Inactivation rates in pathogens: lab vs in situ

Managed aquifer recharge (MAR) schemes involve water being recycled and stored underground in an aquifer for reuse and relieve water shortages. Microbial pathogens can be present and survive in the recharged water, which subsequently cause health hazards. The potential survival times of enteric pathogens can be assessed to alter the MAR scheme or determine the need for additional treatment. In this study the authors have evaluated the effectiveness of using in situ diffusion chambers for assessment of health risks associated with MAR, this involved comparing inactivation rates in both laboratory microcosms and in situ diffusion chambers.

Groundwater samples seeded with pathogens and indicators were tested in laboratory microcosms (50ml polypropylene centrifuge tubes) and in situ Teflon diffusion chambers. The chambers contained membranes with pore sizes 0.010µm and 0.025 µm and were suspended in a well for 50 days with the upper chambers 1 m below the water table to intercept flow of groundwater. The pH, temperature, redox potential, dissolved oxygen (DO) and EC were measured in situ while dissolved organic carbon (DOC) was analysed by a commercial laboratory. These parameters remained almost constant during the in situ study.

To tackle the issue of potential clogging in groundwater mentioned in previous studies, the chambers were recovered and Rhodamine WT (RWT) a stable fluorescent dye was used to measure diffusion of water across the constructed diffusion chambers and to determine whether clogging of membranes has reduced the flow of water across the chambers. No biological growth or biological clogging was observed on the membranes, so investigating the potential effect of clogging on water flow requires further studies that involve greater turbidity and nutrient concentrations.

All microorganisms tested in this study were observed to decay both in laboratory microcosms and in diffusion chambers. The bacteriophage MS2 was found to decay at much faster rate than adenovirus, which suggests that MS2 is not a suitable indicator for enteric virus inactivation in groundwater. The results are simplified in figure 3, showing that the inactivation times of seeded bacteria were lower when 0.010µm membranes were used compared to 0.025 µm membranes. The higher inactivation time found in the smaller pore size (0.010µm) suggests that reduced rate of water flow across the membranes does influence the rate of bacterial inactivation. The laboratory microcosm inactivation rates were similar to the 0.025 µm membranes, yet significantly different compared to the 0.010µm membranes.

Previous studies have used laboratory microcosms and in situ diffusion chambers however in situ studies are preferred for accurate assessment of pathogen inactivation. Reasons for this preference seem to be laboratory microcosms can cause an underestimation of inactivation rates. I agree with this preference because an in situ model is likely to simulate the amount of time the recharged water is contained  within the aquifer, as well as the conditions it is subjected to. The authors provided several possible factors (for example, low water flow or water temperature) that could have influenced the inactivation rates of the pathogens, but they were unable to explain several  mechanisms behind some of the variations in inactivation rates which suggest that further research into the factors associated with diffusion chambers is needed. 

Sidhu, J.P.S. and Toze, S. (2012), Assessment of pathogen survival potential during managed aquifer recharge with diffusion chambers. Journal of Applied Microbiology, Volume 113, pages 693–700.