Creating biosolids blends using Class A materials from San Francisco


Sally Brown, & Ryan Batjiaka,

More and more communities are considering the potential to develop biosolids based soil products that can look, feel and act like compost but don’t have to go through the time, temperature and permitting process that are typically required.  Composting is perhaps the classic treatment for converting Class B cake to a Class A product that is both legally and physically suitable for general use.  While tried and true, composting requires a significant time commitment for both the active and curing phase.  The composting process is regulated to assure pathogen destruction.  Composting sites require permits.  When the biosolids have achieved Class A during treatment, it is possible to skip the composting step by identifying the appropriate blending ingredients.  These mixes may require a limited curing time but may also be ready to go right off the Royer (a machine that works well for mixing and screening soil blends).  Tagro and the line of Tagro soil products is the gold standard for this approach.  

The City of San Francisco is one example of a community trying to go this route.  A new treatment system will bring their biosolids to Class A.  For his MS thesis, Ryan tested the potential for different materials +/- sand to mix with the biosolids to create a marketable product.  As reported earlier here, mixes were made and left to ‘cure’ for 30 days.  He measured a range of parameters on the mixes immediately after they were made and at the end of the 30 days.  These included measures of marketability (odor and appearance), chemical measures (pH, carbon to nitrogen ratio, electrical conductivity (EC), and nitrate and ammonia content), and biological measures (CO2 evolution to test the reactivity/stability of the mixtures).  Mixes were used in a seed germination trial and a growth study with petunias as a test plant.  

A number of the mixes did very well, even with using Tagro as the basis for comparison.  The results from the odor measures for each of the different treatments are shown below.  All treatments except for the Almond fines and Walnut Char had generally acceptable odor ratings that improved over time.  

odor comparison

Odor ranking for those two amendments also ended up as a good indicator of performance in the plant growth trials.  The Almond fines were highly biologically active and not well aerated.  The mixture became moldy after a few days and was one of the few treatments that saw a decrease in appearance ratings over time.  The Walnut char treatment had very high pH and EC, both of which can impact plant growth.  The high pH can result in high ammonia emissions.   The description of the odor from this treatment is proof in point.  The relationship between some of the tested variables and seed germination is shown below.  High EC and ammonia, and high rates of active decomposition were responsible for germination rates below 80% in cucumber and radish germination assays.  The walnut germinationshell char (WSC), almond fines (AF), biochar (BC) are unsuitable for use as a potting soil or a topsoil substitute without additional curing and stabilization.  Too high pH or EC, too active a mixture, and too much nitrogen likely mean that a mixture is not good as a stand- alone soil blend.  

This was also the case for the petunias.  Both the almond and walnut char mixes did not grow pretty petunias.  They were the lowest yielding of all of the blends tested.  Over time, plants in two of the replicates of the almond fines started to recover.   These results should not be interpreted as meaning that these mixtures are inherently toxic to plants.  Instead, it means that they are not suitable as a stand- alone topsoil substitute.

walnut char/biosolids blend
Petunias in the Walnut char/biosolids blend
almond fines/biosolids blend
Petunias in Almond fines/biosolids blend

Many of the other mixtures that we tested worked well.  Perhaps the best performing over all was the Yard waste/ biosolids mixture.  The odor for this mixture was classified as ‘earthy’ and based on the growth of the petunias, it would rank as very nice earth.  

yard waste/biosolids
Petunias in the Yard waste/biosolids mixture

The next step was making larger piles of the best mixtures at one of the treatment plants in San Francisco.  The Yard waste/ biosolids mixture had the best plant growth, a good appearance and odor rating, and was within acceptable limits for the other variables.  It was also a feedstock that is locally available and cost effective.  

blend station

Ryan made the piles and measured temperatures within the piles over 30 days.  One pile was turned and the other left to cure without turning.  There was also a biosolids only pile and a biosolids mixed with sawdust.  Both piles that included yard waste heated up relatively quickly – becoming hot enough to be considered thermophilic.  Unfortunately, in California, if it gets hot enough to compost, even if the regulations do not require time and temperature to reach Class A, the regulations require that you are permitted as a compost facility.  So not clear as to the next steps, but we have a number of options going forth, and Ryan has a Master’s Degree.