Wastewater Treatment Goes Au Naturel
Move over steam stripper! It's time for a little biology lesson.
The "reign of the steam stripper" as the sole alternative for treating high-strength, ever-changing process wastewater finally ended. Now there is an alternative that offers higher performance at lower cost and less operator grief. Recent successful applications of nembrane biological reactors (MBRs) within the specialty chemicals and pharmaceutical industries are making many environmental managers sit up and take notice. There is finally a viable and proven alternative to steam stripping for these high-strength wastes.
What is a Membrane Biological Reactor (MBR)?
An MBR consists of two principal components: (1) a biological reactor tank, and (2) an ultrafiltration or microfiltration membrane to retain biological solids within this tank. The membrane may either be internal or external to the reactor vessel. The membrane provides a barrier to keep microbial solids in the reactor and renders a clarified, solids-free effluent stream for discharge. The microbial solids are essentially the same as those found within activated sludge systems at water treatment plants, but within the MBR, they are weaned to grow on very difficult-to-degrade pollutants. These biological solids use the pollutants in the wastewater as food, creating an even greater quantity of microorganisms within the reactor. The age of the microorganisms in the reactor (i.e., the solids retention time, or SRT) is controlled automatically by the periodic wasting of a small amount of reactor contents to the sewer. To a large extent, the SRT dictates the effluent quality, or treatment level, of an MBR system; the longer the SRT, the greater the treatment level and the more robust the system. The MBR design employs membranes that are suspended within one or more troughs within the reactor itself, so that separation and "recycling" of biosolids occurs in the same reaction vessel. The suspension of the membranes in the trough allows them to be vigorously cleaned in place when needed without physically removing them from the system (U.S. Patent # 6,331,251).
MBR Case Studies
The applications for MBR technology have rapidly expanded to include very difficult industrial wastewaters. One reason for this successful expansion is the ability of the MBR to develop and sustain exceptionally high biomass concentrations and long SRTs, even for specific bacterial populations. This makes the technology an excellent choice for treatment of recalcitrant compounds, high influent concentrations, widely varying influent composition or for applications that demand stringent effluent quality.
Three case studies are presented below to exemplify the new breadth of applicability for this technology. For each case, the management of the plant's wastewater includes simple tests to determine compatibility with the existing microorganisms in the reactor. When testing indicates parameters to be outside established norms, the feed flow is decreased temporarily to allow for acclimation to the new feed source prior to establishing the design flow. Each MBR system has been shown to tolerate a wide range of organic compounds and salt concentrations without detrimental impact on performance.
In September of 1998, the U. S. Environmental Protection Agency (EPA) promulgated the final rules regarding wastewater effluent guidelines and standards for the pharmaceutical industry. These standards are specified in 40 Code of Federal Regulations Part 439 (40 CFR 439) and limit the discharge of 23 organic/organo-nitrogen compounds, ammonia and cyanide.
To meet the new effluent limitation guidelines, a West Coast batch chemical manufacturing plant has installed an MBR treatment system. The chemicals produced by this plant are sold to pharmaceutical companies, which in turn make specialized therapeutic drugs from them. Typical chemicals in the plant?s process wastewater include ethyl acetate, triethylamine, hexane, methylene chloride, various alcohols and acetone. Like most batch pharmaceutical plants, this facility produces a variety of wastewater effluents in terms of volume and composition. The MBR receives wastewater from production operations and other sources that are stored in existing holding tanks and are transported from remote operations via tanker truck. The treated water and reactor solids are sent directly to the Publicly Owned Treatment Works (POTW). The treatment system is designed to process 7,200 gallons per day (gpd) of wastewater at a feed total organic carbon (TOC) concentration of 14,000 milligrams per liter (mg/l). The MBR was started up in October 2001. By the beginning of March 2002, the treated water began consistently meeting the pharmaceutical pretreatment standards (40 CFR 439). The reactor has consistently produced a steady effluent quality independent of changing feed concentration and composition.
Specialty Chemicals Plant
A manufacturing plant located in the Midwest produces specialized chemicals. The facility is a batch plant with highly variable wastewater in terms of both volume and composition. Discharge limits are governed by EPA guidelines for Organic Chemicals, Plastics and Synthetic Fibers (OCPSF) manufacturers, as well as the pharmaceutical pretreatment standards (40 CFR 439) and other local guidelines for suspended solids, biochemical oxygen demand (BOD) and organic nitrogen. Typical chemicals in the facility's wastewater include methanol, isopropanol, isopropyl acetate, acetone, toluene, methylene chloride, chlorobenzene and tetrahydrofuran.
To meet the OCPSF and pharmaceutical pretreatment standards, the plant made a decision to add a biological destruction step to their existing wastewater treatment facility. The initial phase of the project involved the testing of two field-pilot biological treatment systems of different design during the summer of 1998, which resulted in an MBR being selected. The full-scale MBR system was started up in summer 2001. The MBR system is designed to treat 29,000 gpd of wastewater at a feed TOC concentration of 1,000 mg/l. On September 20, 2001, the first sample analyses of reactor influent and effluent showed the plant to be in compliance with EPA regulatory and local POTW limits. All the target compounds that were present in the feed stream were below their corresponding detection limits in the effluent.
A manufacturer of water treatment membranes operates a production plant that conducts polysulfone casting operations. The plant discharges wastewater into the local sewer system containing a mixture of organic reactant residues and solvents that include dimethylformamide (DMF), n-methyl pyrrolidone (NMP) and other chemicals. The daily BOD5 load, specific contaminant concentrations and flow rates all vary depending on production. Influent DMF and NMP concentrations are approximately 30,000 and 7,000 mg/l, respectively. The focus of this project was to upgrade the plant to include pretreatment to meet POTW requirements under these varying conditions. Effluent requirements included reducing BOD5 to less than 1,500 pounds per day, including the solids from the reactor.
The plant conducted an extensive review of technical options, including high temperature/pressure oxidation, chemical and ultraviolet light oxidation and biotreatment options, before concluding that an MBR held the best opportunity for cost-effectively meeting performance requirements. Optimization and pilot plant studies were then conducted to confirm that this complex and varying mixture of contaminants could be biologically treated to meet effluent criteria.
The MBR system is designed to process an average flow of 22,000 gpd at very high organic loads. Start-up was completed in fall 2001, with all effluent requirements achieved. Like the other two examples, the MBR produces a steady effluent quality, even though the feed varies significantly in concentration and composition. When TOC concentrations in the effluent unexpectedly increased in November 2001 and April 2002 due to plant process changes, system performance quickly recovered.
For each of the case studies summarized above, the use of an MBR has eliminated off-site wastewater disposal costs, and the plants that installed the systems are on track for rapid payback on their capital investment. In each case, the MBR proved to be even more robust and flexible than originally anticipated, allowing these plants to continue to meet discharge requirements even during product production changes (i.e., switching from the production of one chemical or product to another), when the chemicals treated by the MBR drastically change.
This article originally appeared in the March 2003 issue of Environmental Protection, Vol. 14, No. 2, p. 43.
This article originally appeared in the 03/01/2003 issue of Environmental Protection.