soil and its capture into a groundwater aquifer. The transformation of aqueous organic matter is one of the central issues in this process. The purpose of this work was to assess the potential of indigenous microorganisms in the source water to contribute in the aqueous organic matter biodegradation. For this purpose, microorganisms were enriched from the source water in a fluidized-bed reactor (FBR) and used for kinetic studies on biodegradation of organic matter at ambient temperature range. Lake water (total organic carbon $5.8\text{mg}{\text{L}}^{-1}$ ) was continuously fed to the FBR containing porous carrier material to support biomass retention. In the inlet and outlet water there were on average $21\pm 6$ and $13\pm 5\times {10}^5\text{cells}{\text{mL}}^{-1}$ , respectively. Biofilm accumulation (as volatile solids) reached $13.1\text{mg}{\text{g}}^{-1}$ dw carrier. In the continuous-flow mode and the batch tests, the highest oxygen consumption rate appeared in the summer, followed by the fall, spring, and winter. At low temperatures, the biodegradation of aqueous organic matter was relatively rapid initially for labile fractions followed by a slower phase for refractory fractions. The average temperature coefficient $\left(Q_{10}\right)$ in the system was 2.3 illustrating a strong temperature dependency of oxygen consumption. The isotopic analysis of dissolved inorganic carbon ${\delta }^{13}{\text{C}}_{\text{DIC}}$ analysis revealed 27 and 69% mineralizations of dissolved organic carbon at 23 and $6°\text{C}$ over 65 and 630 min, respectively. These results can be used to construct additional input parameters in modeling applications of artificial groundwater recharge process. The biological component especially, i.e., the biodegradation, is difficult to predict for on-site applications without experimental proof and thus the interpretation in this study will help formulate design predictions for the process.