IDEA Waste Water Treatment Plants


In this process, a mechanical bar screen is installed to remove all the inorganic material from the sewage and dispose of it to a waste bin.

The screened effluent then enters the Intermittent Decant Extended Aeration (IDEA) tank wherein the biological treatment process of oxidation, nitrification, de-nitrification, sedimentation and aerobic sludge digestion takes place.

After clarification in the IDEA tank the clear liquor from the top of the tank is dosed with sodium hypochlorite for sterilization and then decanted via a float suction weir to the final effluent disposal tank where it is stored prior to disposal.

The Intermittent Decant Extended Aeration (IDEA) Tank is operates in four modes; these are Aerobic, Anoxic, Settle and Decant.  During all modes of operation screened sewage is allowed to enter the IDEA Tank via the inlet baffle. A submersible aeration pump is installed in the centre of the IDEA tank to provide oxygen to the tank’s contents.  The pump is provided with a variable speed drive which automatically ramps up or down based on the required oxygen level in the IDEA Tank. An ultrasonic level transmitter is fitted to the tank to indicates the tank volume, control the decant pump and indicates an alarm at high level. An emergency overflow is connected from the IDEA tank to the final effluent disposal tank for use as a balance tank in the event of excessively high levels in the IDEA tank.

The aerobic period is timer controlled via a PLC and maintains aerobic conditions in the IDEA tank for a 105 minute period.  During this 105 minute period the aerator maintains a minimum oxygen level of 2.5mg/L. The aeration pump is fitted with a VSD (Variable Speed Drive) to maintain the required oxygen level at all times independent of the system inflow.  A dissolved oxygen sensor is located in the IDEA Tank to control the output of the aeration pump based on the current oxygen level and operation mode.

After the 105 minute aerobic period the system automatically initiates the anoxic mode.

The anoxic period is timer controlled via the PLC and maintains anoxic conditions (oxygen level of 0mg/L) in the IDEA tank for a sixty minute period.  During this time the aeration pump operates at 30% of full speed to mix the contents of the tank with zero oxygen input. After the sixty minute anoxic period the system automatically initiates the second aerobic mode when the aeration pump starts.

The aerobic and anoxic modes alternate continuously until such time as the settle mode is initiated.

The settle mode is timer controlled at set time intervals.  It is maintained for one hour. During this time all the solids in the IDEA tank settle to the base of the tank leaving clear liquor on top.  During this period the aeration pump is turned off. After this period the decant mode is initiated.

The decant pump starts and draws the clear liquor from the top of the IDEA tank and transfers it to the final effluent tank for storage.  The decant pump stops at the low level set point in the IDEA tank. The duty decant pump alternates at the end of every decant cycle. Should the duty pump fail to start the standby pump starts.  On the completion of the decant cycle the aerobic mode runs until the timed anoxic and aerobic sequence recommences.

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