Testing biosolids soil blends

By Ryan Batjiaka, University of Washington

Class A biosolids mixed with other ingredients can make the perfect soil amendment.  The biosolids supply organic matter and fertility.  You just need to add some other ingredients to balance out the carbon to nitrogen ratio and take away the ‘yuk’.  The question is what other ingredients and how much of them?    Karri Ving from the San Francisco program tasked Ryan Batjiaka, a graduate student at UW, with finding the ingredients and making some blends.  The City of San Francisco will soon be producing all Class A and is hoping to develop a diversified program through different blended products.  At present most of their material is used on a few farms and for daily landfill cover.  

Ammonia volatilization
The face of ammonia volatilization

Ryan had previously worked at Vision Recycling, a Bay Area composter, and knew some of the readily available local ingredients.  He also worked at Tagro in Tacoma as part of his graduate program and Dan Thompson from Tagro is a member of Ryan’s graduate committee.  Ryan drove down to the Bay Area over the holidays and came back with a car loaded with buckets of potential mix ingredients.  These included different yard waste material, ground nut shells, a residual material high in gypsum from recycled particle board and biochar.  The standard recipe for Tagro classic was used as a starting point.  Biosolids and sand were mixed with the different feedstocks at a volume ratio of 40% biosolids: 40% feedstock: 20% sand.  In addition a few biosolids feedstock mixtures were made without any sand added.  Tagro classic and a locally produced food/yard compost were used as controls.  Mixtures were left to cure for a month and then tested using a germination assay and a petunia growth trial. A range of different tests were performed on the mixtures including total carbon and nitrogen, available nutrients, total nitrate and ammonium, electrical conductivity, and pH.  Mixture stability was measured using total CO2 emissions.  A group of students served as odor panel members as did the team at Tagro.  

A partial list of the treatment mixtures and select properties of the mixes is shown below.  Measurements were taken after 30 days of curing.  Petunia biomass is from plants grown in 100% of each mixture.

Treatment mixturesTreatment mixtures

For the growth trial Ryan used petunias, the reasoning being that the products needed to work well for the home gardener and home gardens often have pretty annuals.

Petunias in the University of Washington greenhouse

Ryan BatjiakaPetunias fit the bill.  The plants were grown in the UW greenhouse in half gallon pots filled with either 100% of each biosolids mixture or 50% mixture and 50% sand. Four replicates were used for each treatment. Soon the greenhouse bench was a sea of purple flowers.  Results varied by mixture.  Some of the mixtures clearly inhibited growth of the plants.  There were some dead petunias and others looking like they were not far from dead.  However the petunias in most of the mixtures were vigorous with healthy leaves and plenty of flowers.  The plants were measured for total chlorophyll using a SPAD meter, number of blossoms, width and height of the plants and dry weight.

Ryan will be writing this up for his MS degree.  He is headed to San Francisco for a six month internship with their biosolids program.  Part of his time there will be spent further perfecting recipes for different biosolids based mixtures and establishing a garden using the mixtures at the treatment plant.