Abstract

Combined sewer overflow (CSO), discharged by sewers during rain events, is a major cause of pollution in surface water bodies. The main goal of this work was to develop a compact CSO treatment process by filtration and cation/anion exchange and to validate it with actual CSO in a real WWTP environment. Preliminary screening of several cation and anion exchange materials led to the selection of a molecular sieve (MS13X) that retained ammonium with an 11.6 mgN g-1 capacity, and a layered double hydroxide (calcined Sorbacid 911) that removed phosphate with a 2.2 mgP g-1 capacity. For both materials, the development of a regeneration procedure allowed to perform several adsorption/regeneration continuous-flow cycles with actual CSO without losses in adsorption performance. The packed bed heights were scale-up to 40 cm (Sorbacid 911) and 60 cm (MS13X) without variations in performance. The combined filtration/cation exchange/anion exchange process was tested at pilot scale in a real WWTP environment, with results detailed in Table 7. The pilot plant testing demonstrated removal rates of 68% for COD, 78% for BOD5, 85% for TSS, 88% for total P, 57% for total N, and 67% for faecal coliforms. Significant removals were also observed for heavy metals (Zn 83%, Cu 89%, Ni 81%) and pesticides (desethyl-atrazine 100%), whereas the process was largely ineffective towards PFAS (12%). While 1-µm microfiltration contributed significantly to overall removals, the cation/anion exchange process was crucial for achieving high removals of specific pollutants, including ammonium (57%), phosphate (95%), and total N (57%). This work represents a relevant step towards the development of a compact CSO treatment process featuring a limited land requirement and the possibility to withstand prolonged periods of dry weather.