CFD MODELING OF MUNICIPAL WASTEWATER DISINFECTION BY PERACETIC ACID (PAA) IN CONTINUOUS FLOW SERPENTINE REACTORS

In phase II of a two-year III Life-Environment EU project (acronym: SELSY, contract # LIFE00 ENV/IT/000090) aimed at investigating the municipal wastewater disinfection performance by peracetic acid (PAA), a serpentine-type pilot contactor was built and operated at the City of Taranto Wastewater Treatment Plant. The objectives of this pilot study were: (a) to determine whether use of PAA for disinfection is practical in a realistic reactor and (b) to ascertain the influence of serpentine reactor hydrodynamics on disinfection performance.Pilot plant hydrodynamics were quantified by means of tracer tests; disinfection performance was assessed batch and pilot tests. Data were used to validate a computational fluid dynamics (CFD) model. The validated CFD model was used to explore PAA process understanding and design under different operating conditions. Use of CFD in tandem with pilot studies provided in-depth understanding of the processes (hydrodynamic, chemical and microbiological) underlying disinfection and offered an inexpensive means for assessing design modifications in pilot studies. Exploration of design modifications of serpentine reactors is merited, given that, for the pilot reactor described in this study, dead/stagnant regions occupied 40% of the overall reactor volume.The CFD model was used to predict E. coli inactivation for different scenarios: (a) variable initial microbial concentration of 100, 1000, 10000 CFU/100mL, (b) variable PAA dosage of 2, 4, 6, 8 mg/L, (c) variable reactor flow rate of 8 m3/h, 24 m3/h. It was demonstrated that secondarily settled wastewater disinfection by PAA represents an economically-sustainable disinfection option if the E. coli effluent concentration is in between 50 and 100 CFU/100mL. Higher PAA dosages (> 8-10 mg/L) are required to get 10/100mL E. coli, which is the Italian prescription for unrestricted wastewater reuse in agriculture, suggesting the use of PAA in combination with other disinfection technologies.