Development of a disinfection efficiency database for bacterial inactivation: A systematic literature review for selected water treatment technologies

Access to safe drinking water is crucial for public health necessitating the use of effective water treatment processes. We conducted a systematic literature review on microorganism removal by physical treatment processes used in drinking water treatment systems with the aim of providing current summary data to update the World Health Organization's Guidelines for Drinking Water Quality (GDWQ) and to reflect on the data available for comparison of treatment technologies. We reviewed peer-reviewed articles reporting original data that were published between 1997 and March 2022 on the following physical treatment technologies: roughing filters, storage reservoirs, bank filtration, conventional and high-rate clarification, dissolved air flotation, lime softening, granular media filtration, slow sand filtration, precoat filtration, membrane filtration, granular activated carbon, ceramic membrane filtration, and soil aquifer treatment. The literature search was conducted in several databases including Web of Science and PubMed. Data from 165 articles were included in the analysis and used to calculate Log Reduction Values (LRVs) for each technology by microbial contaminant type (bacteria, virus, or protozoa). The quantity and quality of data ranged widely for each technology. We found granular media, membranes (microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO)), and precoat filtration to remove the most protozoa with average LRVs of 3.0 (95% CI 2.8-3.3), 5.7 (95% CI 5.4-6.0), and 4.4 (95% CI 4.1-4.7), respectively. Bacteria was removed most effectively by membrane filtration (MF, UF, RO) with average LRVs of 4.5 (95% CI 3.9-5.1) and moderately by dissolved air flotation, lime softening, and soil aquifer treatment with average LRVs of 2.7, 2.6, and 2.4 respectively. Viruses were removed most effectively by reverse osmosis membrane filtration with an average LRV of 4.9 (95% CI 4.0-5.7). This data provides valuable information on pathogen reduction and areas of needed research. The variation in results underscores the importance of further consideration when selecting technologies to use and the need for standardized reporting in both lab and field studies. It is important to consider variables in water quality and technology operation that may impact treatment effectiveness when selecting treatment options for use. The findings contribute to ongoing efforts to revise the WHO's GDWQ, offering updated insights into LRVs for different water treatment technologies.

In 1998, the City of Brantford Water Purification Plant was retrofitted with an ultra-high-rate upflow clarification process and two post-clarification chlorine contact chambers. The objective of the work described herein was to assess the theoretical inactivation of Giardia lamblia cysts by free chlorine disinfection in the new chlorine contact chambers. At the time this research was conducted, Ontario drinking water disinfection requirements were not as comprehensive as they are now, following the Walkerton outbreak. In an effort to remain proactive with respect to potential new disinfection regulations in Canada, the 1989 United States Environmental Protection Agency Surface Water Treatment Rule (SWTR) was used to assess the disinfection capability of the City of Brantford chlorine contact chambers. The Ontario Ministry of the Environment promulgated the Ontario Drinking Water Protection Regulation (DWPR) shortly after this project was completed (using the SWTR as a model), reaffirming the relevance of the approach described herein. This study showed that the new chlorine contactors did not provide sufficient contact time to ensure the minimum 0.5-log inactivation of G. lamblia cysts under all potential adverse scenarios. Contactor modifications were made to correct design deficiencies. The comprehensive assessment of the disinfection strategies presented here may be useful to other utilities in Ontario that must now comply with newly implemented DWPRs as well as utilities in other provinces that wish to be proactive with respect to potential new regulations in Canada regarding pathogen control. Key words: surface water treatment rule (SWTR), disinfection, chlorine, Giardia lamblia, CT concept, tracer studies, T10 contact time, log inactivation.

As a consequence of the 1986 Amendments to the Safe Drinking Water Act (SDWA) the U.S. EPA has issued a Surface Water Treatment Rule (SWTR) for systems using surface and ground waters under the direct influence of surface water. In the Guidance Manual to the SWTR, the EPA recommends C·t values (product of disinfection concentration in milligrams per liter and disinfectant contact time in minutes) for different disinfectants to achieve required levels of inactivation for Giardia lamblia. This paper describes the procedure by which C· values were calculated for Giardia lamblia by chlorine disinfection in the SWTR. A model has been developed which can be used to approximate the C·t values that are embodied in the SWTR. It was found that C·t values increased due to higher pH, the level of inactivation required, and chlorine concentration, and were inversely related to temperature.

In contrast to "frank" pathogens, like Salmonella entrocolitica, Shigella dysenteriae, and Vibrio cholerae, that always have a probability of disease, "opportunistic" pathogens are organisms that cause an infectious disease in a host with a weakened immune system and rarely in a healthy host. Historically, drinking water treatment has focused on control of frank pathogens, particularly those from human or animal sources (like Giardia lamblia, Cryptosporidium parvum, or Hepatitis A virus), but in recent years outbreaks from drinking water have increasingly been due to opportunistic pathogens. Characteristics of opportunistic pathogens that make them problematic for water treatment include: (1) they are normally present in aquatic environments, (2) they grow in biofilms that protect the bacteria from disinfectants, and (3) under appropriate conditions in drinking water systems (e.g., warm water, stagnation, low disinfectant levels, etc.), these bacteria can amplify to levels that can pose a public health risk. The three most common opportunistic pathogens in drinking water systems are Legionella pneumophila, Mycobacterium avium, and Pseudomonas aeruginosa. This report focuses on these organisms to provide information on their public health risk, occurrence in drinking water systems, susceptibility to various disinfectants, and other operational practices (like flushing and cleaning of pipes and storage tanks). In addition, information is provided on a group of nine other opportunistic pathogens that are less commonly found in drinking water systems, including Aeromonas hydrophila, Klebsiella pneumoniae, Serratia marcescens, Burkholderia pseudomallei, Acinetobacter baumannii, Stenotrophomonas maltophilia, Arcobacter butzleri, and several free-living amoebae including Naegleria fowleri and species of Acanthamoeba. The public health risk for these microbes in drinking water is still unclear, but in most cases, efforts to manage Legionella, mycobacteria, and Pseudomonas risks will also be effective for these other opportunistic pathogens. The approach to managing opportunistic pathogens in drinking water supplies focuses on controlling the growth of these organisms. Many of these microbes are normal inhabitants in biofilms in water, so the attention is less on eliminating these organisms from entering the system and more on managing their occurrence and concentrations in the pipe network. With anticipated warming trends associated with climate change, the factors that drive the growth of opportunistic pathogens in drinking water systems will likely increase. It is important, therefore, to evaluate treatment barriers and management activities for control of opportunistic pathogen risks. Controls for primary treatment, particularly for turbidity management and disinfection, should be reviewed to ensure adequacy for opportunistic pathogen control. However, the major focus for the utility's opportunistic pathogen risk reduction plan is the management of biological activity and biofilms in the distribution system. Factors that influence the growth of microbes (primarily in biofilms) in the distribution system include, temperature, disinfectant type and concentration, nutrient levels (measured as AOC or BDOC), stagnation, flushing of pipes and cleaning of storage tank sediments, and corrosion control. Pressure management and distribution system integrity are also important to the microbial quality of water but are related more to the intrusion of contaminants into the distribution system rather than directly related to microbial growth. Summarizing the identified risk from drinking water, the availability and quality of disinfection data for treatment, and guidelines or standards for control showed that adequate information is best available for management of L. pneumophila. For L. pneumophila, the risk for this organism has been clearly established from drinking water, cases have increased worldwide, and it is one of the most identified causes of drinking water outbreaks. Water management best practices (e.g., maintenance of a disinfectant residual throughout the distribution system, flushing and cleaning of sediments in pipelines and storage tanks, among others) have been shown to be effective for control of L. pneumophila in water supplies. In addition, there are well documented management guidelines available for the control of the organism in drinking water distribution systems. By comparison, management of risks for Mycobacteria from water are less clear than for L. pneumophila. Treatment of M. avium is difficult due to its resistance to disinfection, the tendency to form clumps, and attachment to surfaces in biofilms. Additionally, there are no guidelines for management of M. avium in drinking water, and one risk assessment study suggested a low risk of infection. The role of tap water in the transmission of the other opportunistic pathogens is less clear and, in many cases, actions to manage L. pneumophila (e.g., maintenance of a disinfectant residual, flushing, cleaning of storage tanks, etc.) will also be beneficial in helping to manage these organisms as well.

Understanding and managing the risk posed by helminth eggs (HE) is a key concern for wastewater engineers and public health regulators. The treatment processes that produce recycled water from sewage at wastewater treatment plants (WWTPs) rely on achieving a defined log10 reduction value (LRV) in HE concentration during the production of recycled water from sewage to achieve the guideline concentration of ≤1.0 HE/L. The total concentration of HE in sewage reaches thousands of HE/L in developing countries and therefore, an LRV of 4.0 is generally accepted to achieve a safe concentration in recycled water, as this will meet the guideline value. However, in many developed countries with good sanitation and public health standards, the HE concentration in sewage is generally <10 HE/L. Therefore, validation of the sewage treatment process relied on to achieve an LRV of 4.0 can be difficult. Because of these limitations, design equations to predict LRVs from hydraulic retention times (HRT), which are geographically non-specific, are commonly relied on to ensure the production of safe quality recycled water with respect to HE. However, these design equations could be further refined by defining the design and management of the treatment process in greater detail and thus be used more effectively for determining the LRV required. This paper discusses the limitations and possible improvements that could be applied to LRV design equations for predicting HE removal at WWTPs and identifies the data requirements to support these improvements. Several options for LRV design equations are proposed that could be validated experimentally or via the ongoing operation of WWTPs. These improvements have the potential to assist the rationalization of the HE removal requirements for specific treatment options, exposure scenarios and use of recycled water in agriculture.

This article is the fourth in a series examining the Surface Water Treatment Rule (SWTR). This month's column explains how to calculate C x T values and what design and operational factors affect the value. This is one in a series of five articles about the Surface Water Treatment Rule. Other articles discuss “Surface Water Treatment Rule Targets Microbials” (May 1993); “SWTR Disinfection Standards for Filtered Systems” (June 1993); “SWTR – C x T Values Indicate Disinfection Applied” (July 1993); and “SWTR Conditions to Avoid Filtration Are Demanding (September 1993).

The goal of the Surface Water Treatment Rule (SWTR) is to ensure that harmful microbes are either removed or inactivated. This article discusses regulatory requirements, focusing on removal by filtration. Rather than performing difficult tests to monitor for microorganisms, water systems must accumulate a certain number of credits or points by achieving certain treatment goals. A table summarizes filtration credits, which are based on turbidity after filtration. Details on filtration credits and turbidity standards are given for conventional treatment with direct filtration, slow sand filtration, and diatomaceous earth filtration. Finally, some suggestions for improving filtration are given. This is one in a series of five articles about the Surface Water Treatment Rule. Other articles discuss “SWTR Disinfection Standards for Filtered Systems” (June 1993); “SWTR – C x T Values Indicate Disinfection Applied” (July 1993); “Configuration, Operation of System Affects C x T Values” (August 1993); and “SWTR Conditions to Avoid Filtration Are Demanding (September 1993).

The Surface Water Treatment Rule (SWTR) sets disinfection standards for water systems that use filtration. The new standards take effect in June 1993. In many states, water systems (even small systems) have been required to filter when starting to use surface water. The SWTR places additional requirements on these systems. Together with new filter performance standards, implementation of the disinfection standards prevent harmful, disease‐causing organisms from entering the water distribution system. This is one in a series of five articles about the Surface Water Treatment Rule. Other articles discuss “Surface Water Treatment Rule Targets Microbials” (May 1993); “SWTR – C x T Values Indicate Disinfection Applied” (July 1993); “Configuration, Operation of System Affects C x T Values” (August 1993); and “SWTR Conditions to Avoid Filtration Are Demanding (September 1993).

The Surface Water Treatment Rule (SWTR) contains tables of disinfectant CT(concentration × time) values that have been demonstrated to achieve the required degree of inactivation of viruses and Giardia cysts. If the CT product for a water treatment plant meets or exceeds those in the SWTR tables, it is assumed this inactivation has been achieved. The objective of the study was to compare two methods for calculating C and T. Tracer tests were conducted to characterize the behavior of flow through the process units of water treatment plants. Effective CT values, which were calculated based on the residence time distribution and the kinetics of chlorine decay, were compared with the most conservative CT values recommended in the US Environmental Protection Agency guidance manual.

Wastewater reclamation and reuse have been practically applied to water-stressed regions, but waterborne pathogens remaining in insufficiently treated wastewater are of concern. Sanitation Safety Planning adopts the hazard analysis and critical control point (HACCP) approach to manage human health risks upon exposure to reclaimed wastewater. HACCP requires a predetermined reference value (critical limit: CL) at critical control points (CCPs), in which specific parameters are monitored and recorded in real time. A disinfection reactor of a wastewater treatment plant (WWTP) is regarded as a CCP, and one of the CCP parameters is the disinfection intensity (e.g., initial disinfectant concentration and contact time), which is proportional to the log reduction value (LRV) of waterborne pathogens. However, the achievable LRVs are not always stable because the disinfection intensity is affected by water quality parameters, which vary among WWTPs. In this study, we established models for projecting virus LRVs using ozone, in which water quality and operational parameters were used as explanatory variables. For the model construction, we used five machine learning algorithms and found that automatic relevance determination with interaction terms resulted in better prediction performances for norovirus and rotavirus LRVs. Poliovirus and coxsackievirus LRVs were predicted well by a Bayesian ridge with interaction terms and lasso with quadratic terms, respectively. The established models were relatively robust to predict LRV using new datasets that were out of the range of the training data used here, but it is important to collect LRV datasets further to make the models more predictable and flexible for newly obtained datasets. The modeling framework proposed here can help WWTP operators and risk assessors determine the appropriate CL to protect human health in wastewater reclamation and reuse.

A northern California treatment plant installed microfiltration to satisfy design constraints and comply with SWTR requirements.With the enactment of the federal Surface Water Treatment Rule (SWTR) and a state SWTR, San Jose Water Company faced a decision to either abandon or upgrade a small diatomaceousearth (DE) filtration plant. Several environmental, design, operational, and regulatory constraints influenced the decision to replace the existing DE filtration system with microfiltration (MF). At 5 mgd this is currently the largest potable water MF plant in the United States. It has been operating successfully since February 1994. Results to date demonstrate that the plant is producing finished water that exceeds SWTR requirements. This article describes elements of the design, construction, operation, and performance of the new plant as well as factors leading to the selection of MF over other treatment options.

Passage of the Safe Drinking Water Act in 1974 and its Amendments in 1986 (SDWAA) is changing the way water is treated and delivered in the United States. Under the SDWAA the U.S. EPA is required to regulate chemical contaminants and pathogenic microorganisms in drinking water. Emphasis has shifted from a primary concern with treated drinking water to attainment of standards at the point of consumption. Two regulations promulgated under the SDWAA, the Surface Water Treatment Rule (SWTR) and the Total Coliform Rule (TCR) specify treatment and monitoring requirements that must be met by all public water suppliers. The SWTR requires that a detectable disinfectant residual be maintained at representative locations in the distribution system to provide protection from microbial contamination. The TCR regulates coliform bacteria which are used as “surrogate” organisms to indicate whether or not system contamination is occurring. Monitoring for compliance with the Lead and Copper Rule is based entirely on samples taken at the consumers’ tap. The current standard for trihalomethanes (THMs) is 0.1 mg/L for systems serving more than 10,000 people but the anticipated Disinfectants and Disinfection By-Products (D-DBP) Rule may impose the current (or a reduced) THM level on all systems. This regulation also requires monitoring and compliance at selected monitoring points in the distribution system. Some of the regulations promulgated under the SDWAA may, however, provide contradictory guidance. For example, the SWTR and TCR recommend the use of chlorine to minimize risk from microbiological contamination. However, chlorine or other disinfectants interact with natural organic matter in treated water to form disinfection by-products. Raising the pH of treated water will assist in controlling corrosion but will increase the formation of trihalomethanes.

Exposure to pathogens remains the greatest acute health concern related to potable water reuse applications. Here, we implement high-volume sample concentration for both molecular- and culture-based analyses to evaluate pathogen and surrogate removal through a 1-MGD scale coagulation, flocculation, sedimentation, ozonation, and biofiltration treatment train. The reduction of Cryptosporidium and Giardia was quantified across the wastewater treatment plant and the advanced water treatment (AWT) process. Considering the low influent concentrations, only 2 and 4 total log-reduction values (LRVs) could be demonstrated for Cryptosporidium and Giardia, respectively. Adenovirus, rotavirus, norovirus GI, and norovirus GII concentration and reduction were quantified using droplet digital polymerase chain reaction (PCR) across the AWT process. Average enteric virus LRV, through coagulation/flocculation/sedimentation, ozonation, and biofiltration, was shown to be 1.5, 0.3, and 2 LRV, respectively. Both molecular and culture-based nonpathogenic viral surrogates were shown to be representatives of enteric virus reduction by physical removal treatment processes. Due to the low concentration of indigenous pathogens and surrogates, challenge tests were performed on the pilot scale to evaluate the inactivation and removal of pathogens by ozone and biofiltration. These full-scale monitoring data and pilot challenge testing data provide validation of the pathogen LRV credit claimed in ozone-biofiltration-based AWT, which is necessary to protect public health in reuse scenarios.

Issues: Efficient solution treatment methods are necessary for the abatement of pharmaceuticals and pesticides in water, and mitigate environmental impacts. Chemical, physical and biological water treatment processes have been applied for the removal of emerging pollutants from water. However, these methods are not completely efficient due to the formation of secondary pollution, high cost and time of operation. Advanced oxidation processes can overcome these problems on water and wastewater treatment containing emerging pollutants. Major advances: We reviewed in this text catalytic photodegradation processes of pharmaceuticals and pesticides in aqueous media using catalysts incorporated in/on polymer-based porous rigid organic solid supports. Advanced oxidation processes are usually conducted using specific catalysts combined to ultraviolet (UV) radiation emission. Many catalysts have been studied in UV radiation-assisted water treatment techniques, including titanium dioxide (TiO2), zinc oxide (ZnO), tin dioxide (SnO2), cerium (IV) dioxide (CeO2) and tungsten trioxide (WO3), in addition to chalcogenides (CdS, CdSe). The UV radiation emission with wavelength lower than 385 nm generates electron–hole pairs on catalyst structures, inducing the generation of free radicals capable of photo-degrading adsorbed pollutants. The photocatalysis of organic pollutants can also take place after emission of either visible light or combination of UV/visible light. The degradation efficiencies can vary from 61.0 to 99.2% depending on the employed system. Many catalysts have low photodegradation efficiency due to their small surface area and low pollutant adsorption capacity. This problem can be overcome with the immobilization of the catalyst in solid rigid supports. Polymer-based porous composite materials have been demonstrated to be potential organic rigid solid supports to improve the photocatalytic degradation efficiency of organic pollutants due mainly to the increase of surface area. In this sense, we have shown the incorporation of metal oxides on polymer-based porous composite materials for the photodegradation of pharmaceuticals and pesticides contained in aqueous solutions.

Assessing the Basis for Regulatory Crediting of Virus LRVs for Secondary Biological Wastewater Treatment: A Systematic Review