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Fact Sheet Removal and Inactivation of Cryptosporidium by TreatmentIntroductionWater provides an excellent vehicle for the transmission of Cryptosporidium oocysts, which commonly occur in surface and recreational waters through faecal contamination from wildlife, livestock or humans. While significant removal and inactivation of oocysts can occur in terrestrial and aquatic environments (Removal and inactivation of Cryptosporidium from water), oocysts capable of infecting humans can persist and penetrate water treatment processes. Such events can lead to outbreaks of cryptosporidiosis from consumption of contaminated potable water, or via contact with contaminated recreational or inadequately treated re-use waters. Appropriate water treatment provides a key barrier to the entry of Cryptosporidium oocysts into these waters.

This factsheet provides an overview of the engineered processes that contribute to the physical removal or disinfection of oocysts in water and wastewater. While water treatment often uses combinations of these processes, treatment by removal and treatment by disinfection are considered separately herein. Pertinently, many of the largest waterborne outbreaks of cryptosporidiosis around the world have been caused by failures in water treatment systems or sub-optimal treatment; therefore, adequate operation of water treatment is a critical barrier in protecting public health.

Removal by treatment processesCoagulation-Flocculation

Coagulation-flocculation is a chemical water treatment method typically applied prior to sedimentation and filtration to enhance the removal of turbidity and, to a certain extent, natural organic matter (NOM) from waters. Cryptosporidium removal throughout all stages of the conventional treatment process is chiefly influenced by the effectiveness of coagulation, with poor coagulation conditions shown to negatively impact subsequent filtration performance for removing Cryptosporidium (1).

The majority of the data on treatment process performance for removing oocysts from drinking water have been obtained from pilot-tests, with a few studies performed in full-scale

conventional water treatment plants (2). Oocyst removals of between 1-1.5 log10 have been typically reported after coagulation and sedimentation (3, 4). The removal mechanism is related to particle charge and physical entrainment. In the case of the coagulant ferric chloride, oocysts maintain a negative charge and sweep flocculation (physical entrainment in the developing floc) is required for oocyst removal. Contrastingly, oocysts undergo charge neutralisation in the presence of alum, meaning that oocysts can adsorb to alum flocs as well as be removed by sweep flocculation (5). While coagulation-flocculation can effectively remove oocysts, aside from a small amount of mechanical damage, the chemicals used in the process do not appear to reduce oocyst infectivity (6).

Media filtration

Media filtration is commonly used in water treatment after coagulation-flocculation for the physical removal of turbidity and microorganisms. Few studies have examined filtration performance of Cryptosporidium oocysts separately from coagulation/sedimentation, with those studies reporting highly variable removals (0.5–2 log10)

(4, 7-9). Instead, most investigations have examined removals from combined processes. For example, monitoring of source water and drinking water produced by a conventional treatment plant (coagulation-flocculation, sedimentation and rapid filtration) measured a combined oocyst removal of 2.5 – 2.7 log10

(10). Yet, laboratory and pilot-scale experiments simulating a conventional treatment plant with slow sand filters have demonstrated that combined oocyst removals as high as 5 – 6 log10 are possible (11, 12). Based on modelled distribution profiles, removal efficiencies of 23 – 200 log10/m have even been claimed for bank filtration, a pre-treatment used in some parts of the world for drinking water and wastewater (13).

The substantial differences in reported oocyst removals for media filtration are likely to result from the numerous influences that affect filtration efficiency. These factors include, but are not limited to: media type, grain size, presence of biofilm and NOM, predation, ionic strength, and pH (2). Additionally, operational parameters such as hydraulic load, chemical pre-treatments, and backwashing regimes can greatly affect removal efficiency. While media filtration

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can remove oocysts, its efficacy is highly reliant on pre-treatment processes and the many factors shown to affect removal efficiency. Though a number of these factors have been well studied, some, like biological processes, may play a significant part in the removal of oocysts but are not yet well understood or quantified (14).

Dissolved air flotation

Treatment of surface waters and wastewaters by Dissolved Air Flotation (DAF) uses coagulants to develop flocs, which are floated to the surface using aeration, collected from the top of the reactor and disposed of as sludge (15). The effectiveness of DAF, like conventional flocculation processes, is affected by turbidity and natural organic matter. Oocyst removals of 1.7 – 2.5 log10 have been reported, increasing to > 5.4 log10 when combined with filtration (DAFF) for drinking water (16). In the case of DAFF, management of filter backwashing is critical, as backwash recycled to the front of the plant contributes to oocyst build-up in the influent.

Membrane filtration

Membrane filtration systems can provide excellent removal of oocysts, through size exclusion. However, most research on membrane filters in terms of Cryptosporidium removal has been in relation to the concentration of oocysts from large volume water samples, rather than validating the technology for oocyst removal. Full-scale validation of membrane filters more often measures removal of bacteria (such as E. coli) and use calculated log10 removal values (LRV) for smaller organisms to provide a conservative LRV for the larger oocysts (2). In practice, the performance of filters is higher than the maximum LRV that will be credited by health regulators. As a guiding principle, health regulators (at least in Australia) will not provide a credit of more than 4 LRV for any individual treatment barrier. In the case of membranes this is partly driven by limitations in real-time detection of membrane failure; for example, parameters such as turbidity may not detect failures that will allow breakthrough of small particles (such as viruses) while still removing larger particles and producing acceptable levels of turbidity in the filtrate.

Activated sludge processes

Common conventional wastewater treatment processes, such as the activated sludge process (ASP), are designed to reduce nutrients to a level suitable for environmental discharge, with pathogen removal often a secondary consideration. However, being an active biological process, removal or inactivation of oocysts may occur through entrapment in biological flocs or via predation. Oocyst removals quantified at full-scale wastewater treatment plants have been reported to be highly variable, ranging from as low as 0.2 to > 3 log10

(9, 17-19). While different configurations for ASP may have a large effect on removal efficiency, seasonality and inflow variability were recently identified as significantly affecting processes (17). Though oocysts could be removed by secondary treatment processes, no decrease in oocyst infectivity was identified across any of the five ASPs investigated in this study, emphasising the importance of tertiary treatment due to the risk posed by infectious oocysts remaining in secondary treatment effluent (17).

Lagoons / waste stabilisation ponds

Wastewater stabilisation ponds and constructed wetlands provide attractive low technology and low energy solutions for treating contaminated waters such as wastewaters or storm waters, especially in areas where there is sufficient space. Provided that detention times are adequate and there are reduced opportunities for disturbance of sediments, lagoons and constructed wetlands have the potential to achieve excellent removal of oocysts from these waters. Removals in lagoon systems and constructed wetlands have been reported to range from 0.5 – > 4 log10

(17, 20). Variation in removals can be partly explained by differences in configurations; for example, whether single lagoon systems or a series of extensive interconnected lagoons are utilised. Furthermore, oocyst attenuation (removal and inactivation) in lagoons can be highly seasonal, influenced by temperature, predation and solar radiation (17, 21). However, the capacity of these systems to attenuate oocysts has not been fully appreciated until recently, due to the absence of a satisfactory tool to quantify oocyst infectivity from water samples (22). A recent study measuring oocyst number and oocyst infectivity demonstrated that while a large number of oocysts were present in effluent from a lagoon early in the treatment train, the majority had been inactivated.

Potential log10 reduction at different stages in a generalised drinking water treatment process

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Disinfection of Cryptosporidium in waterOxidant-based disinfectionThe resistance of Cryptosporidium to disinfectants such as chlorine and chloramine is one of the primary reasons this organism has been an issue for the water industry and the cause of so many drinking water-related outbreaks. To achieve a 1 log10 reduction in oocyst infectivity, a contact time (Ct) of 1,600 mg.min/L is required (23). While it is not possible to deliver the required Ct for treating drinking water or reuse water, chlorine can be used to inactivate oocysts in swimming pools following a faecal accident (Ct for 3 log10 inactivation 10,400 – 15,300 mg.min/L) (24). Chlorine dioxide has been shown to be slightly more effective (Ct of 1,000 mg.min/L for 2 log10 inactivation) (25), whereas ozone treatment (1 ppm) has been determined to be the best oxidant for inactivating oocysts, achieving 2 log10 inactivation for a Ct estimated between 5 – 10 mg.min/L and >4 log10 inactivation for a Ct of 10 mg.min/L (23). Reports on oocyst disinfection from electrochemically produced mixed oxidants (chlorine plus other uncharacterised species produced by a MIOX unit) have been highly variable, with one report finding no difference between MIOX mixed oxidants and chlorine treatment (26).

Ultraviolet light (UV)

Conventional UV reactors used for treatment of bulk water use UV lamps enclosed in a quartz sleeve and installed either in the centre of a stainless steel pipe, or suspended in an open channel. Water is passed through the pipe or channel and the UV dose delivered is a function of the lamp intensity, flow rate, mixing through the system and the UV transmissivity of the water. There are two types of UV-C lamps used in disinfection systems, medium pressure (MP)-UV lamps that deliver UV light in the full UV-C spectrum (200-300 nm), and low pressure (LP)-UV lamps that deliver germicidal UV-C (254 nm). The most effective wavelengths of UV-C for Cryptosporidium inactivation have been determined to be 250-275 nm (27). Cryptosporidium oocysts are highly susceptible to UV with doses as low as 3 mJ/cm2 shown to cause >2.6 log10 inactivation and 10 mJ/cm2 causing >3.9 log10 inactivation (28). Significantly, ex vivo and in vivo studies have found no evidence for reactivation of oocysts (29-31).

Combined processesSequential disinfection has been shown to improve oocyst inactivation. The combination of ozone followed by free chlorine increased oocyst inactivation by 4 – 6 fold compared

with ozone alone (32). Similar results were obtained using a combination of ozone and monochloramine, although the increase in inactivation was only 2.5-fold (33). Low ozone doses have also been combined with UV disinfection to allow adequate disinfection of water while reducing the formation of disinfection by-products by ozone (34). In terms of the treatment of wastewater to produce reuse water, combined treatment trains such as UV and chlorine disinfection have been recommended to allow control of both chlorine resistant pathogens such as Cryptosporidium and UV resistant pathogens such as adenovirus (35).

Heat

Cryptosporidium are sensitive to heat, making this an effective method for disinfection. Moist heat is highly effective and the temperatures achieved in thermophilic aerobic digestion of sewage sludge (55°C) are sufficient for ready inactivation of oocysts (36). Pasteurisation has been shown to be effective for the inactivation of oocysts in water and milk, but while this treatment can be used to inactivate oocysts in food (37), it has not been used for large-scale treatment of water or wastewater.

Conclusions

Cryptosporidium continues to be a public health concern around the world. However, we have developed a better understanding of the processes required to remove or inactivate Cryptosporidium in drinking water or wastewater. Despite these advances, outbreaks from drinking water will still occur, either due to treatment barrier failures, or insufficient risk characterisation of catchments and resulting inadequate treatment barriers to prevent entry of infectious oocysts into drinking water systems. Although there are advanced treatment options, outbreaks will still occur from recreational waters and swimming pools because it is not possible to prevent transmission when faecal events occur in close proximity to other water users. However, for drinking water, the technology is available to remove the threat of a Cryptosporidium outbreak, although at the cost of additional infrastructure and operating costs.

For a more detailed description on the removal and inactivation of Cryptosporidium by treatment please refer to the book chapter “Removal and inactivation of Cryptosporidium from water” by Monis, King and Keegan in “Cryptosporidium: parasite and disease” (2).

Potential log10 reduction at different stages in a generalised wastewater treatment process

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