Enhanced biological phosphorus removal (EBPR) was observed in high-rate, non-nitrifying plants in the United States that were operated in a plug-flow mode. In facilities designed for nitrification and denitrification, a first-stage anaerobic zone, free of nitrate and nitrite was needed to accomplish EBPR, and this is referred to as the Phoredox (a.k.a. the AO and A2O) process. When a biological mechanism responsible for EBPR was proposed, these treatment configurations were accepted as normal practice, but many later observations showed that more reliable phosphorus removal could be achieved with alternative configurations. This paper discusses the development of alternative configurations for EBPR and the likelihood that a host of phosphate accumulating organisms (PAOs) that react to different environmental conditions might play a much bigger role in reliable and sustainable biological phosphorus removal. The conclusion is that conventional designs might have inadvertently selected for less efficient PAOs, while alternative configurations allowed for the growth of multiple PAO species such as Tetrasphaera, which can ferment higher carbon forms and take up phosphorus under anoxic conditions.
All discoveries associated with phosphorus removal were first made in plants as accidental occurrences. Once observed, they were followed by laboratory and pilot scale work to be studied and explained. While the mechanism of biological phosphorus removal has been well researched and documented to the point where it is now possible to design a plant with a very reliable phosphorus removal process using formal flow sheets, biological phosphorus removal is still observed in a number of plants that have no designated anaerobic zone, which is considered essential for phosphorus removal. It was also not possible to model all these plants with standard simulation packages such as BioWin and GPS-X. This paper will discuss some of the background, and some case histories and applications, and present a simple postulation as to the mechanism and efforts at modeling the results.
With more than 30 years of experience multiple options exist for removal of nitrogen and phosphorus from wastewater. Communities that were exempt from nutrient removal for many years must now comply with imposed nutrient limits, and in areas where technology-based nutrient limits have been in place communities are now faced with more stringent mass-based limits that are becoming more difficult to meet as their populations increase. Recent efforts in the industry have been focused on getting more out of existing plants, or in many cases where land is not available, in intensifying existing processes to increase capacity and/or level of treatment. This paper will discuss some of these methods and the general direction in which biological nutrient removal is developing to address these new challenges.
Biological denitrification of wastewater has been a topic of interest for several decades; however, recent nutrient reduction initiatives and the need to achieve limit-of-technology (LOT) effluent quality have renewed interest in denitrification rate research. Since readily-biodegradable chemical oxygen demand (rbCOD) drives rapid denitrification, it is essential for design efforts to include raw influent and primary effluent sampling, for different seasons if possible. However, the influent rbCOD/TKN ratio is not the only factor to consider, since rbCOD can be easily destroyed by non-optimal design and operating conditions. Two case studies are presented, each having very similar influent rbCOD/TKN ratios but very different full-scale denitrification performance. Furthermore, bench-scale tests that were conducted for the two plants yielded similar denitrification rates, which raises the question: how can engineers apply denitrification rate research to full-scale designs? Activated sludge models provide some insight to the unknown (dissolved oxygen in the mixed liquor recycle) but cannot predict rbCOD destruction due to hydraulic conditions, or fermentation due to heterogeneous mixing. This paper discusses all of the factors that affect denitrification and poses questions for future research.
The radical PO 4 3-is used by all life forms for storing energy in the form of high energy phosphate bonds and can thus not be destructed by bacteria in activated sludge. When effluent containing phosphorus is passed to a water body, algae blooms result through photo-synthesis. 1 kg of phosphorus as P has the potential to grow 138 kg COD in the form of algae that can rot and exert a high oxygen demand (Randall, 1992). Phosphorus can be removed only by either chemical precipitation or uptake in the cells of certain phosphorus accumulating organisms (PAO) occurring naturally in all activated sludge. Removal of the cells with the sludge in the final clarifiers will remove the phosphorus from the liquid stream. It is possible to reduce soluble (ortho) phosphorus to 0.03 mg/L by either chemical or biological means. The reliability of either process depends on the reliability of the mechanical plant and on the operational control. This paper will discuss operational needs for ensuring an effluent soluble phosphorus concentration of less than 0.1 mg/L. The removal of the soluble phosphorus to low levels must be accompanied by effective removal of the solids, which contain particulate phosphorus, in order to reduce the total phosphorus to the required levels of less than 0.1 mg/L.
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