Water and Effluent Treatment Magazine
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10 WET NEWS JUNE 2015 means that over-extraction is a direct threat to global food production. The manufacture of fertiliser from the PHOSPAQ treatment process generates a 100% renewable source of the chemical. According to BlueTech Research, approximately 18% of the EU's total phosphorus requirements could be met if all of the phosphorus in sewage sludge was recycled. The trade liquor stream received at Stoke Bardolph comes from a nearby rendering plant and posed a particular challenge for the site. Animal By-Products Regulations meant that the waste could not go straight to the digester, but had to be received at the head of the works. High in ammonia and COD, the waste requires treatment upstream of the ANAMMOX to remove COD. The waste stream, however, does not contain the metals and phosphates needed by bacteria in biological treatment systems; therefore effluent from the PHOSPAQ is circulated to provide the necessary nutrients for the bacteria in the UASBplus to remove the COD. The Paques process designer identified the BIOPAQ UASBplus reactor as suitable for treatment of the trade waste influent, prior to ANAMMOX treatment. BIOPAQ is a proven technology for efficient removal of high levels of COD; it also produces biogas, a sustainable source of energy. Mixing a 20m3/h extract of the regular liquor stream emanating from the ANAMMOX reactor with the 20m3/h trade waste flow at the head of the works helped achieve a suitable biological composition for BIOPAQ treatment. The bioreactor converts organic compounds into mainly methane (CH4) biogas anaerobically. The gas from the UASB is blended with the biogas generated by the biosolids digesters and is subsequently used for power generation in the combined heat & power (CHP) engines onsite. Overall the CHPs produce approximately 3MWh/ day, with the UASB contributing 7% to this energy neutral site's total gas output. Ammonia removal Following removal of phosphorus and COD, the flows from both reactors transfer to the third process - ANAMMOX - for high- rate ammonia removal. Severn Trent has already carried out a successful ANAMMOX installation of at Minworth sewage treatment works, its largest, in 2012. In the ANAMMOX reactor, ammonium is converted to nitrogen gas by two co-existing species of bacteria. Nitritation bacteria oxidise about half of the ammonium to nitrite, while anammox bacteria convert the ammonium and nitrite into nitrogen gas. The treated wastewater leaves the reactor by passing the biomass retention system at the top of the reactor. The granular biomass is separated from the cleaned wastewater, assuring high biomass content in the reactor. Together with the dense conversion properties typical for granular biomass, the high biomass content provides for high conversion rates and therefore a small reactor volume. The use of heat-exchange technology means the temperature in the ANAMMOX reactor is self-sustaining under normal operation parameters reducing power consumption by up to 60%, while minimising sludge output. The highly automated approach to mechanical and electrical control of treatment at Stoke Bardolph has been a step- change for the technology provider in the way its systems are operated. All previous installations have been at more heavily manned sites, while in Nottingham the plant runs itself, flagging operational issues through an automated system. NMCNomenca and Paques worked closely to deliver and optimise the operation and control system. One of the challenges was matching ONSITE sEwagE trEatmENt averts struvite-related damage to equipment further along the train. The high concentration of phosphorus in dewatering liquors ordinarily leads to excessive struvite (magnesium-ammonium- phosphate) deposits in pipes, pumps and other equipment, which can cause significant operational and maintenance problems. Conventional chemical dosing treatment for phosphorus-removal raises operational expenditure (opex) costs and can result in elevated P levels in the sludge cake. Damage Magnesium oxide is added to remove phosphate that is recovered as struvite by precipitation. The PHOSPAQ reactors are equipped with separators that retain the struvite, which is harvested from the bottom of the reactor. It is not only at the opera- tional level that the installation at Stoke Bardolph demonstrates extraordinary benefits, but also in terms of financial return and even global sustainability. Sev- ern Trent expects to make an annual saving of some £70,000 a year by reducing maintenance costs incurred by struvite dam- age to plant equipment. In addition, capturing struvite as a resource will deliver about 736t/yr phosphorus for conversion into fertiliser, providing Severn Trent with an additional revenue stream. The Stoke Bardolph scheme is expected to produce around 2t/day of high quality struvite, equivalent to 2,000t/day of mined rock. The fertiliser meets EU regulations and has a vastly reduced carbon footprint compared with mined production. The chemical resource is bought by fertiliser manufacturer Omex. Depletion of naturally occurring phosphate rock reserves is a serious global issue and it is expected that mined reserves will run out in the next 50 to 100 years. Its widespread use as a fertiliser the two BIOPaQ UasBplus reactors tackle a rendering wastewater stream at stoke Bardolph to produce 3mwh/d of mainly methane biogas teresa Jeffcoat magnesium-oxide dosing pumps feed the PHOsPaQ reactors to enable precipitation of struvite