Operational strategies for thermophilic anaerobic digestion of organic fraction of municipal solid waste in continuously stirred tank reactors

Process performance and modelling of anaerobic digestion using source-sorted organic household waste

Three distinctive start-up strategies of biogas reactors fed with source-sorted organic fraction of municipal solid waste were investigated to reveal the most reliable procedure for rapid process stabilization. Moreover, the experimental results were compared with mathematical modeling outputs. The initial inoculations to start-up the reactors were 10, 50 and 100% of the final working volume. While a constant feeding rate of 7.8gVS/d was considered for the control reactor, the organic loading rate for fed-batch reactors with 10 and 50% inoculation was progressively increased during a period of 60 and 13days, respectively. The results clearly demonstrated that an exponentially feeding strategy, considering 50% inoculation relative to final volume, can significantly decrease the alternatively prolonged period to reach steady conditions, as observed by high biogas and methane production rates. The combination of both experimental and modelling/simulation succeeded in optimizing the start-up process for anaerobic digestion of biopulp under mesophilic conditions.

In situ biogas stripping of ammonia from a digester using a gas mixing system

Previous studies have suggested the use of digester biogas mixing systems for in situ ammonia removal from anaerobic digestates. The feasibility of this was tested at moderate and complete gas mixing rates at mesophilic and thermophilic temperatures in a 75-L digester. Experimental results showed that at gas mixing rates typical of full-scale commercial digesters the reduction in total ammonia nitrogen concentrations would be insufficient to allow stable acetoclastic methanogenesis in mesophilic conditions, or to prevent total inhibition of methanogenic activity in thermophilic food waste digestion. Simulation based on batch column stripping experiments at 55°C at gas violent flow rates of 0.032 m³ m^-2 min^-1 indicated that ammonia concentrations could be reduced below inhibitory values in thermophilic food waste digestion for organic loading rates of up to 6 kg VS m^-3 day^-1. These mixing rates are far in excess of those used in full-scale gas-mixed digesters and may not be operationally or commercially feasible.

Biogas stripping of ammonia from fresh digestate from a food waste digester

The efficiency of ammonia removal from fresh source-segregated domestic food waste digestate using biogas as a stripping agent was studied in batch experiments at 35, 55 and 70°C, at gas flow rates of 0.125 and 0.250Lbiogasmin(-1)L(-1)digestate with and without pH adjustment. Higher temperatures and alkaline conditions were required for effective ammonia removal, and at 35°C with or without pH adjustment or 55°C with unadjusted pH there was little or no removal. Results were compared to those from earlier studies with digestate that had been stored prior to stripping and showed that ammonia removal from fresh digestate was more difficult, with time constants 1.6-5.7 times higher than those previously reported. This has implications for the design of large-scale systems where continuous stripping of fresh digestate is likely to be the normal operating mode. A mass balance approach showed that thermal-alkaline stripping improved hydrolysis.

Semi-continuous anaerobic co-digestion of cow manure and steam-exploded Salix with recirculation of liquid digestate

The effects of recirculating the liquid fraction of the digestate during mesophilic anaerobic co-digestion of steam-exploded Salix and cow manure were investigated in laboratory-scale continuously stirred tank reactors. An average organic loading rate of 2.6 g VS L(-1) d(-1) and a hydraulic retention time (HRT) of 30 days were employed. Co-digestion of Salix and manure gave better methane yields than digestion of manure alone. Also, a 16% increase in the methane yield was achieved when digestate was recirculated and used instead of water to dilute the feedstock (1:1 dilution ratio). The reactor in which the larger fraction of digestate was recirculated (1:3 dilution ratio) gave the highest methane yields. Ammonia and volatile fatty acids did not reach inhibitory levels, and some potentially inhibitory compounds released during steam explosion (i.e., furfural and 5-hydroxy methyl furfural) were only detected at trace levels throughout the entire study period. However, accumulation of solids, which was more pronounced in the recycling reactors, led to decreased methane yields in those systems after three HRTs. Refraining from the use of fresh water to dilute biomass with a high-solids content and obtaining a final digestate with increased dry matter content might offer important economic benefits in full-scale processes. To ensure long-term stability in such an approach, it would be necessary to optimize separation of the fraction of digestate to be recirculated and also perform proper monitoring to avoid accumulation of solids.

Anaerobic digestion of selected Italian agricultural and industrial residues (grape seeds and leather dust): combined methane production and digestate characterization

A combined experimental evaluation of methane production (obtained by anaerobic digestion) and detailed digestate characterization (with physical-chemical, thermo-gravimetric and mineralogical approaches) was conducted on two organic substrates, which are specific to Italy (at regional and national levels). One of the substrates was grape seeds, which have an agricultural origin, whereas the other substrate was vegetable-tanned leather dust, which has an industrial origin. Under the assumed experimental conditions of the performed lab-scale test series, the grape seed substrate exhibited a resulting net methane production of 175.0 NmL g volatile solids (VS)(-1); hence, it can be considered as a potential energy source via anaerobic digestion. Conversely, the net methane production obtained from the anaerobic digestion of the vegetable-tanned leather dust substrate was limited to 16.1 NmL gVS(-1). A detailed characterization of the obtained digestates showed that there were both nitrogen-containing compounds and complex organic compounds present in the digestate that was obtained from the mixture of leather dust and inoculum. As a general perspective of this experimental study, the application of diversified characterization analyzes could facilitate (1) a better understanding of the main properties of the obtained digestates to evaluate their potential valorization, and (2) a combination of the digestate characteristics with the corresponding methane productions to comprehensively evaluate the bioconversion process.

Two-phase anaerobic digestion of the organic fraction of municipal solid waste: estimation of methane production

Energy generation from methane (CH(4)) is one of the primary targets of the anaerobic digestion process. Consequently, the focus of this study was to investigate the effect on CH(4) production of total solids (TS) loading (measured as % TS) and hydraulic residence time (HRT) during the treatment of the organic fraction of municipal solid waste (OFMSW). Laboratory-scale, two-phase anaerobic digestion systems were employed with each system consisting of an acidogenic reactor and a methanogenic reactor linked in series. The group A runs in the experiment explored the effect on digester performance of four variations in methanogenic HRT (15, 20, 25 and 30 days) at three different feed TS concentrations (8, 12 and 15%). The group B runs compared the actual methane yield (0.14 to 0.45 L g VSfeed−1)) to that predicted by the Chen-Hashimoto model. Results from the group A runs indicated that acidogenesis improved with an increase in % TS and a decrease in HRT; while, methanogenesis behaved inversely, achieving higher yields at the lower % TS and longer HRT values. In comparison with the group B runs, the Chen-Hashimoto model under-predicted (by an average of 16.5 ± 6.6%) the CH(4) yield obtained from the digestion of OFMSW.

Effect of mixture ratio, solids concentration and hydraulic retention time on the anaerobic digestion of the organic fraction of municipal solid waste

This paper describes how the degradation of the organic fraction of municipal solid waste (OFMSW) is affected through codigestion with varying amounts of return activated sludge (RAS). Solid waste that had its inorganic fraction selectively removed was mixed with RAS in ratios of 100% OFMSW, 50% OFMSW/50% RAS, and 25% OFMSW/75% RAS. The total solids (TS) concentration was held at 8% and three anaerobic digester systems treating the mixtures were held (for the first run) at a total hydraulic retention time (HRT) of 28 days. Increasing amounts of RAS did not however improve the mixture's digestability, as indicated by little change and/or a drop in the main performance indices [including percentage volatile solids (VS) removal and specific gas production]. The optimum ratio in this research therefore appeared to be 100% OFMSW with an associated 85.1 ± 0.6% VS removal and 0.72 ± 0.01 L total gas g(- 1) VS. In the second run, the effect of increasing percentage of TS (8, 12% and 15%) at a system HRT of 28 days was observed to yield no improvement in the main performance indices (i.e. percentage VS removal and specific gas production). Finally, during the third run, variations in the total system HRT were investigated at an 8% TS, again using 100% OFMSW. Of the HRTs explored (23, 28 and 33 days), the longest HRT yielded the best performance overall, particularly in terms of specific gas production (0.77 ± 0.01 L total gas g(-1) VS).

The effect of upper mesophilic temperature and feed-to-seed ratio on batch anaerobic digestion systems

This study investigated the effect of upper mesophilic temperature and feed-to-seed (F/S) ratio on anaerobic digestion using four 3.5 L batch-scale reactors. Initially, eight F/S ratios, ranging from 10/90 to 90/10, were explored at 37 degrees C, using a mixture of primary and secondary municipal sludge as feed. It was observed that the systems with low F/S ratios (40/60 and below) showed a stable performance while those with high ratios (50/50 and above) experienced the effect of organic overloading indicated by reduced removal of volatile solids (VS) in the feed, a drop in pH, volatile fatty acid (VFA) accumulation during the first 10 days of operation, and total gas production markedly lower than the corresponding theoretical values. Subsequently, the effect of temperature, in the 37 to 49 degrees C range, was studied at an F/S ratio of 20/80. Results revealed that an increase in temperature between 37 and 43 degrees C had a rather minimal effect on the process, with the exception of a moderate increase in total gas production. A further rise in the temperature in the 45 to 49 degrees C range however appeared to trigger an adverse effect evidenced by enhanced percent VSS reduction (possibly the result of cell lysis), VFA accumulation and an increase in the non-VFA total organic carbon (TOC) content. Therefore, it can be concluded that an operating temperature in the 37 to 43 degrees C range resulted in a stable and satisfactory reactor performance.