Advanced Multi-step Filtration of Digestate from a Biogas Plant

A commercial scale, agricultural biogas plant with digestate post treatment system was monitored. The plant featured a power of 1MWe, was fed with silages and operated in thermophilic regime (52°C). The AD system was represented by a 600 m3 first stage and two 850 m3 second stage digesters. The digestate treatment was composed by a multistage liquid/solid separation, screw-press and decanter, followed by dissolved air flotation (DAF) and membrane filtration units. The membrane treatment was performed by Ultra-Filtration (UF), and by final reverse-osmosis (RO) stage, performed by composite polyamide membranes. Flocculants were used in decanter and flotation. Three tests were performed in different operating conditions, focusing on per-formance, mass, energy, N and P balances. In general the treatment system showed relevant potential and flexibility in the management of digestate with variable characteristics and flow rates. The treatment capacity resulted between 3.1 and 4.2 m3/h, more than appropriate to face the production of 50 m3/day of digestate. The outputs are represented by screw-press and decanter solids, con-taining 31-42% of input N, and concentrate from RO, containing 17-28% of in-put N, with the variations due to different characteristics of digestate, use of chemicals and setting of the system. Energy demand resulted between 8.6 and 10.6 kWh/m3 of input digestate, interesting result in comparison to other systems implementing membrane filtration. Flotation, along with the decanter, proved to be the key step of the treatment chain, but requiring precise management.