Yorkshire Water tries new sludge disposal method

A new method for sewage sludge disposal suitable for small and medium-sized works is being investigated

Yorkshire Water has begun a trial of pyrolysis to thermally treat sewage sludge at its Esholt works near Bradford. 

The pilot pyrolysis plant built by UK firm Intervate has been leased to Yorkshire Water for the 18 month trial. The Environment Agency has issued a regulatory position statement allowing the trial to occur without an environmental permit.

The plant heats sludge to about 600°C in an oxygen-free atmosphere. This produces syngas, a mixture of carbon monoxide and hydrogen, which is then burnt in a 180 kilowatt gas engine to generate electricity. Biomass-derived charcoal (biochar) is a by-product.

The process takes a range of feedstocks with minimal pre-treatment. It will be able to handle 1,500 tonnes of dry solids per year but with minimal alteration could take 5,000tds/yr, making it suitable for a medium-sized sewage treatment works. Larger sites would need multiple units.

The feedstock is currently dried sewage sludge pellets from Yorkshire Water’s Hull treatment works. Intervate director Bill Lilly told ENDS the plant works best with a dry, homogenous feedstock. But there are plans to use digested sludge cake from Esholt, using heat from the pyrolysis process to dry the sludge.

The plant will reduce Yorkshire Water’s reliance on grid electricity, generating £150,000-200,000 per year, based on electricity savings and income from renewable obligation certificates (ROCs). Energy firms buy the ROCs to meet their renewables targets.

The biochar by-product could be used to generate additional syngas or sold as a soil improver. Yorkshire Water will be collaborating with several universities to analyse the char’s potential to improve soil and sequester carbon (ENDS Report 408, pp 15-16).

If successful, the pyrolysis process will add to the portfolio of options available to water companies for sludge disposal, especially at small and medium-sized works where costly sludge incinerators are not viable. It also reduces dependence on sludge recycling to agriculture, which faces threats from public and farmer acceptability (ENDS Report 404, p 19).

Several water companies have experimented with sludge gasification, a similar process that allows a little air into the mix and produces ash as an end product. Anglian (ENDS Report 307, p15) and Northumbrian (ENDS Report 318, 20-24) have tried the process, but it appears not to have gained widespread application.

ROC approval

Other firms using advanced thermal treatment technologies have been struggling to claim ROCs (see p 19). Energy regulator Ofgem is requiring continuous monitoring to prove the calorific value of the syngas. Intervate will be able to do this as its system continuously monitors gas composition, it says.

The main problem encountered at Esholt relates to tar in the syngas output, which must be removed before feeding the gas engine – a problem common to all pyrolysis processes.

Intervate have designed a gas recirculation and condenser system which aims to break down the tar without wasting the energy it contains.

Mr Lilly said the two companies were keen not to over-promise with the trial. But he added: “The project at Esholt is very exciting and could be ground-breaking for the UK water industry… The early results are very encouraging.”

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