In-series digestion – experimental studies

Edgington, R., Gray, B. and Hyde, S., United Utilities, UK



There are numerous pre-treatment options and digestion enhancements available to improve on conventional mesophilic anaerobic digestion; some which give an enhanced product, others which achieve greater volatile solids destruction (with the consequences of more biogas and hence more energy generation) and others that deliver both. To understand in-series digestion performance, as an enhancement process to conventional digestion,  a literature review has been undertaken.

In-series digestion is being considered as an option to upgrade the existing mesophilic anaerobic digesters at Burnley Sludge Treatment Centre. To generate understanding of in-series digestion, experimental work has commenced and is still on-going. Three experiments, in small reactors, are being investigated. Firstly, temperature phased anaerobic digestion (55OC reactor followed by 35OC reactor). Secondly, an acid phase reactor in front of a temperature phased process. Finally, a conventional mesophilic anaerobic digester reactor is being run to act as the control to benchmark the other two.

The success criteria for the experimental work were based upon the ability to produce an enhanced treated sludge and the volatile solids destruction.


In-series digestion, thermophilic, mesophilic, acid phase digestion


United Utilities produces about 200000tsdapa of raw sludge. By 2015 over 90% of this sludge will be digested either by conventional (mesophilic anaerobic digestion (MAD)) or advanced digestion technologies. One of the conventional digestion sites, at Burnley Sludge Treatment Centre (STC), has been included in the current Asset Management Plan to be upgraded to produce an enhanced treated sludge to meet the internal drivers of:

  • widening the land bank for recycling of the sludge to agriculture,
  • enabling the Company to meet the nitrate vulnerable zone regulations with the ability  to store a caked product,
  • increasing the CHP capacity and reducing United Utilities’ carbon footprint.

Earlier work for Burnley, identified that in-series digestion could be an option to produce this enhanced treated sludge. In-series digestion is defined as operating the digestion process in tanks one after each other compared to the more conventional method of operating in parallel. The theory of in-series digestion was discussed in detail (Barber 2006) and concluded that the process performance was likely to be improved and to achieve a more stable operation when compared with parallel operation.

Recognising these opportunities, further work was done to assess the process variants that were operated elsewhere in the world, then could be tested at laboratory scale and then be considered for full-scale application at Burnley STC.

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