Project Focus: Phosphate and energy recovery at Stoke Bardolph WWTW
An innovative three-stage treatment process at a Severn Trent wastewater treatment plant incorporated ammonia and phosphate removal, with energy and resource recovery in a £6.8M project
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.”
by Teresa Jeffcoat, Project Manager, NMCNomenca
An innovative three-stage biological sewage treatment process is cutting costs and improving efficiency at Nottingham’s Stoke Bardolph wastewater treatment works. The Severn Trent Water (STW) plant serves a population equivalent of 650,000 and includes a stream of difficult-to-treat trade waste from a local rendering plant.
A new phosphate consent along with the need to replace ageing assets at Stoke Bardolph provided STW with the opportunity to reduce the physical footprint of the process, target improved resource recovery and allow for the possibility of future raised ammonia consents. The £6.8M system at the Nottingham plant, which replaces a conventional activated sludge process (ASP), also presented particular challenges in terms of process and control for the project partners.
An early feasibility study recognised the potential of carrying out efficient pretreatment of the high-strength liquors. Severn Trent Asset Creation & Service Delivery and civil engineering, electrical and mechanical subcontractor NMCNomenca worked with Netherlands’ company Paques BV to introduce three suitable and complementary technologies to the site in a first for the UK.
The combined use of Paques’ PHOSPAQ, BIOPAQ UASBplus and ANAMMOX processes has made it possible to recover phosphate fertiliser, generate biogas and deliver efficient ammonia and phosphate removal at the site. It has also halved operational costs, made possible a 40% saving on Capex and reduced the plant’s physical footprint by three-quarters.
The sludge dewatering liquors from the municipal sewage stream are first treated in a phosphorus removal reactor. The trade waste stream is first treated in an upflow anaerobic sludge blanket (UASB) reactor for biogas production and chemical oxygen demand (COD) removal. The two streams are then combined and treated in an ammonia-removing reactor.
The two PHOSPAQ reactors at Stoke Bardolph are the UK’s first installation and only the third in Europe. The process takes place in an aerated reactor, which provides oxygen for the biological conversion of the COD. Aeration also provides optimal mixing 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/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.
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 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 coexisting species of bacteria. Nitritation bacteria oxidize 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 their 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 treatment with the characteristics of the waste stream itself, which changed as the project progressed.
A pilot test was carried out a few years ago to prove the process. This showed that fundamentally the process would work at Stoke Bardolph.
“It’s by far the most complex control system we’ve worked with,” says Simon Kuitert, Paques process engineer. “We’ve never automated Phospaq like this before. It’s been a learning process.”
The new plant stacks up some major benefits. In terms of footprint, the highly efficient pre-treatment process brings it in at 25% of the size of a conventional ASP plant.
This has removed the necessity for a further ASP plant and associated treatment tanks, achieving major savings on the total expenditure (totex). A saving of 40% capex was achieved compared to the alternative solution.
Further, the installed system demands only 50% of the air of a standard ASP, so power consumption and energy costs are slashed too. The plant also uses surplus heat from CHP production and the exothermic process within the ANAMMOX to recycle its own internal heat. Some 1,000MWh per year is produced on site.
Further operational expenditure (opex) savings are made when the new processes are compared with conventional nitrification and denitrification. Operational costs are reduced by over 50%, as are CO2 emissions.
Each element of the innovative technology is suitable for use either independently or combined, where similar waste streams exist. This is applicable on many of the major treatment plants across the UK particularly in areas with industrial waste streams and those with phosphorus discharge consents.
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