Activated sludge method is being widely used for the treatment of household waste water and sewage treatment. Activated sludge is a group of microorganisms which decompose the organic matter in the waste　water. This method is called activated sludge method. Activated sludge grows in a large number while the treatment of waste water and it is to be necessary to discharge the excess activated sludge out of the system. This activated sludge is a huge burden of the environment as well as economy. We are using ferrite particles’ motion controlled by the magnetic field to treat the excess sludge and later return this treated sludge the plant where the activated sludge purify the water. Thus a zero emission waste water treatment plant is proposed and we have found it is working well at laboratory scale. The miniature plant of our lab does not produce any excess sludge but does purify the waste water.