Q. What do you mean by a Sewage treatment unit and its use onboard?
Ans : The Sewage Treatment unit is a self contained system for the treatment of sewage from ships or rigs before it is discharged overboard and so prevent the pollution of harbour and coastal waters and inland waterways_ The system uses the aerobic principle of sewage digestion, coupled with treatment of the final effluent, and is generally accepted as the most compact, efficient and flexible system for use on board ship.The unit operates satisfactorily on salt, fresh or brackish flushing water.
Q. Define the other parts of a Sewage treatment unit?
Ans : Basically the Super Trident Sewage Treatment unit comprises a tank, divided into three water tight compartment. An aeration compartment, a setting compartment, and a chlorine contact compartment.
The incoming sewage enters the aeration compartment, where it is digested by aerobic bacteria and micro organisms which are promoted in the sewage itself by the addition of atmospheric oxygen.
From the aeration compartment the sewage flows into the settling compartment where the aerobic bacteria floc, known as activated sludge, is settled out producing a clear effluent which passes through a chlorinator and into the chlorine contact compartment before being finally discharged.
Q. Describe the purpose of its different compartments and fittings?
Ans :
Aeration Compartment
In this compartment of the treatment unit, aerobic bacteria, that is those bodies requiring dissolved oxygen to exist, reduce the influent waste material which mainly comprises Carbon, Oxygen, Hydrogen, Nitrogen and Sulphur into Carbon Dioxide, Water and new bacteria cells. The Carbon Dioxide is emitted throughout the vent system whilst the water together with the bacteria cells are displaced into the settling compartment.
Air is supplied to the sewage from a rotary compressor through a number of fine bubble diffusers, located at the bottom of the tank but removable from the side for ease of maintenance.
The air provides the life giving oxygen to the aerobic organisms and also keeps the contents of the tank intimately mixed with the incoming raw sewage and the returned settled activated sludge.
Settling Compartment
In the settling compartment the bacteria settle out and are returned to the aeration compartment by the airlift tube. This takes its supply from the bottom of the compartment and discharges to the aeration compartment, via a visual indicator pipe which enables a check to be made on the sludge return. The settling compartment of the unit is of the hopper type. The sloping sides prevent the sludge from accumulating and direct it to the suction side of the air lift. The effluent enters the compartment through a stilling chamber and rises through the clarifier, to discharge to the chlorine contact compartment through a weir at the top of the clarifier. A surface skimmer is provided to skim off and return surface debris back to the aeration tank – a second airlift is used for this purpose.
Chlorine Contact Compartment
The effluent is stored in this compartment after chlorination to allow time for the chlorine to kill off any harmful bacteria. On units fitted with a discharge pump two level regulators (or float switches) are fitted to control the operation of the pump, see Electrical Controls.
An additional float switch is 'fitted which operates an alarm signal should the level rise above the normal 'high' position.
An emergency overflow pipe connection is also provided through which only treated effluent can pass.
The Chlorinator
Tablet disinfection (standard). This is of the tow-through type and all of the effluent passes over the specially formulated tablets and absorbs the required amount of chlorine before flowing into the chlorine contact tank.
One or two tubes depending upon test results (see Section 4 – Determination of Residual Chlorine Content) are filled with tablets and placed in position in the chlorinator with the cut-away sections at the lower end. The effluent flows around and past the lower end of the tube and comes into contact with the tablets.
At the outlet end of the chlorinator is a control weir arranged so that as the effluent flow increases the level rises and more tablets come into contact with the effluent In this way the chlorine uptake is always sufficient for sterilisation of the effluent.
Discharge Pump
These pumps are horizontal close coupled centrifugal units with a 2-bladed open impeller of the non-clog type. They are designed for capacities up to 4D m3/11 and operate at 2-pole motor speeds.
The medium head pump is suitable for heads up to 18 metres. The high head pump is suitable for heads up to 25 metres.
The pumps are particularly suited for effluent discharge, sump drainage and handling dirty water containing solids up to 25rnm. diameter.
Air Compressor
One rotary vane air compressor is fitted as standard.
The rotary air compressor motors, are operated by manual switches on the control panel. When a discharge pump is supplied it is controlled by float switches fitted in the final effluent tank with an overriding HANDIAUTO switch on the control panel.
Float Switches
These are of the magnetic reed type with three 'floats mounted on a common stem and wired to the control panel. The two lower floats operate the discharge pump{s}, and the upper float operates the high level alarm circuit should the level rise above normal.
Q. Explain its Operation?
Ans : When the system is fitted with a discharge pump and the control switch is in the AUTO position, the typical sequence of operation is as follows:
When the liquid reaches the "High' level float switch, the pump motor is started and the liquid begins to discharge from the compartment The pump continues to run until the liquid drops to the level of the "Low' level float switch when the motor is switched off
Should the 'High' level alarm float switch fail to operate for any reason, the level of the liquid will reach the 'High' level alarm float switch, and actuate the alarm circuit