The effluent from Greencore is treated by a hybrid AD facility which incorporates UASB granular sludge bed and fixed film media technology. The plant is not designed to treat fats, oils and greases and the presence of fats, oils and grease (FOG) has two detrimental effects on plant performance, namely;
- FOG’s coat the UASB sludge granules causing then to float and be lost from the system. It is therefore impossible to sustain a sufficientsludge bed to treat the design load
- FOG’s form hard stabilised material (up to 50mm in diameter) which has blocked the media in the uppermost part of the digester. This further reduces the treatment capacity but also increases the velocity through remaining pathways in the media, thus increasing the granular sludge loss.
The plant can now no longer treat the maximum design load, which not only risks prosecution from breaching the discharge consent but also massively increases operating costs due to the rescued biogas yields that the plant can generate. An assessment of the digester has concluded that the degree of fouling with hardened fat material is such that this will never degrade under normal operating conditions and action must be taken to remove it. Additionally, a strategy to cope with future discharges of FOG’s is required.
The cost of opening the digester and physically removing and replacing the media has been estimated to be in the region of £120k. Aqua Enviro have previously been involved in the site and identified the causes of the operating problems. In order to help identify a solution Aqua Enviro have provided an initial trial of a couple of products which can solubilise FOG’s and thus render them treatable. This has been carried out at no cost to assess whether thisis a potentialsolution. The laboratory trials have indicated thisto be a viable option to provide the following benefits:
- The chemical has been shown to solubilise the hardened material blocking the filter media. Dosing may be able to clear the filter media thus removing the need to spend £120k refurbishing the digester
- An ongoing dosing regime at times of FOG discharge could protect the digester from granule loss and media blockage as well as providing significantly enhanced biogas yields for use in the CHP plant.