Increased production of biogas through industrial symbiosis – co-digestion of steam pretreated straw and sewage sludge

Straw has an untapped potential for domestic biogas production of at least 4.3 TWh.

The potential could be realised in a socio-economically profitable way through industrial symbiosis between agricultural straw production, combined heat and power plants that supply steam, for pre-treatment of straw, and adjacent wastewater treatment plants (WWTPs) with sludge containing macro- and micronutrients suitable for digestion with energy-rich but nutrient-poor straw.

This co-digestion can lead to a more easily dewatered and manageable sludge with lower quotas of copper, nickel and zinc in relation to phosphorus, which improves the quality of the sludge when spreading on arable land. In the event of thermal destruction, the calorific value of the sludge increases in comparison with mono-digestion of sewage sludge.

Existing infrastructure in Malmö, digesters at Sjölunda WWTP and steam production at Sysav’s household waste incineration plant, can minimize the investment costs for biogas production from straw.

The project is part-financed by the Swedish Energy Agency.

The project will:

  • Show whether biogas can be produced from straw at a net cost of no more than 600 SEK per MWh by pre-treatment with steam from existing steam production and digestion of pre-treated straw and sludge at adjacent WWTP.
  • Demonstrate whether the dewatering properties of digestate are the same or better after co-digestion of straw and sludge compared to mono-digestion of sludge and whether the addition of polymers can be reduced or eliminated.
  • Demonstrate process performance for digestion in terms of biogas production, organic load, stability and hydraulic residence time for straw and sludge digestion compared to mono-digestion of sludge.
  • Evaluate selected properties of the digestate from the co-digestion of sludge and straw compared with mono-digestion of sludge for sludge spreading on arable land and sludge incineration.