Municipal wastewater treatment plants continue to adopt submerged membrane bioreactor (MBR) technology at a rapid pace. Two advantages of MBRs compared to some other biological wastewater treatment systems are their improved effluent quality, and a substantially smaller footprint. When coupled with membranes, biological and chemical phosphorous-removal methods can be employed to meet total phosphorous effluent limits of less than 0.1 mg/L. Phosphorus precipitation combined with the positive barrier provided by the membrane significantly reduces the amount of phosphorus and suspended solids in the plant effluent.

Controlling phosphorous discharge is a key factor in preserving surface water quality. Recent legislation and regulations strictly limit phosphorous discharge into sensitive waters. Typical phosphorus removal processes using biological and tertiary treatment steps are capable of providing effluent total phosphorous levels in the range of 0.5 to 1 mg/L.

Removal options

Enlarge this picture Figure 1

The biological removal process utilizes phosphorous accumulating organisms (PAOs) in an alternating anaerobic/aerobic environment. An anaerobic environment, combined with the presence of adequate volatile fatty acids (VFA), promotes the release of stored phosphorous by the microorganism. An aerobic environment subsequently promotes the luxury uptake of phosphorous by the PAOs.

The process involves stressing the bacteria in the anaerobic zone (absent nitrate and oxygen), causing the release of phosphorus. New bacteria are formed, utilizing the phosphorus that was previously released in the anaerobic zone, in addition to extra phosphorus that is taken back into the cells' mass. The additional phosphorus that is removed by the bacteria is commonly referred to as a "luxury uptake."

Chemical phosphorous removal can be employed as an alternative method or as a supplemental treatment downstream of the biological removal process. However, the biological process has the advantages of reduced chemical costs and less sludge production. In a chemical removal process, metal salts are combined with the various forms of phosphorus, forming an insoluble precipitate. Aluminium sulphate (alum), calcium (lime) and ferric chloride (ferric) are metal salts commonly used for chemical removal of phosphorus.

In some applications, gravity clarification and filtration separation methods cannot remove all biological and chemical solids, thus allowing phosphorus to pass into treated water. Membranes provide a physical barrier that captures nearly all of the suspended solids. Membrane filtration is utilized as the liquid/solids separation method in an MBR to capture solids resulting in low phosphorous levels in the treated water.

Pilot study

Figure 2: A block diagram of the pilot treatment system. Other steps could be added if needed for the study.

A pilot study, conducted from January to June 2007 by a large environmental consulting firm, examined the nitrogen and phosphorus removal capabilities of MBRs for one wastewater treatment plant. Operating a system at a mixed-liquor suspended solids concentration of 10,000 ppm, the study focused on whether or not an MBR could meet stringent, anticipated effluent quality requirements (see Figure 1).

The pilot system employed submerged membrane technology from Koch Membrane Systems Inc., Wilmington, Mass. The MBR was a module design that used reinforced hollow fibers. This was a change from earlier-generation MBRs that employed a so-called "double-header" design, or one that fixed the top and the bottom ends of the hollow fibers. The changes meant the newer system would have no top header (hair and other fibrous debris were often trapped on this header, causing the clogging of the membrane module). In the newer model the tips of the hollow-fiber membrane were designed to move freely with a seaweed-like action.

The MBR directs low-pressure air to the center of each fiber bundle, where the system can then scour the entire membrane bundle. This air-scouring feature creates coarse bubbles that shake the membrane and cleans the outside of the hollow fibers, removing accumulated debris. The method allows for a cyclical air supply, reducing energy consumption, and diminishing sludging within the membrane fiber.

Enlarge this picture Figure 3: Effluent results with the addition of acetic acid to anoxic zone.

The study also employed a biological phosphorous removal process that used anaerobic, anoxic and aerobic zones prior to the solids separation step performed by the membrane module. A chemical removal process could have been added prior to the membrane if additional phosphorous removal was required, but the pilot study omitted this step (see Figure 2).

The system demonstrated nutrient removal capability that met or exceeded the test's hypothetical regulatory limits. The system reduced total phosphorus from 8 mg/L to 0.04 mg/L, with the addition of acetic acid in the anaerobic zone. The supplemental acetate was required to boost the BOD/P ratio to the optimal performance conditions of between 10 and 25 BOD/P (see Figure 3).

Also critical in the study was the reduction of the solids retention time, also called "sludge age." Keeping an eye on the sludge age is important in managing phosphorus removal, because as it increases, the microbes age and perish, releasing the contents of their cell mass (including some phosphorus) into the solution. The pilot study demonstrated that lower effluent phosphorus levels could be achieved with an MBR system in place of a biological removal process. PE