Black waste tanks size

[Peter]

Whats a good size for a black waste water tank, as big as possible, can go up to 60 gallons (a lot of ****). Fitting the boat out for long distance crusing, Med , USA and other places so size might be important for long stays in port. Any ideas on how to calculate how long it takes to fill a tank ie loo use per tank gallon or other measure. In fitting one of these tanks are pump facilities really available and therefore is it worth putting in a pump out fitting to the tank. Material, looking a the Tek Tank range or can s/steel be used. At this time should I be looking at fitting a grey water tank as well.

Peter

<hr width=100% size=1>

 

[ZuidWester]

Make it big enough..

and you can cook using the gas )

This is surely a tricky question. I'm about to get one fitted, but at the moment we don't use the heads if there's a loo in the habour.

Make it big if you have the space I guess.

<hr width=100% size=1>

 

[Talbot]

The Headmistress is the fount of all knowledge in this area. I was asking a similar question last year, and found a USCG site which gave some guidance on amounts per person, but this is a false estimate, cause it will totally depend on two factors:
How much water you pump through
How long is the pipe from Head to Tank.

The Headmistress gave me the following advice:

<font color=blue>What Size Holding Tank Do I Need?

There really aren’t any hard and fast answers to this question. There have been numerous and sometimes arcane attempts to arrive at one, but what it really comes down to 99% of the time is: how much space there is in the boat to put one--or (more often than not) more than one if the boat has two or more heads. The hose run from either head to the tank shouldn't be longer than 6-8'...10’ is the absolutely maximum. Any longer, and you'll always have waste sitting in the hose to permeate it. So, if the location you’ve picked means it has to be longer, find another location for the tank that's closer to the head. If you have two or more heads, you may need two tanks...or--unless you're in "no discharge" waters--a Type I or II MSD on the head you use the most and a tank on the other one.

Here's MY rule of thumb--and it's based on the size of the boat:

under 30': 12-20 gallons.

30-35': 20-30 gallons.

35-40': at least 30 gallons...

40' plus: at least 40 gallons total, more if you have room.

There are just too many variables to do it any other way, different types of toilets use different amounts of flush water and people are different, but the following information may help you decide. Just remember that no one has EVER complained that his holding tank was too big!

Vacuum toilets and electric macerating toilets that draw off the onboard pressurized freshwater system use the least flush water--1-3 quarts/flush.

Manual toilets use about a gallon/flush if pumped long enough to completely rinse the sewage out of the head discharge line. There are ways to cut down on that--rinse out the line once a day instead of pumping enough water through with every single flush, but the average will still be at least half a gallon, whether the toilet has a piston/cylinder pump or a diaphragm pump.

Electric macerating toilets that have integral raw water intake pumps need 1-3 gallons per flush, because there's no way to switch to "dry bowl" without frying the impeller in the intake pump. A couple of brands/models have separate pumps using separate motors, which cuts the water consumption considerably, but you still have to run enough water through it to clear everything out of the macerator and pump unless you want to deal with clogs.

The further the tank is from the head, the longer ANY toilet has to be flushed to move the bowl contents from the head to the tank.

Those are the parameters for the toilets. Now we need to factor in people: Women go to the bathroom more often than men (women also tend to use about 5x the toilet paper that men do, but that's another issue). Children tend to fall somewhere in between women and men. The average urine "deposit" from an adult is about a cupful (6-8 oz)...somewhat less for children, depending on their ages because their bladders are like the rest of their bodies--they grow as the child grows. Some people drink more liquids than others...some sweat more of it out than they process through their kidneys...and that can vary with the time of year.

Keep track of how many times you have to use the toilet each day...multiply that by the average volume...then by the amount of flush water your toilet needs, add at least one flush/person/day for solids too (double the flush water volume for that) and you'll come out with the same answer I always do: it's totally impossible to find enough space--at least space you're willing to sacrifice for tankage--for a tank on your boat that will last 4 people a week without a pumpout--and it's unlikely that even two people won't fill it in 4-5 days--unless the lee rail gets a LOT more use than the toilet.
</font color=blue>

<hr width=100% size=1>

 

[HeadMistress]

It's unlikely that you'll need a gray water tank. I would carefully research any areas you plan to visit to determine whether one is required anywhere before installing one, because gray water tanks are MUCH harder to maintain than black water tanks. Odor is harder to eliminate...hair, food particles etc that end up in it don't dissolve, they become sludge that can clog the discharge and the discharge pump.

As for your black water tank, the recommended material is thick-walled polyethylene, so you'll want to go with one from Tek-Tank. You do NOT want one made of ANY metal...urine is so corrosive that it typically eats through a weld at a seam or a fitting within 2-5 years and turns the tank--even the thickest 316 stainless--into a collander in about 10 years

As to the size...waste and water weigh 8.333 lbs/gallon (US...I don't have the calculation for Imperial gallon). So you want to keep the weight down while also having adequate storage to meet your needs. As for calculating how long it takes to fill up a tank: the average adult uses the toilet 5x/day. Each flush of most manual toilets is approximately .6 gal...so that would be an average of 3gal/day/person. That means 2 adults continuously aboard would fill up a 60 gal tank in 10 days. I don't think you need that much waste storage, though. I need a bit more information before advising you...

How many people will be aboard during your cruise? Average consecutive days you expect to spend either anchored or moored in coastal waters where direct overboard discharge is a no-no, or cruising in those close-in coastal waters (that would be the whole US, btw)?

Would you consider installing a Type I MSD--device that treats waste and discharges it overboard legally, and is accepted in most of the world for use in all waters except those specifically designated "NO discharge," which are far fewer than most people realize--and a small tank for use only in those "no discharge" waters?

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

 

[Peter]

Thanks for the replies, one question to the headmistress, what is a Type I MSD tank and where can I get the information on one?

Thanks

Peter

<hr width=100% size=1>

 

[HeadMistress]

There are several on the market. The most popular and most widely accepted is the Raritan Lectra/San...It's not a tank, it's a "flow through" device the toilet flushes into where waste is treated and discharged a flush at a time...and can be used with any toilet. It does not require the use of any chemicals, but instead creates hypochlorous acid (a type of chlorine) by charging the ions in salt water with electrical current. The beauty of that is that the solution reverts back to salt water when the stimulus (electrical current) is removed, leaving no free chlorine in the environment. You can read all about it here: http://www.raritaneng.com/products/waste_treatment/lectrasan.html The current draw appears a bit scary at first...but the actual power consumption for two people is only about 10-15 A/day...none at sea where it can be bypassed.

The other one that's equally environmentally friendly is the Groco ThermoPure. It uses heat to kill the bacteria. However, it's designed to process waste from a tank instead of directly from the toilet--which means it wouldn't solve your problem...and also needs 115vAC current, which makes itimpractical on anything but 50-65' yachts (65' is the max LOA for a Type I...anything larger has to use a Type II) with generators that run pretty much continuously to supply the power for it. It also costs at least twice as much as a Lectra/San. So I don't think it's the right one for your needs...I mention it only because it's the only device except the Lectra/San that doesn't require the use of toxic chemicals to break down solids and paper and kill bacteria.

IMO, a Lectra/San and a 15-18 gallon holding tank for use only if you should visit a "no discharge" port would be ideal solution for you. As for cost, list here in the US is about $1100...a dealer in FL (who also sells on Bay, btw) sells 'em for $699. I'd have to look up the current rate of exchange to know how that compares with UK prices when you add shipping from the US.

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