Discarding sewage produced onboard a ship is one of the few tasks on a ship that should be taken utmost care of if one wants to save him and his shipping company from a heavy fine. The sewage generated on the ship cannot be stored on the ship for a very long time and, for this reason, it has to be discharged into the sea.
Though sewage can be discharged into the sea, we cannot discharge it directly overboard as there are some regulations regarding discharging sewage that needs to be followed. Sewage on the sea is generally the waste produced from toilets, urinals, and WC scuppers. The rules say that the sewage can be discharged into the seawater only after it is treated and the distance of the ship is 4 nautical miles from the nearest land.
But if the sewage is not treated this can be discharged 12 nautical miles away from the nearest land. Also, the discharged sewage should not produce any visible floating solids nor should it cause any discoloration of the surrounding water. The details of the sewage discharge regulations can be found in MARPOL Annex IV.
There are different methods of treating sewage available in the market, but the most common of them is the biological type for it occupies less space for a holding tank, unlike those of the other methods. Moreover, the discharge generated from this plant is eco-friendly. It is to note that each sewage treatment system installed onboard has to be certified by a classification society and should perform as per their requirement and regulations.
The most preferred type of sewage treatment plant is that involving aerobic bacteria. Anaerobic bacteria are equally capable of decomposing and breaking down the sludge but during the process, they generate and release harmful gases such as H2S and methane which are toxic and dangerous for aquatic organisms.