Long-Term Lab Scale Studies of Simulated Reclaimed Water Distribution: Effects of Disinfectants, Biofiltration, Temperature and Rig Design
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As demand for alternative water sources intensifies, increased use of reclaimed water is important to help achieve water sustainability. In addition to treatment, the manner in which reclaimed water is distributed is a key consideration as it governs the water quality at the point of use. In this work, simulated reclaimed water distribution systems (SRWDSs) were operated for more than two years to examine the role of system design, biofiltration, residual disinfectant type (i.e., chlorine, chloramine, no residual) and temperature on important aspects of chemistry and microbial regrowth under laboratory-controlled conditions. Turbidity decreased to 0.78 NTU after biofiltration and chlorinated treatments from 10.0-12.6 NTU for conditions with chloramine and no residuals. SRWDSs were susceptible to sediment accumulation, which occupied 0.83-3.2% of the volume of the first pipe segment (1 day of hydraulic residence time), compared to 0.32-0.45% volume in the corresponding chlorinated SRWDSs. The mass of accumulated sediment positively correlated (R2 = 0.82) with influent turbidity. Contrary to experiences with potable water systems, chlorine was found to be more persistent and better at maintaining biological stability in the SRWDSs than chloramine, especially at the higher temperatures >22°C common to many water scarce regions. The severe nitrification at the warmer temperatures rapidly depleted chloramine residuals, decreased dissolved oxygen, and caused elevated levels of nitrifiers and heterotrophic cell counts. A metagenomic taxonomic survey revealed high levels of gene markers of nitrifiers in the biofilm samples at 22°C for the chloraminated system. Non-metric multidimensional scaling analysis confirmed distinct taxonomic and functional microbial profiles between the chlorine and chloramine SRWDSs. Reflecting on multiyear experiences operating two different SRWDSs reactor designs, including thin tubes (0.32-cm diameter) and pipe reactors (10.2-cm), illustrated strengths and weaknesses of both approaches in recreating key aspects of biochemical changes in reclaimed water distribution systems. It is clear that approaches deemed successful with drinking water distribution systems may not always directly transfer to simulating reclaimed distribution systems, or to proactively managing full-scale reclaimed systems that have long periods of stagnation and where minimally-treated wastewater with high levels of nutrients and turbidity are used.
General Audience Abstract
Increasing water scarcity is creating an impetus for creating more sustainable water supplies. Wastewater effluent is increasingly viewed as in important resource that can reduce both water and energy demand. Reclaiming moderately to minimally-treated secondary wastewater effluent for non-potable reuse (NPR) applications; such as agricultural irrigation, landscaping, and toilet flushing, helps reduce demand for higher quality potable water sources. NPR presently accounts for more than 50% of total reuse and is projected to become increasingly important. While NPR is attractive, important knowledge gaps remain in terms of managing water quality and safety as it is transported through distribution pipes to the point of use. A comprehensive literature review revealed that NPR distribution systems are distinct from conventional drinking water distribution systems (DWDSs) and that it is doubtful if our current understanding of DWDSs would directly transfer to NPR systems. Unlike drinking water systems, NPR systems are currently unregulated at the national level and corresponding state-to-state regulations vary widely. The levels of water treatment can vary from simply distributing untreated effluent from wastewater treatment plants to very high-level treatment with membranes that produces water of equal or even higher quality than many existing tap waters. A common treatment train for minimally-treated NPR involves biologically activated carbon (BAC) filtration and the use of disinfectants (e.g., chlorine or chloramine) to control microbial water quality to the point of use. Prior studies from DWDSs have demonstrated water quality degradation in terms of disinfectant loss, bacterial growth, and aesthetic problems, with the settling of trace particulate matter producing sediment within pipe distribution systems. In particular, accumulated sediment can become a hotspot for water quality deterioration. Considering that minimally-treated reclaimed water can have much higher levels of particulate matter and nutrients than drinking water, it was predicted that NPR distribution systems could suffer from faster water quality degradation than corresponding drinking water systems, especially at the warmer temperatures common in water-scarce regions. This work was the first multi-year attempt to examine the effects of disinfectant (i.e. free chlorine, chloramine, no residual), BAC filtration versus no filtration, water age (up to 5-d versus 28-min), and temperature (14°C, 22°C, 30°C) in different types of lab-scale reactors. Two simulated reclaimed water distribution systems (SRWDSs) including 4-in. diameter Pipe SRWDSs versus 1/8-in. diameter Tube SRWDSs, were designed to study key aspects of full-scale NPR systems and were operated for more than two years to study chemical and microbial changes as distributed water traveled through the two systems. The Pipe SRWDSs were designed to assess the impacts on final water quality after long-term operation that allowed sediment to slowly accumulate, whereas the complementary Tube SRWDS design did not allow sediment to accumulate and only held the water for 28 minutes. Water was sampled regularly to track the trends of key water quality parameters, including disinfectant residuals, dissolved oxygen, nitrogen compounds involved in nitrification reactions, and various types of bacteria of interest. Sequencing of the biological genetic materials on selected samples was conducted to understand the types of bacteria present and their functions under the different circumstances. High levels of sediment were found to accumulate near the beginning of the Pipe SRWDSs, which caused loss of oxygen and disinfectants at the bottom of the pipes. Chlorine was more persistent and better at preventing bacteria growth as water traveled through the distribution system. In contrast, a type of bacteria that used ammonia as a nutrient (i.e., nitrifying bacteria) were observed in the pipes with chloramine (i.e., ammonia plus chlorine) as the disinfectant. The nitrifying bacteria caused rapid depletion of chloramine residuals, especially at temperatures above 22°C. At 30°C both chlorine and chloramine were almost immediately consumed in the pipe reactors. Nitrification is known to trigger water quality problems in chloraminated DWDSs, and we expect that chloraminated RWDSs would be even more susceptible to nitrification and associated water quality degradation issues in Compare the Tube SRWDSs to the Pipe SRWDSs, aside from heavy accumulations of sediment in the pipes versus no sediment in the thin tubes, the tubes clogged repeatedly from formation of thick biofoulants in the systems treated with no disinfectant and chloramine, whereas they remained relatively free of biofoulants and clogging in the tubes with chlorine. Even in just 28 minutes, it took water to move from the start to the end of the tube, both chlorine and chloramine were almost completely consumed in the tubes, due to the unrealistically high pipe surface area to the small flow volume inherent to this reactor design. As NPR becomes increasingly common to help achieve water sustainability, it will be important to deploy laboratory simulations, that are capable of testing and revealing key chemical and microbial processes that affect the operation of these systems and water safety at the point of use. The insights from this first long-term effort of simulating RWDSs highlight some unique characteristics and challenges of RWDSs, and reveals key concepts to help guide future research.
- Doctoral Dissertations 
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