Estimating the carbon footprint of waste stabilization ponds

Abstract

Waste stabilization ponds (WSPs) are natural treatment systems extensively employed for wastewater treatment because they efficiently remove BOD, COD, organic matter, and pathogenic microorganisms from domestic wastewater. Wastewater treatment generates significant amount of greenhouse gases, primarily carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). In this study, identification and estimation of direct and indirect emission of greenhouse gases (GHGs) from WSP in a series of facultative pond 1 (primary facultative pond), facultative pond 2, and maturation pond for treatment of municipal wastewater at Hikkaduwa are presented and compared with hypothetical anaerobic pond, which comprising of typical WSP system. In addition, GHG emissions in the entire WSP are compared with the hypothetical activated sludge extended aeration (ASEA) treatment process with the same wastewater characteristics in similar climate conditions. All the calculation approaches of GHG emissions and emission factors are based on literature studies. GHG emissions during the wastewater and sludge treatment are known as direct GHG emissions, and indirect GHG emissions are generated due to the power consumption of plant operation, which were discussed. The high rate of CO2 and CH4 emissions were observed at the primary facultative pond (FP1) and significantly low in FP2 and MP at HSTP and the amount of production of CO2 was more significant than that of the production of methane in a series of WSP. However, methane is the main source of carbon footprint in WSP system, which follows the two facultative ponds and maturation pond due to high global warming potential (GWP). It is obvious that WSPs are the minor source of nitrous oxide emission, although GWP over 100 years of N2O is 298. The results showed that the FP1 was the major direct GHG emission source for HSTP and approximately 75% of GHG emitted that of WSP. The results further showed that the anaerobic pond was the main source of direct GHG emissions, 40% higher than the FP 1, followed by the FP 2 and the MP. Indirect emissions had the highest contribution to the total carbon footprint in the ASEA treatment process than direct emission. The ASEA system contributed the highest to the total carbon footprint (3.5 times) than WSP, although direct GHG emission is significantly higher in WSP (72%). The results showed that carbon footprints are highly dependent on the treatment process of WWTPs.