Sewage treatment

[Graham_Wright]

Wife and I have just returned from a fantastic liveaboard diving holiday in the Red Sea.

In facy, it ain't red, its crystal clear.

That is baffling as the boat owner told me that the black (and grey) water on board went over the side and was not stored. He claimed there was no macerator or biological action but that the treatment involved sodium hypochlorate.

Although we did have difficulty with translation (he is Arabic), he was adamant that his explanation was correct. He told me that this is an approved method for the charter boat operators. The discharge was crystal clear.

Anybody able to fill in the gaps in my understanding?


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[SteveA]

As an ex Merch Engineer I have had the dubious honour of looking after ships sewage treatment plant. The very last part of this was to treat the overboard water with Sodium Hypochlorite. This simply kills off any remaining bugs. It's crazy to think that treating raw sewage with it would then make it crystal clear!

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[simonfraser]

just shows you what a load of ** all this sewage treatment for boaties is, surely the fish etc. produce fare more !

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[FrayBentos]

Sodium Hypochlorite is bleach - not suprising the discharge is clear

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[HeadMistress]

How big was the boat? Do you have any idea what the name of the device might be?

There is a Type I MSD made by Raritan called the Lectra/San that creates hypochlorous acid (another type of chlorine) by charging the ions in salt water with electrical current. However, it's only designed to treat black water, not gray, and is only legal on boats under 66' LOA. While the discharge is not crystal clear (in fact, I don't know how ANY device practical for onboard use on any vessel under 50-60' can accomplish that, because it requires a considerable amount filtration), it's very pale and highly diluted by flush water...resembling skim milk that's been cut 2:1 with water....but since the discharge is also only about a half-gallon at a time, it's further diluted so quickly in the water under the boat, that unless the discharge thru-hull is very close to the waterline, it would be unnoticeable except to anyone who was diving under the boat next the thru-hull.

While I doubt that this was the device on your dive boat, it's entirely possible that very similar technology was used.

<hr width=100% size=1>Peggie Hall
Specializing in marine sanitation since 1987

 

[Graham_Wright]

Now that rings bells! There was a continuous overboard considerable discharge from an above water outlet around 150mm in diameter. Another visitor (an ex ships engineer now inspector) though it was to do with the water maker. The outlet certainly had a slightly milky sheen to the discharge.

I can't believe that at least a macerator was used although, in common with many other countires, toilet paper was deposited in a bin and not flushed down the toilet.

The boat owner was aboard (an ex chief engineer) and impressed me with his dedication to the environment. After all, his livelihood depends entirely on the welfare of the sea. He was extremenly solicitous about the well-being of his "guests" and I believe his claim that this form of sewage treatment was approved by the authorities. Anyone who has been to a Red Sea dive centre will be aware of the very high density of dive boats. The harbour with about 25 movements an hour and continuous generator use was probably the cleanest I have ever seen. With a depth of around 15m, it was possible to see the bottom clearly and there was no trace of diesel on the surface.

I must confess, I did not expect such a level of professionalism (apologies to the Egyptians) but they all seem to be recognise that if they don't look after it, it will go away.

I have swum in the aforementioned outfall and could detect no taste or odours whatsoever.

The language barrier coupled to my pathetic sense of hearing may have contributed to not understanding the principle.

The boat was 35m overall but if the principle could be scaled down to our boat sizes it could be worth investigating.



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[Trevor_swfyc]

Also one animals waste is another animals food, when discharging in the med at the regulation distance from shore we were surprised at how many fish were feeding in our wake. I wonder if it had sodium hypochloride mixed with it, if it would have been quite so attractive.

Trevor



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[HeadMistress]

Graham, the continuous outfall above the waterline was most likely from the air conditioning water pump. Unless all the black and gray water went into a tank, it's unlikely that there'd be any black or gray water discharge except while water was actually running down a drain or a toilet was flushed.

Trevor. the beauty of the system that creates hypochlorous acid by charging the ions in salt water with electrical current is that the "chlorine" only exists while the current is on...when the stimulous is removed, the solution reverts to salt water again...putting no free chlorines into the water. It's the MOST environmentally friendly means of treating waste and killing bacteria...so it's not surprising that there'd be plenty of fish around.

I'd love to know what the system on that boat was!

<hr width=100% size=1>Peggie Hall
Specializing in marine sanitation since 1987

 

[Graham_Wright]

Yes;- you're probably right about the air con; I hadn't thought of that. The system was centralised with air outlets distributed round the boat.

I have a very good relationship with the dive agency and will pass on the queries to them. The response may take a while as the boat is nearly always at sea save for a few hours on a Sunday.

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[HeadMistress]

I will really appreciate your efforts to find out the name and mfr of the system, and any other information about it that might be available anywhere in English, Graham....'cuz I'm VERY curious to know more about it.

<hr width=100% size=1>Peggie Hall
Specializing in marine sanitation since 1987