Water & Wastewater Treatment Magazine
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3MWh/d of methane biogas www.wwtonline.co.uk | WWT | JUNE 2015 | 15 • Perspectives Dylan Thorley, Severn Trent Water Site Manager at Stoke Bardolph: "It's a win-win for STW. The 7% extra gas production from the UASBplus reactor gives us a better yield on site, which has saved us having to bring in diesel oil. "We're getting rid of some ammonias currently going through the head of works by treating them through the ANAMMOX plant, and we're going to remove struvite – and we can actually sell that back to Omex as a fertiliser once commissioning is complete. "The partnership with NMCNomenca and Paques has been great. It's always difficult when you've got people on site, but the easier you make it, the better it is for everyone – getting them involved is key." • Innovations ● PHOSPAQ reactor: provides combined phosphate and chemical oxygen demand (COD) removal. Phosphate is recovered from effluents as struvite (magnesium-ammonium-phosphate), which provides an excellent slow-release fertiliser comprising nitrogen, phosphorus and magnesium ● BIOPAQ UASBplus reactor – COD is converted into biogas anaerobically, reducing effluent discharge costs and generating renewable biogas energy for use in power generation ● ANAMMOX reactor – This anaerobic ammonium oxidation process is a cost-effective, robust and sustainable way of removing ammonium from wastewater and from waste gas. day of high quality struvite, equivalent to 2,000t/day of mined rock. The chemical resource is bought by fertiliser manufacturer Omex, and produces a fertiliser which meets EU regulations and has a vastly reduced carbon footprint compared with mined production. Depletion of naturally occurring phosphate rock reserves is a serious global issue, so this 100% renewable source of the chemical is welcome from a sustainability perspective. Tackling rendering waste 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 reactor 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 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. STW has already carried out 3MWh/d of methane biogas conditions for struvite formation. Using the PHOSPAQ reactor as the first stage of treatment for the 120m3/h sludge dewatering liquor at Stoke Bardolph effectively removes phosphorus, produces a phosphorus fertiliser and 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. 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. Severn Trent expects to make an annual saving of some £70,000 per year by reducing maintenance costs incurred by struvite damage to plant equipment. In addition, capturing struvite as a resource will deliver approximately 736t/year phosphorus for conversion into fertiliser, providing STW with an additional revenue stream. The Stoke Bardolph scheme is expected to produce approximately 2t/