Taste and odor problems associated with drinking water are a pervasive problem for many municipalities. Suffet et al.1 reported that 22% of the water providers surveyed in a national study reported taste and odor problems in their source waters. Taste and odor problems have been traced to both plank tonic and entice algae in surface impoundments and in water supply and distribution networks, including canals (Izaguirre et al.2; Means and McGuire3; Izaguirre and Taylor4). Municipalities in the Phoenix Metropolitan area have experienced taste and odor problems for many years, but the problems seem to be increasing, especially beginning during the late summer and extending well into the late winter.
Water treatment costs for taste and odor problems alone have become exorbitant and consumers are more outspoken about expectations of receiving water that tastes and smells good, as well as is safe to drink. Two compounds, 2-methylisoborneol (MIB) and geosmin, are most commonly cited as imparting unpleasant earthy/musty tastes and odors to water. The source of these and other compounds associated with taste and odor is primarily blue-green algae (cyanobacteria) and certain fungi (actinomycetes). Research objectives were achieved through collaboration between operators of the City of Chandler Treatment Plant and researchers at Arizona State University, in cooperation with the Salt River Project. The on-going and focused effort was instrumental in an attempt to:
- identify the organisms causing the taste and odor problems, and
- to explore the use of citral to mitigate taste and odor problems.
Water samples were collected from two sites along the South Canal and five sites along the Consolidated Canal, including the intake into the Chandler Water Treatment Plant. Each sample was collected and stored on ice in 16-ounce plastic Whirl-Pak storage bags in the field and transferred to 125-ml plastic screw top bottles in the laboratory. Bottles were subsequently kept on storage racks at room temperature under light conditions similar to those in the natural environment. Aliquots of each sample were streaked on agar plates (1.5%) of Bold's Basic Medium (BBM; Carolina Biological Supply Co.) with an inoculating loop. After approximately two weeks, isolated colonies were transferred to culture tubes with 10-ml of liquid BBM. Additional streaks were performed when multiple organisms appeared in the same colony. This procedure was performed repeatedly in an attempt to isolate the maximum number of organisms from each sample.
Sampling and subsequent isolations indicate that taste and odor compounds most likely originate in "hotspots" along the canals rather than being distributed uniformly throughout the supply system. Detectable earthy/musty odors occurred intermittently along the supply system, as opposed to similar intensities at each site. Approximately 40 organisms (algae) have been isolated. Three of these isolates, two Oscillatoria spp. and Pseudanabaena sp. appear to be MIB and/or geosmin producers. These results must be verified using GC/MS after individual organisms have been cultured in larger quantities.
The results obtained are significant because the cost of the current approach of applying chemical treatments to the entire system to eliminate taste and odor problems may be reduced substantially if treatment efforts can be concentrated at supply system "hotspots". OPA panel results suggest that a citral concentration of 100 nl/L renders 50 ng/L of each MIB and geosmin undetectable and concentrations as low as 10 nl/L make 50 ng/L of each MIB and geosmin barely detectable to the human sense of smell. GC/MS results revealed that citral masks the tastes and odors associated with MIB and geosmin.
At the concentration used, citral also imparted a "citrus" aroma to the water. MIB, geosmin and citral were all detected at their original concentrations (100 ng/L) when water was spiked with all three compounds. There was neither an indication that citral initiated a chemical change in MIB or geosmin, nor that citral increased the binding efficiency of PAC. Despite the initial promise of using citral as a cost efficient alternative or supplement to PAC, citral was found to be an oily compound that was not readily miscible in water.
At the present time, additional research is required to determine whether citral may be mixed thoroughly enough with supply water during water treatment to make its use feasible and cost effective as it relates to taste and odor control. The propensity for citral to float on the water surface currently renders its use a non-viable solution for treating taste and odor problems. Due to the oily characteristic of citral, we were unable to determine the optimum citral:PAC ratio for maximum reduction of taste and odor compounds.