Combining biosolids with carbonaceous materials to mitigate nitrogen-losses

Abstract

Biosolids are the solid by-product of wastewater treatment plants. Humanity produces some 50 kg/person/year, with global output exceeding 10 ×10⁶ t/year. Disposal of biosolids costs New Zealand (NZ) around 33×10⁶ dollars/year. Most biosolids are either burned or placed in landfills, which is not a sustainable solution. Moreover, burning requires energy and results in greenhouse gas emissions. Biosolids are mostly organic matter and contain high concentrations of plant nutrients. Biosolids can also contain pathogens and contaminants, which is why they are not typically applied to NZ’s high value soils. However, in NZ and elsewhere, biosolids are used to rebuild degraded soils for the production of non-food crops such as timber. Applying biosolids to soil improves plant growth, but may result in high levels of nitrate (NO₃⁻) leaching and can introduce contaminants into the food chain. I aimed to determine the effect of mixing biosolids with carbonaceous materials (sawdust, biochars, and lignite) on NO₃⁻ leaching from biosolids-amended soil. Sawdust/wood-waste was derived from Pinus radiata (D. Don), a common forestry species. Biochar was made by pyrolysis as P. radiata, waste at temperatures between 350°C and 550°C. Low-grade lignite coal, which is blended with high-grade coal for its disposal, was obtained from Solid Energy, NZ. The capacity of the amendments to sorb ammonium (NH₄⁺) and NO₃⁻ was measured using batch experiments. Solutions containing 100 mg/L of NH₄⁺ or NO₃⁻ were separately mixed with the amendments in 1:10 materials: solution ratio. Leaching of biosolids mixed with these amendments was determined using column leaching experiments. Columns (4 cm height × 4 cm diameter) containing biosolids mixed with biochars, lignite or sawdust in a 1:1 ratio were irrigated with 5 mL of deionised water and the resulting leachate was collected weekly. Large lysimeters (70 cm height × 50 cm diameter) were filled with intact columns of the Lismore Stony Silt Loam (low fertility soil). There were three replicates of the following treatments: control (no amendment), biosolids added at a rate equivalent to 400 kg N/ha, biosolids + lignite (1.5:1 by weight) and biosolids + biochar (1:1 by weight). All NH₄⁺ and NO₃⁻ concentrations were determined using Flow Injection Analysis (FIA). Batch experiments revealed that none of the amendments adsorbed significant amounts of NO₃⁻. Biochar and lignite adsorbed significant amounts of NH₄⁺, giving sorbed/solution NH₄⁺ concentration quotients of up to 33 and 4.4 respectively. No nitrification occurred during this long-term sorption. The time to reach equilibrium of the lignite NH₄⁺ mixture was in the order of 6 hr, while the biochar took some 150 hr to reach equilibrium. Increasing pyrolysis temperatures resulted in charcoals with an increased ability to sorb NH₄⁺. Unpyrolyzed sawdust did not adsorb significant amounts of NH₄⁺, however, sawdust almost eliminated NH₄⁺-N leaching and reduced NO₃⁻-N leaching by >40%. Low temperature biochar reduced NH₄⁺-N leaching from the columns by 40 - 80%. Overall, dry sawdust and low temperature biochar are the potential carbonaceous materials to mitigate N leaching from biosolids.