Method and arrangement for continuous filtering of particles out of a liquid

A method for continuous filtering of particles from a contaminated liquid by means of two or more separate filtering units (14) in order continuously to feed purified liquid to a consumer, where each filtering unit (14) comprises a rotatable, perforated drum (16) with a particle-catching filter means (18) on the outer surface of the drum, which method comprises the steps of holding the filtering units (14) immersed in a tank (10) of liquid to be purified so that the latter can flow through the filter means and into the drum with the aid of hydrostatic pressure, particles to be separated becoming caught on the filter means and in a coating layer built up thereon by particles caught previously, and of purified liquid being fed from the drum (16) to the consumer by a respective pump (26) and an associated filtrate outlet (24, 28, 32), the method comprising:

• circulating liquid which has passed through the filter means (18) on the drum (16) to the tank (10) until a first predetermined degree of coating with particles has been reached on the filter means of the drums;
• feeding purified liquid inside all except at least one of the drums (16) of the filtering units (14) to a consumer of purified liquid;
• keeping at least one filtering unit inactive in a standby state by shutting off the associated pump (26), interrupting the feed of purified liquid from a drum (16) to the consumer when a second predetermined, higher degree of coating with particles has been reached on the filter means on the drum, activating and maintaining at least one filtering unit (14) in a standby state as a replacement for the filtering unit or units from which the feed of purified liquid has been interrupted by means of its pump (26) being started up and its filtrate outlet (28, 32) being kept open, cleaning the filter means (18) of each shut down, filtering unit;
• passing liquid in the tank through the cleaned filter means on the drum of the shut-down filtering unit and circulating the liquid back to the tank (10) until the first predetermined degree of coating with particles has been reached on the filter means (18) of the drum, and shutting off the pump (26) of the cleaned shut-down filtering unit to put the filtering unit in a standby state.