Is Biosolids Incinerator Ash a Safe and Effective Phosphate Fertilizer?.

Is Biosolids Incinerator Ash a Safe and Effective Phosphate Fertilizer? James E. Crants, Kathryn Blake, Matthew McNearney, and Carl Rosen The bulk of Twin Cities biosolids is incinerated for heat and electricity, and the resulting ash is landfilled. Research has shown that biosolids ash is a viable P source for crops, but historically, heavy metals raised the potential for environmental and safety concerns. Due to strict enforcement of industrial waste disposal, the biosolids ash presently produced in the Twin Cities has lower metal concentrations than that used in earlier studies. To determine whether biosolids ash produced by current methods is a safe and effective P source for crops, we evaluated Twin Cities biosolids ash as a P source relative to dried biosolids pellets, struvite (another wastewater treatment byproduct), and triple super phosphate (TSP) using greenhouse-grown corn and lettuce. Each P source was applied at 56, 112, or 224 kg ha-1 P2O5 equivalent, with a zero-P control included. Plants fertilized with biosolids ash yielded biomass similar to those fertilized with TSP or struvite and similar to or greater than those fertilized with pelletized biosolids. Corn and lettuce plants fertilized with biosolids ash took up at least as much P as those fertilized with TSP or pelletized biosolids, but no more than those fertilized with struvite. Soil pH and salinity in pots fertilized with biosolids ash were not different from pots receiving other treatments. Biosolids ash yielded higher plant-available soil Cu and Zn concentrations than struvite or TSP, but not pelletized biosolids. This translated into higher tissue Cu and Zn concentrations in plants fertilized with biosolids ash than TSP or struvite, but sludge-ash-fertilized plants did not generally have significantly higher tissue Cu or Zn than the zero-P control. Based on these results, Twin Cities biosolids ash may be a safe and effective source of P for crops. Long-term studies are needed to evaluate crop responses and soil chemical properties under field conditions.