National Institute for Water Resources, Water Resources Research Institute Program
03/01/06 - 02/29/08
Membrane bioreactors (MBRs) are a relatively new wastewater treatment technology which combines several typical unit operations (primary sedimentation, activated sludge aeration and sedimentation, and tertiary media filtration) into a single treatment step. This allows production of water suitable for recycling (unrestricted use) in a simple and compact system consisting of only a fine screen, an MBR, and a reduced-strength disinfection system. Like other membrane systems but to a greater degree, MBRs are susceptible to biofouling. Biofouling is not well understood but does increase operating pressure, reduce maximum flux (water passed through the membrane), increase recovery cleaning requirements, and possibly reduce total membrane life.
All of these effects of biofouling have adverse effects on either initial capital cost or ongoing operation and maintenance costs for MBRs. The overall goal of this research was to obtain a better understanding of biofouling in MBRs and methods to control said fouling in order to improve the economics of water recycling. The study includes long-term operation of two bench-scale MBRs under different conditions and evaluating membrane biofouling as a function of microbial population analysis using genetic fingerprinting. Biofouling was simultaneously related to various water quality and operational parameters such and transmembrane pressure (TMP), biofilm thickness, soluble microbial products (SMP), extracellular polymeric substances (EPS), viscosity, particle size distribution (PSD), soluble COD, and sludge filterability. Pilot scale MBRs operated for a separate project were also sampled as well as full-scale conventional activated sludge systems for comparisons.
The various findings were evaluated and relationships determined between microbial speciation, and resultant water quality parameters and membrane fouling states. This information was integrated into a chart/decision-tree format useful for predicting fouling conditions as a function of operating parameters and based on dominant bacteria species. The 20-year life-cycle MBR system costs associated with different fouling states were estimated and integrated into a separate chart/graph which were then used to determine the most economical operating conditions for water recycling.