Introduction to Biological Vacuum Sewage Treatment Plant
The biological vacuum sewage treatment plant consists of an integrated vacuum generator and a biological sewage treatment plant. In the sewage treatment plant an aerobic process of the biopopulation convert organic substances existing in waste water to carbon dioxide and water without danger of methane gas production.
All waste water, both black water (toilets, urinals, hospital) and grey water (galley, showers and sinks) can be treated in the biological vacuum sewage treatment plant. The treatment plant can operate in all vessels which are sailing in salt, brackish or fresh water areas.
The biological vacuum sewage treatment plant is fully automatic in normal operation and they require low maintenance.These plants are designed to fulfill the requirements of IMO MARPOL 73/78 Annex IV and USCG 33 CFR Part 159 for inspected vessels which specify discharged waste water from sewage treatment plants.
The effluent quality is as follow:
• BOD5 < 50 ppm
• Suspended solids < 50 ppm (tested on shore) or 100 ppm (tested aboard ship)
• Faecal coliforms < 250 pcs/100ml M. P. N.
Figure below shows biological vacuum sewage treatment plant normally used on ships.
Black sewage water is collected by a vacuum ejector system as shown above. Incoming black water must be diluted with technical, grey or sea water. There is a separate dilution/gravity inlet connection built in chamber I. All waste water is led to the aeration chamber I by vacuum from vacuum toilets and by gravity from wash basins. Bacterial growth is stimulated by oxygen of air supplied by the blowers. Required air is produced by the air blowers and led to aeration chambers I and II via air distributors. The aerobic treatment process starts in the aeration chamber I and continues in the aeration chamber II. If treated waste water is coming from a vacuum toilet system, required dilution water must be added into the aeration chamber I.
An activated sludge is separated in the settling chamber by gravity and clarified water floats to the disinfection chamber. The activated sludge is pumped back to the aeration chamber I by an air driven ejector (also called as air lift). A chlorine based disinfection chemical is added by the dosing pump into the clarified water in the disinfection chamber to meet IMO’s requirements regarding presence of coliform bacterias in treated water. Treated water is pumped to sea or ashore by a discharge pump. Disinfection can be also done by UV on some models. A mineralized sludge from activation chambers I and II is pumped at given intervals sea, to on shore facilities or to the vessel’s sludge storing tank. The discharge pump is equipped with an integrated cutting device to prevent blocking.
The vacuum generation unit creates vacuum in sewage piping by means of an ejector. The vacuum generation unit consist of ejector, ejector pumps, pressure switch, vacuum gauge, shut-off valves and antifoaming dosing unit. Vacuum generation is controlled by the vacuum switch.
Aeration and blowers
Waste water is led to the aeration chamber I by gravity and vacuum. Bacterial growth is stimulated by oxygen of air. Required air is produced by the air blowers and led to the aeration chamber I and II via installed aerators. An aerobic process continues in the aeration chamber II. Inorganic solids (for instance plastic) are stopped in the aeration chambers I and II. Air flow can be adjusted between chambers I and II using air valves.
Settling of Sludge
Activated sludge is separated in the settling chamber by gravity and clarified water flows to the disinfection chamber. Activated sludge is pumped back to the aeration chamber I by an air driven ejector pump (also called as air lift). Air flow for the sludge ejector is adjusted so that the sludge return from the chamber III is about 1/3 of the pipe section (A transparent hose above tank). Rest of the air flow produced by the blower is used for aeration and divided equally between tanks I and II. Sludge has to be removed frequently from the process to maintain a good biological balance in the sewage treatment unit. Sludge content in process (in chamber I) must be kept between 100ml/l and 500 ml/l. Sludge is removed by the discharge pump.
Disinfection chemical (e.g. Sodium hypochlorite (NaClO), solution, active chlorine 10%) is added to the clarified water in the disinfection chamber to meet IMO’s requirements regarding presence of coliform bacterias in treated water. Residual chlorine must be kept between 2 ppm and 5 ppm. Residual chlorine can be adjusted by the dosing pump’s settings and/or timer settings. Treated water can be pumped to sea or ashore by the discharge pump.
Materials of Construction
- Tank material: Steel S355 EN10025
- Ejector body: Acid proof steel EN 1.4404
- Welded flanges, pipe bend: Acid proof steel EN 1.4404
- Loose flange: Al
- Nozzle: PVC
- Flap: Nature Rubber NR
- Gaskets: Rubber EPDM
The biological vacuum sewage treatment plant may require additional components for optimal operation.
Because the combined treatment plant is processing black water from vacuum toilet system, dilution water must be added into incoming sewage. There are several system options to control and monitor dilution water:
• Flow meter and pressurized fresh or technical water
• Grey water buffer/holding tank and constant flow pump
• Grey water buffer/holding tank and dilution water feeding by gravity with flow meter
Screen or Catcher
If there is a risk that a lot of non organic solids obstacles are entering in the treatment plant, a screen or catcher must
be installed before the treatment plant.
Grease is harmful for treatment process. If there is galley water entering to the treatment unit a grease separator must
be installed before treatment plant.
Reference: EVAC Environmental Solutions Marine Sector Operation Manual