Biosolid stockpiles are a significant point source for greenhouse gas emissions

Highlights

• Biosolid stockpiles emit large amounts of greenhouse gas emissions.

• Nitrous oxide and carbon dioxide were the dominant greenhouse gases emitted.

• Younger stockpiles had greater emissions indicating decay of labile organic material.

• Emissions did not show any seasonality, stockpiles likely create their own climate.

• Managers of biosolids should assess alternate storage and uses for biosolids.

Abstract

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.

Introduction

Greenhouse gas (GHG) concentrations of methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂) have increased in the atmosphere due to direct and indirect human activities (Dalal et al., 2003). The waste and wastewater industry sectors contribute approximately 3% to the global anthropogenic emissions of GHG (IPCC, 2007), which has led to increased public awareness and the need to better estimate or measure GHG emissions from the sector (Ishigaki et al., 2005). The wastewater sector emits CH₄, CO₂ and N₂O directly through nutrient transformations during the wastewater treatment process and degradation of organic matter (Boldrin et al., 2009, Czepiel et al., 1996) and indirectly through fossil fuel consumption for pumping, transport, operations and processing (Monteith et al., 2005). In all wastewater treatment plants large quantities of sewage sludge are produced (Brown et al., 2008). The dried sludge is called biosolids and still contains significant quantities of organic matter, macro-nutrients and trace elements (Bright and Healey, 2003). The quantity of biosolid produced in Australia is around 360,000 dry Mg per year (Pritchard et al., 2010) and in the state of Victoria it is approximately 68,250 dry Mg (AWA, 2013). These biosolids are often stored in large stockpiles for many years within the wastewater treatment site, before they are moved off-site and used as a resource.

Currently, GHG emissions from waste and wastewater treatment plants are often based on simple emission factors or mathematical models developed from kinetic relationships of mass and energy balances (Yerushalmi et al., 2013). One of the main uncertainties in GHG accounting is associated with the method of calculation, which is frequently not based on, or validated by, direct GHG flux measurements. For example, waste industries use minimum data to predict the GHG emissions based on population, waste generation rate, solid waste composition and methods of solid waste disposal (El-Fadel and Massoud, 2000). Another uncertainty confronting GHG accounting in the wastewater industry is access to long term monitoring of GHG emissions and under a wide range of climatic and management conditions (Cowie et al., 2012, Bogner et al., 2008, Czepiel et al., 1993). Hence, to reduce these uncertainties it is important to obtain reliable long-term data to better understand the magnitude of direct GHG emissions and the variation over seasonal or inter-annual time frames. There are many studies investigating GHG emissions from landfill waste (Bastian et al., 2013, Stanisavljevic et al., 2012, Goldsmith et al., 2012), stockpile composting (Chan et al., 2011, Ahn et al., 2011, Boldrin et al., 2009) and stockpile manure management (Petersen et al., 2013, Wood et al., 2012). However, GHG emissions from stockpiled biosolids have not yet been measured in detail.

The overall objective of this study was therefore to investigate the direct GHG emissions from various ages of biosolids stockpiles at a wastewater treatment plant. The specific objectives were to:

• measure the flux of CH₄, CO₂ and N₂O from different ages of biosolid stockpiles across different seasons and investigate GHG concentrations (CH₄, CO₂) at two different biosolid stockpile depths,

• investigate the relationship between GHG emissions from biosolid stockpiles and environmental variables,

• calculate the cumulative magnitude of annual CH₄, CO₂ and N₂O fluxes from different aged biosolid stockpiles and compare this against modeled emissions.