By Jill Bibby, Waterlink, Inc.
When a Wisconsin meat processing plant wanted to reduce the operational costs associated with their wastewater treatment system, its head operator searched for a process that would be both cost-effective and reliable. The existing plant consisted of a chemical pre-treatment system with high chemical and operating costs, along with the surcharges from local municipalities for treating the high volume of wastewater transported to publicly owned treatment works (POTW). Operation costs totalled approximately $10,000 (US) per month.
In addition to reducing operating costs, a new wastewater treatment system had to be capable of treating an average volumetric loading rate of 168,000 US gallons per day, with surge capabilities to 280,000 gallons per day. Average influent soluble BOD and soluble COD concentrations were 1,367 mg/L and 1,989 mg/L, respectively.
The head operator was introduced to the Moving Bed Biofilm Reactor Process, also known as MBBR, on the basis that this system was ideal for limited space installations or retrofits.
The MBBR is a biological aerobic degradation of organic pollutants. The process utilizes millions of tiny, polyethylene biofilm elements that provide a high surface area as a home for a vast, highly active bacteria culture. This fixed film process features a flexible reactor design, the ability to handle load increases without the need for extra tankage, and remains stable under large load variations, including temperature, strength or pH. Like the activated sludge process, the MBBR process utilizes the whole volume of an open tank. Unlike an activated sludge reactor, it does not require sludge return to operate effectively.
The original wastewater treatment system consisted of a chemical pre-treatment system, followed by an anaerobic system, which in turn was followed by an activated sludge process. To minimize the amount of wastewater being sent to the POTW and increase the amount of treated water sent to a nearby creek, a two-stage MBBR system replaced the chemical pre-treatment system.
The two MBBRs were installed between the existing equalization tank (EQ) and the activated sludge system. Total reactor volume of the MBBRs is 9,357 cubic feet, giving 10 hours hydraulic retention time at average flows and six hours at peak hydraulic flow. When there is no flow from the production facility, the system pulls out of the EQ tank to maintain the system. Effluent from the second MBBR is sent to a dissolved air flotation unit, which removes 70-90% of the solids generated, prior to being discharged to the activated sludge system. The old anaerobic tank was converted to a sludge storage and digestion tank.
Replacing the chemical pre-treatment system with the MBBR process resulted in a 70% reduction of wastewater being hauled to the POTW. The MBBR reactors reduced the incoming organic load from an average soluble BOD of 1,367 mg/L down to 59 mg/L, and an average soluble COD of 1,989 mg/L down to 226 mg/L. This reduction allowed the activated sludge process to treat the ammonia-nitrogen within the wastewater in a more efficient and effective manner, allowing a higher volume of treated water to be discharged to the creek.
The MBBR process is also designed to allow them to increase organic loads in the future without requiring additional tankage to the existing wastewater treatment system by simply adding media, up to 65% of the reactors' water volume, to the existing MBBR tanks, which are currently filled at 50% with media.
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