Biosolids and Biochar: A Slow Burn
Biochar is the end product of pyrolysis or gasification. Both are terms for a combustion process where organics are kept at high pressure and temperature in a low oxygen environment. This results in the production of three products: a high carbon gas, a liquid carbon and biochar (which is similar in many ways to wood ash or charcoal). The relative abundance of each of those products depends on the burn temperature, length of the combustion process and the pressure maintained during combustion. Pyrolysis is an old technology that has come back into vogue for a number of reasons. From a soils perspective, biochar as a soil amendment is all the rage as a way to sequester carbon over the long-term. From an energy perspective, pyrolysis is viewed as an alternative source of green energy with both the gas and the liquid having value as an energy source. Pyrolysis or gasification is also frequently talked about in the residuals community as a green alternative to combustion. To date, there is one WWTP in Florida that is using this process, with more either in discussion or on the drawing board. A few operational plants pyrolize manures. According to Craig Frear from WSU these are having their share of operational and economic issues. For this library, we are going to concentrate not on the efficiency or feasibility of the process itself, but on the residuals from the process. And within that, we are going to focus on nitrogen. One of the reasons that I have had major concerns about pyrolysis as a treatment method for biosolids is that I thought that heating the biosolids would result in the loss of most of the N in the biosolids and so decrease their value. For those looking to bone up on N in biosolids let me direct you to a NBMA publication on the topic, “Managing Nitrogen in Biosolids,”which, although not in this library, is available through the WA DOE website: www.ecy.wa.gov/pubs/99508.pdf . We talked about biochar once before in the July 2009 blurb. Since that time there has been a lot more research on these materials. The first article in the library is an introduction to char in soils and also to a special section of the Journal of Environmental Quality on this topic. For this library, you only get the overview. The article details results to date on biochar in soils, noting that outcomes are often contradictory. Biochar may increase the soils’ capacity to bind NH3. This will depend on the burn temperature of the biochar, as material cooked at higher temperatures will lose the ability to supply acidic binding sites. The authors note that the question still remains of whether this bound ammonia is plant available. Char may suppress N2O release, potentially by improving soil properties or maybe by increasing soil pH. However, one study showed higher N2O emissions from poultry char amended soils than the control. There are some indications that char will increase the rate that ammonia is transformed into nitrate, but this may depend on age of the material and whether some microbially toxic compounds have been formed during the combustion process. In other words, if you read this article critically, you will see that there is much more uncertainty on the interactions between char and N than there is clear research. The second article does not qualify as an easy read but it has the most important information on N in biochar. The authors cooked up some manure and some biosolids at different temperatures and then characterized the N in the final products. Table 1 here shows the %Yield of the char at the different pyrolysis temperatures. The hotter the oven, the lower the yield. While the total C and N concentrations (ppm) stay relatively consistent, at the hottest temperature (550 C), you have lost about 50% of the volume of both. Measurements of extractable ammonia and nitrate in the biosolids/sand or biochar/systems reveals that available N in the biochar is reduced by at least 66% over the biosolids. The next two papers deal with plant response to biosolids based biochar addition, first with rice and then with tomatoes. Both papers talk about metal availability in biochars. Both papers also note a significant growth response to the plants grown with biochars. Neither paper compared growth to biosolids amended soils. Both papers also added fertilizer with the biochar… Notable though with the rice paper is that they observed decreased N2O emissions in the biochar amended rice paddy. And that leads to the final paper. Here authors have noted that biochar added to soils reduced nitrate leaching from soils. These last two observations may actually provide a useful tool for biosolids application. Biosolids is great because it is such a good slow release source of plant nitrogen. The bad part about biosolids is that excess N can leach over time and, in some cases, volatilize as N2O. It seems clear from these papers that using biosolids as a feedstock for pyrolysis results in the loss of valuable N. However, perhaps if we used almond shells for the biochar production, and used the char in combination with the biosolids, we could reduce the potential for nitrate movement and N2O emissions while still keeping the fertilizer value of the biosolids.
Sally Brown September 2013