BIOLOGICAL & CHEMICAL SEWAGE TREATMENT PLANT - PRINCIPLE, OPERATION, BOD, MARPOL, MAINTENANCE

Principle and operation (with sketch):

• A biological/ chemical sewage treatment plant treats the vessel's sewage (black water) before discharge. It uses a combination of biological (aerobic bacterial) treatment and chemical treatment/ disinfection.
• The plant typically consists of: a primary (aeration) tank, a settling/ clarifier tank, and a disinfection/ chemical treatment stage.
• Operation: Sewage enters the aeration tank, where air is supplied (by a blower/ air diffusers) to promote the growth of aerobic bacteria. The bacteria break down the organic matter (BOD) in the sewage. The mixed liquor then passes to the settling tank, where the sludge settles and the clear liquid (effluent) overflows. The effluent is disinfected (by chemical treatment, e.g. chlorine, or by UV) before discharge. The settled sludge is returned to the aeration tank or removed.
• The plant operates continuously, with the aeration and settling maintaining the biological treatment.

• BOD (Biochemical Oxygen Demand) is the amount of oxygen required by micro-organisms to break down the organic matter in the sewage/ effluent over a specified time (usually 5 days, BOD5) at a specified temperature. It is a measure of the organic pollution load of the sewage/ effluent.
• Significance: BOD indicates the strength/ pollution load of the sewage and the effectiveness of the treatment. A low BOD in the effluent indicates good treatment (the organic matter has been broken down); a high BOD indicates poor treatment. BOD is used to assess compliance with the discharge standards.

• MARPOL Annex IV regulates the discharge of sewage. The discharge criteria (for treated sewage) include:
• The effluent must meet the standards for faecal coliform (e.g. not more than 250 faecal coliforms per 100 ml, or as per the standard) and for BOD (e.g. not more than 25 mg/l BOD5, or as per the standard) and suspended solids (e.g. not more than 35 mg/l, or as per the standard).
• Discharge is permitted at a distance from land (e.g. more than 3 nautical miles from the nearest land for treated sewage, or more than 12 nautical miles for untreated sewage, with the sewage comminuted/ disinfected), and in accordance with the flag/ port requirements.
• The sewage treatment plant must be approved/ certified (IMO type approval) and the discharge must comply with the standards.

• Regular inspection and cleaning of the tanks, screens, and the aeration/ settling compartments.
• Checking and maintaining the air blower/ diffusers (clean/ renew the diffusers).
• Checking and maintaining the pumps (sewage/ transfer pumps) and the valves.
• Checking the chemical treatment/ disinfection system (chemical level, dosing, and the disinfection effectiveness).
• Checking the sludge removal/ return system.
• Monitoring the effluent quality (BOD, coliform, suspended solids) and the plant's operation.
• Cleaning the plant and removing sludge/ deposits as required.
• Checking the electrical/ control system and the alarms.