Aqueous solutions of nicotinic acid (NA) were treated using a low consumption UV light source and the potassium persulfate reagent (UV/KPS) in a falling thin film photo-reactor. Under determined optimum conditions of [KPS] = 1400 mg/L and pH 4, degradation of NA reached to 82.2% after 90 min. Presence of conventional water contained bicarbonate, chloride and nitrate anions caused unfavorable effect; however, adding only 0.5 mg/L of Cu2+ ion, as promoter, gave rise to the system capability to 97.1% degradation. Enhancement in degradation efficiency could also be achieved by using 28 kHz ultrasonic waves in combination with the UV/KPS process. The intermediates of formic acid, malonic acid and oxalic acid were identified by GC/MS analysis and a reaction pathway was proposed. The scavenging role of ethanol and tert-butanol reagents was followed and it was revealed that HO center dot and SO4 center dot- radicals had, respectively, 53.7 and 46.3% contribution in the process degradation. In addition, the reaction kinetic was followed and a pseudo first order reaction was appropriate within conventional temperature range of 15-40 degrees C. Meanwhile, the electrical energy consumption was determined and the total operating costs, for one order of magnitude degradation, was estimated as 2.92 $/m(3).' (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.