The disinfection effect of a novel continuous-flow water sterilizing system coupling dual-frequency ultrasound with sodium hypochlorite in pilot scale

In this paper, a self-designed novel continuous-flow water disinfection system coupling dual-frequency ultrasound (US) with chemical disinfectant sodium hypochlorite (NaClO) was tested in a pilot scale using a simulated effluent containing Bacillus subtilis (B. subtilis), one of the indicators of water treatment efficiency. A suspension having a B. subtilis concentration of approximately 10⁴CFU/mL was introduced into the system to (1) investigate disinfection efficiency of US pretreatment with NaClO (US+NaClO) and simultaneous US and NaClO (US/NaClO) disinfection under different single frequencies; (2) further examine the disinfection efficiency of these two processes with dual-frequency US; and (3) identify dosage reduction of chlorine in this disinfection system. The results demonstrated that lower dual-frequency (17kHz+33kHz) US pretreatment with NaClO disinfection and simultaneous higher dual-frequency (70kHz+100kHz) US and NaClO were beneficial to bacterial inactivation in terms of sterilizing efficiency. It has also been observed that US pretreatment with lower combination of 17+33kHz frequencies showed better enhancement in which log reduction reached to 3.82 after 10min chlorine reaction (chlorine alone was 0.22 log reduction), nearly 1 log reduction higher than single frequencies at the same constant power. Consequently, at equivalent power dissipation levels, US of lower frequencies combination pretreatment with NaClO disinfection performed such a promising process that one-thirds (from 12mg/L NaClO reduced to 8mg/L NaClO) of the required NaClO dosage was reduced for the ideal disinfection efficiency of 4 log reduction, namely 100% disinfection. And the utilization efficiency of NaClO was increased from 37.67% to 85.25% in 30min of treatment time using an optimized combination of pretreatment and chlorination.