Pathogen and Particle Associations in Wastewater: Significance and Implications for Treatment and Disinfection Processes

Abstract

Disinfection guidelines exist for pathogen inactivation in potable water and recycled water, but wastewater with high numbers of particles can be more difficult to disinfect, making compliance with the guidelines problematic. Disinfection guidelines specify that drinking water with turbidity ≥1 Nephelometric Turbidity Units (NTU) is not suitable for disinfection and therefore not fit for purpose. Treated wastewater typically has higher concentrations of particles (1-10NTU for secondary treated effluent). Two processes widely used for disinfecting wastewater are chlorination and ultraviolet radiation. In both cases, particles in wastewater can interfere with disinfection and can significantly increase treatment costs by increasing operational expenditure (chemical demand, power consumption) or infrastructure costs by requiring additional treatment processes to achieve the required levels of pathogen inactivation. Many microorganisms (viruses, bacteria, protozoans) associate with particles, which can allow them to survive disinfection processes and cause a health hazard. Improved understanding of this association will enable development of cost-effective treatment, which will become increasingly important as indirect and direct potable reuse of wastewater becomes more widespread in both developed and developing countries. This review provides an overview of wastewater and associated treatment processes, the pathogens in wastewater, the nature of particles in wastewater and how they interact with pathogens, and how particles can impact disinfection processes.

Keywords: Disinfection; Particles; Pathogens; Reuse; Treatment; Wastewater.

Figure 1

Major sources of wastewater contamination.

Figure 2

Schematic of a typical wastewater treatment.

Figure 3

Illustration showing the most common designs of wastewater sedimentation tanks (clarifiers): (A) rectangular or horizontal flow clarifier and (B) circular or radial flow clarifier.

Figure 4

Schematic of a modified activated sludge process that promotes biological removal of nitrogen and phosphorous.

Figure 5

Schematics of two common variations of standard pond systems. (A) One primary facultative pond with no pretreatment and (B) pretreatment using an additional anaerobic pond.

Figure 6

Simplified representation of the operating principles of a nephelometer. Light is directed from a light source to the sample through a narrow slit and the reflected light is collected by a detector and analyzed.

Figure 7

Comparison of the size distribution of different types of particles in wastewater.

Figure 8

Bacterial Floc: a typical structure of a bacterial floc held together by extracellular polymeric substances (EPS) associated with inorganic clay particles.

Figure 9

An environmental scanning electron microscope image of a mixed liquor particle in the size range of 90–106 μm highlighting its structure. Arrows indicate different compartments outlined by fibrils.

Figure 10

Attachment of Escherichia coli to organic and inorganic particles: Scanning electron microscopic image of (A) growth of E. coli attached to a diatom in a biofilm (B) E. coli attached to a clay particle. Scale bars indicate 1 μm.

Figure 11

Particle associated viruses: Transmission electron microscopy images of MS2 (left panels) and T4 bacteriophage (right panels). (A and B) Phage free in suspension; (C and D) phage associated with kaolin clay particles; (E and F) phage associated with humic acid flocs; (G) MS associated with a bacterial flagellum; (H) T4 associated with a sludge particle. Arrows indicate the bacteriophage.

Figure 12

Wastewater particle structural pathways: various interstitial diffusive layers of a wastewater particle.

Figure 13

Graph of chlorine inactivation of microorganisms illustrating a first-order disinfection curve (dashed line) and disinfection with tailing (solid line).

Figure 14

Typical UV inactivation curve for microorganisms comparing log inactivation versus UV dose, highlighting the steep inactivation slope representing inactivation of free microorganisms and a shallow slope representing tailing.

Figure 15

Limitations of UV radiation: different protective effects of particles on inactivation of pathogens by UV radiation.