Effect of DNA sizes and reactive oxygen species on degradation of sulphonamide resistance sul1 genes by combined UV/free chlorine processes.

•UV-Cl2 and UV/Cl2 processes generated synergetic effects for sul1 gene deactivation.•Difference of DNA size affected gene deactivation by individual UV and chlorination.•UV/Cl2 and UV-Cl2 used corresponding reactivity of single processes to generate synergy values.•Formation of HO· contributed to up to 45 % of the synergy values in UV/Cl2 process.•The contribution of Cl· to sul1 gene deactivation in UV/Cl2 process was negligible.