The solids are maintained in suspension in a reactor vessel and mixed with nutrients and oxygen. If necessary, an acid or alkali may be added to control pH. Microorganisms also may be added if a suitable population is not present. When biodegradation is complete, the soil slurry is dewatered. Dewatering devices that may be used include clarifiers, pressure filters, vacuum filters, sand drying beds, or centrifuges.
Slurry-phase bioreactors may be classified as short- to medium-term technologies.
Typical Bioreactor Process
Bioremediation techniques have been successfully used to remediate soils, sludges, and sediments contaminated by explosives, petroleum hydrocarbons, petrochemicals, solvents, pesticides, wood preservatives, and other organic chemicals. Bioreactors are favored over in situ biological techniques for heterogenous soils, low permeability soils, areas where underlying ground water would be difficult to capture, or when faster treatment times are required.
Slurry-phase bioreactors are used primarily to treat nonhalogenated SVOCs and VOCs in excavated soils or dredged sediments. Ordnance compounds may also be treated.
Slurry-phase bioreactors containing cometabolites and specially adapted microorganisms are both used to treat halogenated VOCs and SVOCs, pesticides, and PCBs in excavated soils and dredged sediments.
Sequential anaerobic/aerobic slurry-phase bioreactors are used to treat PCBs, halogenated SVOCs, pesticides, and ordnance compounds found in excavated soils or dredged sediments.
Factors that may limit the applicability and effectiveness of the slurry-phase biotreatment process include:
Excavation of contaminated media is required, except for lagoon implementation.
Sizing of materials prior to putting them into the reactor can be difficult and expensive. Nonhomogeneous soils and clayey soils can create serious materials handling problems. In the case of free phase contaminant, precluded removal is mandatory.
Dewatering soil fines after treatment can be expensive.
An acceptable method for disposing of nonrecycled wastewaters is required
Although a specific organic substance might have been shown to be amenable to biodegradation in the laboratory or at other remediation sites, whether it degrades in any specific soil/site condition is dependent on many factors. To determine whether bioremediation is an appropriate and effective remedial treatment for the contaminated soil at a particular site, it is necessary to characterize the contamination, soil, and site, and to evaluate the biodegradation potential of the contaminants. A preliminary treatability study should be conducted.
Important contaminant characteristics that need to be identified in a bioremediation feasibility investigation are their solubility and soil sorption coefficient; their volatility (e.g., vapor pressure); their chemical reactivity (e.g., tendency toward nonbiological reactions such as hydrolysis, oxidation, and polymerization); and their biodegradability.
Aerobic bioslurry was used to reduce TNT, HMX, and RDX concentration in soil at Joliet Army Ammunition Plant, IL. Slurry phase bioremediation demonstrated a removal rate over 99% and a high degree of mineralization. Studies in support of a feasibility study at Iowa Army Ammunition Plant developed designs and cost estimates for full scale application of aerobic and anaerobic bioslurry processes. Demonstrations of three different bioslurry processes are underway: the Simplot Anaerobic bioremediation (SABRE) process, the Anoxic / Aerobic bioslurry process developed by AEC in conjunction with Argonne National Lab, and a 'generic' anaerobic bioslurry process.
Mobile treatment units that are quickly moved into and out of the site are available. Residence time in the bioslurry reactors will vary depending on the nature of the contaminants, their concentrations, and the desired level of removal. Residence time is typically 5 days for PCP-contaminated soil, 13 days for a pesticide-contaminated soil, and 60 days for refinery sludge.
Treatment costs using slurry reactors range from $130 to $200 per cubic meter ($100 to $150 per cubic yard). Costs ranging from $160 to $210 per cubic meter ($125 to $160 per cubic yard) are incurred when the slurry-bioreactor off-gas has to be further treated because of the presence of volatile compounds.