Sewage testing is necessary to confirm that the biological sewage treatment plant is working as per the requirements of IMO.

Sludge Testing

It is important to keep a sludge content within certain limits i.e. a good biological balance for the treatment process in the sewage treatment unit.

Sludge content test, aeration chambers I and II

• Measure the sludge levels in aeration tanks I and II using test cocks while blowers are running.
• Fill the glass funnel with 1000ml and throw the first “sample” water away immediately. This action will “clean” the sample valve and increases test result significantly.
• Fill the glass funnel (imhoff glass) with 1000ml of waste water from the test cock of aeration chamber I or aeration chamber II while the blower is running.
• Let the sludge settle for 30 minutes in this glass.
• Observe the color of the sludge and odor. If the biological process is running well, sample color is grey or brown and sample will not smell significantly. If the sample color is black and/or it smells, it is a indication that biological
  process does not work properly.
• Read the sludge quantity from the side of the glass.

Interpretation of Result

• Sludge content measurement below 300 ml/l = sludge content too low for biological process.
• Between 300 ml/l and 600 ml/l = normal
• Above 600 ml/l = sludge content too high, discharge sludge.

Disinfection and Residual Chlorine Testing

When sailing in areas where chlorine disinfection of treated waste water is required, the chemical dosing system should be prepared as follows:
• Check that the chlorine container is filled. Fill it if needed.
• Check that dosing pump is on and adjusted correctly.
Chlorine concentration must be monitored regularly. Residual chlorine should be 2-5 ppm in effluent. If chlorine test shows that residual chlorine is higher than 5 ppm or lower than 2 ppm, adjust chlorine pump’s stroking rate.

Disinfection chemical

Recommended disinfection chemical is sodium hypochlorite (NaClO), technical quality, concentration 10% active chlorine.

The storage of chlorine solution should be in a dark room, temperature in between 10-15 °C otherwise it will loose free chlorine. The maximum storage time is 180 days at +10°C. The maximum storage time depends on storing temperature i.e. the maximum storage time is only 50 days at +25°C if redundancy is 50%. See a chart below.

Residual chlorine test

Take a discharge water sample from a test cock after discharge pump while a discharge pump is running. Measure the free chlorine content from the effluent with a test kit.

Test procedure:

1. Fill the sample glass and throw the first “sample” water away immediately. This action will increases test accuracy
   significantly.
2. Fill the sample glass from the test cock after discharge pump while the discharge pump is running.
3. Immerse test paper in effluent.
4. Read the residual chlorine by comparing test papers color to the reference color map. For detailed information see
   the test kit’s instructions.

Reference: EVAC Environmental Solutions Marine Sector Operation Manual

Related Links:

Biological Vacuum Sewage Treatment Plant on Ships

Maintenance of Biological Vacuum Sewage Treatment Plant on Ships

Troubleshooting of Biological Vacuum Sewage Treatment Plant on Ships

Working and Maintenance of Air Ejector, Pressure Switch and Level Switch

Sewage Testing in Biological Vacuum Sewage Treatment Plant

Precautions in Operating Biological Vacuum Sewage Treatment Plant