TECHNICALLY SPEAKING
mainly Acinetobacter, had the ability to store phosphorus within the cells as long chains of inorganic polyphosphate. They explained that:
n Under anaerobic conditions, energy associated with the polyphosphate bonds could be used to take up short chain volatile fatty acids (VFA), especially acetic acid, and store it as poly-b-hydroxybutyrate (PHB), while phosphorus is released n When organisms reach the aerobic zone of the plant, they metabolize the PHB and use the energy to take up all available phosphorus in the feed, and store it as energy-rich polyphosphate chains, which in turn serve as the energy source for VFA uptake in the anaerobic zone n Removal of excess activated sludge, enriched with phosphorus, eliminates it from the liquid fraction
As the main driver for the reaction, the VFA content of wastewater, varies in time within the same plant, and from one plant to another, the performance fluctuates. Marais et al (1976) established that readily biodegradable COD (rbCOD) in the influent can easily be converted to VFA in the anaerobic zone. However, nitrates will inhibit this fermentation process. If on the other hand the rbCOD is converted to VFA beforehand, the effect of nitrates inhibiting phosphorus removal will be less pronounced. In most situations the influent wastewater does not contain sufficient VFA or rbCOD. The addition of acid-fermented primary sludge supernatant as a source of VFAs to the anaerobic zone was first proposed and applied to the Kelowna plant in British Columbia, Canada (Barnard 1977). There are various approaches to accomplishing carbon augmentation: n Primary sludge can be accumulated in the primary tank and recycled to elutriate the soluble VFA formed from acid fermentation in the sludge blanket. The elutriated VFA are then contained in the primary effluent stream. n Construct an over-sized thickener that can retain about eight times the daily sludge production. Most of the VFA formed through acid fermentation in the sludge blanket will be transferred to the supernatant which can be fed to the anaerobic zone. n Construct a stirred basin ahead of the thickener for holding and fermenting the sludge. The sludge is then passed to the thickener and elutriated with effluent. The supernatant is fed to the anaerobic zone. n A supplemental carbon source such as a yeast waste, brewery waste, or a sugar waste may be discharged to the anaerobic zone. While there may be a problem with the growth of unwanted
40 Water & Wastewater Treatment April 2011
Table 1: Biological phosphorous removal achieved in some plants Plant
Clarifier Orto-P mg/l
Bonnybrook, Calgary AB Canada
Pinticton, BC, Canada1 Kalispel MT US2
McDowell Creek, Charlotte NC, US
Pinery Water, CO, US Kelowna BC, Canada1 Westbank BC, Canada1 Durham OR, US5
0.1-0.2 0.1 0.1-0.2
0.17 0.2
0.07
1 Rabiniwitz & Barnard (1996), 2 Emerick & Abraham (2002), 3 Goins et al (2003), 4 Clarke (2002), 5 Stephens H (2004)
organisms such as glycogen accumulating organisms (GAO), it can be avoided if brewery waste or molasses are fed to a fermentation stage for conversion to VFA before the anaerobic zone, as realised at the Reading plant. n Some mixed liquor or RAS could be fermented. A suitable odour control system must be installed when using on site fermentation
In Table 1 is listed some of many plants achieving good phosphorus removal consistently, even in cold conditions and with diluted wastewaters. In all these plants, there is a controlled way of adding or producing VFA, mostly through primary sludge fermentation, except for: n At McDowell Creek, a waste soft drink product with high sugar content is added to the anaerobic zone n At the Pinery wastewater treatment plant in the US, near Denver CO, a portion of the mixed liquor from the anaerobic zone is fermented in one of the anaerobic basins
The Bonnybrook plant in Calgary, AB, Canada, achieves an annual average total phosphorus (TP) concentration of 0.5mg/L without filtration. (Wilson et al, 1994). This requires that the effluent suspended solids should average about 5mg/L. Sand or cloth filtration is normally required for removal of particulate phosphorus from the effluent in order to achieve very low effluent values. Yet some applications of Bio-P removal technology produced mediocre results with treated effluent total phosphorus values, well in excess of 1mg/l even when there seem to be sufficient rapidly degradable COD. A number of reasons for this are outlined below. This can occur in an oversized anaerobic zone when all the rbCOD and VFA are used up, or in an oversized anoxic zone when all nitrates are consumed, or in the final clarifier in the absence of nitrates. The solution is to reduce the SRT of the plant to a value just sufficient to ensure nitrification, restrict the anaerobic zone to that
which is indicated by a phosphorus release test and increase the MLR rate to reduce the actual retention time in the anoxic zone, or increase the sludge recycle rate to ensure that no sludge blanket is formed in the final clarifiers. Nitrates in the feed will usually be from an industrial source or from seepage in the groundwater. Allowing nitrates to enter the anaerobic zone in either the RAS stream or other internal recycle streams will prevent the necessary anaerobic conditions for VFA formation and uptake by the PAOs. In order to have distinct zones, partitions between zones should not allow any back- mixing and have to avoid recycling oxygen-rich mixed liquor to the anoxic zone. Partitions should be solid, have structural strength to withstand some differential pressure, should not have large openings near the floor, and have mostly discharge over the top. Flexible partitions wear out and allow back-mixing. There have not been many instances of inhibition of phosphorus removal, but in one BNR plant for an abattoir waste, biological phosphorus removal was inhibited by urea from the pen where the cattle were kept. At the Noosa plant in Australia, phosphorus removal was inhibited by discharges of hydrogen peroxide used to prevent odours in the sewers. There are apparent contradictions in practice concerning the ability of plants to remove phosphorus; some plants appear to have the correct conditions to favour phosphorus removal but fail to do so, and other plants remove phosphorus for no apparent reason. To understand the reasons for failure, a proper analysis of the process is necessary. Phosphorus profiles may show excessive release in the anaerobic zone, secondary release in anoxic zones or in the final clarifiers. When biological sludge from a BNR plant is digested, phosphorus will be released and returned to the plant, so a balance needs to be undertaken, and the opportunities for phosphorus harvesting and recovery by adding chemicals to the return streams should be considered. nnn
0.28 0.3
0.12 0.1
0.03
Filtered effluent TP mg/l
FE - Chem. Chemical treatment
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