Protease and xylanase activities and thermophilic populations as potential process monitoring tools during thermophilic aerobic digestion

Abstract

Thermophilic aerobic digestion (TAD) of potato process waste slurry was carried out in a batch process in a continuously stirred tank reactor at 55• C for a total of 156 h. The pH of the slurry was either unregulated, or regulated at 6.0, 7.0, 8.0, 9.0 and 9.5, and aeration rate was 0.5 vvm. The effect of aeration rate on the digestion process was studied at 0.1, 0.25, 0.5 and 1.0 vvm at pH 7.0 and 55• C. The development of thermophiles (55 and 65• C populations) and hydrolytic enzyme activities (protease and xylanase) in the process were monitored. Thermophiles developed rapidly to reach peak populations in 24h or earlier, and remained stable at all fixed pH reactions. The thermophile population was only minimally affected by the aeration rate, and they were present mostly as spores after 96h of digestion. Xylanase enzyme appeared rapidly, reached a peak in approximately 60h, and declined rapidly thereafter. Highest activities were produced in neutral reactions and higher aeration rates. Aeration rates affected protease activity profoundly. The profile of both enzymes closely reflected the development of microbial activity and the overall progress of TAD, in a medium where their substrates were not predominant and not the preferred carbon sources. The relatively simple process of measuring the activities of these enzymes is potentially a more direct measure of the progress of TAD than enumeration of the microbial populations and thus, has greater potential in process monitoring.

Application of thermophilic aerobic digestion in protein enrichment of high strength agricultural waste slurry for animal feed supplementation

“…Other participating populations were identified as B. stearothermophilus with growth temperature optimum at 65 °C and maximum extending up to and above 70 °C. The population types and relations of the thermophiles did not appear to be significantly influenced by the pH or aeration rate employed in the digestion process, even though total population declined slightly at pH 9.5 relative to the lower pH values 4, 5. The population relation observed appears understandable considering that as spore formers, populations could sporulate (but remain counted) if the pH became less conducive to activity.…”

“…In this study, waste stabilization efficiency in terms of removal of TSS deteriorated as the pH moved either side of neutrality. This may be understandable, given the neutrophilic nature of thermophilic populations involved in TAD 4, 13…”

“…Thermophilic aerobic digestion (TAD) is a process that employs the heat generated during microbial aerobic metabolism to raise to thermophilic levels (≥45 °C) the temperature of waste undergoing treatment in an insulated system 1–5. It is a versatile but relatively new process, which is increasingly being accepted for waste treatment, either in standalone processes, or as part of integrated two‐stage, aerobic–thermophilic/anaerobic–mesophilic, or aerobic–thermophilic/anaerobic–thermophilic processes 6.…”

“…New studies are emerging that seek to achieve proper biochemical characterization of TAD to enhance its applicability to the different potential uses 4, 5, 21, 22, 32–36. However, to our knowledge, no empirical studies have been published on protein enrichment, including the pattern or extent of protein accretion in biomass during TAD, which should help in the assessment of the economics and potential applicability of the process for protein enrichment of agricultural wastes.…”

The effect of phosphate buffer on improving the performance of autothermal thermophilic aerobic digestion for sewage sludge

“…A few research works have also been done about the effect of pH on aerobic digestion, including aspects of nitrication, 20 biological phosphorus removal, 21 treatment efficiency, evolution of volatile fatty acids 22 and so on. Nonetheless, no effort has been made to illustrate the inuence of pH on ATAD process.…”