Full-scale membrane filtration system for the treatment of digestate from a co-digestion plant

Recently, membrane filtration systems have been proposed for the treatment of animal manure and digestate to concentrate nutrients, especially nitrogen, in liquid and solid fractions. A full-scale plant, adopting ultrafiltration (UF) and reverse osmosis (RO) after mechanical screening and centrifugation, was monitored in order to assess the effectiveness of the system. The total flow rate of the plant was 3,600 kg/h of digestate from a biogas plant, which was fed with swine and poultry manure and corn silage. Digestate was characterized by Total Solids (TS) content of 71.0 g L-1 Volatile Solids (VS) content of 53.1 g L-1 and Total Nitrogen (TN) content of 4.96 g L-1. Mechanical screening, centrifugation and reverse osmosis produced solid fractions and concentrate representing 9%, 26%, and 17% of input mass, respectively. The permeate from RO represented the final clean water output, which amounted to 48% of digestate. The first treatment stage separated 11% to 13% of TN, referred to input digestate, in the solid phase. The centrifugation was better by reducing 57% to 61% of TN. The membrane stages (UF and RO) separated 24% to 28% of TN. Significant phosphorus (P) reduction was achieved (>99% of input P) with centrifugation and UF phases. RO removed the remaining 1% of P. The permeate met the criteria imposed by Italian regulation for discharge into surface water (D. Lgs. 152/2006) for pH (pH = 8.1, allowable range 5.5-9.5) and for COD (COD<10 mg/L, limit 160 mg/L). However NH4+, exceeded the limit (NH4+ = 25 mg/L, allowable limit 15 mg/L). This fraction could be used to dilute solid biomasses feeding the digesters or as cleaning water for livestock facilities. A further removal of NH4+ could also be achieved with zeolites.