Estimation and prediction of greenhouse gas emissions from individual septic tank and imhoff tank

Abstract

Domestic wastewater treatment system is one of the sources that emit greenhouse gases (GHG) to the environment mostly due to the microbial breakdown of organics in human waste. This study aimed to estimate and predict GHG emissions for selected on-site domestic wastewater treatment systems in Malaysia, which are Individual Septic Tanks (ISTs) and Imhoff Tanks. In this study, the GHG inventory was developed for on-site septic systems, including 25 Imhoff Tanks and 2 ISTs situated at one of the earliest residential areas in southern part of Peninsular Malaysia. Direct GHG emissions include emissions of methane that can be biological produced, emitted in sewers and during wastewater treatment. Whilst indirect GHG emissions include emissions of carbon dioxide from the usage of electricity to operate the treatment facility and nitrous oxide emissions from wastewater treatment effluent that is discharged into river. Referring to the Intergovernmental Panel on Climate Change (IPCC) (2006) method and adaptation of several widely-used accounting procedures, the GHG emissions from Imhoff Tanks were estimated based on wastewater data provided by Imhoff Tanks operator, Indah Water Konsortium (IWK), Skudai Unit, Johor. However, for IST, the primary wastewater data was analyzed in-situ. In order to establish GHG emission inventory for certain period and to estimate per-capita basis, the procedures to estimate GHG emissions from the selected onsite septic systems, were developed, which can be a guideline for other case studies in Malaysia. The inventory of GHG emissions for one-year period shows that under different Population Equivalent (PE), wastewater with higher Biological Oxygen Demand (BOD) concentrations produced more methane compared to wastewater with lower BOD concentrations. Furthermore, the anaerobic treatment process in Imhoff Tank contribute to the highest direct GHGs emissions, compared with the indirect emissions from electricity consumption and wastewater treatment effluent that is discharged into river. Based on trendline and R-squared value analysis of six-year estimation, data showed that Imhoff Tanks (without pumping station) treating wastewater from the lowest PE (260) has GHG emissions of 471 tCO2-eq/year and the highest PE (2,120) has GHG emissions of 50,838 tCO2-eq/year. For the Imhoff Tanks (with pumping station) treating wastewater from the lowest PE (214) has GHG emissions of 421 tCO2-eq/year and the highest PE (2,840) has GHG emissions of 86,264 tCO2-eq/year. The six-year data of GHG emission for 25 Imhoff Tanks were then analyzed using MATLAB® version 2017 to identify the parameters that significantly influence the GHG emissions based on linear regression model. The analysis seemed that PE and BOD influent concentration significantly influenced GHG emission. By using two functions in Excel® version 2016, which are FORECAST and TREND, prediction of future GHG emission was then done for Imhoff Tank (without pumping station) that treats wastewater from 640 PE and Imhoff Tank (with pumping station) that treats wastewater from 520 PE. Hence, the estimation of GHG emission from Imhoff Tanks in other areas in Malaysia can be predicted based on wastewater from the same population. GHG emissions from the two ISTs were also successfully estimated for per-capita basis using IPCC (2006) method and adaptation of Sasse (1998) model. The overall GHG emission results show that anaerobic treatment process in the studied domestic wastewater treatment systems release methane which has greater Global Warming Potential (GWP) impact to environment.