Disinfection of Escherichia coli in the presence of primary sludge particulates

Fecal contamination is a problem in both primary sludge (PS) and combined sewer overflow (CSO), both of which contain significant amount of primary sludge particulates (PSPs) to complicate any disinfection efforts. In this work, disinfection of Escherichia coli (E. coli) in the presence of PSPs using power ultrasonication (20 kHz), electrochemical (EC), and ultraviolet (UV) techniques was studied.;The role of PSPs in ultrasonic disinfection of E. coli was investigated. Entrapment of E. coli by PSPs was directly observed through scanning electron microscopy (SEM) after E. coli and PSPs were incubated together in water for 24 hours at 35 °C. An entrapment coefficient was proposed for the first time to reflect the ability of PSPs to entrap E. coli and was estimated as 1.4x10 3 CFU/mg PSPs under experimental conditions. Ultrasonication (20 kHz) of different E. coli-PSP solutions showed that the entrapped E. coli cells were protected by PSPs from ultrasonication and the unentrapped cells were not. However, the protection of entrapped E. coli cells gradually decreased as ultrasonication proceeded suggesting the ability of power ultrasonication to deprotect the entrapped E. coli cells. SEM studies suggested a two-step mechanism for ultrasonic (20 kHz) disinfection of entrapped E. coli: breakdown of the protective PSPs refugia followed by disinfection of the exposed E. coli cells.;EC residual disinfection of E. coli in the presence and absence of PSPs was investigated. Kinetics for EC residual disinfection of E. coli followed a first-order rate law. When PSPs were absent, the EC residual disinfection rate coefficient (k) increased linearly with EC pretreatment energy (EC, 0-2.3 kJ/L). However, in samples that contain 143 mg PSP/L, k first increased linearly with EC (0 to 1.0 kJ/L) and then decreased linearly with EC (1.0-1.5 kJ/L). A significant amount of H2O2 was detected only in PSP-free samples during EC pretreatment and the concentration of H2O 2 (CH) correlated with EC (0-3.0 kJ/L) linearly. Chloride concentration (CC) changed in two patterns during EC pretreatment of aqueous PSPs (143 mg PSP/L): decrease of CC when EC increased from 0 to 1.0 kJ/L and increase of CC with EC further increased from 1.0 to 1.5 kJ/L. In each of the two patterns, k was correlated to the initial post-EC chloride concentration (C CI) in an inverse linear relationship. The two-stage change of both CC and k was caused by a combination of two reactions: anodic oxidation of chloride and the reaction of chloramines with excess chlorine.;Direct and residual UV disinfection in the presence of PSPs were investigated. Our results indicate that the reciprocal of first order direct UV disinfection rate coefficient (k) was positively correlated with PSPs concentration by a linear function. An interior wall made of UV reflective DRPRTM (diffuse reflective product, manufactured by W. L. Gore & Associates, Inc.) improved the performance of direct UV disinfection. Optimal size and dosage of suspended DRP discs exist for the best performance of direct UV disinfection. UV pretreatment (254 nm for 600 s) of PSP-free aqueous samples generated a significant residual disinfection effect and presence of PSPs (143 mg/L) inhibited the residual disinfection significantly. When H2O2 (10 mg/L) was added during UV pretreatment (254 nm for 600 s), presence of PSPs (143 mg/L) resulted in significant UV-H2O2 synergetic disinfection; however, no positive UV-H2O2 synergetic disinfection was observed when PSPs were absent.;This work evaluated disinfection through ultrasonication, EC technique, and UV technique in the presence of PSPs, and proposed feasible strategies for disinfection of PS and CSO.