Biosolids Metals that Nourish The library this month is all about metals. We’re continuing with the Whole Foods agenda here. But even without the Whole Foods issue, I find that I am asked about metals in biosolids again and again. This always surprises me because metals in biosolids were an issue over 25 years ago. I was young (relatively speaking) 25 years ago. I am not young now and metals in biosolids are not an issue. Here is a library to bring that point home.
If you notice above, the table shows the 503 metal concentrations in the King County, WA, biosolids from each of their three treatment plants. I am using the King County biosolids for several reasons. One: I help to make them. Two, I like to use them, and three I knew I could find these numbers on line. I would guess that the metals in your biosolids are not dissimilar from these values. We can use this table as a reference point for the papers in the library.
The first three papers center on Cu and Zn. If you look at the data above, you’ll notice that Cu and Zn are by far the biggest numbers in the biosolids, big enough to make people nervous. What the people tend to forget with this is that both copper and zinc are necessary plant and animal nutrients. The first paper focuses on metal concentrations in animal feeds and animal manures. The authors note that Cu and Zn concentrations in poultry and pig feed are high and that as a result the concentrations of these elements in the corresponding manures are also high. The reported concentrations are very similar to those in the King County biosolids. I ventured to guess that the concentrations of these elements in our diets are high as well. So I checked. I looked on line and found a site that tells you what foods to eat to maximize your zinc intake as zinc is an essential nutrient.
(http://www.healthaliciousness.com/articles/zinc.php). Concentrations are given on a wet weight basis, but a guess puts the Zn concentrations in beef at about 600 ppm.
The second and third articles also have information on Cu and Zn. The 2nd paper focuses on metal concentrations and speciation in pig slurry. Again, Cu and Zn are the big numbers here. Again, they are very similar to the King County biosolids. The authors looked at metal partitioning between solids and liquids and then used a sequential extraction to look at metal associations. They found that the greatest concentrations of Cu and Zn were in the organically bound or carbonate fractions. This is the type of research that went on with biosolids metals back when metals were a concern.
From there we go to paper #3 and Cu and Zn in biosolids amended soils. Biosolids were applied to dryland wheat in 1991 and again in 2002. Here metal associations were determined using a range of X ray techniques as well as computer modeling. The authors found high binding to organic matter with many bonds including shared electrons. The study includes metal loading rates from each time period and it is clear that metal concentrations (Cd, Cr, and Pb in particular) decreasing significantly over that time frame. The authors also note that the soluble concentrations of metals in the biosolids amended soils were well below drinking water guidelines.
The last two papers provide a contrast between poultry manure and biosolids, and here we can focus on arsenic. It turns out that poultry in the US is often fed roxarsone, a high As supplement that promotes weight gain. The estimated As concentration in poultry manure is 43 - 47 mg kg-1. Take a look at the biosolids from King County. It would seem that the people that live in my neighborhood have opted not to include roxarsone in their diets. In this paper, the authors note that land application of the poultry manure in Arkansas (assuming all birds received the high As supplement) would result in over 56,000 kg of As being applied statewide. The research looked at ways to limit As and Cu runoff from soils that receive the poultry manure and found that alum addition reduced runoff.
The final paper focuses on As and Pb and gives a current perspective on metals and biosolids. Here the authors are looking at the potential to use biosolids and food/yard composts as a way to reduce Pb and As availability in contaminated soils. In fact, this study was done on soils in Seattle and Tacoma. Cedar Grove compost was tested in Seattle, and Tagro biosolids was used in Tacoma. The total As and Pb concentrations in both soil amendments are provided. The Pb in Tagro (9.6 ppm) was about one forth that of the Cedar Grove (41 ppm). Same deal with As -- 3.2 ppm in Tagro and 15 ppm in the Cedar Grove. The authors found that using the biosolids (and the yard/food compost) diluted total metals in the soils. They found that both amendments reduced plant uptake of these elements. Both amendments also reduce the bio-accessible portion of the total lead and arsenic, meaning that if soil is eaten accidentally, less lead and arsenic will be absorbed into the body if the biosolids or compost have been added to the soil.
It is time that we start talking more about the real message about biosolids and metals. That is, biosolids are a tool to make the metals that can hurt us less available, and they are a great source of the metals that nourish us.