Storage management influences greenhouse gas emissions from biosolids.

Biosolids produced by wastewater treatment plants are often stored in stockpiles and can be a significant source of greenhouse gases (GHG). Growing trees in shallow stockpiled biosolids may remove nutrients, keep the biosolids drier and offset GHG emissions through C sequestration. We directly measured methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) flux from a large biosolid stockpile and two shallow stockpiles, one planted with Salix reichardtii (willow) trees, from December 2009 to January 2011. All stockpiles emitted large annual amounts of GHG ranging from 38 kg CO2-e Mg(-1) dry biosolid for the large stockpile, to 65 kg CO2-e Mg(-1) for the unplanted shallow stockpile, probably due to the greater surface area to volume ratio. GHG emissions were dominated by N2O and CO2 whilst CH4 emissions were negligible (<2%) from the large stockpile and the shallow stockpiles were actually a CH4 sink. Annual willow tree growth was 12 Mg dry biomass ha(-1), but this only offset 8% of the GHG emissions from the shallow planted stockpile. Our data highlight that biosolid stockpiles are significant sources for GHG emissions but alternate management options such as shallow stockpiles or planting for biomass production will not lead to GHG emission reductions.

Biosolid stockpiles are a significant point source for greenhouse gas emissions.

The wastewater treatment process generates large amounts of sewage sludge that are dried and then often stored in biosolid stockpiles in treatment plants. Because the biosolids are rich in decomposable organic matter they could be a significant source for greenhouse gas (GHG) emissions, yet there are no direct measurements of GHG from stockpiles. We therefore measured the direct emissions of methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) on a monthly basis from three different age classes of biosolid stockpiles at the Western Treatment Plant (WTP), Melbourne, Australia, from December 2009 to November 2011 using manual static chambers. All biosolid stockpiles were a significant point source for CH4 and N2O emissions. The youngest biosolids (<1 year old) had the greatest CH4 and N2O emissions of 60.2 kg of CO2-e per Mg of biosolid per year. Stockpiles that were between 1 and 3 years old emitted less overall GHG (∼29 kg CO2-e Mg(-1) yr(-1)) and the oldest stockpiles emitted the least GHG (∼10 kg CO2-e Mg(-1) yr(-1)). Methane emissions were negligible in all stockpiles but the relative contribution of N2O and CO2 changed with stockpile age. The youngest stockpile emitted two thirds of the GHG emission as N2O, while the 1-3 year old stockpile emitted an equal amount of N2O and CO2 and in the oldest stockpile CO2 emissions dominated. We did not detect any seasonal variability of GHG emissions and did not observe a correlation between GHG flux and environmental variables such as biosolid temperature, moisture content or nitrate and ammonium concentration. We also modeled CH4 emissions based on a first order decay model and the model based estimated annual CH4 emissions were higher as compared to the direct field based estimated annual CH4 emissions. Our results indicate that labile organic material in stockpiles is decomposed over time and that nitrogen decomposition processes lead to significant N2O emissions. Carbon decomposition favors CO2 over CH4 production probably because of aerobic stockpile conditions or CH4 oxidation in the outer stockpile layers. Although the GHG emission rate decreased with biosolid age, managers of biosolid stockpiles should assess alternate storage or uses for biosolids to avoid nutrient losses and GHG emissions.

Native Australian species are effective in extracting multiple heavy metals from biosolids.

Selecting native plant species with characteristics suitable for extraction of heavy metals may have multiple advantages over non-native plants. Six Australian perennial woody plant species and one willow were grown in a pot trial in heavy metal-contaminated biosolids and a potting mix. The plants were harvested after fourteen months and above-ground parts were analysed for heavy metal concentrations and total metal contents. All native species were capable of growing in biosolids and extracted heavy metals to varying degrees. No single species was able to accumulate heavy metals at particularly high levels and metal extraction depended upon the bioavailability of the metal in the substrate. Metal extraction efficiency was driven by biomass accumulation, with the species extracting the most metals also having the greatest biomass yield. The study demonstrated that Grevillea robusta, Acacia mearnsii, Eucalyptus polybractea, and E. cladocalyx have the greatest potential as phytoextractor species in the remediation of heavy metal-contaminated biosolids. Species survival and growth were the main determinants of metal extraction efficiency and these traits will be important for future screening of native species.