Marine sewage treatment plants are type-approved, but may fail to meet the required performance levels when on ships. Wärtsilä’s Wei Chen explains why compliance monitoring is required
Almost all marine sewage treatment plants (STPs) have been type tested on land, but the majority do not meet the performance standards on ships. Many of them do not even conform to the STP testing guidelines. Unacknowledged and uncorrected, these certified magic boxes and non-conformities have driven a wedge between the IMO’s environmental goal and the type-approval regimes. To an industry accustomed only to type tests, to “confirm the lifetime performance of STPs” (PPR 7/16) this may cause anxiety. Yet, few maritime professionals have wondered how on earth those science-defying magic boxes were ‘successfully’ tested, approved and certified in the first place.
Most type tests are carried out at municipal wastewater treatment plants (WWTP). Some knowledge of WWTP, such as that illustrated by a recent BBC documentary ‘The Secret Science of Sewage’, is useful.
An STP needs to be tested for 10 days, using fresh raw sewage with a minimum total suspended solids (TSS) concentration of 500 mg/l, by adding primary sewage sludge as necessary. Domestic sewage, or urban wastewater, contains grey water from households, groundwater infiltration, and rainwater. Its average TSS is a mere 210 mg/l. To meet the 500 mg/l criterium, primary sludge must be added. If primary sludge is not added, the lab results would bear no relation to the test, and the test is clearly rigged. The tricks involved in such a rigged test could be revealed by an investigation. But there may never be one. Who is to check the checkers?
Nevertheless, 500 mg/l is only a fraction of a ship’s vacuum sewage. One missing ingredient is grey water. Ship’s grey water should be regulated like that on land. It can bring ship’s sewage closer to domestic sewage ashore. Introducing conditioning water can also help, provided STPs are not turned into dilution machines. In any case, to test an STP with one sewage, and use it for another sewage of much higher concentrations does not serve the maritime industry nor IMO’s environmental goal.
Define primary sludge
Primary sewage sludge is a black, smelly slurry with 30,000~50,000 mg TSS/l. The particles that have been settled from the raw sewage are the right material to create the challenging test conditions. It should be noted that the primary settlement tanks of some WWTPs receive surplus activated sludge and produce a ‘co-settled sludge’ that is less challenging than primary sludge and, therefore, unfit for type tests. Defining primary sludge is essential, but it is not enough.
A trick harder to tell is when activated sludge is used in disguise as primary sludge. WWTPs serving our cities, towns and villages use biological treatment, or activated sludge process. Activated sludge is a brownish-coloured pollutant-destroying muddy water, with an earthy smell resembling that of garden compost. It contains naturally occurring aerobic bacteria and larger microorganisms such as protozoa. Treated effluent is readily separated out from this activated sludge by settlement, flotation, or membrane filtration. Hence, adding activated sludge to boost up TSS and organic content of STP influent can easily cheat a type test.
Likewise, when a physical-chemical STP produces a brownish-coloured sludge, the test may be rigged.
“Have ISO17025-approved testing laboratories helped? Evidently not”
Tougher type test can be shooting in the dark
Many other tricks exist. Residual chlorine may not be tested in-situ; coliform samples may not have the residual chlorine neutralised; flow meters may not be installed; STP sludge may not be prevented from entering the STP influent. These tricks have led to magic boxes that are smaller, simpler, cheaper, ‘care-free’, … and popular.
Introducing tougher tests seems logical. However, with these certified magic boxes and non-conformities unacknowledged, do we know how to make a type test tougher? For example, when a 10-day test failed to catch a magic no-sludge batch process that completes in an hour, extending the test to 30 days can be a pointless distraction, such as the need to define primary sludge, to investigate the root causes of the certified magic boxes and non-conformities, to ensure genuine lab results by rooting out fraudulence.
Without evidence, rationale, and justification, introducing tougher type tests can be like shooting in the dark.
ISO approved and independent facilities?
Have ISO17025-approved testing laboratories helped? Evidently not. For years, the EU’s Marine Equipment Directive (MED) has stipulated ‘testing laboratories used for conformity assessment purposes meet the requirements of standard EN ISO/IEC 17025:2005’. But this has not stopped magic boxes and non-conformities from being embellished with the prestigious wheel marks.
For decades, the USCG has required ‘independent’ and ‘accepted’ facilities. But unfortunately, similar magic boxes and non-conformities have been wrongly approved and the performance status of Type II Marine Sanitation Devices (MSDs) is as poor as that of STPs.
The way forward
Independent and approved facilities have merit, not least to ensure the basic conformities and seaworthiness of STPs. Tougher type tests can also have merit, when supported by rationales and evidences. But the reliance on type tests to serve IMO’s environmental aspirations may only be supported when the certified science-defying magic boxes and non-conformities are one day acknowledged for what they are, and correctly accordingly. The only question is, will this day ever come?
There is only one way to protect the environment: compliance monitoring. It is tried and tested by large Alaska-going cruise ships for 20 years, and by the rest of our society for much longer. As alien as it may be, compliance monitoring may finally come to MARPOL Annex IV, and mark the start of a goal-based regulation which would one day stop the magics of type tests from pulling the wool over our eyes.
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