SUBSTRATE LOADING CONSIDERATIONS
JRW Bioremediation L.L.C. provides substrates and nutrients for anaerobic bioremediation. The substrates provided include highly soluble materials such as WILCLEAR® sodium and potassium lactate, SoluLac® ethyl lactate, and Wilke Whey® whey powder and slowly soluble substrates including LactOil® soy microemulsion, and ChitoRem® chitin complex. The underlying principle of adding a carbon substrate to a system is to establish and maintain an anaerobic environment capable of promoting reductive dechlorination for a period of time sufficient to completely dechlorinate the contaminants of concern.
Since all sites are unique, site geochemistry, hydrogeology, contaminant distribution, and project goals play large roles in determining the type of substrate appropriate for the situation. In addition, each substrate has specific properties related to the fermentable fraction, solubility, carbon release profile, longevity, and ease of application. All of these factors impact the amount and type of substrate appropriate for each situation. JRW has developed a simple process to estimate substrate loading rates that are applicable to a large percentage, but not all, of chlorinated solvent remediation projects. The process is based on adding a specific amount of fermentable material as a fraction of the liquid within the pore space of the aquifer. Since most substrates contain some percentage of water, the water fraction of a substrate is not included in the loading calculation. This process of substrate loading is volume dependent and should not be confused with injection rates.
Approach to Substrate Loading: The ITRC Technical/Regulatory Guidance Document Remediation Technologies for Perchlorate Contamination in Water and Soil published in March 2008 references a substrate loading rate of 300 milligram per liter (mg/L) for highly soluble substrates and up to 500 mg/L for slowly soluble substrates for the remediation of perchlorate. Although it is generally recognized that perchlorate remediation requires less of an anaerobic environment than the reductive dechlorination of chlorinated solvents, it can be considered a minimum baseline loading for most sites.
For these reasons, JRW recommends that 1,000 mg/L of the fermentable amount of a readily soluble substrate be used as the minimum target substrate concentration for most sites. This loading is generally sufficient to promote reductive dechlorination on sites with total chlorinated solvent contamination of less than 1 mg/L. As a general rule, for each 1 mg/L of chlorinated solvent concentration, 1,000 mg/L of fermentable amount of a substrate is recommended.
Field data has indicated that high levels of substrate may promote fermentation pathways that lead to the production of ketones or other undesirable products. This phenomenon has been observed on sites where the loading of the fermentable fraction of a highly soluble substrate was greater than about 5,000 mg/L. In order to reduce the likelihood that undesirable metabolism is promoted, a maximum fermentable fraction of a readily soluble substrate loading of 4,000 mg/L of the pore space is recommended.
Due to their physical and chemical makeup, substrates dissolve at different rates. The rate at which a substrate dissolves is also important in determining if a specific substrate mass is capable of providing sufficient carbon to establish an environment that can support reductive dechlorination for the desired duration. Although dissolution rate relationships are not necessarily linear, substrates with slower release profiles need to be added at significantly greater loadings to provide a similar mass of dissolved carbon per unit time. For JRW’s slowly soluble substrates LactOil® soy microemulsion and ChitoRem® chitin complex, loading rates of between 2,000 mg/L and 8,000 mg/L of the fermentable fraction of a substrate within the pore space is recommended for most sites. Again, as a general rule, for each 1 mg/L of chlorinated solvent concentration, 2,000 mg/L of fermentable fraction of a substrate is recommended for slowly soluble substrates.
It should be noted that substrate loadings are highly site specific and the recommended loadings are only general guidelines for sites with a minimal or moderate mass of competing electron acceptors, low to moderate contaminant concentrations, and relatively low to moderate velocity aquifers.