Over the past few years, several wastewater utilities have experienced elevated pollutant concentrations in their biosolids resulting from the release of legacy pollutants during sewer system cleaning projects.   In some cases, the elevated concentrations adversely affected the management of the biosolids end-product.

As clean water agencies continue their work to rehab and clean their existing collection and interceptor systems, this scenario may become more common.  This Regulatory Alert is intended to ensure the NACWA membership is aware of this potential problem and to provide insight on preventative measures that some agencies are taking.  For more information, contact Chris Hornback, NACWA at 202/833-9106 or This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

NACWA Members Cautioned to Evaluate Cleaning Project Impacts on Biosolids

NACWA is aware of two specific instances where sewer cleaning operations have been linked to elevated biosolids pollutant concentrations – one involving mercury and the other polychlorinated biphenyls (PCBs).  Pollutants such as mercury are especially prone to accumulation in grit and solids in the collection system and may be re-suspended during cleaning operations.  While PCB production in the United States ended in the 1970s, their widespread use prior to the ban often resulted in the settling of solids with elevated PCB concentrations in the older portions of many collection systems.  The two brief case studies below are only intended to raise awareness of the potential issue for the NACWA membership.

Mercury
The agency that experienced elevated levels of mercury in its biosolids does not land apply, distribute or market its biosolids.  The agency conducted a series of monitoring efforts and determined that mercury levels in interceptor samples collected downstream of cleaning operations were often one to two orders of magnitude higher than upstream samples.  Influent mercury concentrations at the treatment plant were elevated, but a corresponding increase in the effluent was not evident.  This was most likely due to the fact that the mercury was predominantly particulate mercury that settled out during the treatment process.  A monitoring program is now in place to track potential impacts of cleaning programs on biosolids mercury concentrations.

PCBs
The agency that experienced elevated PCB concentrations in biosolids as a result of sewer cleaning projects manages biosolids through land application and distribution/marketing practices.  Significant expense was incurred by this agency in addressing the impacts associated with elevated biosolids concentrations.  This agency believes that due diligence requires a reasonable search and inquiry (by the owner of the sewer) to identify and quantify persistent pollutants, such as PCBs, in the solids of the pipes and manholes before commencing action that disturbs the solids. The agency has now voluntarily developed its own requirements for its crews and collection system owners discharging to the system to analyze representative samples of bottom sludges for reservoir sources of PCBs before commencing sewer cleaning.  Those parts of the sewer system constructed after PCB production ended or serving largely residential areas would not be suspected to contain PCB contamination and are not part of the sampling program.

If you or your agency has additional information that may provide added insight for the NACWA membership on this issue, or if you would like additional information on either of these case studies, please contact Chris Hornback, NACWA at 202/833-9106 or chornback@nacwa.org.