The dark side of virus removal by waste stabilisation ponds

Adequate sewage treatment is vital for maintaining New Zealand’s economy (aquaculture, tourism) and lifestyle (mahinga kai and recreation). Viruses are particularly problematic because they can be infectious at very small doses, shellfish concentrate them as they filter-feed, and they can survive for long times in the environment. Virus removal mechanisms in waste stabilisation ponds (WSPs) are likely to be complex. Sunlight is an important mechanism, but the focus of this study is what removal occurs in the absence of direct sunlight, at night and at depths in the pond where there is no sunlight penetration. Here, we report on the development of two mesocosms which allowed us to compare the removal rates of Escherichia coli with those of MS2 phage, the latter being commonly used as a virus model, with and without solar radiation (referred to as ‘light’ and ‘dark’ conditions). The second mesocosm configuration, which had efficient exchange of dissolved oxygen and pH between the light and dark sections, resulted in similar levels of removal of MS2 and E. coli and in both the light and dark sections. The level of removal was 1 and 2 log, respectively.