The biochemical relationships in anaerobic digestion after thermal hydrolysis at Davyhulme
Jolly, M.1, Belshaw, D.2 and Telfer, J.3, 1Black & Veatch, UK, 2United Utilities, UK, 3Aqua Enviro Limited, UK
(free)
The commissioning of the largest thermal hydrolysis plant in the world at Davyhulme, Manchester involved detailed analysis of the digestion process. The plant consists of 8 digesters and 20 thermal hydrolysis reactors with a maximum throughput of 121000tds/year. The hydrolysis process converts long chain polymers (such as proteins and carbohydrates) into acids and this is followed by methanogenisis which converts the acids into methane gas. The gas is used to power the engines in the plant to produce electricity and heat. As part of the commissioning of the digestion plant many parameters were monitored and relationships between them analysed. The biochemical analysis included relationships between the following variables:
- Feed rate,
- Bacterial seed material,
- Acid, alkalinity, ammonia concentration and pH,
- Biogas production and methane concentration,
- Salmonella and E Coli.
This paper gives an assessment of the digester operational parameters and performance during commissioning.
The purpose of the Davyhulme Sludge Balanced Asset Programme (SBAP) project was to provide a central sludge processing facility for United Utilities at Davyhulme (Manchester, UK) waste water treatment works (WwTW).
The throughput of the sludge processing facility was increased from 39 000 tonnes dry solid per year (tDS/a) to 121 000 tDS/a (equivalent to a maximum throughput of sludge produced by 4.4 million people). The project also sought to improve biogas production by installing a thermal hydrolysis pretreatment plant upstream of the 8 existing mesophilic anaerobic digesters. Sludge is fed to the thermal hydrolysis plant via two routes:
1. Liquid sludge from Davyhulme WwTW is thickened to around 25% dry solids using 4 dedicated centrifuges and transferred to a storage silo. The centrate is pumped directly back to the head of the sewage works.
2. Sludge cake is imported to Davyhulme from 7 satellite sites operated by United Utilities. The cake (between 18% and 32% dry solids) is tipped into one of two cake reception conveyors and transferred to one of two storage silos.
The sludge cake is pumped from the storage silos and diluted to 16.5% dry solids using heated final effluent before being fed to one of four streams of the thermal hydrolysis plant. The sludge then undergoes thermal hydrolysis by heating with steam to 165°C for a reaction period of 30 minutes in one of five batch reactors per stream before being cooled and fed to one of two 7500m3 digesters (2 per stream). The combination of the high temperature and change in pressure during the thermal hydrolysis process ruptures cells in the sludge, thus making more of the organic matter available for digestion and rendering it pathogen free.
Digested sludge overflows the digesters and is dewatered to greater than 28% dry solids using 2 dedicated centrifuges before being conveyed to two storage silos for export by trucks. Centrate from the dewatering process passes to a Dissolved Air Flotation (DAF) unit and is then returned to the head of the sewage works.
Biogas produced by the advanced digestion process is passed to 2 gas holders and then is fed to the 5 combined heat and power (CHP) engines on site with a maximum installed power of 12MW.
The design maximum organic loading rate to the digesters was 4.16 kg volatile solids (VS)/m3 digester volume/day.
Keywords: Thermal hydrolysis, digestion, biochemistry, ammonia, VFA/alkalinity, methane
