Acidification of non-medicated and oxytetracycline-medicated cattle manures during anaerobic digestion

Abstract

Possible adverse effects of a commonly used veterinary antibiotic, oxytetracycline (OTC), on acidogenic phase of anaerobic digestion of cattle manure along with optimum operating conditions were investigated. A standard veterinary practice of 50 ml OTC solution (20 mg/kg cattle weight) was injected into the muscles of cattle and then manure samples were collected for 5 days following the injection. The 5-day samples were equally mixed and used throughout digestion experiments. Preliminary batch tests were conducted to obtain the optimum pH range and observe volatile fatty acids (VFAs) production. In this regard, different sets of batch digesters were operated at pH ranging from 5.2 +/- 0.1 to 5.8 +/- 0.1 at mesophilic conditions with total solids content of 6.0 +/- 0.2%. The pH of 5.5 +/- 0.1 was found to be the optimum value for acidification for both non-medicated and OTC-medicated conditions. Under predetermined conditions, maximum total VFA (VFAtot) of 830 +/- 3 mg (as acetic acid)/L was produced and maximum acidification rate was evaluated as 11% for OTC-medicated cattle manure, whereas they were 900 +/- 6 mg (as acetic acid)/L and 12% for non-medicated manure. Digestion studies were further continued in a semi-continuous mode at pH 5.5 +/- 0.1 and SRT/HRT of 5 days. VFAtot concentrations and maximum acidification rate increased up to 2181 +/- 19 mg (as acetic acid)/L and 29% for non-medicated cattle manure. For OTC-medicated cattle manure, lower acidification rate of 18% was observed.

The fate of oxytetracycline in two‐phase and single‐phase anaerobic cattle manure digesters and its effects on microbial communities

“…After this start‐up period, enzyme‐added digesters were operated semi‐continuously for another 60 days with SRT/HRT of 5, 15, and 20 days for acidogenic, methanogenic and single‐phase digesters, respectively. Comparatively higher SRT/HRT for acidogenic phase than stated in the literature was chosen based on the findings in our previous studies, in which hydrolysis and acidogenesis seemed to be the rate limiting steps due to cattle manure's hardly‐biodegradable content . In this regard, poor acidification rate of cattle manure was enhanced via hydrolytic enzymes and subsequently higher acidification was observed between 4th and 5th days following the enyzme addition…”

“…Comparatively higher SRT/HRT for acidogenic phase than stated in the literature was chosen based on the findings in our previous studies, in which hydrolysis and acidogenesis seemed to be the rate limiting steps due to cattle manure's hardly-biodegradable content. 12 In this regard, poor acidification rate of cattle manure was enhanced via hydrolytic enzymes and subsequently higher acidification was observed between 4th and 5th days following the enyzme addition. 32 wileyonlinelibrary.com/jctb Effluent characteristics of the digesters are summarized in Table 3.…”

“…TS content of the digesters was maintained at 7 ± 0.2%, in which 80-85% of that was VS. pH of acidogenic digesters were adjusted to 5.5 ± 0.1 since the highest VFA concentrations both in the absence and presence of OTC were observed at this value in our previous study. 12 Methanogenic and single-phase digesters were operated at pH 7.2 ± 0.1. pH increased to 6 in acidogenic digesters; however, it remained at neutral range in methanogenic and single-phase digesters. Due to high buffering capacity of cattle manure, alkalinity concentrations were sufficient and there was no need to add any extra buffering materials.…”

“…The presence of such antibiotics and/or their metabolites in manure can adversely affect the microbial communities in anaerobic digestion systems, since anaerobic digestion is a reliable technology to treat animal manure . Fate and inhibitory effects of OTC in anaerobic digestion processes have been commonly studied and despite some adverse results, negative effects of OTC on biogas and methane yields in digestion systems have been reported mostly without causing complete systems failures. These studies highlighted that the degree of inhibition is highly dependent on environmental and operational parameters.…”

Antibiotic Inhibition on Anaerobic Digestion of Animal Manure and Controlling Strategies: A Short Review

“…Separation of the acidogenesis and methanogenesis phases could contribute to a more robust process performance by increasing the removal rates of antibiotics, thereby reducing their adverse impacts on the AD system . Fermentation at weakly acidic environment (pH 5.5) was reported to greatly accelerate the acidification of oxytetracycline‐medicated cattle manure, contributing to higher VFAs yield . By studying the structure of microbial community in different reactors, Akyol et al suggested that the bacterial diversity in acidogenic reactor and the archaeal diversity in methanogenic reactor was higher than those in single‐stage digesters during the AD period.…”