Slurry-based Reactors
Background
Slurry-phase biological treatment is performed in a reactor to remediate a mixture of water and excavated soil. The soil is mixed with water to a concentration that is determined by the proportions of the contaminants in soils, the rate of biodegradation, and the physical nature of the soils. If the soil is prewashed, the contaminated fines and washwater are treated in the reactor. The slurry contains between 5 and 40% solids by weight depending on the nature of the biological reactor. The soil is suspended in a reactor vessel and mixed with nutrients and oxygen. Microorganisms, acid, or alkali may be added depending on treatment requirements. When biodegradation is complete, the soil slurry is dewatered [1,2,3].
Applicability
Biological treatment in slurry-phase reactors is usually a batch process and has been successfully used to remediate soils, sludges, and groundwater contaminated by hydrocarbons, petrochemicals, solvents, pesticides, wood preservatives, and other organic chemicals. Bioreactors are more suitable for heterogenous soils, soils with low permeability, soils belonging to areas where groundwater would be hard to capture, or scenarios requiring relatively short treatment times [1,2].
Limitations
There are several factors that may limit the applicability and effectiveness of the technology: (1) excavation of contaminated soils is required; (2) the need to size materials prior to putting them into the reactor can be difficult and expensive; (3) non-homogeneous soils can create serious materials handling problems; (4) dewatering soil fines after treatment can be expensive; and (5) an acceptable method to dispose of non-recycled wastewaters is required [4].
Performance
Performance data for various slurry-based reactor treatments are summarized in Table 1.Table 1. Performance data of slurry-based reactors [1,4].
Site Characteristics
Beginning Levels
Levels Attained
Acetic acid (2,4-D)
1,500 ppm
To 10 ppm in 13 days
BTEX-contaminated soil and groundwater treated simultaneously
NA
Treated to drinking water standards
Creosote- and PAH-contamination
NA
96% PAH removed
in 2 weeksExplosive contamination
1,300 ppm TNT
10 mg/kg of soil in 15 days
NA: not available
BTEX: benzene, toluene, ethylbenzene, xylene
PAH: polycyclic aromatic hydrocarbon
TNT: 2,4,6-trinitrotoluene
Data Requirements
Important contaminant characteristics that need to be identified in a bioremediation feasibility investigation include solubility and soil sorption coefficient. Volatility (e.g., vapor pressure), chemical reactivity (e.g., tendency toward non-biological reactions such as hydrolysis, oxidation, and polymerization), and biodegradability also need to be determined [4].
Cost
Soil bioremediation using slurry-phase reactors can cost between $130 and $200 per cubic meter. Costs will increase to $160 to $210 per cubic meter if the slurry-bioreactor off-gas requires treatment to contain volatile compounds. The cost for slurry bioremediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil may include slurry preparation ($50-$60 per ton), biological treatment ($40-$50 per ton), and dewatering processes ($20-$30 per ton) [1,4].
Status of Technology
Slurry-phase reactors have been successfully applied to the decontamination of solids and sludges. Slurry reactor designs may differ in the mechanisms of oxygenation and mixing of the solid suspension. Slurry treatment systems also can be constructed as a lagoon facility [1,2,3,4].
References
1. Cookson, J.T. Jr, 1995, Bioremediation Engineering Design and Application, McGraw-Hill, Inc., New York, NY.2. EPA, 1990, Slurry Biodegradation, Engineering Bulletin, EPA/540/2-90/016.
3. EPA, 1991, Pilot-Scale Demonstration of Slurry-Phase Biological Reactor for Creosote-Contaminated Wastewater, EPA RREL, Series includes Technology Demonstration Summary, EPA/540/S5-91/009; Technology Evaluation Vol. I, EPA/540/5-91/009, PB93-205532; Applications Analysis, EPA/540/A5 91/009; and Demonstration Bulletin, EPA/540/M5-91/009.
4. Office of Research and Development, EPA, ATTIC Downloadable Documents, available at http://www.epa.gov/bbsnrmrl/attic/documents.html.