Mobile solution for digestate transformation to high added-value products

Abstract

Digestate liquid fraction derived from four biogas plants fed with different organic sources (animal manure, mixed waste, agricultural waste and urban sewage sludge) was treated using a series of processes that were integrated on a mobile platform. These processes included microfiltration and ultrafiltration for the removal of solids and large organic molecules, selective electrodialysis for the recovery of nutrients, UV ozonation to selectively oxidize pharmaceuticals and reverse osmosis for clean water production. Results showed that 48–58% of the digestate could be recovered as water that can be used for irrigation or process needs, while 51–67.8% of nitrogen could be recovered by the selective electrodialysis process and used for fertilizer production tailored to farmers’ soil and crop requirements, aiming to replace fertilizers from non-renewable minerals. Applying UV ozonation for the oxidation of pharmaceuticals before the nutrient recovery step was found to be twice as effective as applying it after nutrient recovery. The solid fractions of various digestates were applied as fertilizers for lettuce grown in pots, with the digestate from agricultural and food waste origins giving some of the best yields with the lowest GHG emissions. The solid fraction from sewage sludge digestate produced the highest yields in lettuce growing, but also resulted in much higher GHG emissions than the other fertilizers. A business model for the implementation of the developed technology in three operational scenarios concluded that a mobile solution shared between small anaerobic digestion plants in a region could have a five-year return on investment of 41.6% and a payback period of 1.96 years.