(a) Describe a vacuum sewage system;
(b) With reference to the system described in (a)
(i) State ONE advantage of the system;
(ii) State why a holding tank may be required to be fitted to the system;
(iii) State the problems resulting from the retention of untreated sewage in a holding tank.
Structured for DG Shipping MEO Class II examination scoring criteria.
A vacuum sewage system is an innovative method for transporting sewage that does not rely on gravity. Instead of using large volumes of water and the natural downward flow of gravity, it uses a pressure differential (vacuum) to pull sewage through the pipes. Water is only used for rinsing the toilet bowl, not for moving the waste itself. This allows for both limited vertical lifts and long horizontal transportation of sewage.
The system works by using a vacuum created by an eductor, which pulls the sewage into a sewage tank. A pressure switch controls the system by automatically starting and stopping a centrifugal pump to maintain the vacuum in the lines. The water in the sewage tank acts as the driving force for the eductor.
One significant advantage of a vacuum sewage system is the reduced consumption of fresh water. Since only a small amount of water (around 1.2 litres per flush) is used for rinsing the bowl, the overall water usage is much lower compared to traditional gravity-fed systems. This also leads to less corrosion in the tank and piping since less fresh water is used.
A holding tank is required in situations where the direct discharge of treated or untreated sewage is prohibited, such as when a vessel is in a port or within restricted coastal waters (e.g., within 12 miles of the coast). This tank holds the sewage until it can be discharged to shore reception facilities or pumped overboard when the vessel is in unrestricted waters.
When untreated sewage is held in a tank, it creates an environment where anaerobic bacteria thrive. These bacteria break down the waste without oxygen, leading to several problems: