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Old 29-05-2019, 14:59   #1
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Combine black and grey water?

Hi all,

Can anyone think of problems that might occur if I use the same storage tank for black and grey water?

Aside from making absolutely sure I cannot pollute the grey water system with back-flow from the tank, what else can go wrong?

For the record, the boat will be sailed mostly in Australian waters and bits of the South Pacific, so regulations from other countries, though interesting, may not apply.

Matt
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Old 29-05-2019, 15:04   #2
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Re: Combine black and grey water?

You'll fill the holding tank a whole lot faster.
Soap scum may be deleterious to the tank biology. (Insert Peggy Hall's expert advice here)
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Old 29-05-2019, 15:24   #3
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Re: Combine black and grey water?

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You'll fill the holding tank a whole lot faster.
Soap scum may be deleterious to the tank biology. (Insert Peggy Hall's expert advice here)
Tank biology is a good point, now that you mention it.

(Tank capacity, I HOPE, will be more than adequate though.)
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Old 29-05-2019, 18:27   #4
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Re: Combine black and grey water?

Why would you want to do this?
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Old 29-05-2019, 18:37   #5
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Re: Combine black and grey water?

Thanks, Gord...I got this now...



Not sure what the legalities regarding combining black and gray water in the same tank are in OZ, but it's illegal in the US because of the danger of e-coli and other waste borne bacteria finding their way into sinks contaminating whatever may be IN the sink. In fact, here, waste and black water tanks can't even share any plumbing.

So before you route your sink drains into your black water tank, it would be a good idea to find out whether your laws allow it.

Whether they do or not...maintaining a gray water tank is a LOT harder than maintaining a black water tank because the only thing that goes into a black water tank is human body waste and a little quick-dissolve TP, whereas all kinds of things go down sink drains--galley grease (even a little from plates is enough to matter), bits of food (it takes meat and veggie bits forever to dissolve, unlike body waste solids), body oils, toothpaste, soap scum...and ALL of it stinks when it sits in a tank..wafting odors out the vent and sink drains.

So unless AU law requires that you hold gray water, you'll be a LOT happier without a gray water tank.

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Old 29-05-2019, 18:49   #6
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Re: Combine black and grey water?

Addendum I forgot to mention: If you're concerned about the aesthetics of "polluting" a pristine anchorage with all the stuff that goes down sink drains, put strainers in the sinks and always add a little water and detergent dishwashing liquid to the water before draining the galley sink to prevent an "oil slick."



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Old 29-05-2019, 19:30   #7
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Re: Combine black and grey water?

Thanks Peg.

Yes, I was worried about sink discharge, and shower too. I figured it would be nicer to hold these until I was either well away off shore or, better still, at a pump out station.

I will double check the regulations for my planned cruising grounds, but from what you are saying this would not be a good idea.

Matt
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Old 29-05-2019, 19:41   #8
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Re: Combine black and grey water?

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Originally Posted by peghall View Post
Not sure what the legalities regarding combining black and gray water in the same tank are in OZ, but it's illegal in the US because of the danger of e-coli and other waste borne bacteria finding their way into sinks contaminating whatever may be IN the sink. In fact, here, waste and black water tanks can't even share any plumbing.
Peggie,

What law is this exactly?

I have never heard of such a "law" reflecting boat systems. And it certainly is not true of household systems, where gray and black water are of course all the same piping.

Irrespective of it being a good idea or not... just want to learn about a legal requirement I have never heard of.
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Old 29-05-2019, 20:37   #9
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Re: Combine black and grey water?

You'll find it in USCG regs, specifically USCG Marine Safety Manual MSD,
section (2)(c) Holding Tanks (Type III)

Verify the following:

(a) Capacity is adequate for the time the vessel will have to retain sewage and flushwater.

(b) Tank is used solely to store sewage and flushwater. In older ships, toilet drains may be combined with deck, sink and shower drains, to help flush the overboard drains. This installation was never intended to store sewage and may not be plumbed to prevent the back-venting of fumes from a holding tank.

(c) Tank does not receive gray waters or galley wastes. Adding these wastes can greatly increase the hazards of putrefied material accumulating in the tank. (emphasis mine)

Further down, paragraph (f) sayeth: Vents from the MSD do not cross-connect with other vents.

I can't post a link to this because my only copy of this USCG Safety Manual page is a document in my files. I'll be glad to attach a copy of the whole document to a reply if you want to send me an email.

It's not surprising that you'd never heard of this regulation...gray water tanks are very rare on US boats because, except on a VERY few closed inland lakes and the FL Keys Marine Sanctuary waters, gray water can go directly overboard in all US waters.

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Old 29-05-2019, 21:15   #10
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Re: Combine black and grey water?

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You'll find it in USCG regs, specifically USCG Marine Safety Manual MSD,
section (2)(c) Holding Tanks (Type III)


--Peggie
Thanks! Learn something new everyday... I have read those rules many times, and just skipped over that part as not important to me at the time!

The rational, however, is not contamination of the sink, which would need a trap for odor control even if draining into a dedicated graywater tank, but rather clogging of the holding tank. Makes me wonder if any of the USCG reg writers ever lived in a house with a septic tank?

I have run into a few marinas that insist on gray water holding tanks for liveaboards, but I suspect that is as much to restrict the liveaboards as much as it is to keep the waters clean.

On our boat we don't have a gray water tank, but we do have a sump where graywater collects before pumping overboard. It does need cleaning from time to time (more or less annually) as grease and soap scum accumulate and "putrify." It is a great way to eliminate thruhulls for every sink drain.
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Old 30-05-2019, 09:30   #11
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Re: Combine black and grey water?

We have a boat in Switzerland where the law prohibits discharging gray water into the lakes. Dealers and manufacturers routinely plumb sink drain hoses through a one-way pump to the black water holding tank. Presumably the pump acts as a check valve to protect agains back flow and there’s usually a siphon loop above the holding tank.
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Old 30-05-2019, 09:42   #12
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Re: Combine black and grey water?

what the OP said: you will need something to prevent gasses (or liquids) from flowing up the drains. but on land we always combine black and gray water so not sure why it would be a problem in a boat .. unless it is against the law. the idea is not to put garbage down the sink. eventually you would probably need to pump out the sludge in the holding tank.
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Old 30-05-2019, 10:55   #13
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Re: Combine black and grey water?

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Originally Posted by gonesail View Post
what the OP said: you will need something to prevent gasses (or liquids) from flowing up the drains. but on land we always combine black and gray water so not sure why it would be a problem in a boat .. unless it is against the law. the idea is not to put garbage down the sink. eventually you would probably need to pump out the sludge in the holding tank.
Boat drains do not have a P trap like house drains do. If you have ever been in House that has had the water turned off for an extended time long enough for the P taps to evaporate you can defiantly smell the sewer. Not sure how well a P trap would work on a heeling boat.
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Old 30-05-2019, 11:58   #14
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Re: Combine black and grey water?

Recommend you ascertain the regulations as to plumbing of waste waters in Australia before connecting the grey and black water systems to the same holding tank. In the USA that is not allowed, the in vessel reasons as to restricting such are not really compelling, but most likely so as to avoid the holding tank from more rapid filling and thus needing to be emptied more frequently, so more a method of lessening the possibility of the boat discharging blackish water especially in zero discharge zones Less water, less likelihood of need to discharge. Again in the USA it is you aren't allowed to, thus moot as to an issue of whether you should.

References for Australian regulations:

GBRMPA - Recreational vessel sewage requirements

GBRMPA - Recreational vessel sewage requirements




Grade C is the lowest level of treatment
Grade B is a higher level of treatment
Grade A is the highest form of sewage treatment and these systems allow the greatest flexibility for vessel operators to comply with the various sewage discharge requirements.
For a definition of Grade A, B and C treated sewage, refer to the Great Barrier Reef Marine Park Regulations as described in Regulation 93B and sub-regulation 135(3).

Untreated sewage

The Great Barrier Reef Marine Park Regulations 1983 requirements for vessels that hold untreated sewage are:

All fixed toilets on vessels need to be fitted with a macerator to reduce the sewage into a fine slurry so as to be sparsely visible and speed up the breakdown of the waste in the receiving environment. Most electric toilets contain macerators or they can be purchased separately.
If you carry 16 or more people on your boat you will need to store your sewage and may discharge it at least 1 nautical mile or 1.852 km seawards from the nearest reef, island, mainland or an aquaculture facility.
If you carry 15 or less people on your boat, you may discharge untreated sewage in the Marine Park if you are outside of a boat harbour or marina or more than 1 nautical mile or 1.852 km from an aquaculture facility.
Maritime Safety Queensland has introduced nil discharge requirements for untreated sewage in Queensland waters.

For more information on these and Queensland vessel-based sewage regulations for coastal waters please visit the Maritime Safety Queensland website.
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Old 30-05-2019, 12:14   #15
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Re: Combine black and grey water?

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Boat drains do not have a P trap like house drains do. If you have ever been in House that has had the water turned off for an extended time long enough for the P taps to evaporate you can defiantly smell the sewer. Not sure how well a P trap would work on a heeling boat.
Boats that have grey water tanks utilize P traps in their drain lines so as avoid the tank's sewage gases and smell from returning to the drain. Boats that do not have grey water tanks and which discharge directly overboard from their grey water drains typically do not have P traps but they could have P traps. Traps are good for not just inhibiting foul gases, they also act as catchers for items that inadvertently get dropped into the drain. P traps also inhibit exhaust fumes from piping into the boat, such as happens if the drain discharge is located near the wet exhaust. When a P trap evaporates one has the option to refill the trap with clean water or to pee into it,
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