Two-fraction anaerobic fermentation of grass waste

Waste from public green areas represents large quantities of grassy phytomass. The grass is usually utilised by composting, combustion or anaerobic fermentation. However, the classical composts are time-demanding, the quality of accelerated composts is low, combustion is under increasing criticism and conventional anaerobic fermentation requires high investment.

A new method of two-fraction anaerobic fermentation of grass waste consisting from a hot maceration, up-flow anaerobic sludge blanket reactor, steam explosion, horizontally stirred batch anaerobic fermentors and a charcoal kiln, all run on waste heat from a co-generation unit was investigated on a commercial scale.

The results shows that due to faster energy utilisation the two-fraction technology requires smaller fermentors and hence the technology is approximately one-third less costly than conventional systems, with 4 years shorter payback time. Additionally, huge amounts of charcoal are produced. However, the process control and optimisation is more demanding.