Land application of sludge

By Sally Brown, University of Washington

This year we celebrated our 30th year of Biofest.  For the last research story of this year it seems appropriate to take a trip back 30 years and see what the research world was like, to see how much has changed and what has stayed the same.  The big publication in 1987 was the brown book, coming close on the heels of the blue book.  These color names refer to books put out by the W 170 research group, the group that essentially conducted the research that formed the basis of the 503 regulations.  The brown book: Land Application of Sludge was edited by Al Page, Terry Logan and Jim Ryan; three of the leaders of biosolids research.  The book reports on a workshop that was held in Las Vegas in 1985.  The focus of the book is metals.  The impact of soil properties on metals in biosolids, the impact of sludge properties on metals in biosolids amended soils, plant uptake of metals in biosolids, and risks posed to people from metals in biosolids.  Clearly the overriding concern about the safety of biosolids related to the behavior of metals in the material.  PretreatmentIt is important to remember that 1987 was before the impact of the pretreatment regulations was felt.  Metals in biosolids were miles above what is considered highly contaminated today.  They were also significantly lower than they had been ten years prior.

Cadmium is the metal that received the most attention in biosolids research.  It is the one element that is readily taken up by plants and can cause human illness and even fatalities.  It is a concern for people who are deficient in other nutrients.  The most serious cases of soil cadmium contamination leading to sickness were observed in Japan after WW II.  Many residents who ate milled rice grown in contaminated soils developed weak bones and kidney disease as a result of elevated Cd in their diets.  While not a clear cause for concern with a balanced diet, there are still restrictions on permissible Cd content in different foods.  If we start with cadmium the best place to look is Chicago.  The ‘Nu-Earth’ biosolids produced in Chicago were used as an example of high metal materials.  These materials were shipped across the country so that the behavior of biosolids in different soils and climates could be studied.  Those ‘Nu-Earth’ sludges were an important part of my PhD research in fact.  In 1971 the Cd content in the Chicago sludge was 398 ppm.  By 1977 Cd had gone down to 168, by 1984 a further drop to 121.  These are clearly significant declines but the final levels reported in 1987 are well above regulatory and acceptable levels.  In fact that initial measure is about 100x greater than the Cd in modern day Chicago cake.  Pretreatment regulations have radically altered the quality of the biosolids that are produced today.  Rufus lead researcher in biosolids and food chain researchIn fact, while I still get asked about metals in biosolids those questions come from citizens and not scientists.  In the scientific community much of the work on metals in biosolids revolves around using biosolids to make contaminated soils less hazardous.  Coming full circle, a chapter in the brown book had Rufus Chaney as a first author.  Dr. Chaney was one of the key researchers evaluating the potential risk to human health of Cd in biosolids amended soils.  A paper published in 2017 by Paul and Chaney showed that adding biosolids compost to naturally Cd rich soils can reduce Cd uptake by leafy greens.  So times have changed.  

Working with biosolids sometimes it can feel like you have been answering those same questions again and again and again.  It can feel like a broken record describing the research that has been done and the safety and benefits that have been shown.  It can be very easy to lose sight of how much progress really has been made and how much better things are today.  For me at the University of Washington, I am actually now doing the work that I wanted to do when I first went to graduate school in 1990.  My initial goal then was to use biosolids to enrich soils in farms near cities as a way to make what we now call peri- urban agriculture economically viable.  Back then biosolids for strawberries was unheard of.  Now we have biosolids for backyard gardens in cities across the country.

 

Paul, A.L. and R.L. Chaney.  2017. Effect of Soil Amendments on Cd Accumulation by Spinach from a Cd-Mineralized Soil.  J. Environ. Qual. 46:707-713.