Water And Waste Management On Ships

Author: Raunekk (Marine Engineer)

Need for Water and Waste Management On Board

Water and waste management on ships is as important as on land. We know that due to increasing awareness and the deteriorating environment with every passing day, environmental concerns are affecting the functioning of various industries. The marine industry, which is directly dependent on one of the most widely found natural resource, that is, water leaves its impact on the same. International and national organizations have risen to the occasion and tried to mitigate the effect of industry on this resource and issued detailed guidelines for disposal of waste water on ships. One of the important aspects that need to be dealt with is water and waste management on the ship.

A few years back waste water produced while on board was collected in tanks and pumped into the sea once the ship was beyond the 12 mile distance from the coastline. Today waste water cannot be discharged into the sea without being treated. The water needs to be treated using approved equipments. In case the wastewater is treated it cannot be discharged at a distance less than 3 nautical miles and incase of untreated wastewater at a distance not less than 12 nautical miles. In case waste water is stored in holding tanks, it cannot be discharged instantaneously but rather at moderate rates.

Waste water produced on ships can be classified in two categories:

  • Grey Waster Water
  • Black Waste Water

Grey Waste Water is a term that is to describe water from sinks, laundries and showers, where as Black Waste Water is the term that is used to describe sewage. Various equipments are available for treating waster water.  Grey and Black Waste Water both may be treated using a single equipment or separate equipments. The most commonly used technique that is used for treating waste water is reverse osmosis technique in Membrane Bioreactors known by the acronym MBR.

Working of a Membrane Bioreactor on Board:

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As discussed above, grey water is the water produced from showers, sinks and laundry. The water is directed into a waste water tank. It is vigorously mixed in this tank with the use of rotating paddles. From this tank, the water is pumped through a series of mesh screens. The result is that the solid waste is separated from the liquid waste. The solid waste is sent into the sludge tank. The liquid waste is sent to an aerated bio tank. In this tank water is supplied into it with the help of blowers. This biological process helps in removing organics which are present in the waste water. After the water is treated biologically it is sent into the reverse osmosis tank, which further removes solids and remaining organic compounds.

In case of black waste water, the sewage is pumped into a holding tank. Mixing of the sewage takes place in this tank as a result of continuous supply of liquid and solid waste. From here the waste water is sent into the osmosis treatment tank. Separation of liquid and solid waste, removal of virus and bacteria takes place at this stage.

The sludge in both the cases is pumped into a sludge tank from where it is further processed. It may be dewatered and be got ready to be sent to the shore or pumped ashore in case the ship is in the port. Through out the process the levels of BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), TDS (Total Suspended Solids and Ph levels are continuously monitored according to national and international statutory regulations.

The sludge from the screens and osmosis tank is pumped to a sludge tank. Depending on the treatment plant the sludge can now be dewatered and bagged ready to be sent ashore or, the sludge itself can be pumped ashore for disposal.

We will be studying in detail about Sewage Treatment Plant and other forms of purification equipment used for water and waste management later on.

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