Benchmarking the Sustainability of Sludge Handling Systems in Small Wastewater Treatment Plants in Ontario

Abstract

This project quantitatively benchmarked all aspects of sludge handling in a cross-section of small wastewater treatment plants across Ontario. Using plant operational data and on-site measurements, a variety of sustainability metrics were evaluated: energy consumption, chemical use, biosolids disposition, biosolids quality, and greenhouse gas emissions. In addition, a desktop analysis was conducted to determine the sustainability impact of incorporating innovative technologies into facilities with conventional processes. Parameters from select new technologies within the study sample were applied to plants within the sample that employed conventional processes, and the impact on greenhouse gas (GHG) emissions was calculated. Overall electricity consumption for sludge handling ranged from 0.9 – 3.9 kWh per dry kg of raw sludge. The thermo-alkali hydrolysis and auto-thermal thermophilic aerobic digestion (ATAD) processes consumed the least (0.3 kWh/dry kg) and most (3.8 kWh/dry kg) amount of electricity for stabilization, respectively. Mechanical dewatering processes consumed minor amounts of electricity (2 – 5% of total sludge handling draw), however, associated polymer dosages were found to be higher than literature values in some cases. The disposition fuel requirements for plants with dewatering were up to 85% lower than facilities without dewatering. Biosolids contaminant (pathogen/metals) contents were observed to be substantially below Non-Agricultural Source Material (NASM) requirements. The copper content of the hauled biosolids exhibited the highest concentration relative to the NASM limit among all plants studied, ranging from 14 – 37% among facilities practicing land application of biosolids. Four plants generated a product that met Class A requirements for 𝐸. 𝑐𝑜𝑙𝑖 content, including one facility that generated it through a long-term storage approach (GeoTube™). Carbon emissions ranged from -119 to 299 kg CO2 equivalents per dry tonne of raw sludge. Six of the eight facilities that practiced land application of biosolids exhibited net-negative GHG emissions, as the carbon credits gained from fertilizer production avoidance outweighed the emissions associated with sludge processing and transportation operations. Of these six plants, five employed sludge treatment configurations that are common in Ontario. Given that land application is the most common disposal practice among small treatment plants in Ontario, the findings indicate that current conventional practices can be sustainable with respect to GHG emissions. The innovative technology assessment revealed that existing trucking requirements and polymer dosage are the primary factors that determine whether new technology implementation would improve environmental sustainability. The benchmarking approach developed and information gathered is of value to plant owners and operators who seek to better understand how their utility is performing relative to peers, identify areas of need and further investigation, and improve the long-term sustainability of their operations.