Abstract
To achieve a high rate of nitrification in contact aeration tanks, it is important to provide appropriate countermeasures for the mass propagation of large invertebrates such as Asellus and Physa. Because they consume a large quantity of biofilm.
Herein, experiments were carried out to examine the countermeasures for abnormal growth of large invertebrates, and the following findings were obtained.
The invertebrates tended to increase in number in autumn to winter and spring, and the trend was more evident in Asellus. Nitrification was decreased in such seasons. Therefore, countermeasures must be more important in those seasons.
The lower the wastewater loading for aeration tanks was, the more nitrification was achieved under the abnormal growth. Therefore, in actual plants, the decrease of loading for aeration tanks might be a good countermeasure in the proper management for anaerobic filter tanks. The employment of contactors having a larger specific surface area in aeration tanks might be also worthwhile.
In model aeration tanks, 5mg/l of Cartap, an insecticide, for Asellus and 20mg/l or more of AlCl3 for Physa were effective in decreasing their number. The Cartap effect was also confirmed in the actual plant. The treatment must be done in the order of aeration tanks in which Asellus generated. Otherwise, the inflow of new seed Asellus from the front aeration tanks might shorten the effective period of Cartap.
