To Chris Benjamin,
I've read your Sustainable City columns, "Bio-not-so-solids" (May 8) and "Poop scoop," (April 24).
There are three ways to dispose of sewage sludge (if you don't include ocean disposal): incineration, landfilling and application as a soil amendment and fertilizer. Each option has pros and cons. The option society does not have is to do nothing. If you want a clean harbour, then you must deal with solids.
Incineration of biosolids is the most costly option environmentally and economically. Incinerators are complex and expensive to build and run. An analysis from the University of California at Berkeley showed total environmental costs---including greenhouse gas emissions and other air pollutants---of incineration are greater than other options.
Landfilling is also costly in terms of environmental impacts. We did an analysis comparing the greenhouse gas emissions from composting sewage sludge vs landfilling it. And we found---as others have---that while the fossil fuel and electricity consumed by an active, enclosed composting operation is greater than that consumed by trucking sewage sludge to a close-by landfill, the total greenhouse gas emissions are likely to be far greater from landfilling---no matter how good the landfill is. This is where Maureen Reilly is wrong: Untreated sewage sludge will quickly produce methane; when landfilling, this methane release will occur before the landfill can capture the gas. Even after a landfill is capped and methane is being captured, there is still leakage. Methane is 23 times stronger as a greenhouse gas than CO2.
While use of treated sewage sludge (biosolids) involves the concerns Maureen Reilly raises, decades of research show that, with proper treatment and management, use of biosolids on soils presents negligible risks. Biosolids contain mostly organic molecules from food and human waste. Significantly less than five percent of biosolids are pathogens, heavy metals and trace chemicals. There are fewer pathogens in treated biosolids than there are in typical animal manures commonly used on farms. Heavy metals are tested for and must be below strict standards; animal manures also have heavy metals in them, as do natural soils. And organic chemicals are sequestered and/or broken down in the biologically-rich environment of biosolids and soil. Properly managed biosolids in soils present little risk.
Biosolids are rich in carbon---recent photosynthetic carbon---and applying these to soils builds the soil. Some carbon is held for years. Some estimate eight percent of carbon applied to soils with biosolids compost is sequestered in the soil for 100 years. This carbon does not contribute to CO2 in the atmosphere and global warming. Also, by using biosolids, farmers can reduce use of chemical fertilizers. Chemical nitrogen fertilizer production requires much energy and natural gas---and uses energy for transporting the fertilizer. And then there are the basic soil- and crop-enhancing benefits of biosolids: they improve soils and increase crop yields. And they are usually less expensive than chemical fertilizers.
Biosolids application is part of the cycle of nutrients: from soils to cities and people, and back to the soil. For sustainability, we have to find ways to make this work. A wastewater and biosolids management program works to protect the biological processes at the treatment plant from any significant toxins in the waste stream, and works with the public and businesses to educate all about what should and should not go into sewers.
Managing biosolids through application to soils is often the best option. I think it's the best option for HRM. I encourage interested people to learn more, take a tour of the facilities, and see how biosolids are created and managed at nebiosolids.org.
By Ned Beecher