By Sally Brown, University of Washington

Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members. 

  1. Biochar from Biosolids Pyrolysis: A Review 
  2. Physicochemical Properties of Biochars Produced from Biosolids in Victoria, Australia 
  3. Pyrolysis methods impact biosolids-derived biochar composition, maize growth and nutrition 
  4. Direct and residual effect of biochar derived from biosolids on soil phosphorus pools: A four-year field assessment 
  5. Impacts and interactions of biochar and biosolids on agricultural soil T microbial communities during dry and wet-dry cycles

Happy spring to all, hopefully a shot in the arm both literally and figuratively.  A year ago the library was all about COVID 19.  This year we are back to a more mundane and annoying topic; pyrolized biosolids or biosolids based biochar.  I was asked to review an analysis of this treatment option recently and declined but the request did show that the interest in this option is still out there.  I’ve also been told by a highly reliable source (Andrew Carpenter) that there is a biosolids pyrolysis plant that is in operation in California.  So however much I want this to go away, my wishes have not yet been granted.  Proponents for this stabilization technology argue that the finished material can be used as a soil amendment.  I checked to see if there was any literature on the topic.  There is.  But not that much.  Here is what I found.

The first paper in the library has Jorge Paz-Ferreiro as a first author.  He is from Australia and is faculty in the school of engineering.  He is a co- author of a number of papers on this topic.  This one is a general review.  You can skip much of the first part- a general review of the basics of biosolids with a focus on hazards.  The focus goes to pyrolysis starting with section 2.3.  You can (in my opinion) see that he comes to this with a clear bias- stating early on that pyrolysis is a lower carbon footprint option.  Obviously, he hasn’t looked at our BEAM model.  However, the review provides a good summary of the process, factors that can impact the output and characteristics of the final product.  Some key points- higher burn temperatures increase P and K content of the material (good) but decrease N (bad).  Higher temperatures also decrease salts (good) but increase heavy metals (bad).  There is also a table of growth studies.  Here a basic take-home is that the biosolids char doesn’t work as well as the biosolids for plant growth.  Reduced total and available N is generally to blame.  Phosphorus in biosolids chars seems to be plant available with total concentration not reduced by the cooking process.  There is also a discussion of the impact of pyrolysis on organic contaminants in biosolids with studies showing removal of antimicrobials and some PAHs.  This implies that this process might alter PFAS but that implication is in my brain rather than in the paper.  The authors point out that some of the heavy metals that have historically been a concern (mercury and arsenic) volatilize during the process- becoming the atmosphere’s problem.  While others concentrate, metals in the biochar matrix typically have low availability.   He talks about using this material as a component of compost mixtures to reduce N loss.  Also mentions a study that tested biosolids char as a replacement for a portion of the peat used in horticultural applications.  The world is the oyster here. 

Second paper includes the same author but instead of a review of the literature he/ they present original data.  They took biosolids from a number of plants near Melbourne, Australia and analyzed them as is and post pyrolysis at 500 C and 700 C.   The paper has a few tables of data that show you what happens.  Here are the results in an easy to read form: 


For the third paper we will leave Jorge Paz-Ferreiro for some other authors.  Most authors of paper #3 come from Brazil with some help from Nick Comerford from the University of Florida.  Nick is a former president of the Soils Society, so I am hoping this work is well done.  The authors here tested two types of methods to make char and the effect of the two materials on corn germination and growth.  In both cases, biosolids were used as the feedstock.  The first batch was made in a kiln- to be representative of small- scale operations with the second batch made in a muffle furnace, as would be done on a treatment plant scale.  The authors then tested each product for seed germination and plant response.  Application rates for each material were 0,5,10, 20 and 60 Mg ha.  All treatments got supplemental fertilizer.  You read that right- with biosolids biochar, they added extra fertilizer.  The bottom line on the growth trial here was that they were all the same, with one exception.  The small scale biosolids biochar added at the highest rate grew smaller corn than all of the other treatments.  The large- scale char did the same as the control across all application rates.  In other words, making char from biosolids takes away the biosolids magic that farmers have come to appreciate. 

The 4th paper marks the 3rd and final appearance of Paz- Ferreiro with a continuation of the Brazilian influence.  Here the authors focus on P availability in tropical soils amended with biosolids biochar.  Tropical soils have very high concentrations of iron- that gives them the red color.  The same way that adding Fe to wastewater treatment removes P from solution, having so much Fe in the soil makes it really good at binding P.  Before the study here even started, they added P and K to the whole field twice.  This is a very different scenario from fields on less weathered soils where excess P is a concern.  The two biochars they used were cooked at 300 and 500 C.  At 300 C there was no loss of N with only slight loss observed at 500 C.  So a much better source of fertility than in the 2nd study in the library.  They found big increases in soil P as a result of the char application in comparison to the fertilizer.  The corn wasn’t so sure. 


Across all 4 growing seasons, corn yields were as good as the fertilizer for the 300 C char and almost as good for the 500 C char.   Better than the first corn study for sure.  Gold medal quality, not quite but in this case, a good option. 

The final paper in the library compares the impact of biosolids and char on soil microbial communities.  The char is from walnut shells and the biosolids that were tested was the tried and true commercial material from Wisconsin- Milorganite.  This work was done at UC Davis and has Kate Scow as the last author.  You can trust this paper.  This is a little different from the other papers as the char isn’t from biosolids.  However, many have touted the benefits of char for the soil microbial community.  With increasing focus on soil health, it seemed good to add this in.  Here the authors looked at the impact that biosolids alone and biosolids + char had on the soil microbial community as it was subjected to different wet and dry cycles to mimic drought stress.  They used a PFLA (phospholipid – derived fatty acid analysis) to characterize the response to different groups of bugs.

They found that the char alone only increased microbial biomass during one measurement interval.  Biosolids increased microbial biomass across all moisture regimes by 55% over the 12 weeks of the study.  The biosolids also helped to mitigate the impact of the wet/ dry cycles.  They talk about biochar as a good combination with biosolids but not so much as a tool on its own. 

The take home here- biosolids based biochar seems to fall into the doesn’t do much harm category.   Not clear that it does much good though.  Go this way if you have money to burn but for me- I still like the cake.