Valorization of digestate
The valorization of organic fraction of municipal solid waste answers to realization of a circular economy model, aimed to recovery of materials and energy by treatment and transformation of the former.
Methanation process, that allows generation of biomethane by anaerobic digestion of these kind of waste, however, produces a liquid waste, the digestate, that needs in turn a treatment for removal of pollutant substances as ammonia nitrogen, COD, BOD, boron, chlorides, sulfates, heavy metals as copper and zinc.
The system for treatment of digestate from organic fraction of municipal waste designed by VENZA group permits to recover and recycle many parts of it through application of different kinds of technologies.
The process starts with a treatment of separation in a centrifuge for removal of suspended solids. The solid fraction, with a dry matter content of 15-25%, can be employed as secondary material or, after drying, sent to combustion units for generation of thermal energy or to composting plants for production of biocompost.
The liquid fraction is treated in a high energy efficiency mechanical vapor recompression evaporator. The machine is able to concentrate polluting substances until to solubility product, separating at the same time ammonia in vapor phase; by chemical product dosage it is then transformed in liquid solution of ammonium sulfate.
The concentrate can be disposed or – mixed together with solid fraction separated from centrifuge – used as secondary material after a drying treatment or sent to composting plant.
The recovered ammonium sulfate (containing 6-8% of nitrogen, equivalent to a dry percentage of 30-40%) can be commercialized as liquid fertilizer. After the treatment process, that in some cases can include also MBR and reverse osmosis plants, parameters of water will be in compliance with limit values for discharge fixed by national law (in Italy Legislative Decree 152/06).
The design of each plant is preceded from a technical and economic feasibility study. The evaluation of specific case is aimed to determine the convenience and the efficiency of treatment. The calculation of return on investment needs an analysis of operating expenditures. The concentration of pollutants volume allows a reduction of liquid waste disposal fees (that can vary from 50 to 150 €/m3 according the location). The solid and liquid fractions recovered by treatment can be recycled or commercialized as secondary materials or compost.
The possible reuse of a part of treated water for washing of vehicles and soils can limit withdrawal of clean water. The mechanical vapor recompression evaporation technology, furthermore, has low energy consumptions and can access to white certificates as established by E.U. Directive (received in Italy with Ministerial Decree 158/18).