Features
2 Small-scale domestic wastewater treatment systems with a submerged membrane unit
Kazuyuki Honda
KUBOTA Corporation

1. Properties and benefits of the activated sludge process with membrane separation
The advantages of the activated sludge process with membrane separation are: 1) Installation requires only a small area. 2) Sludge does not need to be concentrated as the johkasou operates at 1 ? 1.5% activated sludge concentrations. 3) Maintenance and management is easy because monitoring of filtration pressure and activated sludge concentrations are the only significant maintenance items. 4) It is strong against load variation of influent sludge. 5) The water quality is extremely high and transparency is highly stable. 6) Coliform bacteria and cryptosporidium can be prevented.
The disadvantages are: 1) if the activated sludge process with membrane separation is used except for the purpose of small-scale johkasou and advanced treatment, the initial cost and a running cost are comparatively high. 2) It is difficult to respond to a large excess of sewage quantity and not feasible for the purpose of extreme variation of influent water. 3) It is necessary to ensure regular activated sludge concentrations from the start of an operation. These are future challenges.

2. Membrane types
The membranes used for domestic wastewater treatment are mainly hollow fiber membranes and flat membranes. Positioning of the membrane for use in a johkasou system is shown in Fig. 1. It is classified as micro filtration and its hole diameter is 0.1 ? 0.4μm. When compared to the most general membrane in Japan, the hole diameters of hollow fiber membranes and flat membranes are 0.4μm and they have almost the same specifications.

Fig. 1 Positioning of the membrane for use in a johkasou system
Fig. 1 Positioning of the membrane for use in a johkasou system

3. Principle of activated sludge process with membrane separation
The activated sludge process with membrane separation is the process used to obtain treated water after water treated by activated sludge has been filtrated through a membrane. For the process of filtration, a cross flow filtration as shown in Fig.2 is employed. Installing the unit directly on the aeration tank and the process that produces a cross flow of membrane using air needed for aeration are both commonly found practices. The power costs for this model are drastically lower than older models.

Fig. 2   Conception diagram of a cross flow filtration
Fig. 2   Conception diagram of a cross flow filtration

4. Practical small-scale membrane separation johkasou
4.1 KUBOTA Corporation KM type and DAIICHI POLLUTION CONTROL EQUIPMENT CO.,LTD. FM type

The Kubota KM type and FM type、one of its OEM (Original Equipment Manufacturing) products, both have a bigger flow equalization tank to suck peak influent flow of domestic wastewater: NUD (Number of Users for Design) 5 at 613 L, NUD 7 at 811 L and NUD 10 at 1,112 L. Flow sheet of KM type / FM type is shown in Fig. 3.

Fig. 3 Flow sheet of KM type / FM type
図3 KM型・FM型のフローシート

The membrane employed is a flat membrane (KUBOTA Submerged Membrane separation) and performance is BOD 5mg/L or less, COD 10mg/L or less, SS 5mg/L or less and T-N 10mg/L or less. The filtration is performed by the self-suction pump. The operation or stop of the pump is controlled by the float switch installed on the denitrification tank. The control is performed by the control unit which employs the programmable controller. It has three operation modes which respond to a different load. The program can be rewritten.


4. 2 TMG type of Tokai Maintenance CO., LTD.
The TMG type is the device to convert the installed flush toilet wastewater treatment system into a domestic wastewater treatment system by using the installed flush toilet wastewater treatment system as a flow equalization tank. The membrane employed is a flat membrane and performance is BOD 5mg/L or less. With sales structure coupled with maintenance management, performance is guaranteed. A flow sheet of the process is shown in Fig. 4.

 

Fig. 4 Flow sheet of the TMG type
Fig. 4 Flow sheet of the TMG type

The TMG type is a half-underground type johkasou and has a structure which focuses on maintenance management. Filtration is performed by the self-suction pump. The pump along with the control panel is installed on the wall surface of the main body. The sludge storage tank is prepared as an option. Because a flow equalization tank is separated from the main body, the flexibility of the layout when installed is high.

5. Maintenance guidelines of the Ministry of the Environment
The guideline for the membrane separation johkasou for 5 to 10 NUD is defined by the Ministry of the Environment. The frequency of maintenance check must be once or more in three months and cleaning frequency must be once or more in six months as well. Cleaning must be done within a week after a maintenance check. The frequency of membrane cleaning chemicals must be once or more in six months. The first maintenance check must be performed within one week before the operation starts. At this time acclimation is also required. Thus, the guidelines indicate the primary treatment equipment, bioreactor, membrane separation equipment, sludge storage tank and disinfection tank must be checked regularly.


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1 Domestic wastewater treatment facility(Small-scale Gappei-shori johkasou)
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3 Development of nitrogen and phosphorus removal with the johkasou