Synergistic effects of urban tributary mixing on dissolved organic matter biodegradation in an impounded river system

Comparing effects of untreated and treated wastewater on riverine greenhouse gas emissions

“…This large amount of labile DOM contained in the untreated wastewater discharged from the city centre of Phnom Penh might be partially responsible for large GHG evasion from the downstream Mekong River. The magnitude of BDOC and CO 2 concentrations measured for the untreated wastewater from Phnom Penh has rarely been observed in river systems receiving treated wastewater such as the Han River, Korea (Yoon et al, 2017;Begum et al, 2019) and rivers in North America (Sickman et al, 2007;Hosen et al, 2014).…”

“…Wastewater can emit large amounts of GHGs directly from the source to downstream rivers (Alshboul et al, 2016) or indirectly via degradation of labile DOM contained in the discharged wastewater (Yoon et al, 2017;Jin et al, 2018). Furthermore, the discharged wastewater can also exert a synergistic effect on OM processing when it is mixed with the downstream river water (Begum et al, 2019). As shown in Figure 4, the treated wastewater predominantly produces new OM upon mixing with the river water, which lacks in nutrients and other resources for immediate microbial degradation.…”

“…Samples were filtered through a 0.2 µm polycarbonate filter (Whatman, USA) and a three-day incubation was conducted in the laboratory, following the methods modified from Servais, Billen, and Hascoët (1987) and Begum et al (2019). Three replicates of filtered samples (60 mL) were incubated in 120 mL glass bottles, sealed with butyl septa and aluminium crimps to measure the change in dissolved CO 2 in addition to measuring biodegradable dissolved organic carbon (BDOC).…”

“…The concentration of BDOC was calculated as the net change in DOC concentration over the three-day incubation, and %BDOC was calculated as relative proportion in the initial DOC concentration. Dissolved gas samples were collected from each bottle at the beginning and end of the incubation, using manual headspace equilibration method (Begum et al, 2019). CO 2 production from biodegradation was calculated as the net change in dissolved CO 2 during the incubation.…”

“…Recent studies have reported that concentrations of dissolved organic carbon (DOC) and dissolved nutrients increase substantially downstream of large metropolitan cities with treated and untreated wastewater discharged to the river (Sickman, Zanoli, & Mann, 2007;Hosen, McDonough, Febria, & APN Science Bulletin, Volume 9, Issue 1 (2019): 88-94 89 Palmer, 2014). Enhanced bioavailability of nutrients and labile OM derived from wastewater, especially after mixing with the mainstem river, has been associated with increased emissions of GHGs from downstream rivers (Begum et al, 2019).…”