UPGRADING EXISTING SLUDGE DIGESTERS, TECHNOLOGIES USED AND IMPROVEMENTS ACHIEVED
Gareth Howell1, Gary Stephensonr2 and Andy Gillighan1
1Earth Tech Engineering , 2Yorkshire Water Services
(free)
Current sludge disposal to farms as an organic soil conditioner has recently become a cause for concern for Water Companies, Farmers and Supermarkets in view of the potential risk of transmitting pathogenic micro-organisms into the food chain.
These concerns and the possibility of a food scare have led to discussions between the stakeholders (ADAS, politicians, farmers, food industry and environmentalists) and the ADAS Matrix (Safe Sludge Matrix) [1] has been developed. The safe sludge matrix prevents the disposal of raw untreated sludge to land and indicates a minimum sludge treatment requirement for specific crop applications.
The Department of the Environment Code of Practice [2] defines a “conventionally treated sludge” as having a 2 log reduction of E-coli and also achieve an E-coli level of 105 MPN/gDM. Yorkshire Water Services have carried out trials that show that a well-operated digester followed by sludge storage and with conditioning as part of a sludge handling scheme can achieve the 2 log reduction of Ecoli.
The treatment of sludge by anaerobic digestion has advantages in that it can substantially stabilise raw sludges as well as inactivating pathogenic micro-organisms in the digesting sludge. The use of anaerobic digestion produces biogas as a by-product that can be used for the generation of electricity.
Yorkshire Water requested ETM, a joint venture company between Earth Tech Engineering and Morrison Construction, to undertake feasibility studies for 18 digestion sites treating a total of 54,000 tds/a. The aim of the feasibility study was to check the compliance of Yorkshire Water’s existing digestion sites with the safe sludge matrix. On completion of the study, recommendations were provided on how all of the digestion sites could achieve compliance with the safe sludge matrix.
As part of the AMP3 partnering agreement, contracts were awarded to undertake the necessary modifications to 12 of the digester installations in line with the recommendations of the feasibility study. The modifications included the installation of new mixing equipment (including a novel mixing approach), thickening facilities, boilers, gas holders and flare stacks.
This paper reviews the performance of the digesters prior to the modification works, the modifications made to the digestion plants and the performance of the digesters after the completion of the modifications.
KEY WORDS Digestion, Mixing, Performance Improvements, Sludge.
