Poop (i.e. human waste) contains millions of bacteria, viruses, and other disease-causing organisms called pathogens. Quite conveniently, in today’s modern world, our poop is easily flushed down the toilets. But the story does not end there. Pathogen-laden faecal waste (alongside wastewater from homes, businesses, schools, industrial facilities etc.) finds its way through the wastewater network into wastewater treatment plants where it is subjected to some form of treatment. The treated wastewater is thereafter discharged into receiving waterways where it is used again for other purposes, such as for recreation, irrigation and sustenance of aquatic life.
Reflecting on the Quantitative Microbial Risk Assessment (QMRA) projects that I have completed in recent years, the question that resonates loudly with me relates to the efficacy of the wastewater treatment plant (WWTP) process. What pathogen log removals are WWTPs required to achieve before discharging treated wastewater into receiving waters to ensure that there are no observable adverse effects in terms of attributable public health risk (i.e. additional health risk caused by the discharge at identified recreational swimming and food gathering areas).
A preliminary answer is: it depends. Yes, it depends on a whole lot of other determinants:
the proximity of the receiving environment site to the discharge outlet;
exposure pathways that allow the pathogen in the treated wastewater plume to reach people and cause infection (through the air, through ingesting polluted water, consuming shellfish, etc.);
range of pathogen concentrations in treated wastewater;
the discharge volume of treated wastewater;
environmental fate of microbial contaminants in the freshwater or marine receiving environment, considering the effects of dilution;
how much water a child or adult will ingest over a period of time during a particular recreational activity;
and the amount, frequency, length of time of exposure, and dose-response relationships for specific pathogens in the treated wastewater.
Quite complicated isn’t it?
While this myriad of factors play a huge role in determining whether a treated wastewater discharge will increase illness risks beyond the no observable adverse effect level, one crucial issue is that a high level of treatment requirement must be met. “Log”, short for logarithm, is a power to which a base (e.g. 10) can be raised to produce a given number. Let's do the math! Let's assume a concentration of 1,000,000 cells per litre of raw wastewater. In terms of reduction of pathogens, a reduction of 1 log (90%) reduces pathogens in wastewater from 1,000,000 cells/L to 100,000, 2 log (99%) reduces 1,000,000 to 10,000, and 5 log reducing 1,000,000 down to 10 cells/L.
Now, let’s assume that 4-log reductions (i.e. 99.99%) in pathogen levels is achieved by a WWTP. In quite simple terms, a 4-log pathogen reduction would still retain 100 cells/L in the treated wastewater from the initial 1,000,000 present in the raw wastewater. Having 100 cells/L in the treated wastewater can be both good and bad news. If a three-dimensional hydrodynamic modelling reveals that there is enough dilution in the receiving environment, there may be no cause for alarm. If otherwise, depending on the dose-response relationships for specific pathogens in the treated wastewater (and other factors already stated above), there may be some attributable health risk as a result of the discharge.
Now back to the question that led us here. What pathogen log removals are WWTPs required to achieve before disposing treated wastewater into receiving waters to ensure that there are no observable adverse effects in terms of attributable public health risk? The absolute answer is: only a QMRA can tell. Using carefully calibrated mathematical models that incorporates information on the above-listed determinants, QMRAs unravel these intricacies in a way that informs regulatory limit setting geared towards improved public health outcomes. QMRAs do not only provide an assessment of the health risks associated with wastewater discharge, they also provide an indication of the wastewater pathogen log removals required to alleviate those risks. To cut a long story short, a QMRA provides a crucial feedback loop to the WWTP treatment requirement.