Using advanced bioreactor systems in wastewater treatment
Since the passage of the Clean Water Act (CWA) (33 United States Code Section 1251) in 1972, discharge criteria associated with the National Pollutant Discharge Elimination System (NPDES) program and other CWA provisions have become increasingly stringent. Pending regulatory initiatives suggest that this trend will continue.
So what does this mean to industry? Many industrial treatment plants were constructed in the 1970s and 1980s. Twenty to 30 years later, process plant expansions often outstrip treatment capacity. Initial compliance was achieved by installing facilities that performed primary treatment of wastewater that involved using screens and sedimentation tanks to remove most of the materials in the wastewater that float or settle.
As subsequent discharge criteria were tightened, secondary treatment became necessary to meet NPDES permit limits. In secondary treatment, bacteria are used to consume the organic parts of the wastewater. Treatment is accomplished by bringing together waste, bacteria and oxygen in trickling filters or the activated sludge process.
Now, many facilities are considering installation of tertiary treatment facilities to comply with the latest permit parameters. Tertiary treatment processes go beyond conventional secondary treatment. These advanced treatment processes remove excess nutrients such as nitrogen and phosphorus, as well as recalcitrant organic compounds. Hydraulic loading to these treatment plants has been reduced in many cases, but organic loading has typically remained the same on a mass basis. Organic concentrations have increased because of hydraulic reductions. Industry has turned to the following alternatives:
- Construction of new, conventional wastewater treatment plants;
- Expansion of the existing conventional treatment facilities; and
- Transportation of wastewater off-site for treatment or disposal.
It has been known for some time that biological treatment can be a highly cost-effective approach to the treatment of difficult aqueous wastes, particularly where at-source treatment can be applied. However, such systems must meet stringent performance requirements, often within a limited space. Today, the combination of advances in microbiology coupled with innovative bioreactor designs has resulted in bio-based systems for treatment of wastewater and groundwater that provide high quality effluent for difficult-to-treat wastestreams at low cost and in a small footprint. Examples of such systems are the membrane biological reactor (MBR) and the fluidized bed reactor (FBR), which provide viable, cost-effective options for the treatment of wastes previously considered to be very difficult or impossible to biodegrade, such as methyl tertiary-butyl ether (MTBE), complex mixtures or very high-strength wastes.
Fluidized bed reactor
The main component of the FBR system is a fixed film reactor where microbes are immobilized onto individual particles in a hydraulically fluidized bed of media. The media most often used are either granular activated carbon (GAC) or sand. Figure 1 provides a simplified process flow diagram of the FBR system. The individual particles of the fluidized media provide a vast amount of surface area for biofilm growth. Consequently, large inventories of biomass can be maintained, and reactor sizes can be minimized. Use of GAC as the fluidized media enables the integration of the removal mechanisms associated with biotreatment and physical-chemical adsorption into a single reactor configuration. The fluidized GAC enhances the ability of the reactor system to treat more recalcitrant organics, and mitigates microbial inhibition due to various toxic inputs.
Numerous FBR systems have been designed and installed for treatment of a variety of contaminants in both groundwater and process wastewater. The contaminated influent stream, as shown in Figure 1, is mixed with nutrients, oxygen and pH control chemicals. This conditioned influent is then fed into the lower portion of the reactor, which operates in a plug flow fashion. As the microbes grow on the fluidized particles, the diameter of each particle increases and the effective density is reduced, resulting in a bed expansion beyond that due to fluidization of the unseeded media. The thickness of the biofilm is controlled to prevent the density of the bioparticles from decreasing to the point where bed carryover occurs. Oxygen needed for the metabolism of the organic contaminants during aerobic operation is supplied by a packaged pressure-swing absorption system. Liquid nutrients and pH control chemicals are metered into the reactor from an ancillary feed system.
Membrane biological reactor
The MBR system consists of a suspended growth bioreactor, either aerobic or anaerobic, coupled to ultrafiltration modules mounted within the bioreactor, as shown in Figure 2. In contrast to the fixed-film FBR, maintaining a high biomass concentration in the suspended growth reactor portion of the MBR depends on the performance of the downstream solid/liquid separation step and subsequent biomass recycle. Following biotreatment, the treated water, or permeate, is drawn through the ultrafiltration membrane module using a vacuum pump. The biosolids remain in the reactor.
Because of the efficient solid/liquid separation achieved by the membrane, the biomass concentration in the bioreactor can be up to an order of magnitude higher than in conventional activated sludge systems. This translates into smaller reactor sizes and the ability to treat high-strength wastes, e.g. greater than 100,000 milligrams per liter (mg/L) alcohols. The ability of the MBR system to control solids retention times is also key to the development and maintenance of specific microbial populations. Solids retention time, or microbial growth rate, is controlled by removal of excess biomass directly from the reactor, normally disposable by sewer discharge. Precise control of retention time prevents loss of selected microbes that have the ability to degrade a specific compound or that grow at low contaminant concentrations. With no clarifier in the process, gravitational settling is no longer a problem, and the system footprint is significantly reduced.
Choosing the right bioreactor system for a particular application is governed by several factors:
- Influent flow rate;
- Mix and concentration of organic contaminants;
- Buildup and retention of biomass;
- Required effluent quality; and
- Available space.
Figure 3 provides a graphical representation of some of the criteria used for bioreactor selection. The FBR is useful over a broad range of influent flowrates and low to moderate contaminant levels. Single train units can be configured for systems approaching 1,500 gallons per minute (gpm) of lightly contaminated feed. The MBR, on the other hand, produces better cost-effectiveness for lower flow rate streams that have high organic concentrations because costs for the membrane portion of the system do not have the same economies of scale as the FBR system. Both reactor designs handle influent hydraulic and organic variations very well. Ultimately, selection of the proper system is site-specific.
When properly applied, the FBR has been shown to significantly reduce project life cycle costs (capital plus operating and maintenance costs over the life of the project) as compared to air stripping and dry carbon adsorption, wet carbon adsorption, and ultraviolet (UV) oxidation systems. Envirogen, a company in Lawrenceville, N.J., designed and installed a 250 gpm FBR system on a site in the northeastern United States to treat groundwater contaminated with aniline and nitrobenzene. The total influent contaminant concentration is approximately 80 mg/L. The system is currently operating at a contaminant removal efficiency consistently greater than 99.5 percent.
In another application, a train of four FBRs was installed to treat 4,000 gpm of groundwater contaminated with ammonium perchlorate. These FBRs were set up for anaerobic operation. The perchlorate concentration in the influent groundwater is 8 mg/L. The treated effluent perchlorate concentration has been as low as 4 micrograms per liter (µg/L).
Two MBR reactor systems are currently being designed for a municipal wastewater treatment district in southern New Jersey. One of these MBR systems will be used to pretreat landfill leachates shipped to the facility from the surrounding area. The effluent from the pretreatment system will then be polished in the existing municipal wastewater treatment plant. The design influent flow to the MBR system is 400,000 gallons per day (gpd) with a chemical oxygen demand (COD) of 10,000 mg/L. COD is the measure of the amount of oxygen required to oxidize organic and oxidizable inorganic compounds in water. The COD test is used to determine the degree of pollution in water.) The footprint of the system is approximately 7,000 square feet (2,000 ft2 for reactors and membranes and 5,000 ft2 for pumps, blowers, and other auxiliary equipment).
The second MBR system is being designed as a mobile publicly owned treatment works (POTW). It will be capable of treating 80,000 gpd with a BOD5 of 625 mg/L. This system will have phosphorous removal and disinfection capabilities built in. The footprint for this system is approximately 640 ft2. The system is trailer-mounted (two 40-foot-long-by-8-foot-wide skids) and will be highway transportable.
An MBR system has been designed for a petrochemical company located in southeast Texas to treat three high-strength industrial wastes, including alcohols and sulfur-containing compounds. The design was based on a field pilot test conducted by Envirogen. One wastestream consisted of approximately 60 percent isopropanol by weight. The other streams contained light hydrocarbons and organic sulfides. The influent COD to the MBR was 25,000 mg/L. Removal efficiencies averaged 90 to 95 percent, thereby allowing the plant to cost-effectively stay within regulatory limits. The three streams treated accounted for less than 2 percent of the plant's hydraulic wasteload, but over 70 percent of the organic wasteload.
The examples discussed above demonstrate that high performance biological treatment systems using advanced microbiology can achieve cost-effective wastewater/groundwater compliance, in efficient and compact systems. Now, more than ever before, installation of at-source treatment systems is a technically feasible, cost-effective alternative to construction of new, or expansion of, existing central treatment facilities.
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This article originally appeared in the 09/01/1999 issue of Environmental Protection.