Evaluation of membrane bioreactor process capabilities to meet stringent effluent nutrient discharge requirements.

A six-stage membrane bioreactor (MBR) pilot plant was operated to determine and demonstrate the capability of this process to produce a low-nutrient effluent, consistent with the nutrient reduction goals for the Chesapeake Bay. Biological nitrogen removal was accomplished using a multistage configuration with an initial anoxic zone (using the carbon in the influent wastewater), an aerobic zone (where nitrification occurred), a downstream anoxic zone (where methanol was added as a carbon source), and the aerated submerged membrane zone. The capability to reliably reduce effluent total nitrogen to less than 3 mg/L as nitrogen (N) was demonstrated. A combination of biological (using an initial anaerobic zone) and chemical (using alum) phosphorus removal was used to achieve effluent total phosphate concentrations reliably less than 0.1 mg/L as phosphorus (P) and as low as 0.03 mg/L as P. Alum addition also appeared to enhance the filtration characteristics of the MBR sludge and to reduce membrane fouling. Aeration of the submerged membranes results in thickened sludge with a high dissolved oxygen concentration (approaching saturation), which can be recycled to the main aeration zone rather than to an anoxic or anaerobic zone to optimize biological nutrient removal. Biological nutrient removal was characterized using the International Water Association Activated Sludge Model No. 2d. The stoichiometry of chemical phosphorus removal was also consistent with conventional theory and experience. The characteristics of the solids produced in the MBR were compared with those of a parallel full-scale conventional biological nitrogen removal process and were generally found to be similar. These results provide valuable insight to the design and operating characteristics of MBRs intended to produce effluents with very low nutrient concentrations.