Introduction to Ships Wastewater Treatment Processes
Wastewater produced on board ships is required to be treated before being discharged overboard into the sea.
The wastewater comes in the form of grey and black water and their treatment can be carried out separately or by using the same equipment to process both grey and black wastewater.
There are several modern types of equipment for treating the wastewater, and these are supplied as a complete unit, ready for installation in the engine room.
When I was a young engineer at sea, all wastewater was collected in a tank (which I was told contained beasties) and pumped overboard if we were outside the stipulated 12 mile distance from any coastline.
Nowadays however, international law quite rightly prohibits this practice, requiring wastewater to be treated to acceptable levels before being pumped over the side, to prevent pollution of the seas.
This is an article on the processing of ships wastewater to comply with International Maritime Organization (IMO) legislation. We shall examine the sources of grey and black wastewater and its treatment; we begin then with a look at the sources.
Categories of Ships Wastewater
The two basic categories of wastewater are grey and black in which there are several sources.
Grey Wastewater Sources
- Sinks – from galley sinks and sinks in crews cabins and toilets
- Showers – from showers in crews accommodation
- Laundry – from washing clothes
Black Wastewater Source
- Sewage – from crews accommodation
- Medical – from ships hospital
Treatment of Grey Waste Water
Treatment of Grey Waste Water
There are a number of wastewater systems, either to process just grey water or total wastewater management where both grey and black wastewaters are processed, with many maritime governing bodies requiring this.
Modern wastewater processing plants are supplied as a compact unit containing all the tanks, pumps and piping required.
One of the most efficient units uses a Membrane Bioreactor (MBR), a reverse osmosis technique that is described below.
The grey water from showers, sinks, galley and laundry is piped into a wastewater tank, where it is mixed by rotating paddles.
It is pumped from here through a set of mesh screens which remove any rubbish, piping this to the sludge tank. The effluent is piped into an aerated bio-tank where air supplied by blowers is injected to the liquid to aid the biological processing of removing organics from the liquid.
From here the liquid is pumped into the osmosis tank where reverse osmosis removes solids and any remaining organic compounds or viruses from the liquid. This clean water can then be pumped overboard without any threat to the environment.
The discharge from the osmosis process is constantly monitored to ensure that levels of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Suspended Solids (TDS), and Ph. levels remain within current legislation.
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.
Black Wastewater Treatment
The sewage can also be treated using reverse osmosis in a Membrane Bioreactor and the process follows.
The sewage is piped from the accommodation into a holding tank where it is mixed due to the constant influx of liquid and solid sewage. A submerged pump discharges the liquor into the osmosis treatment tank. Here the liquids and solids are separated, coliforms and other viruses being removed by reverse osmosis.
Once the osmosis treatment is complete, the resultant clean water can be safely pumped overboard without any threat to the environment.
Throughout the process, instruments monitor the levels of BOD, COD, TSS and fecal coliforms.
The sludge is pumped to a sludge tank for further on- board processing or pumped ashore when in port.