Development of a combined filtration and ion exchange process for the treatment of combined sewer overflow

Combined sewer overflow (CSO), discharged by sewers during rain events, is a major cause of surface water pollution. This work aimed at developing a compact CSO treatment process by filtration and cation/anion exchange and validating it with actual CSO in a real environment. Preliminary screening of several cation and anion exchange materials led to selecting a molecular sieve (MS13X) that retained ammonium with a 13.3mgN g-1 capacity, and a layered double hydroxide (calcined Sorbacid 911) that removed phosphate with a 2.2mgP 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 adsorption performance losses. Packed bed heights were scaled-up to 60cm without variations in performance. The combined filtration/cation exchange/anion exchange process, tested in a real wastewater treatment plant, led to removals ≥87% for COD, BOD5, suspended solids, total P, total N, faecal coliforms, metals (Zn, Cu, Ni) and pesticides (desethyl-atrazine, desethyl-terbuthylazine). 1-μm microfiltration gave the highest contribution to the overall removals. The cation/anion exchange process was essential to reach high removals of ammonium (72%), phosphate (70%), and total N (92%). The design of a full-scale filtration/ion exchange unit resulted in a surface requirement of just 0.20m2 for each m3 h-1 to be treated, and in a 2.7 € mCSO-3 operational & maintenance cost. This work represents a relevant step towards the development of a compact CSO treatment featuring limited land requirement and the possibility to withstand prolonged periods of dry weather.