N-plant, Japan

Kubota helped demonstrate that the combination of high-rate filtration and MBR was the most suitable solution for renewing the East treatment line by using the existing civil works.

The previous treatment train consisted of primary sedimentation, conventional activated sludge with aeration biological tanks, secondary sedimentation, and disinfection. The new treatment train consists of high-rate filtration instead of primary sedimentation and MBR instead of CAS. Secondary sedimentation and disinfection are no longer needed.

Submerged membrane units were installed in the existing aerobic tanks, converting them into membrane bioreactor tanks and enabling high-quality treated water production in a more compact layout. High-rate filtration was installed ahead of the MBR to strengthen wet-weather treatment capacity and improve CSO control. Because the floating filtration media removes nearly all debris, it also helps protect the downstream membranes from clogging and physical damage.

Keeping energy consumption of the renewed plant as low as possible was an important part of the project. Kubota combined several measures in one integrated concept. The SCRUM system controls the supply of supplemental air for biological treatment using an ammonium sensor, and also controls the scouring air needed to keep the membranes clean using a transmembrane pressure prediction model. In practice, the MBR requires two types of air: supplemental air for biological treatment and scouring air for membrane cleaning during operation. By helping dose the actual air flows required, the SCRUM system also helps reduce the energy consumption of the blowers, which account for more than 75% of the plant’s total energy demand.

Additional energy savings come from the use of air lift pumps to recirculate sludge from the membrane tanks to the anoxic tanks, using 95% less power than conventional recirculation pumps, and from siphon filtration for permeate extraction, using 99% less power than conventional membrane filtration pumps. Together, these measures help keep the operating energy demand of the renewed plant as low as possible.

The combined high-rate filtration and MBR system is designed to respond differently under dry weather and wet weather conditions.

Under dry weather conditions, the average inlet flow of 40,000 m3/day passes first through the high-rate filtration system and then through the MBR.

Under wet weather conditions, the average inlet flow can increase to 96,000 m3/day. In that situation, the same 96,000 m3/day passes through the high-rate filtration system and can then be treated by the MBR for a maximum of 4 hours per day.

At peak wet weather conditions, a maximum daily inlet flow of 576,000 m3/day can pass through the high-rate filtration system. Of this, a maximum of 96,000 m3/day can be treated by the MBR for up to 4 hours per day. The remaining 480,000 m3/day can be bypassed as combined sewer overflow, but with an important advantage: the water quality is significantly better than the effluent from a conventional primary sedimentation process. This helps reduce pollutant loads during wet weather.

N-plant is a strong example for municipalities that need to renew aging wastewater infrastructure without expanding the site, while also improving effluent quality and strengthening wet weather performance. It shows that high-rate filtration and MBR can work together as a practical renewal route when footprint, enhancing water quality, wet weather control, and operating efficiency all matter.