Effect of anaerobiosis on indigenous microorganisms in blackwater with fish offal as co-substrate

• Complete elimination of faecal streptococci in two out of three anaerobic samples.
• Constant reduction of coliphages in the anaerobic samples.
• Escherichia coli was strongly and equally reduced in both anaerobic and aerobic samples.
• Strongest reduction of tet-resistant bacteria in sample with highest CH4-production.
• E. coli and faecal streptococci in some of the samples resuscitated during recovery.

The aim of this study was to compare the effect of mesophilic anaerobic digestion with aerobic storage on the survival of selected indigenous microorganisms and microbial groups in blackwater, including the effect of addition of Greenlandic Halibut and shrimp offal. The methane yield of the different substrate mixtures was determined in batch experiments to study possible correlation between methanogenic activity in the anaerobic digesters and reduction of indigenous microorganisms in the blackwater. By the end of the experiments a recovery study was conducted to determine possible injury of the microorganisms. In both anaerobic and aerobic samples, survival of Escherichia coli was better in the presence of Greenlandic Halibut offal when compared to samples containing blackwater only and blackwater and shrimp offal, possibly due to more available carbon in the samples containing Greenlandic Halibut offal. Reduction of faecal streptococci was large under both anaerobic and aerobic conditions, and the results indicated a complete removal of faecal streptococci in the anaerobic samples containing blackwater and a mixture of blackwater and shrimp offal after 17 and 31 days, respectively. Amoxicillin resistant bacteria were reduced in the anaerobic samples in the beginning of the study but increased towards the end of it. The opposite pattern was observed in the aerobic samples, with a growth in the beginning followed by a reduction. During the anaerobic digestion tetracycline resistant bacteria showed the least reduction in the mixture of blackwater and shrimp offal, which had the lowest methane yield while the highest reduction was observed in the mixture of blackwater and Greenlandic Halibut, where the highest methane yield was measured Reduction of coliphages was larger under anaerobic conditions. Addition of fish offal had no effect on survival of coliphages. The results of the recovery study indicated that a fraction of the E. coli in the aerobic blackwater sample and of the faecal streptococci in both the anaerobic and aerobic samples containing blackwater and Greenlandic Halibut were injured only, and thus able to resuscitate during recovery.The use of anaerobic digestion in the Arctic is limited to substrate types like those tested in this study because of absence of agriculture. The results indicate that anaerobic digestion of wastewater could benefit from the addition of fish offal, with respect to both microbial reduction and energy production.

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