Breathing into the Hori River August-December 2002

Breathing into the Hori River August-December 2002

The demonstration that began in Nagoya's Hori River is a straightforward system of pumping up river water, mixing it with microbubbles, and returning it to the river. The bubbles are approximately 0.03 to 0.10 mm. The bubbles are estimated to be the equivalent of 24 liters per minute, and it is calculated that the level of dissolved oxygen will reach a level at which fish can live in about 30 hours after the system is put into operation. The system will be in operation until the end of the year. It was a light first step to tackle the bad odor and stagnation of urban rivers not through major civil engineering surgery, but through intervention in the water itself.

In December 2001, the 5th Water Quality Control Regulation was introduced, and nitrogen and phosphorus were newly regulated in addition to COD in the Tokyo Bay, Ise Bay, and Seto Inland Sea basins. The aeration of the Horikawa River is a perfect example of this. Aeration of the Hori River is an attempt to meet the demands of the times.

Looking at related technologies, biological treatment combining aerobic nitrification and anaerobic denitrification is the mainstream for nitrogen, while for phosphorus, in addition to biological dephosphorization as typified by the A2O method, pressure flotation, coagulation and precipitation of iron salts, and collection in adsorption columns are combined. The use of reclaimed water and the control of sludge generation were also requirements, and the optimization of equipment and operation was a simultaneous challenge.

From the viewpoint of resource recovery, the method of extracting phosphorus as crystals was advanced. For example, a system that recovers phosphorus in the form of magnesium ammonium phosphate granules has achieved a recovery rate of over 90% and also contributes to the partial removal of nitrogen components. This is an ingenious way to realize the interface between the reduction of the load on the ecosystem and resource recycling in the field of sewage and industrial wastewater.

In this way, the microbubbles in the Horikawa River were a breathing intervention to reduce the load upstream of the watershed and to mesh with the advancement of sewage treatment, while checking the practical barriers of flow conditions, water temperature, electricity, and maintenance, one by one. Moving water, delivering oxygen, and making time our ally. The river restoration of 2002 began its story with a series of quiet bubbles.