Kraft Lignin Grafted with Polyvinylpyrrolidone as a Novel Microbial Carrier in Biogas Production

Abstract

The objective of this study was to verify the effect of kraft lignin as a microbial carrier on biogas/methane yield. An anaerobic co-digestion test process was carried out, in which confectionery waste was used with sewage sludge. At the first stage of the study pure lignin and lignin combined with polyvinylpyrrolidone (PVP) were subjected to an extensive physicochemical analysis. Their morphology, dispersion and adsorption properties were determined. The two materials were also subjected to thermal, spectroscopic and elementary analysis. The anaerobic digestion of the two substrates was carried out with and without the addition of the carrier, under mesophilic conditions and in periodic operation. The monitoring and analysis of the two essential parameters, i.e., pH and volatile fatty acids/total alkalinity (VFA/TA) ratio, revealed that the process was stable in both tests. Microbial and biochemical analyses showed intensified proliferation of eubacteria and increased dehydrogenase activity in samples prepared with the lignin + PVP material. The cell count increased by 46% in the stuffed wafers (WAF) + sewage sludge (SS) variant with the carrier, whereas the enzyme activity increased by 43%. Cell immobilisation noticeably improved the process efficiency. The biogas production increased from 722 m3 Mg−1 VS to 850 m3 Mg−1 VS (VS – volatile solids), whereas the methane production increased from 428 m3 Mg−1 VS to 503 m3 Mg−1 VS (by about 18%). The research proved that lignin could be used as a very effective microbial carrier in anaerobic digestion (AD).

The Use of Lignin as a Microbial Carrier in the Co-Digestion of Cheese and Wafer Waste

“…The carrier was added to the mixture at an amount based on data in reference publications [32,33]. The authors of this study made partial physicochemical analysis of the lignin+PVP material in earlier tests [25,26], which showed that it met essential requirements for the microbial carrier in the AD process.…”

“…The biogas efficiency for individual substrates was calculated by subtraction of the volume of biogas produced from the inoculum alone from the volume of biogas produced from the samples. The equation presented by the authors in earlier studies was used to calculate the cumulative amount of biogas generated from the inoculum (digested sewage sludge) in the bioreactors with a given substrate or mixture of substrates [25,26].…”

“…The content of total solids (TS) was measured by drying at 105 • C (Zalmed SML dryer, Zalmed, Łomianki, Poland). The content of volatile solids (VS) was measured by combustion at 550 • C (MS Spectrum PAF 110/6 furnace, MS Spectrum, Warszawa, Poland) [25].…”

“…The porous structure parameters of the lignin and lignin+PVP material were measured using an ASAP 2420 instrument (Micromeritics Instrument Co., Norcross, GA, USA). Before performing the analysis, the samples were degassed in vacuum at a temperature of 90 • C. The BJH (Barrett-Joyner-Halenda) method was applied to measure the total volume and pore size distribution [25].…”

“…The digestate samples were incubated for 24 h in darkness, at 37 • C. Next, the preparations were analyzed with a fluorescence microscope equipped with an AxioCam MRc5 color digital camera. The image was analyzed by means of the AxioVision 4.8 software (AxioVision LE 4.8 2.0, Carl Zeiss Microscopy, LLC, White Plains, NY, USA) [25].…”