Chemically Enhanced Solid–Liquid Separation of Digestate: Suspended Solids Removal and Effects on Environmental Quality of Separated Fractions

Abstract

Chemically enhanced solid–liquid separation (CES) of digestate can improve its membrane filterability but potentially influence the environmental features of the separated solid fraction, thus hindering its possible agricultural reuse. In this study, the effects were assessed of different dosages of polyaluminum chloride (PAC), epichlorohydrine-dimethylamine with ethylendiamine (DEED) and polyacrilamides (PAM) on CES of digestate from biowaste in terms of Total Suspended Solid (TSS) mitigation in the liquid fractions and resulting environmental quality of the solid fractions. Results from lab-scale trials showed that applied chemicals significantly increased the centrifugation efficiency with achieving minimum TSS concentration of 2347 ± 281 mgTSS/L (up to 90% improved TSS mitigation). Also, performed treatments led to almost complete removal of P and Heavy Metals (HMs) from the liquid fractions after centrifugation. Conditioned solid fractions showed higher Al (reaching 20 g kg−1 TS), organic carbon and nitrogen content (up to 324 mgTOC kg−1 TS and 44.1 mgTKN kg−1 TS, respectively) due to residual PAC, DEED and PAM. However, achieved concentrations of HMs guaranteed full consistency with EU regulation limits established for agricultural reuse of organic soil amendments. Further, leaching tests performed on the treated solid fractions indicate higher chlorides and soluble Al concentrations in the water extracts (up to 4.6 g L−1 and 2.3 g L−1, respectively), but lower HMs leachability from the digestates undergone CES. Nevertheless, water extracts from treated biosolids were characterized by higher phytotoxicity, likely related with direct Al toxicity and increased salinity due to chemicals addition. Accordingly, the effects on soil–plant system should be better investigated when agricultural reuse of CES-treated solid fraction of digestate is foreseen. Graphic abstract

Technologies for Environmental Safety Application of Digestate as Biofertilizer

“…Recent research of new types of chemically enhanced solid-liquid separation (CES) treatments indicated that polyaluminum chloride (PAC), epichlorohydrine-dimethylamine with ethylendiamine (DEED) and polyacrilamides (PAM) such as cationic and anionic polyacrylamide (CPAM, APAM respectively) in different ratio had a potential to improve the environmental quality of the solid fraction. The highest Total Suspended Solid (TSS) removal rate (up to 90 ± 1% with respect to the control) in the liquid fractions separated after centrifugation was achieved using PAC4 (3.6 g•L -1 of PAC and 0.2 mg•L -1 of CPAM), PACDE5 (3.2+4.2 g•L -1 of PAC + DE and 0.32 mg•L -1 of APAM) and PACDE6 (3.2+4.2 g•L -1 of PAC + DE and 0.32 mg•L -1 of CPAM) [Beggio et al, 2021]. Besides of this, such treatment provides the predominant transfer of phosphorus and heavy metals (cadmium, lead, copper, nickel, zinc and chromium) into the liquid fraction, regardless of the type of reagent, while organic nitrogen is concentrated in the solid phase.…”