Taste-and-odor problems in drinking water have been occurring globally, and much of the cause is an earthy-musty odor. In October 2014, even in Oita city, a serious musty-odor problem occurred in drinking water. The cause was found that musty-odor-producing cyanobacteria proliferated in the multipurpose dam located upstream of the intake of the water purification plant, and 2-methylisoborneol (2-MIB) concentration increased. The urgent task was to provide information to customers and to reduce the concentration of 2-MIB in drinking water. Therefore, the cause-cyanobacteria surveys, organization response, odor removal treatment at the water treatment plant were carried out, and the convenience and comfort of drinking water restored. These series of countermeasures against the earthy- musty odors in tap water were discussed.

Recently Tama river, the water source of Tokyo, has a problem with 2-MIB caused by Phormidium autumnale. In August 2013, 220ng/L of 2-MIB was detected from raw water of the purification plant. In high water temperature period in summer, 2-MIB concentration was very high although there were a small amount of cyanobacteria. In contrast, in low temperature period in winter, 2-MIB concentration wasn’t so high although there were a large amount of cyanobacteria. From these results, it is suggested that production and release of 2-MIB of cyanobacteria was affected by water temperature. Based on the result of a hydraulic experiment model, we introduced intermediate chlorination to take measures against high concentration 2-MIB. As a result, the removal capacity of 2-MIB was doubled.

In the Kanagawa prefectural water service, which is the source of the Sagami River system, it is problem that nuisance algae growth in reservoirs, Lake Sagami and Lake Tsukui, has been adversely affected on water purification treatment and water quality. Among these biological problems, the most serious is a taste and odor problems caused by a musty odor. In recent years, it has been observed that this taste and odor problems occur not only in the upstream water source lake but also in the main stream of the Sagami River. This paper outlines the taste and odor problems in the Sagami River water system, furthermore, reports the case of limnetic nuisance algae growth and the investigation of the epiphytic and periphytic algae existence in the middle stream of the Sagami River.

Phenylacetic acid and nonanoic acid are components found in the living environment that give off an unpleasant odor. Although these substances are very common in our daily environment, they have not been recognized as the cause of unpleasant odors, because they have a relatively high boiling point and are subject to less general evapotranspiration. We proposed to measure the odor threshold by the Triangle Odor Bag Method and evaluate the value of odor quality; research participants sniffed four gasbags at predetermined concentrations.

The results are below:

1.　The odor threshold of phenylacetic acid is 7.3×10^－7 (ppm) and nonanoic acid is 7.1×10^－4 (ppm). That is equal or lower than the offensive odor substances such as fatty acids and aldehydes in the previous studies, and the existence of odor was distinguished in a low concentration.

2.　The odor quality was evaluated by 138 research participants. The evaluation items are odor intensity and pleasant/unpleasant level, and so on. The results show the score of odor intensity and level of unpleasantness become higher as the odor concentration increase in both substances.

3.　Factor analysis based on the kind of odor by the SD method plotted a factor score on the extracted axis, for the increase of unpleasantness, solid feeling and acridity score with odor concentration.

4.　Odor index of phenylacetic acid is 17 and nonanoic acid is 21, which is calculated at the 20% of non-acceptance rates. These were both higher than the odor of restroom and personal odor of the past study and it was understood these were allowable levels of odor at a high concentration when they existed in the air.