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Sustainability of Substrates in Sulfate-Reducing Bacteria Bioreactors
Primary Issue Addressed: Acid Drainage/WaterSecondary Issue Addressed: Sustainability
Project Site: Currently, a site with acid mine drainage (AMD) draining to the North Clear Creek in the town of Black Hawk, Colorado, is being considered for project implementation. Originally, the McClelland Tunnel site located in Clear Creek County, Colorado, was designated as the project site. However, due to an extended drought period, the metal load carried by the AMD at this site decreased to the level that made it unsuitable for the project investigations. Several other AMD sites located in the proximity of Golden, Colorado, were then considered and subsequently eliminated mostly because of problems with obtaining site access agreements from their owners.
Collaborating Entities: The project is implemented by MSE Technology Applications, Inc. (MSE) in cooperation with Colorado School of Mines (CSM) in Golden, Colorado.
Cost Share: EPA Region 8 has provided in-kind support, particularly with site selection/access.
Project Description
The key step in sulfate-reducing bioreactor systems is the conversion of sulfate to sulfide and the subsequent precipitation of metal sulfides. The treatment of metal-containing waters in systems that rely on sulfate reduction include wetlands, bioreactors, and permeable reactive barriers. Long-term sustainability has been a key issue for systems based on sulfate reduction.
Recent advances in understanding critical components of sulfate-reducing systems has allowed the Mine Waste Technology Program to develop a better sulfate-reducing bioreactor system that considers the issues of sustainability of sulfate-reducing activity, flow distribution, maintenance, and placement at remote sites. A modular reactive cartridge (RC) design developed by MSE will be evaluated during this project. Specifically, testing will focus on the sustainability of flow through cartridges and the functionality of a newly-formulated, organic substrate consisting of walnut shells and corn stover (the plant residual remaining after harvest of the grain).
Important factors affecting sustainability of the organic substrate are
the chemical forms in which organic carbon is present in the substrate.
In general, organic carbon may be present in forms of organic acids; pectin,
starch, or polysachharide; cellulose; or lignin. It is expected that tracking
depletion of organic carbon from the organic substrate as a function of
the form of organic carbon will give important information on biodegradability
of the substrate in the presence of sulfate-reducing bacteria (SRB).
Since the project was designed to test the performance of a modular
bioreactor system and the sustainability and functionality of
a newly formulated organic substrate, a four-module treatment
system will be constructed. This system (see Figure 3) consists
of four RSc (modules) configured to operate in parallel. Each
RC is an 8-foot-diameter-by-7-foot-tall plastic tank with a removable
cover and appropriate piping to control the AMD flow and facilitate
periodic maintenance.
The RCs are filled with 5-gallon bags containing an organic substrate composed of corn stover and walnut shells. The bagging material is plastic mesh with 0.5-inch openings not to inhibit the flow. For testing, two RCs will use substrate prepared as a 1 to 1 ratio (by volume) mix of corn stover and walnut shells. Two other RCs will be filled with a 1 to 2 ratio mix of corn stover and walnut shells.
The main advantages of using an organic substrate composed of corn stover and walnut shells are listed below.
Figure 3. Process flow diagram.
- Walnut shells are more recalcitrant to biodegradation; thus, they support good long-term operation of a bioreactor.
- Walnut shells provide a solid matrix structure because individual shells actually rest on each other. This structure minimizes time-driven compaction (settling) and works toward preservation of the initial permeability of the medium.
- Walnut shells contain a high percentage (56%) of total organic carbon (TOC).
- Corn stover, a widely available crop residual that remains in the field after harvesting corncobs, contains a high percentage of organic carbon in the form of cellulose that is used by cellulose degrading bacteria to produce substrates easily accepted by SRB.
- Corn stover includes nitrogen needed for healthy microbial activity.
- The easy biodegradable corn stover provides for a quick start of bioreactors and constitutes a relatively sustainable source of organic carbon for SRB, while walnut shells will provide organic carbon in the late stage of the bioreactor’s operation.
Performance of the bioreactor system will be monitored by collecting and analyzing aqueous and solid matrix samples and by field measurements collected at the locations shown in Figure 3. The systems will be checked weekly for influent and effluent pH, alkalinity, rate of flow, and hydraulic head differential at inflow and outflow locations. Air and water temperature will be monitored weekly, and temperature profiles within the reactor systems will be measured monthly. Liquid samples will be collected monthly to evaluate the performance and effectiveness of the system as indicated by solution chemistry including concentration of dissolved and total metals. Solid phase samples of the organic substrate will be collected quarterly to monitor the decreases in solid phase of organic matter, TOC and its speciation, and changes in the microbial community structure.
Status
The implementation of the project is behind schedule due to climatic driven changes in the AMD composition at the original project site at McClelland Tunnel, and subsequent difficulties with obtaining site access agreements from the owners of alternative sites.