Suspended Growth Bioreactors Dolloff F. Bishop and Richard C. Brenner Office of Research and Development, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, OH Introduction Suspended growth bioreactors are standard technology for treating organic contaminants in aqueous and waste sludge systems. The reactors use microbial metabolism under aerobic, anaerobic, or sequential anaerobic/aerobic conditions to biosorb organic compounds and biodegrade them to innocuous residuals. The microbial activity in the systems produces biomass that is removed by gravity sedimentation, with a portion of the settled biomass recycled to maintain a desired mixed liquor suspended solids concentration in the bioreactor. The excess biomass is wasted to a sludge disposal process. Reactor configurations include sequencing batch reactors (SBRs), completely mixed activated sludge systems, plug flow activated sludge systems, and aerobic and anaerobic digestors. The reactor systems used to efficiently treat <a href="/keyword/hazardous-wastes/" ><a href="/keyword/hazardous-waste/" >hazardous waste</a> s</a> in aqueous streams or sludges require sufficient amounts of organic carbon in the stream or sludge to support a stable microbial culture in the bioreactor (i.e., at least 5 to 10 pounds influent biochemical oxygen demand [BOD] per day per 1,000 cubic feet of bioreactor volume and at least 100 pounds influent volatile suspended solids [VSS] per day per 1,000 cubic feet of aerobic or high-rate anaerobic digester volume) (1). Conversely, influent concentrations and/or loadings of <a href="/keyword/hazardous-wastes/" ><a href="/keyword/hazardous-waste/" >hazardous waste</a> s</a> high enough to cause inhibitory effects and process performance disruption must be avoided. Typical loading ranges for suspended growth processes (1) are shown in Tables 1 and 2. The restrictions noted above limit application of suspended growth reactors in <a href="/keyword/hazardous-waste/" >hazardous waste</a> biotreatment, although addition of powdered activated carbon to a bioreactor (1) may expand the application area. Thus, ground water or leachates contaminated with low levels of BOD often will not be efficiently treated at the contaminated source by onsite suspended growth bioreactors without the addition of supplemental organic carbon. With this limitation, an alternative approach for treatment of dilute <a href="/keyword/hazardous-waste/" >hazardous waste</a> streams in suspended growth bioreactors can be considered. The dilute waste stream can be discharged to a central wastewater treatment plant (with plant management approval) for combined offsite treatment with municipal wastewater. Representative Reactor Systems A typical system for onsite treatment (2) of aqueous waste streams (Figure 1) for leachates or highly contaminated ground water includes an equalization tank, a splitter box, and a contact stabilization activated sludge process with a secondary clarifier. Ancillary processes include a waste sludge digester with supernatant return to the equalization tank and a volatile organic compound (VOC) stripper for unproved management of poorly degradable VOCs in the aqueous effluent. This Seminar Series on Bioremediation of <a href="/keyword/hazardous-waste/" >hazardous waste</a> Sites: Practical Approaches to Implementation 14-1 relatively complex biosystem may also require tertiary treatment processes such as sand filtration and/or carbon adsorption to meet effluent discharge standards. Carbon adsorption may also be applied to VOC stripper air discharges, if required. The alternative approach of discharging the <a href="/keyword/hazardous-waste/" >hazardous waste</a> stream to a central wastewater treatment plant (3), if available, offers more cost-effective biotreatment. U.S. Environmental Protection Agency (EPA) evaluated such an approach in two pilot clarification/activated sludge systems (Table 3) typical of continuous plug flow municipal wastewater treatment plants. One bioreactor was operated at a sludge retention time (SRT) of 4 days, the other at an SRT of 8 days. The municipal wastewater fed to the systems was spiked with up to 28 hazardous organic compounds. The spiked concentrations in the wastewater were less than or equal to 0.25 mg/L and less than or equal to 0.5 mg/L for the 4- and 8-day SRT systems, respectively. Finally, the sludges produced in the municipal pilot system receiving wastewater with 0.5 mg/L of spiked contaminants were treated in pilot anaerobic digesters to evaluate the impact of the hazardous contaminants in the wastewater sludges on the anaerobic digestion process (4). Three completely mixed pilot-scale digesters (Figure 2) maintained at 35.5EC with a 30-day solids retention time were used to simulate typical digester operation. Two of the digesters were fed contaminated primary and secondary sludges from the pilot study. The third digester (used as a control) was fed similar sludges without the hazardous organic contaminants. Performance and Conclusions The onsite activated sludge system achieved moderate to high removal efficiencies (Table 4) of benzene, toluene, ethyl benzene, and xylenes (BTEX) and low to high removals (Table 5) of chlorinated solvents (2). The performance of the complex onsite system suggests that tertiary treatment may be necessary if stringent effluent discharge standards are required. Alternative fixed film bioreactors, in general, would provide superior and more cost-effective bioremediation. The alternative approach, evaluated by EPA, of discharging contaminated ground water or leachates to a central wastewater treatment plant generally resulted in high removals (Tables 6 and 7) of the influent hazardous contaminants (3). Removals were superior to those provided by the onsite activated sludge system. The two treatment systems were not identical, however, and did not treat the same contaminants. The superior performance at the central plant may have been related to more effective biomass generated by the large amount of easily degradable organic substrate in the municipal wastewater. In any event, the complex onsite system will exhibit substantially increased costs per unit of contaminant removed when compared with costs at central treatment plants. The performance of anaerobic digestion on the contaminated sludges from the pilot study evaluating the central treatment plant alternative was compared with that of a control digester (4). Gas production and solids reduction for digestion of contaminated sludges and control sludges were nearly identical. Degradation of the hazardous contaminants (Table 8) was apparent. Twelve chemicals appeared consistently in the digester treating contaminated sludge, and, at steady state, Seminar Series on Bioremediation of <a href="/keyword/hazardous-waste/" >hazardous waste</a> Sites: Practical Approaches to Implementation 14-2 contaminant degradation or transformation ranged from 93 to 98 percent. Sorption into the digester solids also was an important removal mechanism, especially for aromatics. EPA generated an integrated model for predicting the fate of organics in wastewater treatment plants (5), which includes components for stripping or volatilization, sorption on solids, and biodegradation. The biodegradation component (6) includes a structural activity group contribution method for estimating contaminant biodegradation kinetics. The experimental data generated by the EPA studies described above were used to successfully validate the integrated model. References 1. Metcalf &amp; Eddy. 1991. Wastewater engineering: Treatment, disposal, and reuse, 3rd ed. In: Tchobanoglous, G., and F.L. Burton, eds. New York, NY: McGraw-Hill. Nelson, C., et al. 1993. Reactors for treatment of solid, liquid, and gaseous phases. In: Proceedings of Seminars on Bioremediation of <a href="/keyword/hazardous-waste/" >hazardous waste</a> Sites: Practical Approaches to Implementation. EPA/600/K-93/002. Washington, DC. Bhattacharya, S.K., et al. 1990. Fate and effects of selected RCRA and CERCLA compounds in activated sludge systems. In: Proceedings of the Fifteenth Annual Research Symposium--Remedial Action, Treatment, and Disposal of <a href="/keyword/hazardous-waste/" >hazardous waste</a> . EPA/600/9-90/006. U.S. EPA, Risk Reduction Engineering Laboratory, Cincinnati, OH. Govind, R., et al. 1991. Fate and effects of semivolatile organic pollutants during anaerobic digestion of sludge. Water Res. 25:547-556. Govind, R., et al. 1991. Integrated model for predicting the fate of organics in wastewater treatment plants. Environ. Prog. 10:13-23. Desai, S.M., R. Govind, and H. Tabak. 1990. Development of quantitative structure-activity relationships for predicting biodegradation kinetics. Environ. Toxicol. Chem. 9:1,092-1,097. 2. 3. 4. 5. 6. Seminar Series on Bioremediation of <a href="/keyword/hazardous-waste/" >hazardous waste</a> Sites: Practical Approaches to Implementation 14-3 Suspended Growth Reactors Dolloff F. Bishop or Gregory Sayles Office of Research and Development National Risk Management Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH Suspended Growth Bioreactor Configurations l Completely mixed activated sludge systems (continuous wastewater feed) Plug flow activated sludge systems (continuous wastewater feed) Sequencing batch reactors (batch wastewater feed) Aerobic digesters (batch or continuous sludge feed) Anaerobic digesters (batch or continuous sludge feed) l l l l Table 1. Activated Sludge Loading Ranges Reactor Configuration Plug flow (conventional) Completely mixed Step feed Contact stabilization Extended aeration SBR Detention Time (hr) 4 8 3 5 3 5 1.5 3 18 36 12 50 Volumetric Loading (lb BOD/day/1,000 ft3) 20 40 50 120 40 60 60 75 10 25 5 15 Table 2. Sludge Digester Loading Rates Sludge Digester Type Aerobic Waste activated sludge (WAS) 10 15 Primary + WAS Standard-rate anaerobic High-rate anaerobic 15 20 30 60 15 20 100 300 100 300 40 100 100 200 Retention Solids Loading Time (day) (lb SS/day/1,000 ft3) Applications of Suspended Growth Reactors l Figure 1. Onsite Activated Sludge System VOC Stripper F F Onsite applications limited to moderate or high strength leachates or ground water Inhibitory concentrations of <a href="/keyword/hazardous-wastes/" ><a href="/keyword/hazardous-waste/" >hazardous waste</a> s</a> can prevent onsite application PAC addition to activated sludge reactors can extend onsite inhibitory waste applications Alternatively, ground water and leachates can be routed to and processed at central wastewater treatment plants Inlet Splitter Box Contact Tank Reaeration Tank Activated Sludge l Equalization Tank Clarifier Holding Tank F l Supernatant Digester Waste Sludge l To VOC Stripper and Tertiary Filter for Further Treatment Seminar Series on Bioremediation of <a href="/keyword/hazardous-waste/" >hazardous waste</a> Sites: Practical Approaches to Implementation 14-4 Table 3. Conventional Operating Performance of Pilot Systems* % Removals 4-day SRT Component Continuous Intermittent** 8-day SRT Continous Intermittent** Figure 2. Pilot Digester System TSS COD NH4-N 97 82 76 97 81 81 95 88 88 94 87 98 *Feed to systems was Mill Creek municipal wastewater at the EPA Test and Evaluation Facility in Cincinnati, OH **Continous or intermittent hazardous contaminant addition Table 4. Representative Onsite Activated Sludge System Performance for BTEX Compounds Compound Benzene Toluene Ethylbenzene Xylenes (total) Influent Conc. (ppb) 120 1,000 270 700 % Removal 78 89 94 95 Table 5. Representative Onsite Activated Sludge System Performance for Chlorinated Compounds Influent Conc. (ppb) 180 31 250 100 21 Compound Chlorobenzene Methylene chloride Trichloroethane 1,2-Dichloroethane 1,2-Dichloropropane % Removal 78 100 80 56 67 Table 6. Representative Removals in Acclimated Pilot System Operating at 4-Day SRT Influent Conc. (ppb) 284 175 255 201 228 Table 7. Representative Removals in Pilot System Operating at 8-Day SRT Influent Conc. (ppb) % Removal 428 391 425 431 655 96 95 56 98 85 Compound Toluene Xylenes (total) Chlorobenzene Trichloroethane 1,2-Dichloropropane % Removal 99 99 99 97 77 Compound Di-n-bytylphthalate 1,4-dichlorobenzene Lindane Naphthalene 1,2,4-trichlorobenzene Seminar Series on Bioremediation of <a href="/keyword/hazardous-waste/" >hazardous waste</a> Sites: Practical Approaches to Implementation 14-5 Table 8. Fate of Representative Organics in Digesters Compound Di-n-bytylphthalate 1,4-dichlorobenzene Lindane Naphthalene 1,2,4-trichlorobenzene Feed Fate Mechanism (% Distribution) mg/kg Sol. Vol. Sorpt. Biodeg. 270 275 490 230 750 1 4 0 4 3 0 16 0 4 5 3 68 2 65 66 96 13 98 27 26 Model for Predicting Fate of Organics in Wastewater Treatment l l Primary sedimentation mass balances Mass balances in secondary treatment n n n n Biodegradation Sorption Volatilization (diffused aeration) Stripping (surface aeration) l Group contribution method for estimating biokinetics Seminar Series on Bioremediation of <a href="/keyword/hazardous-waste/" >hazardous waste</a> Sites: Practical Approaches to Implementation 14-6