Greenhouse Gas Emissions from Blackwater Septic Systems

Abstract

Septic systems are potentially a significant source of greenhouse gases (GHGs). The present study investigated GHGs from the blackwater septic systems that are widely used especially in low- and middle-income countries. Ten blackwater septic tanks in Hanoi, Vietnam, were investigated using the floating chamber method. The average methane and carbon dioxide emission rates measured at the first compartment (65% of total capacity) of the septic tanks were 11.92 and 20.24 g/cap/day, respectively, whereas nitrous oxide emission was negligible. Methane emission rate was significantly correlated with septage oxidation–reduction potential (ORP) (R = −0.67, p = 0.034), chemical oxygen demand mass (R = 0.78, p = 0.007), and biochemical oxygen demand mass (R = 0.78, p = 0.008), whereas it was not significantly correlated with water temperature (R = 0.26, p = 0.47) and dissolved oxygen (R = −0.59, p = 0.075) within the limited range: 30.6–31.7 °C and 0.03–0.34 mg-O2/L. The methane emission rates from septic tanks accumulating septage for >5 years were significantly higher than those at 0–5 years (p = 0.016). These results suggest that lower ORP and higher biodegradable carbon mass, in association with longer septage storage periods are key conditions for methane emissions. To the best of our knowledge, this is the first study to characterize GHG emissions from septic systems.

Non-Sewered Sanitation Systems’ Global Greenhouse Gas Emissions: Balancing Sustainable Development Goal Tradeoffs to End Open Defecation

“…Additionally, where data did not exist, this analysis proved useful in the identification of the need for further experimental-based research in these areas. For each sanitation technology identified in scenarios 1a-3b, Table 3 summarizes the IPCC EF [26] for each GHG, as well as the revised values and corresponding study [15,[17][18][19]23,[35][36][37][38][39][40][41][42][43] that have been used in this analysis. Supplementary data that further support this work are available.…”

“…However, using improved experimental EFs improves this to only an 84% increase and lowers these estimations within each respective SSP scenario on average by 53%. Table 5 compares the estimated annual 100-year GWP CO 2 eq results for each scenario in 2030 using the IPCC default EF values to the results that used the improved, experimentally collected, averaged values from literature [15,[17][18][19]23,[35][36][37][38][39][40][41][42][43].…”

“…Decentralized (on-site) sanitation technologies are the focus of this assessment as they are generally the most scalable, effective and equitable adaptation measure within the sanitation sector and are typically the first accessible improved sanitation option for those that currently have no access to sanitation [2,13]. They have a lower economic burden on the household user [14,15], some forms have been proven to be effective in reducing negative climate impacts [16][17][18][19], and most importantly, they can provide access to improved sanitation for the most vulnerable populations [13,15,17]. Specifically, this assessment focused on the three different service levels of sanitation (i.e., improved, unimproved and no service), as defined by the Joint Monitoring Programme (JMP) [20], as they pertain to three Shared Socioeconomic Pathways (SSPs), as described in the framework presented by O'Neill et al (2014).…”

“…Sanitation Service Ladder Technologies IPCC and Average Improved GHG Emission Factors. Number of Studies used in Improved Average Values in square brackets[15,[17][18][19]23,[35][36][37][38][39][40][41][42][43]. Where NR = Not Reported.…”

Non-negligible greenhouse gas emissions from non-sewered sanitation systems: A meta-analysis

“…Four major recent studies reported emissions that are directly measured from NSSS, and GHG emissions show very high variability. For example, Huynh et al (2021) only measured the CH 4 emission in the first chamber of the septic tanks, ignoring the second and third chambers, thereby leading to significantly lower results than other studies. The CH 4 emissions from Dubber and Gill (2014) and Truhlar et al (2016) were 164.5 kg CO 2 e/cap/year and 100 kg CO 2 e/cap/year, which are consistent with the above range of emissions obtained through meta-analysis.…”