Updated: July 2002

1. Outline

There is an ever-growing demand for water as a result of enhanced standard of living and industrial development. From this, water supply shortage is expected because development of the water resource hardly catches up with the demand. This requires not only a saving but also reutilization of water as much as possible.
The MSR System (Membrane sewage Reclamation System) is a wastewater reutilization system (water reclamation system) combining physical treatment by the ultrafiltration method with biological treatment by the activated sludge method. As shown in the flow sheet, the ultrafiltration membrane (UF membrane) separates solids from the liquid, and eliminates the sedimentation tank after the aeration tank, thus reducing the space for the installation. The function of main equipment for the system is as follows.

- Primary screen: Removes rough and large solids and foreign matter from the raw wastewater.
- Secondary screen: Prevents fine foreign matter from flowing into the aeration tank to avoid the failure such as clogging of the ultrafiltration membrane.
- Ultrafilter: A separator of activated sludge from the treated water.
- Chemical washing device: Periodically washes the membrane.
- Disinfectant tank: Uses chlorine disinfectant.
- Activated carbon adsorption tank: Removes the color when wastewater is mixed in the raw wastewater.

2. Features

1) Compact size for easy maintenance
As clearly shown in the system components, this system is quite compact compared with the general wastewater treatment systems that have large equipment such as the sedimentation tank and sand filter. For the maintenance, it eliminates the work of controlling the quantity and quality of sludge (such as bulking) as required for the activated sludge method.
2) High quality of water and stable treatment
The ultrafiltration membrane helps obtain quite a transparent type of water by completely stopping the undecomposed high polymer, colloidal particles, and microorganisms. It is also sanitary by removing the colitis germs. The microorganisms checked by the membrane are returned to the aeration tank, to increase density of microorganisms (sludge) in the tank. Thus, it not only improves the treatment efficiency but also ensures a stable treatment against the fluctuating feed rate and quality of the raw wastewater.
3) High recovery rate and reduction of sludge generation Being shut up in the aeration tank, the microorganisms repeat propagation within the tank. Finally, however, the propagating speed reaches an equilibrium with the speed of self-digestion of the microorganisms, so that the sludge density comes to an even level. Therefore, by increasing the sludge density, the amount of self-digestion is increased to reduce the excess sludge. At the same time, the quantity of discharged water is also reduced so that the treated water in almost the same amount of the inflow raw wastewater can be obtained.

3. Solid-liquid Separation by Membrane

The ultrafiltration membrane is used to separate and remove the substances such as organic and inorganic substances and colloidal particles that are dissolved or suspended in the wastewater, according to their size. It is generally used to separate substances having a molecular weight of about 500 (approximately 10^-3µm) or more. The membrane because of its aeolotropic structure allows penetration of substances smaller than its hole diameter along with the water, while checking those larger than the hole diameter, thus preventing in most cases the clogging from occurring on the membrane.
However, when the checked substances on the membrane are accumulated on the surface, a density gradient is formed to reduce the speed of solid-liquid separation.
That is why a cross-flow filtration method is adopted.
By the cross-flow method, the filtrate flows in a direction crossing that of the raw wastewater flow. This prevents the membrane from being clogged by allowing the raw wastewater to pass over the membrane surface at a high speed, to flush the sediment.