Abstract
The aim of the article was to present the effects of lignin grafted with polyvinylpyrrolidone (PVP) as a microbial carrier in anaerobic co-digestion (AcoD) of cheese (CE) and wafer waste (WF). Individual samples of waste cheese and wafers were also tested. The PVP modifier was used to improve the adhesive properties of the carrier surface. Lignin is a natural biopolymer which exhibits all the properties of a good carrier, including nontoxicity, biocompatibility, porosity, and thermal stability. Moreover, the analysis of the zeta potential of lignin and lignin combined with PVP showed their high electrokinetic stability within a wide pH range, that is, 4–11. The AcoD process was conducted under mesophilic conditions in a laboratory by means of anaerobic batch reactors. Monitoring with two standard parameters: pH and the VFA/TA ratio (volatile fatty acids-to-total alkalinity ratio) proved that the process was stable in all the samples tested. The high share of N–NH4+ in TKN (total Kjeldahl nitrogen), which exceeded 90% for WF+CE and CE at the last phases of the process, proved the effective conversion of nitrogen forms. The microbiological analyses showed that eubacteria proliferated intensively and the dehydrogenase activity increased in the samples containing the carrier, especially in the system with two co-substrates (WF+CE/lignin) and in the waste cheese sample (CE/lignin). The biogas production increased from 1102.00 m3 Mg−1 VS (volatile solids) to 1257.38 m3 Mg−1 VS in the WF+CE/lignin sample, and from 881.26 m3 Mg−1 VS to 989.65 m3 Mg−1 VS in the CE/lignin sample. The research results showed that the cell immobilization on lignin had very positive effect on the anaerobic digestion process.
Highlights
The dairy industry generates significant amounts of waste at various stages of the production chain, especially in cheese production processes [1]
The goal of the study was to verify the amount of biogas/methane produced in anaerobic co-digestion of waste cheese (CE) and waste wafers (WF) with the addition of lignin combined with polyvinylpyrrolidone (−[CH2 CH(C4 H6 NO)]n −, PVP), and without the carrier—as the control variant
As far as the value obtained in this study is concerned, calcium will only favor the formation of cell aggregates
Summary
The dairy industry generates significant amounts of waste at various stages of the production chain, especially in cheese production processes [1]. Anaerobic digestion (AD) is thought to be the most sustainable solution in waste management and energy recovery, in the dairy sector [3]. This is an environment-friendly and promising method, its implementation may cause problems, mostly due to the chemical composition of dairy waste (i.e., acidic pH, high concentration of mineral salts) [4]. Polymers 2019, 11, 2073 anaerobic digestion [3] In this case, the accumulation of volatile fatty acids (VFA) originating from the coexistent fat may occur frequently. Confectionery waste is the material that potentially meets the abovementioned conditions regarding cheese waste
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