Role of Biomass Recycling in the Successful Completion of Enhanced Reductive Dechlorination (ERD) Systems

Abstract

Introduction Stimulation of microbial populations capable of degrading chlorinated compounds during the implementation of enhanced reductive dechlorination (ERD) is achieved by the addition of a carbon substrate into engineered reactive zones. This can be performed using either continuous or batch injection systems. Biodegradation of chlorinated compounds is facilitated by dehalorespiring bacteria (e.g., Dehalococcoides) that use acetate or hydrogen as an electron donor during the reductive dechlorination process. In most cases, the carbon substrate added to the groundwater must undergo fermentation to produce the electron donors required by the dehalorespirers. However, the addition of a carbon substrate also stimulates the growth of a broad community of organisms beyond the dehalorespiring bacteria, which include fermentative organisms and multiple species of heteroand lithotrophic organisms that can utilize hydrogen and acetate (e.g., acetogens, methanogens). Microbial ecological data from active reductive dehalogenating communities indicate that dehalogenating bacteria make up less than 1% of the total microbial biomass (Lee et al. 2004). If at all possible, designers of in situ ERD systems would prefer electron donor delivery to be selectively available to the dehalogenators alone. However, the complexity of the microbial community that grows in response to carbon substrate addition coupled with the low percentage of dehalogenating bacteria makes the indiscriminate consumption of carbon and the fermentation products by the nontarget organisms unavoidable. This inherent scavenging of electron donors by competing bacteria results in the overwhelming generation and stockpiling of high levels of biomass and subsequent recycling within the reactive zones, which can provide considerable quantities of electron donor to be utilized later by the dehalorespirers. The buildup and subsequent recycling of biomass resulting from multiple carbon injection events has been hypothesized to be a primary factor in preventing rebound of dissolved contaminant levels following completion of the active treatment phase (Adamson and Newell 2009). This mechanism can have a significant advantage for source zone biological treatment where active injection is often most aggressive and biomass growth can extend treatment beyond the active treatment phase. This hypothesis is supported by lab-based studies where the recycling of biomass, or endogenous decay, has been shown to maintain reductive dechlorination processes long after carbon substrate addition was stopped (Sleep et al. 2005). In this paper, we discuss the concept of endogenous decay within in situ reactive zones, evaluate the generation and dissipation of biomass during the active injection phase, and present site data demonstrating how long this enhanced attenuation process is expected to last. We also discuss the practical implications of biomass recycling on the design of in situ ERD systems and how operational management can be used to leverage these mechanisms to improve remedial efficiency.