MPN testing excluded in USCG draft policy letter

The US Coast Guard (USCG) has released a draft policy letter on evaluating methods that allow the enumeration of organisms using nonviable-organism testing protocols for ballast water management systems (BWMS), as required by the Vessel Incidental Discharge Act (VIDA).
Nonviable-organism testing protocols are particularly important for UV-based BWMS that rely mainly on rendering organisms unable to reproduce, effectively limiting their invasive risk and achieving the requirements of the Ballast Water Management Convention and other local regulations. A well established method, the most probable number (MPN) method, has long been used to make that assessment, and is accepted by IMO as one of the methods valid for evaluating the efficacy of BWMS during type-approval testing, and later during installation controls.
To date, the USCG does not accept MPN as a method to evaluate the efficacy of BWMS during type-approval testing; instead, it requires testing facilities to use staining methods, which require organisms to be effectively dead and not unable to reproduce, in order to check if the water treated by the BWMS manages to meet the D-2 standard or the US Ballast Water Discharge Standard.

Industry expectations
VIDA required the USCG to evaluate BWMS using nonviable organisms testing protocols. And when VIDA passed, the industry expected the USCG to put forward the MPN method within its accepted methods and that by the end of 2019, testing facilities could start using this method for USCG type-approval.

Why it is reasonable to expect the USCG to acknowledge MPN?
For several years, an expert group from around the globe had been working under the umbrella of the US EPA (including the USCG), to evaluate MPN as an acceptable method under the currently used ETV protocol. The years of work and comparative testing in the foremost testing facilities in the world led to a comprehensive dossier submitted to the EPA and the USCG, in support of using MPN to evaluate the efficacy of BWMS.
In addition to the above, IMO (of which the US is a member and the USCG leads key committees at IMO), has discussed and approved MPN based partly and substantially on the experiences of the ’best of the best’ in the world when it comes to testing BWMS.
It is therefore very surprising to read the following in the draft policy letter proposed by the USCG:
"At the time of this publication, the Coast Guard does not know of any type-approval testing protocols for BWMS that render nonviable organisms in ballast water that are based on best available science. Therefore, this policy letter establishes the process developed by the Coast Guard for accepting type-approval testing protocols".

What this draft policy letter effectively says
By not accepting the MPN method which is already approved by IMO, the USCG in my view is clearly breaching the VIDA text asking for "best available science". This is what this draft policy letter effectively says.
Clearly, if the testing facilities that the USCG has delegated testing to, announce to the USCG and IMO that they recommend MPN, then this must be "best available science".
Peer reviewed articles, in the 100s, on the MPN method date back decades and prominent professors in marine biology (American and others) have published multiple articles on the subject in recent years. If all this is not good enough, what is good enough?

Practicalities of the ’policy letter’ approach
In practice, by the time the policy letter is published (December 2019?), it will take years before any nonviable-organism testing protocol is approved by the USCG. By that time, the vast majority of BWMS will be type-approved, at a cost of several million dollars, and the intentions behind the text in VIDA not met.

What should have been done instead?
Since the USCG and the EPA have already had a rigorous report presented to them by the vast majority of the current BWMS testing facilities in favour of MPN, this method should have been annexed to the draft policy letter as one acceptable method, and comments received on that. Alternatively, the USCG could have just referred to the circular of the IMO where accepted methods for type-approval testing of BWMS are listed.
We expect the USCG to publish a similar circular to that of the IMO, where all accepted methods are listed in detail for public access.