By Terry Keep of TrojanUV, Said Abou Abdallah of Arcadis, and Dr. Dean Reynolds, Department of Water Treatment City of Alliance, Ohio
The City of Alliance Ohio’s water system has experienced annual Taste and Odor (T&O) events since the mid 1950’s, when the first of two reservoirs, Deer Creek Reservoir, was placed into service. Nutrient contaminants, in particular phosphorous, in the watershed accumulate in the reservoirs causing algal blooms. The blue-green algae, also known as cyanobacteria, produce two aesthetically-offensive secondary metabolite compounds, 2-methyisoborneal (MIB) and trans-1,10-dimethyl-trans-9-decalol (Geosmin). Although both compounds occur in high concentrations in the reservoirs, frequent testing showed that MIB is the primary compound causing T&O problems in the city’s drinking water. MIB can be detected by a few very sensitive persons as low as 7 ng/L (parts per trillion, ppt), but most people cannot detect MIB until it is greater than 20 ppt.
A particularly bad T&O event occurred in the winter of 2009 when the MIB level reached 340 ppt. The City was flooded with complaint calls. The Alliance Water Treatment Plant (WTP) fought the problem with the available treatment processes. The main process used for removing MIB was powder activated carbon (PAC). In addition, 24 in. of granulated activated carbon (GAC) capping the eight filter beds in the plant had shown some seasonal success at removing MIB. Concentrations for a typical 12-month period are presented for 2010 and 2011 in Figure 1. The highest MIB concentration during that time was 1100 ppt.