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
Highlights
The EU-28 yearly production of digestate was estimated in almost 180 million tons, derived from the Anaerobic Digestion (AD) of both the source separated biowaste and the agro-industrial residues [1]
Despite the type of involved chemical, each treatment led to a significant increase of Total Suspended Solid (TSS) removal if compared to the control TSS (i.e., 22.447 ± 1.150 mg/L), ranging from DEED4 with 32 ± 1% to PACDE5 with 90 ± 1%, corresponding to the TSS concentrations of 15,330 ± 575 and 2,347 ± 281 mg/L, respectively (Fig. 1)
Reported TSS concentrations in the liquid fractions derived from Chemically enhanced solid–liquid separation (CES) tests with only polyaluminum chloride (PAC) and PAM dosages are characterized by lower values if compared to all the liquors undergone dimethylamine with ethylendiamine (DEED) and PAM application, except for
Summary
The EU-28 yearly production of digestate was estimated in almost 180 million tons, derived from the Anaerobic Digestion (AD) of both the source separated biowaste and the agro-industrial residues [1]. Digestate processing technologies consist primarily of solid–liquid separation, performed to achieve i) reduced volumes, improving handling and decreasing transportation and storage costs, and ii) substances fractionation in concentrated solid and liquid fractions together with the complete purification of the remaining liquid fraction [4, 6, 7] This latter could be reused in situ according to the concept of zero water discharge when applied on AD plant design. The relative influence of the three mechanisms depends on the specific chemical-physical features of the substrate to be treated, e.g., electroconductivity, specific concentration of suspended particles (TSS) and charge distribution of the suspended particulate matter For this reason, and as reported by Hjorth and Jørgensen, (2012) for manures, the optimal combination between the type of suitable chemicals and their dosage can vary widely between different typologies of digested residues, as derived from a multitude of input feedstocks and the different set of AD technologies. To the best of our knowledge, no previously published study investigated the efficiency of CES when applied on digestates derived from a full-scale biogas plant treating separately collected biowaste
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