Performance of production and component of biogas

The biogas production of TPAD system was confirmed it had a large potentiality due to high VS reduction, and both NT-TPAD and AT-TPAD systems have a large number of biogas comparing with traditional single-stage reactors.

NT-TPAD produces more biogas in the thermophilic reactor than in mesophilic because the former is the main performance of this two-stage system. However the responsibility for producing biogas in AT-TPAD system is mainly by the post-treatment reactor, mesophilic methanogenic reactor, because the thermophilic reactor is responsible for generating the precursors of methane.

In treating sewage sludge, many studies indicated that high temperature first-stage improved the decomposition of sewage sludge, particularly waste activated sludge (WAS), thus resulting in more biogas production. Han et al. (1997) pointed out that TPAD system achieved a methane production rate approximately 30 to100% higher than single-stage mesophilic reactor. A full-scale TPAD system in Germany also supported this result, researchers found temperature-phased operation increased 16.5% biogas production than past mesophilic operation although the gas yield was still low due to few organic fraction in the raw sludge (Oles et al., 1997).

Vandenburgh and Ellis (2002) found both thermophilic and mesophilic biogas production increased with feed sludge concentration, and interestingly, when TS concentration exceeded 4.9%, the biogas production of mesophilic reactor was higher than thermophilic reactor. From their VFAs data, VFAs concentration was below 1000 mg/L when sludge concentration was below 4.9%, therefore, thermophilic reactor could consume these organic acids and no VFAs accumulation problem. But thermophilic reactor couldn’t consume VFAs immediately at high sludge concentration. As a result, follow-up mesophilic reactor was responsible for degrading VFAs producing from thermophilic reactor and then led to much biogas in mesophilic reactor. This procedure made NT-TPAD system similar to AT-TPAD system since the high OLR resulted in VFAs accumulation as well as a drop pH.

The biogas performance of AT-TPAD system is still high than single-stage

may be unsatisfying and make system at unstable status (Rubio-Koza and Noyola, 2010; Coelho et al., 2011). Even if VFAs accumulation appears at high OLR, the HRT of thermophilic reactor doesn’t be recommended due to reducing system efficiency. Riau et al. (2010b) could verify this view from their study, it suggested that the efficiency of the thermophilic reactor was lower than the mesophilic reactor if operated at the same long HRT.

In treating cattle waste, Dugba and Zhang (1999) found the methane production at first thermophilic stage of all systems was higher than second mesophilic stage, indicating mesophilic stage could be operated at high OLR or we could reduce the volume of the mesophilic reactor. Sung and Santha (2003) increased solids concentration to adjust system OLR. They found methane production rates from thermophilic stage were higher than the mesophilic reactor due to high VS removal in the thermophilic stage. Compared with thermophiloic reactor, the methane yield of mesophilic reactor was larger at all OLR, suggesting the thermophilic reactor didn’t converted intermediates to methane effectively.

Methane and carbon dioxide are the main composition of biogas in AD process, and with other small amount of gas like nitrogen and hydrogen sulfide. According to previous studies, the difference of biogas composition wouldn’t be significant whether researchers used thermophilic AD or mesophilic AD treatment, however, methane content of thermophilic AD was a little less than mesophilic AD. The most important factor that affects methane content is composition of the substrate.

The biogas composition of treating sewage sludge by TPAD system was that the thermophilic reactor had a methane content of 41-68%, carbon dioxide of 27-30%, nitrogen of 3-5%, and hydrogen sulfide of 150 ppm; the mesophilic reactor had a methane content of 53-72%, carbon dioxide of 24-27%, nitrogen of 2-5%, and hydrogen sulfide of 25 ppm (Han et al., 1997; Li, 2004; Song et al., 2004; Santha et

al., 2006; Riau et al., 2010b; Rubio-Loza and Noyola, 2010; Coelho et al., 2011).

And the biogas composition of treating cattle manure at varying OLR by TPAD system was that the thermophilic reactor had methane content of 58-61%, and hydrogen sulfide of 500-1300 ppm; the mesophilic reactor had a methane content of 59-62%, and hydrogen sulfide of 125-700 ppm. Like other studies, carbon dioxide was the second only to methane (Sung and Santha, 2003). It seemed that treating sewage sludge and cattle manure had similar results, but noting the hydrogen sulfide concentration in digesting cattle manure was higher than in digesting sewage sludge suggesting livestock wastes have a large number of proteins, and lead to a higher hydrogen sulfide concentration.

Besides substrates are the major influence of biogas composition, the reactor operation is also critical. For instance, a modified TPAD system treating co-digestion of sewage sludge and confectionery waste was investigated, researchers found the average methane content of mesophilic methanogenic reactor was about 44-82% with mesophilic HRT decreasing from 15-day to 8-day, the thermophilic reactor was a pre-treatment stage which HRT was fixed at 4-hour (Lafitte-Trouqué and Forster, 2000). Furthermore, Şentürk et al. (2010) studied the performance of treating potato-chips wastewater by thermophilic anaerobic contact reactor, and they found the methane content declined gradually from 89% to 68%, while the reactor OLR rose from 0.6 to 8.0 kg COD/m³/d.