What is sewage treatment plant and its purpose?

Sewage is defined as drainage and other wastes from any form of toilets or urinals, drainage from medical premises via wash basins, wash tubs and scuppers located in these premises, drainage from spaces containing live animals and other waste waters when mixed with drainages defined aforementioned.

The sewage treatment plant is a biological unit, which works on the aerobic activated sludge principle and constructed to meet the rules and regulations in MARPOL 73/78 Annex IV and should be approved according to IMO MEPC 2(VI) which specify the following effluent values:

  • Suspended solids <1000mg/liter
  • Coliform bacteria <200 off/100 ml
  • BOD5 < 50 mg/liter

The plant will treat black and grey water and is, in general, fully automatic in operation.

Example of a sewage treatment plant

The sewage treatment plant consists, usually of a tank with three or four main compartments: aeration/activation compartment, activation compartment, settling compartment and sterilization and discharge compartment.

Sewage plant schematic

The sewage treatment plant is located in the machinery space and sewage from the accommodations runs through pipelines to the sewage treatment plant, aided by flushing water and gravity or by vacuum generated by vacuum pumps. On the latter the sewage treatment plant is under atmospheric pressure and the vacuum is only generated in the collection pipes between the toilets and vacuum pumps. The vacuum pumps are started and stopped according to the vacuum in the collection pipes and operate on a lag/lead principle as long as they are both switched on automatic mode. Normally only one pump is in operation but, if it is unable to keep the vacuum, the stand-by one will start. There are vacuum systems with only one pump and no stand-by pump available.

Example of vacuum pump arrangement
Sewage system vacuum pump with macerator

The vacuum pipe is connected via a network of pipes to the toilet bowls throughout the ship and when a toilet is flushed the flushing valve at the back of the toilet will open to discharge the contents and the surrounding air will be drawn into the bowl. A synchronized valve will allow fresh water into the bowl for flushing and for sealing the main discharging valve. Although the system is expensive and requires a lot of maintenance and expensive parts, it provides to be effective and uses about 1 liter of water per flush. The vacuum pump macerates the waste from the sewage pipe and discharge it to the first stage of the sewage treatment plant.

After treatment in the sewage plant the effluent is pumped overboard, if permitted, by means of the discharge pumps. If the discharge is not permitted due vessel being in a special protected area or to close to the shore line, the treated effluent is stored into the sewage holding tank which can be pumped overboard at a later stage while the vessel is en route and at a distance more than 12 nm from the nearest shoreline if the effluent is not treated and disinfected and minimum 3 nm if the effluent is treated and disinfected.

Example of a sewage holding tank

All the black water from the toilets, urinals and infirmary black water are collected and discharged to the first compartment aeration (activation) chamber of the sewage treatment plant. The incoming effluent material mixes with the activated sludge already present in this compartment. Air is supplied by means of a blower and distributed though the tank by aerators. The gases produced during the bacterial action which takes place are vented to atmosphere. Oxygen is essential for the aerobic activity, the organisms require oxygen for digesting the raw sewage and it also assists in mixing the incoming sewage with the water, sewage sludge and bacteria already present in the compartment.

The grey water from washbasins and showers flow separately to the first chamber and can also be directed to the sewage holding tank or directly overboard. The grey water from the infirmary flows to the aeration chamber through a separate line and cannot be directed to the sewage holding tank or overboard.

After a period of time in the aeration chamber the bacterial action will have reduced the biodegradable organic matter to water, gas and sludge. The treated effluent is then discharged to the settling chamber where the solid sludge settles out. The sludge contain biomass (bacteria) which increases due to the bacterial action in the aeration/ activation chamber. The biomass is returned to the aeration chamber by an air lift where reacts with the incoming raw sewage. The clean effluent flows from the settling chamber into the clean water/disinfection chamber via a weir.

In the disinfection chamber the pathogens (coliform bacteria) are destroyed, generally, thru chlorination. A dosing pump, located at the front of the unit, transfers the required dose of chemical (sodium hypochlorite) to the compartment in order to sterilize the effluent prior to discharge. The compartment has float operated switches which activate the discharge pump when the high level is reached and stop the pump when the compartment is nearly empty. The dosing pump is activated when the discharge pump stops and so the necessary quantity of sterilizing chemical is added to the compartment before the water flows into it. There should be no need to adjust the dosing rate of the pump under normal circumstances. If adjustment is needed the manufacturer’s instruction manual must be consulted with respect to adjustment of the pump stroke. The amount of sterilizing chemical added to the final discharge tank must be carefully controlled in order to ensure correct sterilization, as too much chemical results in pollution of the sea and destruction of marine organisms. Some local state authorities do not allow the discharge of sodium hypochlorite.

Sewage treatment plant explained. Source and credit: Marine Insight

As mentioned above, the sewage treatment plant works automatically once it is set but periodic attention is required and the unit must be monitored for correct operation. As sludge will build up in aeration/activation and settling chamber, this must be diluted in water (fresh or sea water) and removed approximately every two months, by discharge overboard or to approved shore facilities. It is very important that you never discharge both chambers at the same time, as this will result in the loss of the biological organisms from the plant.

Sewage tank during inspection and maintenance
View inside disinfection chamber

Rules governing the discharge of raw sewage must be complied with all times and the discharge of raw sewage overboard should be done when the sewage plant is unserviceable. The bacterial action in the sewage treatment plant requires a regular supply of raw sewage and the discharge of raw sewage overboard can impair effective bacterial action.
A sewage treatment plant should satisfy the following effluent standards when tested for its certificate of type test: Fecal Coliform Standard (the geometric mean of the fecal coliform count of the samples of effluent taken during the test period should not exceed 250 fecal coliform/100ml as determined by a multiple tube fermentation analysis or an equivalent analytical procedure); Suspended Solids Standard (where the equipment is tested on shore the samples of effluent taken should not exceed 50 mg/l; where the equipment is tested aboard ship the sample effluent should not exceed 100mg/l).

As part of the maintenance, the responsible engineer must check as following:

  • On a daily basis that the sludge is being returned to the activation chamber from the settling chamber, and that the discharge pump and blower are working and that there is sufficient liquid into the dosing tank.
  • Weekly change over the duty discharge pumps as this will avoid blockage in the discharge lines.
  • Every two months check that the air flows are correct and that the compartment vents are clear; Check the sludge content by filling a 1 litre glass container, or similar, with water from aeration chamber and allow the sediment to settle for 30 minutes. A sludge content of 300ml to 800ml is satisfactory. A sludge content in excess of 900ml requires the sludge to be discharged to the sea or to a shore facility. Check the chlorine content of the effluent being discharged using the test kit provided. The chlorine content should not exceed 2ppm.

In conclusion, every marine engineer should be aware of the following before and when operates the sewage system:

  • All crewmembers involved in operating sewage treatment plant must be familiarize with the operation and maintenance of the plant along with the requirements regarding discharge.
  • The documentation related to the plant must be kept in a relevant spot, easy to locate. and this documentations should include: Name of manufacturer, type and model, date of manufacture, any operational and installation limits, manual detailing the operational and maintenance procedures for the plant, test and certificates (type approval + International Pollution Prevention).
  • Discharge overboard should not take place within 12 nautical miles of the coast, so it need to ensure that maintenance takes place when at least 12 nm from nearest land.
  • The discharge overboard of treated sewage effluent must take place with the permission of the bridge. There must be effective communication between the bridge and engine room so that the engine room is immediately aware when the vessel enters waters where such discharge is prohibited.
  • Rules governing the discharge of raw sewage must be complied with all the times and the discharge of raw sewage overboard must be considered only when the sewage plant is unserviceable.
  • Only approved chemical cleaners must be used in lavatory and toilet bowls as biocidal cleaners will destroy the biomass in the sewage treatment plant.
  • Never discharge both tank chambers at the same time, as this will result in the loss of the biological organisms from the plant.
  • The dosing chemical contains chlorine and great care must be taken when handling the chemical. The appropriate protective clothing must be worn, including gloves and safety goggles. The manufacturer’s safety and health instructions should be read before the chemical is handled.

For vessels with only holding tank (without treatment of any kind) the engineers must ensure that a table approved by the Administration giving the maximum rate of discharge for various speed / draft combinations is available onboard ref MEPC 157 (55). This is as per the following:

DR(max) = 0.00926 V d B, where
DR(Max) = Maximum permissible rate of discharge (m3/hr); V = Ships average speed over period (Knots); d = Draft of vessel (m); B= Breadth of vessel (m)

Ensure that the maximum rate of discharge for untreated sewage for the corresponding ships speed is not exceeded. This is irrespective if a pump is used for disposal or done by gravity. Where a pump is to be used, the designated engineer officer is to ensure that the rate of discharge (rated pump capacity) is at or below the corresponding vessels speed as calculated with the above formula. When the gravity is used, the engineer in charge must adjust the discharge (by throttling the discharge valve) so not to exceed the rate corresponding to the vessels speed at that time.

When untreated or comminuted-disinfected sewage is discharged, relevant records are maintained in the engine log book. As a minimum these shall include:

  • Date / Time / Vessel Position at the Start of Discharge
  • Date / Time / Vessel Position at the End of Discharge
  • Vessels speed during discharge
  • Calculated rate of discharge (ONLY for UNTREATED SEWAGE)

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