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Water recycling or reuse
According to the guidelines of water reuse (USEPA, 2012), recycled water is “municipal wastewater that has been treated to meet specific water quality criteria with the intent of being used for a range of purposes. The term ‘recycled water’ is synonymous with ‘reclaimed water’. Municipal wastewater is composed of water, salt, organics and nutrients at different concentrations. The wastewater quality depends on the type of population waste and the type of industry or hospitals present for example. Wastewater treatment is composed of a series of processes which permit to remove contaminants (Wilf, 2010). Briefly, preliminary treatment removes large solids and grit by physical processes such as screening. Primary treatment removes total suspended solids (TSS) and some biochemical oxygen demands (BOD). Secondary treatment removes colloidal and soluble organic contaminants. Advanced and tertiary treatments increase the removal of nutrients, pathogens and sometimes metals. Finally, disinfection is the last treatment before discharge of the water into the environment and permits to avoid the spreading of waterborne diseases.
Water recycling is not a new concept. Indeed, in ancient Greece, wastewater was already reused to irrigate agriculture by the elaborate design of sewerage systems (Angelakis and Spyridakis, 1996). In the 19th century, catastrophic epidemics of waterborne diseases took place due to the lack of adequate water and wastewater treatment. As a consequence, engineering solutions have been developed for alternative water sources and filtration systems have been progressively installed (Barty-King, 1992). However, it is only at the end of the 20th century that the USA and the European Union had accepted more broadly the idea to use wastewater as supplementing water resources (Asano and Levine, 1996). Nowadays, wastewater recycling has attracted worldwide interest due to the reduction of usual water supplies, global warming, urbanisation, population growth, and environmental problems due to the discharge of inadequately treated sewage effluent (Radcliffe, 2004).
Recycled water can be produced at different water quality depending on its end-use (Bastian, 2006):
– Non-potable water reuse for industry, agriculture, landscape irrigation (residence, golf club, parks and school grounds dual reticulation systems);
– Potable water reuse to increase drinking water supplies via direct or indirect potable reuse.
Literature review
Direct potable reuse projects may be put in place temporally due to extreme circumstances such as severe drought; but it is the category of water reuse least accepted by population. An example of a continuously operating direct potable reuse plant is the one in Namibia. The Windhoek’s Goreangab reclamation plant treats water and blends it with potable water distribution network to provide up to 25% of the Windhoek city consumption since 1968 (du Pisani, 2006). In contrast, different indirect potable reuse (IPR) schemes have been successfully implemented in the USA (e.g. California, Water Factory 21 and Orange County Water District Council), Europe and Asia and are presented in Table 1.1 (Rodriguez et al., 2009; USEPA, 2012).
In Australia, there are some projects considering the use of IPR through aquifer recharge or dam supplementation in Perth (Western Australia) and South East Queensland (SEQ), but none is implementing potable reuse as yet for a variety of reasons, among them concerns related to community acceptance. The city of Toowoomba (Qld) is a good example for demonstrating the importance of public opinion. Indeed, in this case the development of a potable water recycling project has not been fulfilled because of the opposition of the local community to this project (Hurlimann and Dolnicar, 2010). Hurlimann and Dolnicar (2012) demonstrated the power of the media on the public acceptance to water recycling in Australia and concluded on the fact that the media should use scientific evidence and be impartial in their statement. Nevertheless, the critical water supply situation in late 2007 and early 2008 in SEQ changed the public opinion towards water reuse as IPR to supplement the Wivenhoe dam. However, as rainfalls increased in late 2008, the community was less supportive and the Queensland Government changed its recycled water policy from continuous use of IPR to emergency use when the dam levels fall below 40% of its capacity (Rodriguez et al., 2009). In order to improve the acceptance of IPR by the population, it is important to demonstrate that the potential risks are well managed by implementing suitable monitoring of the different water treatment processes. In this context, it may seem an interesting observation that the community concerns about engineered potable reuse systems are generally much higher than the in principle similar practice of so-called unintentional potable reuse. This happens for instance along major river systems such as the River Rhine or the River Thames in Europe, where one community abstracts water from the river, uses it, treats it and discharges it back to the river to be used again by the community living downstream (Bixio and Wintgens, 2006).
Risk assessment in water reuse
The use of recycled water poses many risks. One of the main risks associated with recycled water is the potential damage to public health which obliges authorities to draft strict policies in order to protect community health (Radcliffe, 2004). Some of the risks or parameters that must be managed in water recycling are presented in Table 1.2. Recognising and managing these risks are critical to the successful implementation of recycled water schemes.
The transmission of infectious diseases by pathogenic organisms is the most common concern of health professionals in water reclamation and water reuse. Microorganisms associated with waterborne diseases are primarily enteric pathogens, including enteric bacteria, protozoa and viruses. These pathogens can survive in water and infect humans through ingestion of faecal-contaminated water or contact with contaminated surface and food. From a public health and process control perspective, enteric viruses are the most critical group of pathogenic organisms in the developed world due to the possibility of infection from exposure to low doses and the lack of routine, cost-effective methods for detection and quantification of viruses (Asano and Levine, 1996). The definition of the various terms related to pathogens is given in Table 1.3 (NRMMC et al., 2008).
In order to manage these risks, the processes used to produce high quality water have to be validated and monitored. However, there is no universal recycled water policy around the world as the legislation is area dependant. For example, in the USA, each state handles the validation rules independently. Some of them do not have any legislation; others use individual barriers validation as being part of a whole plant such as California which uses the same approach as in Queensland (Australia) (USEPA, 2012). For this reason, the concept of risk assessment presented in this sub-chapter is introduced as defined by the Australian Guidelines for Water Recycling (AGWR) (NRMMC et al., 2008). These principles may be applied and interpreted slightly differently according to the local legislation.
According to AGWR, rotavirus and adenovirus are the reference pathogens for enteric viruses for the following reasons:
– Rotavirus represents waterborne viruses (Khan and Roser, 2007). It is a good candidate for risk assessment because of its high capacity to cause gastrointestinal infection and an established dose-response model (WHO, 2011). However, there is no routine culture-based method permitting the quantification of the infectious units;
– Adenovirus is a virus that can be cultured, found in high numbers in sewage and is renowned for its resistance to UV light inactivation (Gerba et al., 2002; WHO, 2011; USEPA, 2012). However, there is no dose-response model established.
An advanced water treatment plant (AWTP) must monitor equipment and automation to 13 prove the required log rejection and ensure the correct functioning of the processes (NRMMC et al., 2008).
To determine the requirement for virus removal for a specific end-use of recycled water, AGWR used the disability-adjusted life year (DALY). DALY is a common metric for all types of hazard taking into account health outcomes including probabilities, severities and duration of effect (WHO, 2011). For example, the DALY of rotavirus in developing countries is 480 DALYs per 1000 cases (Havelaar and Melse, 2003; WHO, 2011). From this factor and the assumed concentration of a pathogen in the source water, the required log removal value (LRV) is calculated (NRMMC et al., 2006, 2008). LRV is a way to express the removal or inactivate efficiency for a specific target such as an organism, particulate or surrogate (1 LRV = 90% reduction in density of the target organism, 2 LRV = 99% reduction, 3 LRV = 99.9% reduction, etc.) and is calculated as presented in Equation 1.1 (USEPA, 2005; NRMMC et al., 2008; UNESCO and WRQA, 2009).
(1.1) where and are the concentrations of the pathogen in the influent and the effluent, respectively.
From DALYs associated to enteric viruses and their assumed concentrations in sewage, the minimum LRV required for the production of recycled water for potable purposes from sewage has been set to 9.5 in the AGWR (NRMMC et al., 2006, 2008).
Depending on the type of process such as clarification and membrane filtration, and the target such as virus or organic matter, different integrity tests1 are selected. According to Table 1.1, membrane filtration processes are used in the majority of the IPR projects. Low pressure membranes (MF and UF) are difficult to validate continuously and have been widely studied. Moreover, viruses can go through these membranes which give a variable LRV (from 1 to > 7) depending on their pore size (Jacangelo et al., 1995; Asano, 2007).
RO membrane has been proven to be able to remove above 5 log for virus in laboratory and pilot studies (Kitis et al., 2003; Lozier et al., 2003; Mi et al., 2004). However, it has not been possible to prove such performance on full-scale. The assumed difference between the LRV that RO can currently be validated for and its actual performance is therefore largely proved. This is also an important perceived opportunity to increase current LRV associated to this barrier. This is one main reason, why this thesis and consequently the following sections of this literature review focus on RO validation.
Reverse osmosis
RO membranes are commonly used in tertiary treatment for water reuse applications as a physical filtration process. These types of membrane are non-porous and have the capacity to remove salt and other inorganic and organic contaminants (USEPA, 2005). Figure 1.1 presents schematically the principle of the RO membrane (Wilf, 2010). Briefly, a pressure is applied at the feed side and forces water to go through the membrane forming the permeate whereas salt and contaminants are retained by the membrane and remain dissolved in the water of the concentrate.
Table of contents :
Introduction
1. Literature review
1.1. Water recycling or reuse
1.2. Risk assessment in water reuse
1.3. Reverse osmosis
1.3.1. Principal failures in RO process
1.3.1.1. Process failures
1.3.1.2. Membrane impairments and failures
1.3.2. Monitoring RO membrane integrity
1.3.2.1. Direct monitoring
1.3.2.2. Indirect monitoring
1.4. Rejection of virus by membrane filtration process
1.4.1. Virus
1.4.2. Mechanisms of virus rejection by membrane
1.5. Application of virus surrogates and indicators for membrane integrity testing
1.5.1. Surrogates & indicators used to study virus LRV during filtration
1.5.2. Effect of failure modes on testing RO Membrane integrity
1.6. Conclusion and thesis objectives
2. Materials & methods
2.1. Virus surrogate and membrane integrity indicators
2.2 Lab-scale experimental apparatus
2.2.1. Stainless-steel flat-sheet cross-flow set-up (SS flat-sheet set-up)
2.2.2. Plastic flat-sheet cross-flow set-up (plastic flat-sheet set-up)
2.2.3. Spiral-wound module set-up (2.5” module set-up)
2.2.4. Membrane characteristics
2.2.5. Membrane impairment protocols
2.2.5.1. Organic fouling
2.2.5.2. Scaling
2.2.5.3. Ageing
2.3. Analytical methods
2.3.1. Analysis of virus surrogate and membrane integrity indicators
2.3.1.1. MS2 quantification
2.3.1.2. R-WT quantification
2.3.1.3. DOM analysis
2.3.1.4. Sulphate quantification
2.3.1.5. Electrical conductivity
2.3.2. Other chemical analysis
2.3.3. Membrane autopsies
2.3.3.1. Atomic force microscopy (AFM)
2.3.3.2. Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS)70
2.3.3.3. Attenuated total reflection-Fourier transform infrared (ATR-FTIR)
2.4. Statistical data analysis
3. Monitoring RO performance: Conductivity versus EEM
3.1. Reverse osmosis plant description, sampling protocols and general water quality
3.2. Determination of membrane defects through measurement of salt and organics rejection by conductivity and fluorescence profiling
3.3. Understanding the blue-shift of the fluorescence of humic substances from feed to permeate
3.4. Conclusions and implications for practice
4. Effect of membrane impairments: Organic fouling & Scaling
4.1. Organic fouling
4.1.1. Membrane characteristics
4.1.2. Membrane autopsy
4.1.3. Rejection of virus surrogate and membrane integrity indicators
4.2. Scaling
4.2.1. Membrane characteristics
4.2.2. Membrane autopsy
4.2.3. Rejection of virus surrogate and membrane integrity indicators
4.3. Conclusions
5. Effect of membrane impairments: Ageing
5.1. Membrane characteristics
5.2. Membrane autopsy
5.3. Rejection of virus surrogate and membrane integrity indicators
5.4. Conclusions
6. Statistical comparison of experimental set-ups & membrane impairments
6.1. Correlation between the rejection of the different membrane integrity indicators
6.2. Effect of membrane impairment and set-up
6.3. Comparison of the different lab-scale set-ups with the full-scale plant
6.4. Conclusions
7. Conclusions & recommendations for future research
7.1. Conclusions
7.2. Recommendations for future research
References
