Direct discharge of sewage to natural water through illicitly connected urban stormwater systems: An overlooked source of dissolved organic matter

Simultaneous mitigation of multiple pollutants in urban stormwater systems illicitly connected with wastewater systems by an Fe(vi)-based process

“…58,59 Therefore, the harmful N-DBP precursors in downstream drinking water sources may increase due to the discharge of wastewater into the water environment through stormwater pipes in the presence of illicit connections. 6 Traditional coagulation mainly targeted humic-like components, but the removal efficiency of high hydrophilicity protein-like components was not satisfactory. 44 The coagulation process based on Fe(VI) could greatly promote the removal of aromatic protein-like components due to the high reactivity towards nitrogen-containing organic compounds such as amino acids.…”

“…Biopolymers with the molecular weight of 10 kDa or higher were considered to include polysaccharides and nitrogen-containing materials like proteins and amino sugars, serving as the important N-DBP precursors. 61,62 Zhang et al 6 also found that the illicit connections of wastewater pipes resulted in the input of LMW fraction of DOM and high-molecular-weight (HMW) fraction of DOM in the stormwater pipes. The Fe(VI)-based process could effectively remove humic substances and biopolymers and the removal efficiency increased with the increased Fe(VI) dosage, but exhibited limited removal efficiency of building blocks and LMW acids.…”

“…In addition, the construction cycle of drainage systems in developing countries is not synchronized, which causes the illegal connection of sewage pipes to stormwater pipes for separate sewer systems in developing counties. [4][5][6][7] This may pose another potential risk of untreated wastewater entering the natural waters and potentially contaminating drinking water sources through separate stormwater drainage systems. 5 In addition to the large amount of organic pollutants introduced and potential ECs, the charge of dissolved organic matter (DOM) in the urban stormwater drainage system resulted in increased precursors of highly toxic haloacetaldehydes (HALs) and haloacetonitriles (HANs), which may have adverse effects on downstream water quality.…”

“…5 In addition to the large amount of organic pollutants introduced and potential ECs, the charge of dissolved organic matter (DOM) in the urban stormwater drainage system resulted in increased precursors of highly toxic haloacetaldehydes (HALs) and haloacetonitriles (HANs), which may have adverse effects on downstream water quality. 6,8 The current common treatment methods for CSO mainly involved some physical precipitation separation processes and disinfection treatment methods, which have limited treatment effects on ECs and DOM. 9,10 Many unfinished stormwater drainage systems lack proper treatment for the potential wastewater introduced.…”

“…3 In addition, the treatment performance of DBP precursors in CSO and mixed wastewater has not been studied in detail. 6,8 The oxidation efficiency of Fe(VI) can be enhanced by different activation methods such as light irradiation, 23 reducing agents, 24 oxidants 25 or metal ion reactions 26 to produce •OH or other free radicals. Among these methods, Fe(III) activation involves the reaction of Fe(III) with Fe(VI) to produce Fe(V) and Fe(IV) with stronger oxidation capacity as in eqn (1).…”