Assessment of Single- vs. Two-Stage Process for the Anaerobic Digestion of Liquid Cow Manure and Cheese Whey

Abstract

The growing interest in processes that involve biomass conversion to renewable energy, such as anaerobic digestion, has stimulated research in this field in order to assess the optimum conditions for biogas production from abundant feedstocks, like agro-industrial wastes. Anaerobic digestion is an attractive process for the decomposition of organic wastes via a complex microbial consortium and subsequent conversion of metabolic intermediates to hydrogen and methane. The present study focused on the exploitation of liquid cow manure (LCM) and cheese whey (CW) as noneasily and easily biodegradable sources, respectively, using continuous stirred-tank reactors for biogas production, and a comparison was presented between single- and two-stage anaerobic digestion systems. No significant differences were found concerning LCM treatment, in a two-stage system compared to a single one, concluding that LCM can be treated by implementing a single-stage process, as a recalcitrant substrate, with the greatest methane production rate of 0.67 L CH4/(LR·d) at an HRT of 16 d. On the other hand, using the easily biodegradable CW as a monosubstrate, the two-stage process was considered a better treatment system compared to a single one. During the single-stage process, operational problems were observed due to the limited buffering capacity of CW. However, the two-stage anaerobic digestion of CW produced a stable methane production rate of 0.68 L CH4/(LR·d) or 13.7 L CH4/Lfeed, while the total COD was removed by 76%.

Hydrogen and Methane Production from Anaerobic Co-Digestion of Sorghum and Cow Manure: Effect of pH and Hydraulic Retention Time

“…Prior to its use as feeding material, ES was subjected to alkaline pretreatment at 80 • C for 2 h by adding an alkaline solution consisting of 0.5% NaOH and 0.5% KOH (w/w dry matter). The chemical composition of all substrates was in accordance with other studies found in the literature [7,28,42].…”

“…In particular, co-digestion is considered to be an appealing option for boosting the efficiency of the process thanks to its positive synergisms which can significantly increase the economic viability of biogas plants [5]. Furthermore, two-stage anaerobic digestion exhibits various benefits compared to the conventional one-stage process, as it enables the selection and the enrichment of Fermentation 2022, 8, 304 2 of 15 microflora in the reactors, enhancing the stability of the system by (a) the acidification phase regulation in the first reactor and (b) overloading and/or preventing inhibition of the methanogenic population in the second reactor [6,7]. At the first stage of acidogenesis, generation of biological hydrogen and various soluble metabolites takes place, whereas at the second stage the methane is produced [7][8][9].…”

“…Furthermore, two-stage anaerobic digestion exhibits various benefits compared to the conventional one-stage process, as it enables the selection and the enrichment of Fermentation 2022, 8, 304 2 of 15 microflora in the reactors, enhancing the stability of the system by (a) the acidification phase regulation in the first reactor and (b) overloading and/or preventing inhibition of the methanogenic population in the second reactor [6,7]. At the first stage of acidogenesis, generation of biological hydrogen and various soluble metabolites takes place, whereas at the second stage the methane is produced [7][8][9]. Through various manipulation techniques, optimum operational and environmental conditions can be achieved for each microbial community, resulting in significant positive changes in both the production of biogas and in the overall performance of the process.…”

“…Manure is a waste steam in which several groups of microorganisms and significant amounts of volatile organic compounds are included. Moreover, such a substrate is characterized by high alkalinity levels [7], and for this reason is one of the most preferable substrates in cases of anaerobic co-digestion. On the other hand, the energy crops are considered low-input cultures, having low nutrients, protection, work, and water requirements [26].…”

Effect of the Substrate to Inoculum Ratios on the Kinetics of Biogas Production during the Mesophilic Anaerobic Digestion of Food Waste

“…The pH of the inoculum being more than 7 also adds that the digestate is stable and falls within the range of methanogenic bacterial growth [41,42]. The methane composition of the produced gas is very low suggests that the peak of the first three days is of fermentation via acidogenesis and acetogenesis, and largely represents hydrogen production [43,44]. The methane percentage is seen to increase to a maximum of 94.74% on the 22nd day of the tests.…”