Continuous two-step anaerobic digestion (TSAD) of organic market waste: rationalising process parameters

Abstract

Experimental tests on continuous two-stage anaerobic digestion (TSAD) were conducted, to assess its energetic performance, using organic market waste as a substrate. The systems were tested to ascertain the effects of external stressors, which allow the separation into two different microorganism consortia, that is, hydrogen-producing bacteria and hydrogen-consuming bacteria, to be maintained. Two bioreactors were run in series under different operational conditions, including pH, mixing rate, and initial inoculum, and three different decreasing hydraulic retention times were considered, with a fixed ratio of 1:10 in volume between the first bioreactor (hydrogen) and the second one (methane). The performance of the whole system was assessed over > 140 days to monitor the stability of the process, in terms of the reduction of the volatile solids and the energy productivity for each step. Each tested condition was scored using two parameters: efficiency and efficacy. The first corresponds to the fraction of recovered energy of the available (η) and the second (ξ) was used to compare the energy produced by the TSAD with that of one-step anaerobic digestion. The efficiency resulted to be (24-32)%, while the efficacy proved to be around 1.20. The share of energy, under the form of hydrogen, compared to the total energy recovery, was in the (8-12) % range. Finally, the oscillation behaviour of the quasi-steady-state condition was analysed in terms of the Fano factor to establish the most stable conditions.

Energy Sustainability Analysis (ESA) of Energy-Producing Processes: A Case Study on Distributed H2 Production

“…Finally, the third technological option belongs to the biotechnological field and is the utilization of a Two-Stage Anaerobic Digestion (TSAD) system for the production of bio-H 2 and bio-CH 4 , using organic waste as feed [33,63]. The flow diagram of the system is presented in Figure 3c, the process configuration comprehends two CSTR operated in series.…”

“…CH-8708, Manedorf, Switzerland), for bio-CH 4 production with a working volume ratio of 1:10. Both bioreactors were inoculated with adequate microbial consortia for the optimized production of each biofuel [64], the tested substrate was the organic fraction of municipal solid waste (OFMSW), additional information can be found in [34,63].…”

“…In the TSAD case, the main contributions to the direct energy are: (i) the expenses to heat the feed up to the working temperature conditions (35 • C) of the bioreactors (ii) to compensate the environmental thermal losses and (iii) the electrical energy input to the agitators, which is a requirement in both stages (S1 and S2), although in different proportions [63], and to other auxiliary equipment [34,67].…”

Monitoring the Physiological State in the Dark Fermentation of Maize/Grass Silage Using Flow Cytometry and Electrooptic Polarizability Measurements

“…The DF can also be exploited in cascade reactor arrangements, for example coupling DF to AD in two-stage AD (TSAD) [6]. The TSAD optimizes process parameters to produce H 2 and a mixed pool of carboxylic acids in the DF bioreactor (fermentative bacteria), while a second methanogenic bioreactor (fermentative bacteria and methanogens) is fed with the liquid output from the first reactor and targets CH 4 production [7]. Alternatively, the liquid effluent of carboxylic acids and solvents from the DF process can be fed into different processes (see Fig.…”

“…1), thus increasing value addition and improving the sustainability of the process [8,9]. This can be achieved by additional energy production as it occurs in microbial fuel cells (MFCs) [10], algae and microalgae cultivations [11], and other multi-cascade reactor configurations [7], as well as in the synthesis of valuable chemicals [12].…”