Single-stage and multi-stage liquid/solid separation of digestate in full scale biogas plants

Different full-scale separation systems were tested in farms equipped with anaerobic digestion plants. Each biogas plant was fed with agricultural feedstocks and was equipped with single-stage or multi-stage liquid/solid separation of digestate. In particular, one roller separator and three screw presses were tested in a single stage-process. Two multi-step processes were also evaluated, featuring a screw press followed by a centrifuge as pre-treatment before membrane filtration. The objective of the research was the evaluation of the separation efficiency of different systems, with particular attention to mass, solids, nitrogen and phosphorous balances, and energy consumption where possible. Screw and roller separators showed similar performance, while the centrifuges showed higher degree of separation. Referred to digestate input, the roller separator showed a production of solids of 11%, the screw presses of 9.4% (5.8-14.0%), while the centrifuges determined a production of solids of 20.5% (14.0%- 26.3%). The diversion of nitrogen resulted of 13.0% for the roller separator, 14.2% (9.8-18.4%) for the screw presses, and 38.0%N (22.6-55.3%) for the centrifuges, as effect of the high flow, addition of flocculants, and N content in the solid fraction. The roller separator presented a P flow of 40.2%, the screw press 33.2% (20.0-44.4%), while the centrifuge separated 80% of P in the solids. The variability in the results, even among the same category of equipment, can be related to the type of separator, the sieve openings and on the physical-chemical characteristics of digestate. The performance of the centrifuge should be evaluated considering that it was part of a multi-stage process.