Development of Technology for Treatment of Wastewater Digestat From the Food Industry

Abstract

This article proposes a hybrid technological scheme for dewatering the digestate of wastewater from the food industry, further cleaning the obtained filtrate after dehydration with the possibility of returning it to the technological chain for reuse. At least ten different derivative products can be obtained from digestate, including liquid/solid fraction, granules obtained from both solid and liquid fractions, and analogues of commercial synthetic fertilisers: ammonium sulphate, ammonia water, magnesium ammonium phosphate (struvite). The purpose of the experimental studies was to choose an appropriate process for treating digestate of food production wastewater to solve the problem of increasing the amount of sediment and the formation of digestate. The investigated digestate treatment processes include the next: chemical conditioning (dosing of flocculants and coagulants), mechanical dewatering using multi-disc technology (with ESMIL MDQ multi-disc screw press), two stages of membrane separation (using reverse osmosis (RO) membranes, vibrating shear process (VSEP), zwitterionic (ZI) membrane). The cake separated after the dehydration stage can be processed in bulk or pallets with 10–35% dry matter content and used as fertiliser. Due to the high viscosity of the initial digestate, the technological scheme provides for its dilution before the dehydration stage. Dilution occurs with the purified filtrate after the membrane purification stage. The effectiveness of dehydration on cake dryness showed worse results than expected but deserves further investigation if this parameter is significant. Using an additional stage of filtrate purification on RO membranes is efficient and appropriate if high-quality purified water is required. The cleaning process can be stopped at any stage, depending on the cleaning needs. The obtained results regarding the efficiency of the cleaning process after RO membranes showed the following values for the main pollutants: COD – 98.43%, ammonium nitrogen – 99.35%, and phosphates – 99.99%. Keywords: digestate, sludge conditioning, coagulant, flocculent, dehydration, membrane purification, cake, permeate.