Severn Trent Water has applied to the Environment Agency to vary its existing environmental permit for its Stoke Bardolph Sewage Treatment Works – enabling it to construct a new thermal hydrolysis plant (THP), associated combustion/thermal plant and biogas upgrade plant.
Photo: Stoke Bardolph - one of Severn Trent's largest sewage treatment works
The thermal hydrolysis plant (THP) will be used for the pre-treatment of indigenous and imported sewage sludge prior to digestion with the site’s existing anaerobic digesters.
Two of the upgraded CHP engines and boilers will combust natural gas with a biogas option.
The upgraded biogas plant will be used to upgrade the biogas produced within the onsite anaerobic digesters and make the biogas suitable for injection into the National Gas Grid.
Severn Trent says in its application that the Thermal Hydrolysis Plant will utilise the application of heat and pressure to pre-treat sewage sludge prior to anaerobic digestion. Through pre-treating the incoming sludge, the biogas yield will be increased and dewatering of the sludge to form sludge cake will be made easier and make an enhanced sludge product. “As a technology it is increasingly being used, most commonly at larger sewage treatment works, due to the benefits it provides to the overall sludge treatment process”, the application says.
Sewage sludge, comprising either indigenous sludge from within the Stoke Bardolph works, or imported sludge from other sewage treatment works, will be transferred into loading hoppers at the works. The sludge will then be blended and transferred to the reactor vessel where it will be heated to approximately 165°C. The heating will be supplied by steam from the new natural gas fired boilers on site.
The pre-treated sludge will then be transferred to the anaerobic digesters which will be operated in the same manner as present.
The and Two new package boilers combusting natural gas will be provided to supply the THP with a heat input - the boilers are both 4.1 MWth capacity TIBS natural gas fuelled boilers, using natural gas imported from the National Gas Grid. Normal operations will see one boiler operating to provide heat with the other acting as a backup.
The biogas upgrade plant is designed to take biogas produced within the anaerobic digestion processes from the digesters located at the site. Biogas is stored within the existing biogas storage tank/holder at the site and floating roofs in the digesters.
Prior to injection into the National Grid gas grid, the biogas must undergo an upgrading process in order to meet the quality requirements of Gas Safety (Management) Regulations 1996 (GS(M)R).
It is expected that up to 1,800 Nm3/hr of raw biogas will be sent to the gas upgrade plant and the output will be delivered to the National Grid at a rate of up to1,200 Sm3/hr of biomethane.
The biogas upgrade plant will be located to the north west of the permitted anaerobic digesters.
As part of the safety system for the new upgrading plant, an emergency flare will be installed as part of the process.
The Environment Agency has launched a new consultation on Severn Trent’s application to vary its existing environment for the sewage works. Deadline to submit responses to the consultation is 8th October 2020.
Click here to access the consultation and application documents online
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