STP and Ventillation Flashcards
Name 3 types of sewage plant
-Biological
-Collect, hold and transfer
-Membrane Bio Reactor
What gases are commonly associated with STPs and what hazards do they present ?
An aerobic plant may contain hazardous quantities of CO2
An anaerobic plant may contain hydrogen sulphide which is toxic and heavier than air and methane which is explosive and lighter than air.
What are the effects of sewage pollution ?
The biological oxygen demand (BOD) of the organisms in faecal matter will cause de-oxygenation of the water.
The suspended solids level in the water will increase.
Disease producing organisms in raw sewage could enter the food chain particularly via shellfish.
Raw sewage looks horrendous and is a health hazard in public areas like beaches.
Before work is carried out on a sewage system, what preparations should be carried out by Ship’s Staff ?
Ship’s Staff are to ensure the plant is;
• fully isolated,
• emptied,
• thoroughly flushed through with salt water and
• ventilated.
When venting the system and tanks, portable fans should discharge dangerous gases directly overboard.
Do not use ships vent or plant vents.
If the system/STP cannot be emptied due to a blockage, this is to be clearly stated on the WRF.
Where can sewage be ditched ?
Within 3mm of the coast providing an IMO approved STP is in operation.
The discharge shall not produce visible floating solids or discolour the surrounding water.
Between 3 and 12 nm. from the coast,
discharge from an IMO approved system to break up and disinfect the sewage.
Over 12mm. no treatment is necessary,
However, discharge from holding tanks must be done at a steady rate and when the ship is travelling at over 4 knots.
What is the purpose of vent ?
The ventilation system is designed to:
• Cool certain electrical equipment
• Provide fresh air
• Tolerable working conditions
• Tolerable living conditions
It is also used to exhaust:
• Wild heat
• Noxious fumes
• Odours
• Stale air
What part is played by the vent system:
a. in the event of a fire?
b. when closed down to ZA?
In the event of a fire, smoke migration is restricted to defined zones.
Post-fire contamination is cleared with the minimal loss of operational effectiveness.
When ‘closed down’, a positive pressure relative to external atmosphere can be achieved and maintained in the Citadel and Machinery Spaces when required.
Describe the two most common types of fan found in Service.
Centrifugal fans are general-purpose supply and exhaust fans in a trunked system.
Air is drawn into the centre of the fan and then expelled outwards to the tips of the blades by centrifugal force and into the delivery trunk.
Axial fans are fitted inline with the trunking and therefore save space.
They can move large quantities of air against a low resistance.
Why are filters fitted in the vent system ?
Air moving around the ship will pick up impurities such as dust, salt, sand, bacteria etc.
This will quickly clog inlets, heaters and coolers.
Reduces the overall efficiency and performance of the system, equipment and personnel.
Where are odour filters fitted ?
They are fitted in the re-cire trunking of bathrooms. WCs and galleys.
What are odour filters made from ?
Made from carbon granules, carbon cloth or a combination of chemicals.
How long should odour filters last and what can affect their performance ?
The filter should last for 6 months, however humidity in excess of 90% will affect their. performance.
