Currently, the Kyrene Reclamation Plant is one of the largest operating membrane wastewater treatment facilities in North America.

Most conventional reclamation plants in Arizona use a Biological Nutrient Removal (BNR) process followed by secondary clarifiers, granular media filters and a disinfection step to meet Arizona Department of Environmental Quality (ADEQ) standards for Class A+ effluent quality.  In the case of Kyrene, the available 2.55 acre site was too small to accommodate a conventional design. This coupled with the City’s desire for an effluent quality higher than A+ standards led the team to evaluate membrane technology to replace conventional clarifiers and filters.  Membranes are new to the wastewater industry. The membranes used at the Kyrene Plant consist of hollow porous fibers, manufactured by ZENON, with billions of microscopic pores that block passage of solids larger than the pore size. The fibers are assembled into cassettes that are installed in a process tank. A slight vacuum is applied to the fibers to pull treated water (permeate) through the membrane which captures the solids on the membrane surface.  Solids are removed from the membranes using a continuous flow of coarse air bubbles in the tank combined with a periodic internal back pulse of water. In addition, the fibers are periodically chemically cleaned in place to maintain optimum flux capacity. 

One of the significant technological challenges presented by membranes is the need to screen solids larger than 2 millimeters from the influent waste stream. Solids larger than 2 mm can damage the membrane surfaces. Typical raw wastewater flows have significant concentrations of rags, solids and grease that would quickly blind a screen with openings of 2 mm. The construction team devised a solution consisting of a two stage screen facility with the first screen having openings of 3x15 mm and the second stage with 2 mm diameter openings to meet the membrane manufacturer’s requirements.  Over the years the Kyrene Plant has been plagued with a microscopic organism known as bryozoa.  Bryozoa typically grows in clear water with minimal sunlight. The existing covered final clarifiers, filters and effluent channels provided a perfect environment for bryozoa to form sheets of sticky growth on the walls. Periodically the sheets would slough off the walls and quickly plug the effluent filters. No one knows how the Plant was infected, but the operators soon found there was no cure other than methodically cleaning the filters and waiting for the next outbreak.  The decision to use membrane technology was based in part on immunity to bryozoa plugging. 

Biological nitrogen removal is accomplished in an anoxic zone located at the head of the treatment basin where an internal mixed liquor return (IMLR) stream is mixed with raw wastewater influent. In the anoxic zone biological organisms metabolize nitrate in an environment devoid of dissolved oxygen. In the case of Kyrene, the design IMLR rate is five times the influent   flow rate. Due to continuous air scouring of the membranes the IMLR stream was expected to have dissolved oxygen concentrations as high as 5 mg/l. In order to minimize adverse impacts to the anoxic zone the construction team needed to devise a way to remove the oxygen from the IMLR stream. Since there was limited data in the literature, the construction team collected data from an existing membrane plant to evaluate oxygen uptake rates in the IMLR stream. Based on this data, the IMLR wet well was oversized to provide adequate detention time for the organisms in the mixed liquor to metabolize the dissolved oxygen to a concentration near zero without the need for chemical addition. In addition, a bypass line was included that allows operations staff to bypass raw wastewater to the IMLR wet well to increase oxygen uptake in the IMLR stream.

Even though membranes have been used in a number of water and wastewater treatment facilities, questions still remain regarding the durability and length of service life for the hollow fibers. Manufacturers claim the membranes will last 20 years in continuous use in wastewater service, but that has yet to be proven through operating experience. The Kyrene Plant and other membrane facilities around the country form the data base that will be used to verify actual service life of the membranes through operating experience.  Secondly, the use of membranes allows the use of much higher concentrations of Mixed Liquor Suspended Solids (MLSS). Typical conventional BNR plants operate with MLSS in the 3,500 to 4,500 mg/l range with around 4,000 mg/l being common.  Membrane plants typically operate with MLSS concentrations of 8,000 mg/l which allows the use of smaller treatment basins with higher loading rates. The literature contains minimal data on oxygen transfer rates at the higher MLSS concentrations. In the case of Kyrene, the construction team turned to research conducted at the University of Stuttgart to de-rate the oxygen transfer rate to properly size the aeration system. This information coupled with actual operating data from the Kyrene Plant will be most useful to designers of future membrane facilities.  Finally, the previously mentioned research into oxygen uptake rates in IMLR streams will be useful to designers of future membrane facilities.