Fixed-film anaerobic digestion: Mechanisms of pathogen reduction and impacts on virus adsorption to soil
Anaerobic digesters have been used for treatment of various wastewaters. Systems are usually operated at mesophilic (35°C) temperatures and long hydraulic retention times (HRTs) (> 20 days). Pathogen reduction during anaerobic digestion is mainly correlated with operating temperature and HRT. Higher temperatures and longer HRTs result in increased rates of pathogen decimation. Previous studies show that a novel fixed-film anaerobic digester, operated at a low HRT (< 3 days) and ambient temperature (< 28°C) treating flushed dairy manure wastewater, achieved significant reductions of indicator and pathogenic bacteria. Thus, we sought to determine factors that contributed to indicator and pathogenic bacteria reduction during operation at a low HRT and ambient temperature. The presence of indigenous microflora was found to reduce the proliferation of indicator and pathogenic bacteria. Bacteriophages were also inactivated by the presence of indigenous microflora. Also, decimation of Staphylococcus aureus was attributed to starvation, resulting from anaerobic digestion, while not suppressing the other test organisms to the same degree. Furthermore, attachment to the retained biofilm (i.e., fixed-film) contributes to reduction by removing indicator and pathogenic bacteria from the liquid phase during anaerobic digestion. Next, our study examined the impact of anaerobic digestion on virus adsorption to soil following land application of treated wastewater. Anaerobic digestion increased retention of viruses to the soil and decreased mobility of attached viruses through the soil matrix, as compared with untreated wastewater. Anaerobic digestion removed compound(s) that interfered with virus adsorption to soil. Initial characterization showed that these compound(s) were less than 100 kDa in size. These compound(s) interfered with hydrophobic and electrostatic interactions used by the viruses for adsorption to soil. Futhermore, compound(s) less than 10 kDa in size caused inactivation of MS2. Our results show the critical role of indigenous microflora and the retained biofilm during anaerobic digestion in a fixed-film system. The indigenous microflora and attachment to the biofilm causes decimation of indicator and pathogenic organisms from flushed dairy manure wastewater, thereby reducing the environmental load of these organisms during land application. Anaerobic digestion increases the retention of residual viruses within the soil matrix, therefore reducing the likelihood of contaminating groundwater.