WITH WATER SHORTAGES INCREASINGLY becoming the norm in many parts of the country, many communities are looking towards alternative water supplies to supplement their existing water resources. Among these alternative sources is the reuse of municipal wastewater. Water reuse is not a new concept and reuse for nonpotable purposes, such as landscape irrigation, has been practiced for years. However, several communities are considering potable reuse where municipal wastewater is treated to drinking water standards for potable purposes.
Potable reuse can be accomplished through discharging advanced treated water into an environmental buffer such as a reservoir or groundwater basin (indirect potable reuse) or directly into a drinking water system (direct potable reuse). In both cases, advanced monitoring and sampling techniques are needed to ensure that any water produced from a potable reuse system is protective of public health.
A recent study from the Water Environment & Reuse Foundation (WERF), “Monitoring for Reliability and Process Control of Potable Reuse Applications,” with a team led by Dr. Ian Pepper and Dr. Shane Snyder of the University of Arizona investigated the need for monitoring in potable reuse and the several advanced monitoring techniques and sensors (Reuse-11-01).
Potable reuse can be accomplished through a variety of treatment processes including, but not limited to, ozonation, UV-advanced oxidation processes, granular activated carbon, and reverse osmosis. Such processes are typically used in sequence depending on the needs of a facility as part of a multi-barrier approach to treatment. Monitoring before and after each treatment barrier is important to ensure that these treatment systems are operating as intended to remove microbial and chemical contaminants.
For microorganisms, indicator organisms are commonly used to indicate a possible contamination of a water source. However, there is a debate among experts in the field over the suitability of indicator organisms to determine water quality because contamination of drinking water may occur when there were no indicator organisms present. In addition, plate counting and cell cultures used to determine the presence of pathogens and viruses can take about 24 to 48 hours to obtain results. In potable reuse, the response time to a treatment upset is limited and more rapid monitoring techniques are crucial to ensure safety.
In addition to pathogens, pharmaceuticals, personal care products, and endocrine-disrupting compounds are known to reside in wastewaters. While some of these compounds negatively affect aquatic ecosystems, many have not been studied for their potential effect on humans. With the increased utilization of water reuse, there is a growing concern on the impact these compounds have on the human body. Currently, there are over 3,000 pharmaceuticals and several thousand personal care products in circulation in the US. This high number of compounds makes it extremely difficult to monitor each compound individually. However, the use of bulk water quality parameters such as total organic carbon, UV absorbance, and fluorescence excitation-emission spectroscopy have been successful in monitoring trace organic contaminants attenuation through water treatment barriers. Based on these parameters, it can be determined if treatment systems are operating as intended and if chemicals are being removed or transformed to a sufficient degree.
Unlike traditional monitoring practices, online monitoring provides utilities with rapid recognition of natural, accidental, and intentional intrusion of pathogens or chemicals into the water supply. Rapid detection is important to protect users from infection by pathogenic microbial organisms and ensure compliance with environmental regulations. Some of the water quality parameters that can be measured by online sensors include pH, turbidity, conductivity, and total organic carbon to determine if there has been a contamination event in the source water. Low probability, high-impact contamination events of either chemical or microbial nature are prime candidates for an early warning online monitoring system because it allows for a decreased response time to counter a contamination event compared to traditional sampling. This can be especially important for potable reuse where response time is limited.
During the course of this project, there were four laboratory investigations looking at the removal of trace organic contaminants. The treatment barriers for removal included ozonation, UV-advanced oxidation process, granular activated carbon, and reverse osmosis. The general parameters measured were pH, turbidity, and conductivity along with total organic carbon, UV254, and total fluorescence. For the different treatment barriers, the successful parameters could be used to monitor a failure event that can be obtained in real time. The relevant parameters for each treatment process are as follows:
- For ozonation, turbidity was the only general parameter that effectively monitored changes to water quality for this barrier, while UV254 and fluorescence were both good parameters for the decrease of organic containments.
- For UV-advanced oxidation process, none of the general parameters were found to measure changes to water quality after the treatment barrier. The organic parameters that effectively measured UV-advanced oxidation process were UV254 and fluorescence.
- For granular activated carbon, fluorescence and UV254 could be used as online parameters to monitor water quality.
- For reverse osmosis, conductivity was a good parameter to measure the success of the barrier because of its ability to measure salt concentrations, while total organic carbon, UV254, and total fluorescence were all good organic parameters to monitor.
For microorganisms, water quality parameters such as turbidity can be measured by online sensors and advanced monitoring techniques such as vibrational spectroscopy and multi-angle light scattering (MALS) can be used. Turbidity measures the clarity of the water caused by suspended particles. Changes to the baseline turbidity of a water source can indicate contamination by microorganisms and a decreased state of water quality. In practice, unexpected increases in turbidity can be used as an indicator that an incursion of pathogens has occurred.
Vibrational spectroscopy can measure microorganisms in a water source by using a sensor that utilizes a laser or other light source to determine light absorptions and emissions. A specific wavelength is measured (e.g., UV254) to provide an estimate of organic content in test water that correlates to the unique composition of an organism. MALS uses multiple light sources to determine individual microorganisms or particles in a water source based on their unique light scattering patterns. This technology utilizes the light scattering patterns caused by a particle’s characteristics such as shape, size, refraction index, and internal structure and compares it to a database of patterns from known pathogens. These can be an effective technology for detecting particles and pathogens in water and may detect if a pathogen is viable or not. Additional work is needed to develop such sensor technologies further.
During this study, some emerging technologies were investigated to determine the presence of contaminants in real time including biosensors. Biosensors work by using a receptor that senses an analyte and uses a transducer to convert the recognition into a signal that can be measured optically, electrochemically, or by mass-sensitive techniques. There are two types of biosensors available: catalytic and affinity. Catalytic biosensors work by releasing an enzyme or organism that causes a catalytic reaction with an analyte to cause a specific product to be produced. These types of sensors are best used in the detection of chemical toxicity. Affinity biosensors, on the other hand, work by measuring a binding event between an analyte and molecules such as antibodies, DNA, peptides, or lectins with no further reaction occurring. These type of sensors are used to detect pathogenic microorganisms.
Aside from the bench-scale laboratory studies, research was also conducted at pilot-scale and full-scale potable reuse facilities. In completing these studies there were some gaps and issues in sensor technologies that need to be addressed to ensure the integrity of real-time monitoring. First, enhanced sensitivity in contaminant detection and removal would be a major step forward, along with the ability to detect minimal incremental failure. This would provide greater confidence from regulators and the public that treatment systems are operating effectively.
For pathogens, further development of online sensors is needed with immunoassays being a potential technology of interest. Lastly, with the large amounts of data that sensors can generate, better software and data management tools are needed. WERF currently sponsors research in this area in collaboration with Black & Veatch and the University of Arizona to develop better data management strategies (Reuse-14-01). As sensors advance and potable reuse becomes more mainstream, WERF will continue to be engaged through cutting-edge research and facilitating the necessary connections between industry and municipal utilities to ensure public safety from alternative water supplies.
