Thursday, June 13, 2013
Combined sewers receive both water from storm drains and sewage. During heavy rains, the combined sewers can overflow and dump untreated sewage and stormwater into local waterways. Because sewage (treated and raw) is the primary source of PPCPs in the environment, a team of researchers led by Hector Del Rio developed an extensive monitoring plan for seven PPCPs in the combined sewer systems of Santiago de Compostela.
Water samples were taken every three hours over six-day periods of dry weather in the summer and fall. Wet weather samples were collected using an automatic sampling device that was activated by rising stream water. These methods of water collection allowed for PPCP concentrations of fragrances, anti-inflammatory drugs, antiepileptic drugs, an analgesic, and caffeine to be measured over a long period of time and gave researchers a good idea of how these chemicals move through sewer systems.
The quantity of water samples yielded some surprising results. Unlike most pollutants, PPCP levels increased with an increase in water in streams. And, interestingly enough, PPCPs that are most effectively removed by wastewater treatment appeared in the highest levels in combined sewer overflows. The researchers believe that these PPCPs are strongly attracted to sewer system sediments, to which they adhere during dry weather. When a big influx of stormwater flushes out the sewer systems, these PPCP-laden sediments are washed out of the sewer, downstream, and into the automatic sampler.
If, however, there is not enough stormwater to trigger combined sewer overflows and the wastewater makes it to the sewage treatment plant, the PPCPs measured in the study stream are removed by treatment. This explains why stormwater dilution doesn't decrease PPCP levels, and may also indicate that fixing sewer systems to avoid CSOs might also decrease PPCPs in the natural environment.
Written by Corrie Layfield.
Article available at:
Del Rio, H. J. Suarez, J. Puertas, and P. Ures. PPCPs wet weather mobilization in a combined sewer system in NW Spain. Science of the Total Environment 2013; 449:189-198. http://www.ncbi.nlm.nih.gov/pubmed/23425796