Main components of the vacuum sewage treatment plant on board ship are ejector (for creating vacuum in toilets), pressure switch (to maintain vacuum in the system) and level switch for the operation of treated effluent discharge pump. Working and maintenance of air ejector, pressure switch and level switch are explained below.
Operation of Air Ejector
The air ejector consists of a jet tube of stainless steel, a check valve and a nozzle of PVC. The driving water for the ejector is supplied from the collecting tank with the help of a pump and the discharge is fed back to the tank. Sewage is pumped from the tank through the nozzle of the ejector creating a venturi effect whereby air and waste are inducted from the toilet piping, mixed with the sewage jet and passed into the collecting tank. When the pump stops, the rubber flap of the check valve is drawn into the closed position by the vacuum, which is thereby maintained.
Maintenance of Air Ejector
The only moving part in the ejector is a rubber flap in the check valve. Tightness of the ejector flap shall be checked periodically.
Once a year:
- Open and clean ejector check valve
Every five years:
- Change ejector flap
- Clean ejector nozzle
- Check and clean ejector from build-up of deposit
Operation of Pressure Switch
The vacuum level in the system is controlled by a pressure switch. The pressure switch or switches and the vacuum gauge should be installed on a separate manifold, to reduce the chance of impurities entering the switch or gauge. The instrument branch must have 5° slope. Water in the instrument branch will cause malfunctions.
The switch starts a pump when the vacuum level in the system falls below a preset value. If the vacuum level still keeps declining, another pump will be started (provided that there is more than one pump in the system). As soon as the required vacuum level has been reached, the pump will be stopped.
- Set the stop pressure by turning the knob. As you turn the knob, the indicator on the scale will move. It should be set between -0.4 bar and -0.6 bar (40-60% vacuum). This is the vacuum level at which the pumps will be turned off. Note that in each system the optimal setting may vary.
- Set the start pressure by turning the knurled knob, reached by removing the cover of the switch. Turning the knob will set the starting vacuum level somewhat lower than the “stop”level set by the knob. In effect, whenever the “stop”level is changed by turning the knob, the starting level will also be changed.
- The plant can be provided with a low level pressure switch for low vacuum alarm. Set the “off” pressure to -0.3 bar.
- The setting of the switch may vary from system to system depending on the individual requirements.
- The vacuum gauge installed, in each system enables you to check that pressure switches are functioning properly.
Maintenance of Pressure Switch
Once a year:
- Check the operation of both pressure switch and vacuum gauge.
Every five years:
- Clean or change piping and components between switch and gauge.
Operation of Level Switch
The liquid level in the tank is controlled by magnetic level switches, which start and stop the pump. The switches consist of a float, a body, a rubber gaiter, a mounting flange, a switch mechanism in the body, and two permanent magnets in repulsion, one in the float, the other in the switch insert. As the float and its magnets move up and down with the liquid level, the magnet in the switch insert moves correspondingly and either opens or closes the circuit controlling the pump or an alarm function.
Maintenance of Level Switch
- Once a year (or during tank cleaning): Clean the level switch
- Before starting to remove a switch, make sure that liquid level in the tank is below it. If necessary, discharge to required level, but be careful not to let the pumps run dry. Then switch off the electrical supply, remove the securing nuts, and pull out the switch. Clean the float and check that it moves freely.
- Under certain conditions an excessive amount of foam may appear in the tank. Should the level switches react to the surface of the foam, instead of the actual liquid level, malfunctioning may occur in the system. To avoid or eliminate this problem, see under Trouble Shooting of Biological Vacuum Sewage Treatment Plant.
Reference: EVAC Environmental Solutions Marine Sector Operation Manual