Microscopic study of phosphorus removal process in an activated sludge system
USEPA is currently establishing requirements for minimizing discharges of nutrients in the nation's receiving waters. The enhanced biological phosphorus removal (EBPR) process is based on the poly-phosphate accumulating organisms' (PAOs) unique features of luxury phosphate uptake during aerobic conditions and phosphate release in anaerobic conditions. The objective of this study was to investigate the EBPR process at three different levels: full scale and pilot scale level, micro level and molecular level. Two Cincinnati MSD wastewater water treatment plants (WWTPs), the Mill Creek Plant and the Sycamore Creek Plant, as well as a WWTP in Butler County, were chosen to be the research sites. Water qualities were monitored along each treatment process, and activated sludge samples were taken from each plant for micro and molecular level studies. Pilot plants were operated at each plant to evaluate alternative operation modes to achieve the optimum phosphorus removal rate. Operation and design suggestions were made based on the full scale investigation and were confirmed through the pilot plant study.
A phosphate-selective microelectrode was developed in this research and used to obtain P-profiles across activated sludge flocs. This cobalt-based phosphate ion-selective microelectrode has a tip diameter of 5∼20 μm and shows good selectivity towards all three forms of ortho-phosphate ions, H2PO4-, HPO42-, and PO43-. A calibration curve at pH 7.5 with a slope of 31.5 mV per decade change of concentration and a R2 value of 0.99 was obtained. A P peak was found in each P profile of flocs taken from the anaerobic sections at the different treatment plants. This phenomenon could be caused by the DO profile present (lowest DO at the center) or by mass transfer resistance.
Fluorescent in situ hybridization (FISH) was used as a molecular biological tool to show the existence of PAOs in the activated sludge. Three oligonucleotide probes used in this study-PAO651, PAO846 and Rhc0439-were purchased from the DNA core-lab at the University of Cincinnati. The images showed the existence of PAOs at the Mill Creek and the Butler County plant.