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Old 04-08-2010, 16:20   #1
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Join Date: Jul 2009
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Boat: 1984 CS 36
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Sweet Tank Alternative

I'm trying to eliminate holding tank odors, and have two questions:

First, I'm looking at a Groco Sweet Tank system and just can't bring myself to spend $200 on an air pump. I was planning on just tapping a port for an NPT fitting directly into my tank and then attaching a low draw air pump myself (maybe $10). Do you guys have any thoughts on why that might be inadequate? Looking at the specs of the Sweet Tank system, it looks like it's a continuously running pump and nothing more... I'm pretty sure I can make this myself, but I welcome anyone who'd like to talk me out of it. Also if anyone actually has one and can confirm that there's no additional functionality, that'd be helpful.

Second, I'm considering just replacing the tank. It's 26 years old, and I think it might simply be permeated by odor (I've already replaced all the tubing and fittings and there's a lingering smell under the v-berth where the tank is - not the sour hose smell, but just kind of a low level yuck - wafts up from time to time. It's only under the v-berth by the tank - the head and the rest of the boat smells like a sweet meadow after a springtime rain... that or diesel... anyhow...). However, I've always heard it's all but impossible for the tank itself to smell. It's plastic of some sort... Anyhow, have you guys ever heard of the tank smelling? Does the wet rag trick work on the tank too?

I guess I also have secret question number three - part of the motivation for replacing the tank is to get something smaller - we currently have a 40 gal tank, which a) takes up a ton of space and b) invites LOOOONG periods between pump outs. Any recommendation on an optimal tank size for two people?

(insert man on toilet emoticon here)
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Old 04-08-2010, 16:55   #2
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This is the first time I've seen or heard of the "Sweet Tank", and sadly the only boat I have is an O'Day Daysailer III. But I am a plumber, so hopefully that counts for something. I went to the Groco web site to see the diagrams, and I noticed the aerator is at the bottom of the tank to oxygenate the waste in the tank, so I'm not sure if your idea of circulating air through the upper portion of the tank would accomplish anything (could possibly make the smell worse).

If you do try something along these lines, I think I'd use some sort of compression fitting kind of like a through-hull but with at least one heavy duty rubber washer rather than trying to thread something into the side of the tank. Also, there's got to be some kind of check valve in the line between the Sweet Tank module and the outlet to keep raw sewage from flowing up into the module itself. If you build something similar, you'll have to fit a check valve in the line somewhere as well. Finally, notice how the vent tees-off in two directions (port & starboard) on the Groco diagrams. That's an important feature if your pumping air into the tank (while healing over one way or the other)

Good luck!
Ishmael is offline   Reply With Quote
Old 04-08-2010, 19:12   #3
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You can make your own aerator out of a small compressor . The most impt part is taking the air tube the bottom of your tank , and hooking it to a pipe that is horizontal to the bottom. Said pipe should have small holes in it, so it releases bubbles into the sewage ( hence the word aerate, think fish tank). This supplies the oxygen to the bacteria that will eat up or break down the poo. This type of system is used in commercial and residential sewage treatment systems. The advantage of this system is the oxygen bacteria are very good at what they do and with filters can be recycled. or discharged into fresh waterways. . In a regular holding tank a different type of bacteria is at work to break down the matter (works like a septic tank) to a black water which is still full of fecal material just in a liquid form. This water does not meet the requirements for discharge into waterways. Now back to the question: both of these systems put out noxious odors ( go up to the roof of a house with an aeration unit and take a deep breath from the plumbing vent, smells just like all the other systems vents including houses hooked up to a sewer). So if you think you will not get an odor out of an aerated tank , I would think again. We find our 47 gal tank lasts us 1 week (2-full time liveaboards). I think the newer plastic holding tanks (Ronco or Ronko, out of Calif. has a great catalog)don't get permeated with odors. Bottom line aeration is not what you are looking for. Mussings from the ex- largest septic and aerater installer in the central ohio area. Pay me for the dirt.
Mark Zarley is offline   Reply With Quote

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