October 23, 2017
The designations employed and the presentation of material throughout this publication do not imply the expression of any opinion whatsoever on the part of UNESCO concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The ideas and opinions expressed in this publication are those of the authors; they are not necessarily those of UNESCO and do not commit the Organization.
Linden, K. and Murphy J.R. (2017). Physical Agents. In: J.B. Rose and B. Jiménez-Cisneros (eds), Water and Sanitation for the 21st Century: Health and Microbiological Aspects of Excreta and Wastewater Management (Global Water Pathogen Project). (C. Haas (eds), Part 4: Management Of Risk from Excreta and Wastewater - Section: Disinfection), Michigan State University, E. Lansing, MI, UNESCO. https://doi.org/10.14321/waterpathogens.70
|Last published: October 23, 2017|
Disinfection is the critical final process in the management of wastewater and excreta for the protection of human health. Pathogens in wastewater can be inactivated or destroyed by either chemical or physical processes. Physical means of disinfection do not involve the addition of chemicals, but disrupt normal microbial function or cause structural damage to pathogens through physical means. An effective physical disinfection process is safe, energy efficient, consistently effective, and cost-effective at a larger scale.
Physical methods to disinfect water include UV irradiation, heat, sunlight exposure, sonic or hydrodynamic pressure and radiation. These processes inactivate pathogens by denaturing proteins (heat, sunlight), causing mechanical stress to the pathogen that leads structural damage (sonication, hydrodynamic, cavitation), or through nucleic acid (DNA/RNA) and protein damage (UV, sunlight, radiation) to the pathogen. Some of these processes can also create reactive oxygen species within the water matrix that cause oxidative damage to pathogenic microorganisms.
With the exception of UV irradiation, physical disinfection processes are not as commonly used as chemical means for large-scale treatment. However, sunlight irradiation and heat inactivation are important processes for the development and emergency water treatment contexts and are widely practiced on the household scale worldwide.
This chapter describes the most common methods of physical disinfection, their basis of action, the factors that impact their effective application, their effectiveness against pathogens, and basic design considerations.
Physical agents such as light, heat, pressure and radiation can be applied to water to achieve positive changes in water quality, such as disinfection of pathogens or destruction of pollutants. While these physical processes do not involve direct addition of chemicals, they often act by generating reactive chemical species, leading to reactions that transform various constituents in water or breakdown essential physical properties of microorganisms, leading to inactivation. Alternatively, physical agents may directly transform the water chemistry through transfer of heat, pressure or light energy to a receptive chemical in water, leading to transformation and degradation. These same physical agents can directly damage microorganisms via mechanisms such as DNA or RNA damage, cellular membrane damage, and protein or enzyme denaturation. This chapter describes the most common physical agents for disinfection and the factors that impact their effective utilization. Inactivation data for each of the reported physical processes was compiled from the literature and organized into tables of raw data that are available online. Summaries of these data are presented below to illustrate the disinfection effectiveness of each treatment process.
Heat is one of the oldest, simplest, and most effective methods of disinfection. Heat sterilization works by denaturing or coagulating proteins and enzymes within the virus or cell, causing microbial death. Oxidative damage can play a role in heat inactivation of microbes, and cell lysis may also be induced via heating. Dry heat can inactivate microbes through dehydration. Virus inactivation via heat occurs through nucleic acid or capsid protein denaturation (Popat et al., 2010). Heat inactivation of microbes typically follows first order kinetics and the heat inactivation constant varies by organism (Casolari, 1988).
Heat is applied to water or sludge until the minimum temperature required for inactivation is reached, and the heated matrix is held at temperature for a specified period of time. The necessary temperature for inactivation varies based on matrix, target microbe, time of exposure, pH, and elevation. For the heat treatment of wastewater sludge, the US EPA recommends heating to a temperature of 70°C for 30 minutes or more for sludge pasteurization (USEPA, 2003; Aitken et al., 2005). This standard is intended to ensure full inactivation of helminths and enteric viruses, and is therefore, very conservative. Complete inactivation can likely be accomplished in a much shorter time period in a system with good reactor hydraulics (Aitken et al., 2005). Lower temperatures require longer times, whereas high temperatures will be effective in shorter time periods. In cases where no method of thermometry is available, it is recommended that water be heated to boiling for 1 minute, and then cooled to room temperature without the addition ice to ensure complete inactivation in drinking water (World Health Organization, 1993–1998; World Health Organization, 2011), though no method for wastewater is specified. This recommendation applies to all altitudes.
There are several methods of supplying heat for disinfection, including open flame from fuels such as wood, oil and gas, resistive heating, solar heating and microwave irradiation. Solar and microwave heating are discussed briefly below.
For heat, the critical design parameters are temperature and time water is held at temperature. Therefore, some form of thermometry must be employed to efficiently apply heat inactivation. Alternatively, boiling can be used to indicate effective treatment in the absence of a direct measure of heat.
Water or sludge can be heated with solar energy for microbial inactivation. Many types of reactors exist to collect heat from solar irradiation and transfer it to the media to be disinfected (Abraham et al., 2015). Solar heating is often used in conjunction with UV and visible light inactivation but can also be used alone as in cases of black box solar heaters or solar water heat exchangers where the media is not exposed to light (Safapour and Metcalf, 1999). Because sunlight may not supply enough heat for boiling to occur, a thermometer or indicator is required to ensure that the media has reached a temperature sufficient for disinfection (Ray and Jain, 2014). While solar methods can be inexpensive, and do not require fuel, they may suffer from low temperatures, long heating times, inconsistent heating, inefficiencies, no indicator for process performance, and inconsistent availability of sunlight.
Waves of the frequency 0.3-300 GHz applied to a medium are absorbed by polar molecules. Microwaves are lethal to microorganisms in the range of 1-350 MHz with peak lethal effects at 60 GHz (Fleming, 1944; Leonelli and Mason, 2010). The ability for microwaves to affect proteins is highly dependent on the bound water content of molecules. Absorbed energy is converted into heat energy within the medium resulting in increased temperature. There is some debate as to whether the effect of microwave disinfection is purely due to elevated temperatures, or if there is a distinct “microwave microbial effect” (Park et al., 2002; Hong, Park et al., 2004; Park et al., 2006). Microwaves have the advantage of fast heating and lower energy consumption compared to some boiling methods but they require more equipment and technical expertise than other methods of heating and require a reliable supply of electricity. Although microwave processing has been used at the industrial scale for other applications (Leonelli and Mason, 2010), there are no known full-scale microwave reactors for wastewater or sludge disinfection (Foladori et al., 2010).
Heat is a universally effective method of disinfection for all pathogens, although the effectiveness varies depending on the target pathogen for disinfection. Heat tends to be most effective on bacteria, less effective on helminths and viruses. In general, bacteria have low resistance to heat inactivation with spore forming bacteria being more resistant than non-spore forming, gram positive bacteria more resistant than gram negative, and cocci more resistant than rods (Backer, 1996; Mocé-Llivina et al., 2003; Spinks et al., 2006; da Silva Aquino, 2012). Spore forming bacteria exhibit the highest resistance to thermal inactivation of all waterborne pathogens. Protozoan cysts can be inactivated by brief exposure to moderate temperatures (Jarroll et al., 1984). Figure 1 shows the relationship between time and temperature to achieve inactivation of several common pathogens. Figure 2 illustrates the time required for one log10 inactivation of selected pathogens and surrogates under various heating conditions. Comprehensive compiled data on heat inactivation is reported in the tables of raw data. An excellent outline of testing and validation considerations is given in (World Health Organization, 2011).
Figure 1. Susceptibility to heat inactivation varies widely between pathogens, and is a function of both exposure time and temperature (Feachem et al., 1983)
Figure 2. Time for one log10 inactivation of some pathogens and surrogates under various heat conditions (From Table YY – Raw Data tables)
UV irradiation, in the form of photons, inactivates microorganisms primarily by causing damage to nucleic acids and intracellular proteins. The energy from the photons is absorbed by the nucleic acids causing formation of pyrimidine dimers and other photoproducts that inhibit replication and transcription, leading to inactivation. The most effective UV wavelengths for disinfection are those that are most efficiently absorbed by nucleic acids, primarily in the UVC 200 to 280 nm range, with a peak absorbance typically around 260 nm.
UV light is commonly generated from mercury vapor lamps which can be low pressure (LP) lamps emitting primarily at 254 nm or medium pressure (MP) lamps that emit a polychromatic spectra in the 200 to 300 nm range as well as beyond (Figure 3). LP lamps run at about 35% efficiency, whereas MP lamps are around 15% efficient, but with a much higher power output. Some comparisons between LP and MP lamps are detailed in Table 2.
Figure 3. Spectral emittance of a low pressure mercury UV lamp (solid line) and a medium-pressure mercury UV lamp (dashed line) (From Bolton and Linden, 2003)
Alternatives to mercury-based LP and MP lamps include UV light emitting diodes (UVLED), excimer lamps, and pulsed xenon lamps, but these sources are not yet widely commercially available for use in water and wastewater treatment. UVLEDs and excimer sources emit a narrow band of light in distinct wavelength ranges of the germicidal spectra (Oppenländer et al., 1995; Naunovic et al., 2008; Chevremont et al., 2012; Nelson et al., 2013; Wang et al., 2013). Pulsed UV systems are broadband emitters that emit at peak powers higher than conventional UV light sources, which may offer some advantages (Bohrerova and Linden, 2006; Metcalf and Eddy, 2014).
UV lamps are housed in UV disinfection systems, or reactors, operated in continuous flow-through mode, but can be operated with open channel flow or in a closed pipe system. UV lamps are typically encased in a quartz tube, to isolate lamps from water contact and to control lamp temperature. The lamp and tube are immersed in water for treatment. The water treatment time to achieve disinfection is on the order of seconds of UV exposure. Pictures of LP and MP systems for UV disinfection of wastewater are shown in Figure 4 (USEPA, 1986).
Figure 4. Pictures of UV disinfection wastewater systems. Left shows a low pressure lamp open channel system and right shows a medium pressure lamp system
UV disinfection has been effectively employed for wastewater treatment for years, and the US EPA as well as other organizations have published design manuals for UV disinfection of wastewater and for water reuse (USEPA, 1986; Blatchely et al., 2003; Miner, 2004; Bolton and Cotton, 2011; National Water Research Institute, 2012). The UV dose is the measure of disinfectant applied to a water. The UV dose (units of mJ/cm2 or J/m2) is the product of the UV light intensity penetrating the water (in Ww/cm2 or W/m2), multiplied by the time of exposure (in seconds), or hydraulic residence time. Note that the UV light intensity varies throughout the UV disinfection system as a function of distance from the lamp, absorbance of UV light by the water, and any reflecting or refracting surfaces present. UV doses for disinfection are typically in the 20 to 200 mJ/cm2 range (200 to 2000 J/m2). In general, bacteria and protozoan (oo)cysts such as Giardia and Cryptosporidium are more sensitive to UV, while viruses and bacterial spores are more resistant to UV (Hijnen et al., 2006). Ascaris suum (Brownell and Nelson, 2006) and adenoviruses (Hijnen et al., 2006) are two of the more resistant pathogens to UV, requiring a higher dose for disinfection. A very comprehensive review of UV doses required for inactivation of various pathogens and indicator organisms was recently published (Malayeri et al., 2016), and the wastewater relevant data are included in the tables of raw data at the end of the chapter. A summary of the UV doses required for 4 log10 inactivation of select pathogens is presented in Figure 5 and a snapshot of the data specific to UV disinfection in wastewater, taken from the tables of raw data, is presented in Figure 6. Some advantages and disadvantages of UV light disinfection are presented in Table 3.
Figure 6. UV inactivation of select pathogens and indicators in wastewater (From Table YY – Raw Data tables)
 Keller et al., 2003;  Tree et al., 1997;  Gehr et al., 2003;  Jacangelo et al., 2003;  Guo et al., 2009;  Taghipour, 2004;  McKinney and Pruden, 2012;  Li et al., 2009;  Huang et al., 2016;  Hallmich and Gehr, 2010
Water quality parameters important to UV disinfection effectiveness are primarily UV transmittance (UVT) at 254 nm, and particulate concentration. While UV disinfection systems can be designed for any UVT, most wastewaters are over 50% UVT. Wastewaters meeting US EPA secondary standards for suspended solids (30 mg/L TSS) are commonly disinfected with UV light. Particles can interfere with UV inactivation and for high level disinfection, filtration is recommended (Emerick et al., 1999; Loge et al., 2001). Other water components like oils or hardness can foul quartz tubes, reducing UV intensity delivered to the water (Metcalf and Eddy, 2014). Therefore, automated cleaning systems are sometimes used, or lamps are periodically cleaned as part of routine maintenance.
For UV-based processes, the UV dose is the critical design parameter. UV dose will be affected by lamp power, UV absorbance of the water, spatial distribution of the lamps, the water flow rate and hydraulic mixing regime. It is critical to maintain indication of the lamp power status, to control the flow rate, and to periodically monitor the UV transmittance. These parameters will impact the consistent delivery of an effective UV dose to the wastewater. Therefore, a validation procedure should establish set points of these parameters that indicate effective treatment and these points programmed into the system operation. UV transmittance data and flow rate data should be gathered over a time period to include normal variations in wastewater flow and input, including seasonally, to properly design a UV system. An excellent outline of testing and validation considerations is given in (World Health Organization, 2011).
Sunlight uses the action of UVA (315 to 400 nm), UVB (280 to 315 nm), visible light and heat to disrupt microbial activity. There are two recognized pathways by which sunlight can inactivate microorganisms. One is directly, where light (mainly UVB) is absorbed by DNA or RNA, damaging the nucleic acids and blocking replication of the microorganisms, similar to conventional UV disinfection. Sunlight also acts indirectly, when natural or effluent organic matter, humic materials, molecular oxygen and other photosensitive molecules in solution absorb photons to produce reactive oxygen species such as singlet oxygen, superoxide, hydrogen peroxide or hydroxyl radicals. These radical species inactivate cells by damaging cell membranes, transport systems, virus protein capsids, and nucleic acids, and by disrupting respiration and amino acid synthesis. The sensitizers can be present within the cell (endogenous), or in solution (exogenous) (Davis-Colley et al., 2000).
An additional element of inactivation by sunlight is thermal inactivation. As water is exposed to sunlight, its temperature increases and microorganisms can be thermally inactivated (See section on heat inactivation). There are some synergetic effects between optical and thermal effects of solar water treatment (Šolić and Krstulović 1992; Wegelin et al., 1994; McGuigan et al., 1998; Berney et al., 2006; Gómez-Couso et al., 2010; Theitler et al., 2012; Giannakis et al., 2014; Carratalà et al., 2015). Most methods of improving the efficiency of sunlight-based water treatment involve maximizing or concentrating sunlight interaction with water (Saitoh and El-Ghetany, 2002; Rijal and Fujioka, 2004; Mani et al., 2006), increasing temperature of the water (Sommer et al., 1997; Rijal and Fujioka, 2001; Rijal and Fujioka, 2004; Martín-Domínguez et al., 2005), or augmenting the concentration of sensitizers in solution by adding photoinducers, photocatalysts, or oxygenating the liquid matrix (Heaselgrave et al., 2006; Fisher et al., 2008; Harding and Schwab, 2012).
Solar Disinfection (SODIS) is a popular method of using sunlight to disinfect drinking water in developing countries (Wegelin et al., 1994; EAWAG, 2006). While effective for drinking water, this type of small batch treatment is not conducive for wastewater treatment. Several experimental continuous flow reactors have been developed for solar water disinfection, which may also be applied to wastewater. In this type of reactor water enters one end and slowly travels through a long narrow reactor tube while the system is exposed to sunlight. There is typically some form of metallic concentrator behind the reactor tube to increase the amount of sunlight entering the reactor tube (Navntoft et al., 2008; Ubomba-Jaswa et al., 2010; Bigoni et al., 2012; Bigoni et al., 2014; Nararom et al., 2015). Solar mirrors can be applied to batch and flow through systems to increase the radiation absorbed by the water; mirrors can be parabolic, compound parabolic, or V shaped. Specialized Fresnel collectors can also be used although they require a large footprint and capital investment. Because of the long exposure times, and limited reactor size, SODIS is not used on any significant scale.
Waste Stabilization Ponds are a very common, simple and well-established approach to using sunlight to disinfect wastewater. Solar irradiation works along with adsorption, settling, predation and biological inactivation in wastewater ponds to accomplish inactivation (Mayo, 1995; Davies-Colley et al., 2005; Bolton et al., 2010). Because solar ponds are not enveloped in plastic or glass as in SODIS, UVB inactivation is more important for solar inactivation in ponds. However, sunlight is rapidly attenuated as water depth increases, with the UVB portion of the spectrum mostly attenuated within the first 10 cm of depth. Direct solar inactivation likely only occurs at the pond surface (Verbyla and Mihelcic, 2015). Polishing ponds at the end of a treatment train achieve better sunlight inactivation than facultative ponds, which in turn achieve better inactivation than anaerobic ponds, because of greater sunlight penetration through more polished water and higher concentrations of dissolved oxygen in these types of ponds (Bolton et al., 2010).
Inactivation by solar radiation is dependent on many factors. The effectiveness of the sunlight reaching the water and the target pathogens in the water is affected by depth of liquid, weather conditions, the hydraulic retention time, solar intensity, optical quality and turbidity of the water matrix, as well as the water temperature, pH, and presence of potential oxidizing agents. Solar inactivation is highly effective over time against bacteria and has been shown to be effective against MS2 virus (Fisher et al., 2012; Theitler et al., 2012), phiX174 (Mattle et al., 2015) and poliovirus (Heaselgrave et al., 2006; Love et al., 2010; Silverman et al., 2013). Indirect inactivation of viruses through photosensitized reactions has been demonstrated and may be effective for virus inactivation in solar pond systems (Kohn et al., 2007; Kohn and Nelson, 2007; Mostafa and Rosario-Ortiz, 2013; Nguyen et al., 2015; Silverman et al., 2015). Figure 7 illustrates some data for sunlight inactivation of indicator organisms and pathogens in wastewater. These data and others are compiled in the table of raw data.
Figure 7. Time for 1 log10 inactivation of indicator organisms in wastewater under sunlight (From Table YY – Raw Data tables)
For solar-based systems, such as waste stabilization ponds, the solar irradiance, solar duration, depth of the liquid and hydraulic retention time are the critical design parameters. Parameters for solar pond designs are provided in manuals and publications (Middlebrooks et al., 1983; Davies-Colley et al., 2005; Bolton et al., 2010; Nguyen et al., 2015). An excellent outline of testing and validation considerations is given in (World Health Organization, 2011).
Sonication, also known as power ultrasound or acoustic cavitation, involves the generation, growth and collapse of microscopic voids in a liquid matrix. Ultrasound waves at a frequency of 16 kHz-100 MHz are applied to a liquid to create pressure oscillations inside the liquid matrix. When the negative pressure created by the ultrasonic wave exceeds the attractive forces in the liquid, microscopic cavities are formed. Some of the liquid matrix is vaporized and contained inside the cavities, which undergo successive growth and compression cycles until they are eventually destroyed in violent collapse. When these bubbles collapse rapidly, areas of locally elevated temperatures (~5,000 K) and pressures (500-5,000 atm) are created (Doosti et al., 2012). Because these conditions are short lived and occur over very small volumes, the temperature and pressure in the bulk fluid is only slightly elevated. This entire process happens on the scale of milliseconds (Naddeo et al., 2014) and millions of voids are simultaneously present in the reactor.
Inactivation of microorganisms by sonication can occur during the pressure release when the cavity collapses, typically via mechanical failure of the cell wall or membrane, either instantaneously or through mechanical fatigue (Antoniadis et al., 2007). Other mechanisms include shearing forces from microjets (Leonelli and Mason, 2010), formation of highly reactive oxidative radicals, or areas of intense local heat (Thompson and Doraiswamy, 1999).
The main types of sonication systems are either baths or wands. The bath type is a liquid filled batch reactor with one or more ultrasound generators underneath the reactor cavity. Wand type sonicators, also known as horn type sonicators, consist of a sonicator probe that is placed directly into the liquid matrix. Both types of reactors can be operated in batch or flow-through configurations. Power intensities of tens to hundreds of watts per liter of liquid are applied for disinfection. Electricity applied generates ultrasound waves at a frequency of 16 kHz-100 MHz, which create pressure oscillations that generate microscopic cavities inside the liquid matrix. In these cavities, disinfection takes place via the various mechanisms discussed above. Sonication has been used in conjunction with UV to reduce particle size and particle-microbe association, thus decreasing the required UV dose for complete disinfection (Blume and Neis, 2005).
The efficacy of sonication for inactivation of microorganisms depends greatly on the parameters that affect the process performance, and there is not a large body of data from which to draw broad conclusions. Figure 8 illustrates the log10 reduction of indicators and pathogens under various sonication conditions. Other data are presented in the table of raw data. Based on these data, sonication is effective against total and fecal coliforms, and less effective against bacterial spores. Important parameters affecting process performance include, the frequency applied, irradiating area, number of transducers, and the sonication intensity. The properties of the water also impact sonication including vapor pressure, surface tension, temperature, and impurities. Catalysts are often added to increase efficiency by creating more nuclei for cavitation. Sonication can produce water that meets regulatory standards for wastewater reuse but is not considered to be cost effective (Drakopoulou et al., 2009).
Figure 8. Log10 inactivation of indicators and pathogens under various sonication conditions (From Table YY – Raw Data tables)
Sonication processes require electrical input and some allow frequency adjustment. These parameters, in addition to time should be optimally adjusted. The size and shape of the sonication reactor as well as placement of sonicators can also be adjusted to enhance inactivation.
Radiation-based inactivation of microorganisms in wastewater is thought to occur both directly and indirectly, similar to sunlight inactivation. The direct effect is when the radiation causes direct damage to molecules in pathogens. The primary target of direct inactivation is DNA and RNA. Indirect inactivation is caused by radiation interacting with water constituents to form reactive oxygen species (e.g., HO•, O-2), which then oxidize pathogens in solution. Radiation can also neutralize particle charges, which can change the sedimentation behavior of wastewater. Parameters affecting efficacy of treatment include: dose rate, dose distribution, radiation quality, radiation type, exposure patterns, microorganism species, temperature, moisture content and oxygen concentration.
There are three main methods that can be used to apply radiation to wastewater and sludge. The three methods are typically considered equivalent at the same radiation dose.
The advantages of electron beam radiation are there is no radioactive material to store, the beam can be turned off, it operates at a high power per unit of working surface area and it is relatively simple. However, the penetrating power of the beam is very low (~3mm/ MeV) so the sludge or water layer must be constricted into a layer of a few centimeters for effective disinfection (Hashimoto et al., 1986; Cooper et al., 1998). There are currently no known full scale e-beam systems for the treatment of municipal wastewater or sludge in existence (Metcalf and Eddy, 2014).
The high-energy particles emitted from radioactive or e-Beam sources are effective for disinfection of water, sludge, food and other media, but viruses and enzymes are less sensitive than bacteria. Figure 9 reports the dose of gamma radiation required for the inactivation of some bacteria and indicators. Other data are presented in the table of raw data. Literature indicates that an absorbed dose of between 2-4 kGy is enough to inactivate pathogens to safe level (Borrely et al., 1998; Rawat et al., 1998; Tahri et al., 2010; Jebri et al., 2013). Multiple authors have found no evidence of bacterial regrowth in wastewater and sludge when treated with radiation doses as low as 0.5-25 kGy (Sawai et al., 1993; Basfar and Abdel Rehim, 2002; Sabbagh et al., 2014) Radiation can also help in the elimination of odors, improve dewatering and improve conversion of non-biodegradable compounds to more readily degradable compounds (Gazso, 1992; Von Sonntag, 1994; Borrely et al., 1998; Cooper et al., 1998; Gehringer et al., 2003; Tahri et al., 2010; Verde et al., 2016).
Figure 9. Dose of gamma radiation required for the inactivation of bacteria and indicators (From Table YY – Raw Data tables)
 Farooq et al., 1993;  Tahri et al., 2010;  Basfar and Abdel Rehim, 2002;  Verde et al., 2016;  Borrely, 1995;  Hashimoto et al., 1986;  Rawat et al., 1998;  Sawai et al., 1993
Two other technologies that have shown promise for disinfection of complex water matrices in laboratory and pilot settings are Pulsed Electric Field and Hydrodynamic Cavitation.
Pulsed electric field systems utilize two electrodes producing an electric field that generates UV irradiation, reactive oxygen species, acoustic and shock waves. This process is often used in conjunction with chemical disinfectants to reduce the chemical dose requirement. These processes lead to mechanisms of inactivation that include structural fatigue, mechanical stress and induced hydrophilic pores in the cell membrane (electroporation), in addition to UV and oxidative disinfection (Anpilov et al., 2002; Gusbeth et al., 2009; Frey et al., 2013). While it is relative unaffected by particles, it is comparatively expensive (Yadollahpour et al., 2014) and electrodes can decay, requiring regular replacement. Bacterial spores are more resistant to pulsed electric fields than vegetative cells but it has been shown to be broadly effective for disinfection (Gusbeth et al., 2009; Poyatos et al., 2011; Frey et al., 2013). Haas and Aturaliye found that the application of pulsed electric fields could synergistically enhance disinfection by chlorine in river water and phosphate buffer solution, and that chlorine was in fact necessary to achieve appreciable disinfection levels (Haas and Aturaliye, 1999).
Hydrodynamic cavitation follows a similar principle to sonication, but achieves the high pressures and local temperatures by pumping or rapid mixing of the process solution. Types of reactors (Gogate, 2011; Gogate and Pandit, 2011) include shock wave, high-speed homogenizer, low or high-pressure systems, liquid whistle, and steam injection. The process is thought to be about an order of magnitude more efficient than sonication or acoustic cavitation and less expensive because of the simpler operation (Gogate and Pandit, 2011). Like sonication, it uses no chemicals, produces no disinfection byproducts and has side benefits for sludge handling, but operates at low efficiency compared to other disinfection processes.
Plasma can be used for water disinfection, as well as for the removal of certain organic water contaminants. The process typically involves the application pulses of high-energy electrical discharges either into gas or liquid. The process can be configured to apply the discharge either directly to the water matrix or directly above the surface of the liquid matrix. The process disinfects via several different mechanisms. The discharge generates hydroxyl, and other highly reactive radicals (H, HO2, O2, O3, H2O2, H2) in situ, which react with chemical and microbial contaminants in solution. Plasma channels reach temperatures of several thousand Kelvin, disinfecting the matrix thermally. Plasma emissions are accompanied by UV, visible light emissions and shock waves that all contribute to disinfection (Stratton et al., 2015). The mechanisms for microbial inactivation is thought to be irreversible membrane destruction through membrane compression for bacteria, and electroporation (Hamilton and Sale, 1967; Sale and Hamilton, 1967) and RNA and DNA damage in viruses (Mizuno et al., 1990). The relative importance of the different physical and chemical mechanisms is dependent on both the strength of the discharge and the makeup of the matrix (Locke et al., 2006). Plasma disinfection is still in the development stage and is not currently used to treat wastewater at the full scale. The primary limitations of this process are the very high-energy consumption and safety of the process.
Organic matter, in the form of natural organic matter or effluent organic matter, affects physical disinfection processes in both negative and positive ways. It is typical measured and represented by chemical or biological oxygen demand, or by total or dissolved organic carbon, but methods such as UV-vis spectroscopy and fluorescence spectroscopy can be used to understand its character. For sunlight and UV based processes, organic matter can increase the UV or solar absorbance, screening light from target pathogens, and decreasing effectiveness, requiring larger treatment systems. In solar processes, organic matter can benefit treatment by producing reactive oxygen species upon the interaction of sunlight with some types of organic matter, typically effluent organic matter in wastewater systems (Dong and Rosario-Ortiz, 2012). Organic matter has little impact on heat or pressure-based inactivation mechanisms. The presence of biologically degradable organic matter in wastewater is a potential avenue for regrowth of microbes following application of disinfection, if a chemical disinfectant residual is not maintained. Therefore, it is recommended that the presence of organic matter be minimized to optimize the efficiency of some physical disinfection processes.
Minerals, salts and metals in wastewater could cause some interference in the operation of UV disinfection through lamp sleeve fouling (Lin et al., 1999; Blatchely et al., 2003). UV lamp sleeves can be easily cleaned manually or automatically. The presence of some ions can also cause increases in water absorbance. For instance, the presence of iron or nitrate can screen some light wavelengths and decrease their penetration in solar and UV systems. Alternatively, these same ions may also contribute to the indirect inactivation via production of reactive oxygen species (Kohn et al., 2007; Kohn and Nelson, 2007; Kadir and Nelson, 2014). Inorganics have little known effect on the efficacy of heating and pressure systems but could result in scale build-up within the reactor.
Particles are ubiquitous in wastewater and can negatively impact disinfection processes. Particals can shield microorganisms from disinfection through enmeshment in flocs or shade microbes from a lethal dose of sunlight or UV light. Particle size distribution is a measure that can be used to better understand the likelihood of interference. Particle-microbe association is known to impact UV and solar-based processes but the operation of the wastewater system solids retention time can be adjusted to minimize the likelihood of a microorganism being enmeshed in a particle and protected from light-based disinfection (Emerick et al., 1999; Loge et al., 1999). Tertiary filtration is also a very effective means to minimize the effect of particles on disinfection through removal of particles from the water matrix.
Physical disinfection processes have the benefit of not adding any halogenated chemicals, therefore, traditional chlorinated or iodinated disinfection byproducts are not of concern. Furthermore, at the levels of treatment typical of wastewater disinfection systems, there are no expected byproducts or unintended consequences for public or environmental health that would result from utilizing physical disinfection processes.
A summary comparing the various physical disinfection processes discussed above is presented in Table 7.