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RO-Based Desalination Technology Utilizing High-Speed Seabed Infiltration System (HiSIS)

Usual seawater desalination plants using reverse osmosis membranes are introducing raw seawater from conventional open intake facilities where chlorine is dosed as a disinfectant at almost all times in order to prevent growth of marine organisms on these intake facilities. At the same time, some reductive agent must be added, since residual chlorine damages the RO membranes.
Furthermore, these chemicals are said to trigger biofouling of the RO membranes, and the removal of such foulant through repeated cleaning with strong chemicals results in shortening the useful life span of the RO membranes.
A seabed infiltration system (Fig. 1) as one of seawater intake methods does not require these chemical injections. However, in currently operating seabed infiltration systems a very low filtration (infiltration) velocity (approx. 5 m/day) is applied, therefore a much larger and impractical infiltration-intake area becomes necessary in case of a large scale intake rate.
Our advanced RO-based desalination technology utilizing the High-Speed Seabed Infiltration System (HiSIS) we have developed with Nagaoka International Corporation does not require chemical injections and moreover enables a maximum infiltration velocity of 100 m/day. This means that the infiltration-intake area becomes 1/20 smaller than that of a system with an infiltration velocity of 5 m/day (Fig. 2).
The seawater from our HiSIS is further filtrated via UF membranes to remove impurities whose size will be as small as 10 nm (1 nanometer equals one-millionth of a millimeter), and finally desalinated by means of RO membranes.
We are confident that our RO-based desalination technology utilizing our HiSIS will undoubtedly contributes to the stable supply of drinking water and the mitigation of environmental loads in drought and thirsty countries like Middle East, North Africa, etc. who have been experiencing a water supply shortage for prolonged years. At present, we are constructing a demonstration plant on the Arabian Gulf in collaboration with the Abu Dhabi Water and Electricity Authority (ADWEA).

<Environmental Systems & Plant Laboratory, Technical Research Institute>

Fig. 1 RO-Based Desalination System utilizing High-Speed Seabed Infiltration System (HiSIS)

Fig. 2 Comparison between HiSIS and Low-Speed Seabed Infiltration System (Conventional)

Movable Flap Gate-Type Breakwater

As shown in Fig.1, the flap gate type breakwater usually lies down on a seabed, and immediately rises up through the sea surface with buoyancy when a tsunami or storm surge occurs. To achieve the practical use of the flap gate type breakwater, various characteristics of the motion of the gate in waves and currents have been improved by using a hydraulic model experiment and numerical analysis. Fig.2 shows 2-D and 3-D hydraulic model experiments of a flap gate type breakwater.
As shown in Fig.3, we developed the flap gate type rising seawall based on findings from a study of the flap gate type breakwater. This structure is a new type disaster prevention facility with the characteristic of not needing power and artificial manipulation against a tsunami and storm surge.
We will promote research and development of both a flap gate type breakwater and flap gate type rising seawall still more, aiming at both disaster prevention and security around the coastal sea areas such as ports and fishing ports.

<Plant & Energy Solution Technology Group, Technical Research Institute>

Fig.1 Movable flap gate type breakwater

Fig.2 Hydraulic model experiment

Fig.3 Flap gate type rising seawall

Performance Evaluation of Movable Breakwater Using Numerical Simulation Model

Tsunamis and storm surges cause serious damage to coastal areas, especially in Japan, which is surrounded by sea. We have been developing a flap-gate-type movable breakwater to mitigate damage caused by tsunamis and storm surges. The breakwater normally lies on the seabed, as shown in Fig.1. When tsunamis or storm surges occur, it rises up from the seabed because of buoyancy and forms a continuous seawall. We evaluated the performance of the breakwater by performing many hydraulic model experiments.

We have also developed a numerical simulation model that involves the use of the overset grid method, for the performance evaluation of the breakwater. This simulation model can solute both tsunami flow and breakwater motions simultaneously. The overset grid method is particularly effective when flows interact with structures, and it is well suited for performing simulations of the flap-gate. Figure 2 shows an example of a simulation result. By using the simulation model, we can evaluate the tsunami forces acting on the breakwater and obtain useful data for designing the flap-gate.

<Plant & Energy Solution Technology Group, Technical Research Institute>

Fig.1 Flap-gate-type movable breakwater

Fig.2 Simulation result for tsunami flow obtained
by using the overset grid method