--> On a recent visit to the Caribbean we happened to ask our waiter, who, as it turned out, was the owner of the small outdoor restaurant where we sipped our coffee, what local hotels did with their sewage. We were in a developing country and the regulatory framework for enforcement of such things as wastewater management had not kept up with tourism development.
"Well," he said, pausing as if to consider how much to share with us, "the wealthier hotels may hire trucks to haul it away but that is very expensive. Most just put in a pipe." He pointed to the ocean.
It does not escape his notice, or ours, that the more tourists arrive, the more sewage will find its way onto the white sandy beaches where we can see surfers waxing their boards. That won't be good for tourism. The more surfers return home with intestinal infections or skin rashes, the less likely they will be putting raves up on Trip Advisor or WannaSurf. The lack of sewage enforcement is killing the tourism the local government desperately wants.
We have a permaculture solution take that sewage and pyrolyze it. For remote power, a good pyrolysis kit plus installation runs about 20 percent of the cost solar cells or wind turbines. In a place where the standard source of electricity is stand-alone or regional diesel electric generators, sewage-to-biochar serves two problems at the same time: surplus wastes and shortage of electricity.
Actually, it solves many more, because the biochar being produced can then cascade through a sequence of solutions as it moves through water filtration, silage conditioning, feed supplements that eliminate the genesis of disastrous, antibiotic-resistant-bacteria through overuse of pharmaceutical supplements for livestock, and finally on exiting the back end of the livestock – sweeten compost and rejuvenate soils.
But how permacultural is it to turn a nutrient rich asset like humanure into biochar instead of composting it directly? To answer that question we looked to a study published in the peer-reviewed journal, Biology and Fertility of Soils [Soil biochemical activities and the geometric mean of enzyme activities after application of sewage sludge and sewage sludge biochar to soil, Biol Fertil Soils (2012) 48:511-517; DOI 10.1007/s00374-011-0644-3].
The authors, J. Paz-Ferreiro, G. Gascó, B. Gutiérrez, and A. Méndez are in the ag and mining schools of Comunidad de Madrid-Universidad Politécnica de Madrid (UPM Madrid). They first wondered whether pyrolyzing the waste would eliminate the problem of heavy metals, which is one of the things that makes sewage problematic as a soil fertilizer. In 2005 Mendoz and Gascó showed that turning raw sewage into biochar indeed arrested the solubility of heavy metals. It also reduced the cost associated with transportation of sewage sludge.
Paz-Ferreiro brought the team new expertise in measuring key microbial reactions involved in soil nutrient cycling. Rather than focus on soil chemistry, the team wanted to know how sewage and biochared sewage contribute to soil fertility, and that meant understanding soil biology.Biochar from sewage and raw sewage sludges were added to soil at a rate of 4% and 8%. Not surprisingly, the researchers found that "the application of high doses of sewage sludge is harmful for the soil microorganisms resulting in a decrease in soil quality," but "sewage sludge biochar has the potential to improve several soil physical and chemical properties (organic matter content and available water content), while decreasing the solubilization of heavy metals from soil."
What is not answered is our original question: what is the comparison of composted sludge versus pyrolyzed sludge? It seems likely to us that while composted sludge would retain many useful nutrients that are volatized when sewage is pyrolyzed – and unless captured by elaborate gas-scrubbers – the benefit of those nutrients to the soil may be outweighed by the heavy metals that are toxic to the soil microbiota and have the potential to move up the food chain to become toxic to us, as well. Moreover, on a large scale such as municipal treatment, composting has to be a major undertaking, involving energy imports for transportation and large processing facilities. If the same end result of soil fertility enhancement can be qualitatively achieved without composting, the pyrolytic loss of some nitrogen and other elements might be forgiven.
Paz-Ferreiro's group showed the potential to use pyrolysis to transform sewage sludge into a material that can enhance soil biochemical activities without simultaneously spreading dangerous pollutants. That is yet another biochar solution. For hotels and restaurants going into places that have neither sewage plants nor power grids, it is potentially a huge blessing.