Engineering Angle: Are We Paying Enough Attention To Removing Carbon?

Much attention is being given today to the removal of nutrients from wastewaters because of the negative environmental consequences that can result if these are released in excess.  Without a doubt, nutrient reduction is an essential component of our national policy on environmental protection.  But how much attention are we giving to the removal of organic carbon, since its presence in domestic wastewater can be an indicator of the potential for waterborne pathogens.  As population expands, the load of organic carbon released to the environment also expands.  The degree to which this loading is controlled is reflected in the quality of public health and the environment.

Throughout the course of human history, the importance of safe drinking water, particularly in urban areas has been well understood.  This has not been the case, however, with sanitation.  In fact, it was not until the early 19^th Century when it was recognized that public health could be improved by addressing urban wastewaters when developed societies adopted the concept of the “solution to pollution is dilution.”  But it was the 20^th Century, with the development of the biological oxygen demand (BOD) test that transformed wastewater management; the correlation between the “organic strength of a wastewater” and its effects on human health and environment could finally be measured.  From this invaluable data, technologies for purifying wastewaters developed quickly.  By mid-century the number of publicly owned wastewater treatment facilities in the U.S. grew to nearly 12,000, but only 30 percent had the equivalent of secondary (biological) treatment.  By the turn of 21^st Century, that number had increased by 35 percent, with more than 99 percent having at least secondary treatment, and more than 30 percent of those employing tertiary treatment (which included nitrification, biological nutrient removal, and/or some form of filtration).  The result:  BOD_u[1] loadings discharged to the nation’s water bodies declined by nearly 50 percent and both environmental and public health improvements were evident.

But the story doesn’t end there.  About 10 years ago, the U.S. EPA published a critical review of our nation’s treatment performance using BOD loading to the environment as the primary metric.  Though discharged loadings have indeed been in decline, since the mid-1980s, efficiency of treatment technology for removing BOD prior to discharge has been “leveling off.”  As our population grows, more organic load is being produced.  But without concomitant increases in BOD removal efficiency by treatment works, the consequence is a net increase in the total BOD load being discharged to the environment.  When projecting trends of population growth (along with its corresponding BOD load), coupled with trends of “lagging” BOD removal efficiencies, the EPA study found that the mass of BOD loading discharged to the nation’s receiving water bodies in 2025 could equate that which was being discharged in 1972, the year the Clean Water Act came into law.  The implication is profound.  Without an elevation in our attention to BOD removal, we may well again experience similar environmental consequences that we struggled to address 40 years ago. 

How can we do this?  First, BOD removal efficiencies must be increased and at a faster rate.  Facilities employing advanced treatment schemes (beyond tertiary) now are reaching removal efficiencies greater than 98 percent (see table).  We need many more plants commissioning these technologies and the rate at which these are being installed must also increase.  Secondly, BOD loads must continue to be reduced at their points of origin.  While great strides in reducing BOD loadings from industrial and municipal point sources have been made, progress remains slow in reducing the same from rural and urban non-point sources.   Thirdly, though progress has been significant in the past 10 years, much remains to be done in reducing combined and sanitary sewer overflows.

It is clear that technologies for purifying urban wastewaters have made phenomenal strides in the last 50 years.  While today’s focus is primarily on removing nutrients and contaminants of emerging concern are necessary and vital, we must not lose sight of the role organic carbon plays in the overall fundamental objective of protecting public health and the environment.