Long-Term Lab Scale Studies of Simulated Reclaimed Water Distribution: Effects of Disinfectants, Biofiltration, Temperature and Rig Design

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.