Recharge systems for protecting and enhancing groundwate

V
Robustness of microbial treatment
during groundwater recharge
Peter Fox and Roshan Makam
Abstract
The removal of dissolved organic carbon (DOC) has been proposed as a surrogate for the removal of unregulated
contaminants of concern. Since the concentration of most trace pollutants is too low to support growth, trace
pollutants maybe co-metabolized as DOC is degraded. Studies of both SAT and bank filtration systems demonstrate
that the majority of trace pollutants are removed after sub-surface travel times of 2–6 months. From a
regulatory perspective, travel time criteria are based on the potential for virus survival. Travel times of 50 days
and 70 days are criteria used for bank filtration systems in Germany and the Netherlands, respectively while sixmonth
travel times are in the California proposed guidelines for indirect potable reuse. In a biologically active
sub-surface system, the travel time criteria for virus survival might also be suitable for the removal of trace
organics. Since most microbial reactions occur on the surfaces of the aquifer matrix, this study examines
relationships between surface area and travel time. A relationship between the relative importance of surface
area with respect to sub-surface travel time mathematically demonstrates that surface area contact does not vary
significantly for a range of common aquifer materials subject to identical hydraulic gradients.
Keywords
Robustness, organics, surface area, travel time.
INTRODUCTION
Groundwater recharge systems using impaired waters rely on transport through the soil matrix for purification of
the water. Reclaimed waters may contain numerous potentially harmful organic compounds. These compounds
undergo many potential reactions and transformations during transport through the subsurface. Dissolved organic
carbon (DOC) is often used as a surrogate to monitor the removal of organic compounds. DOC is considered to be
removed primarily by biodegradation. Trace organic contaminants may be cometabolized by bacteria during wastewater
recharge which requires time since their concentrations are too low to support growth. Cometabolism refers
to the transformation of a secondary substrate by metabolic reactions that do not directly support the growth of
microorganisms. Trace organic contaminants are usually present at concentrations well below those required to
sustain a microbial growth. Current research has focused on water quality parameters and the biodegradation of
DOC has demonstrated that the majority of persistent DOC resembles natural organic matter (Drewes and Fox,
2001, Quanrud et al., 2003). Numerous studies have also demonstrated the relationship between primary substrate
utilization and biomass growth (Rittmann, 1984; Rittman and Manem, 1992; Rittman and McCarty, 1981). Yet
information regarding the mechanistic rate-limiting step for the removal of DOC during sub-surface transport and
its impact on the removal of trace contaminants has not been clearly elucidated. A portion of this study examined
the mechanistic rate-limiting step for the removal of DOC during sub-surface transport and its potential impact on
the transformation of trace organic compounds.
ISMAR 2005 ■ AQUIFER RECHARGE ■ 5th International Symposium ■ 10 –16 June 2005, Berlin