Adding Active Ventilation to the Holding Tank

Eliminate Foul Odors from the Holding Tank

 The principle behind a nice smelling holding tank is quite simple: Noxious odors in a holding tank are caused by anaerobic bacterial decomposition of the sewage in the absence of oxygen.  The bacteria give off smelly gasses such as hydrogen sulfide and methane which smell much like rotten eggs.  By simply adding oxygen through aeration, the balance shifts to aerobic bacterial  decomposition, which does not produce noxious gasses at all.  In a typical sewage treatment plant, oxygen is added to improve the functioning of aerobic bacteria and to assist them in maintaining superiority over the anaerobes(the smelly guys).

When the head becomes too noticeable in a bad way, the main culprit is typically poor holding tank ventilation.  Simply increasing tank ventilation can have a major aromatic improvement.  Passive approaches such as increasing ventilation size or adding multiple vents have long been the go-to solution.  But active approaches, such as pumping air into the holding tank, have shown to be a viable solutions too. 

As I write this, the Johanna Rose has an oder-free holding tank.  The benefits of a new and never been used system.   Ah, but soon it will be christened, and I expect also in regular use.  With all likelihood, the new holding tank system will operate nicely with passive ventilation, but why not give it a little extra help.   Inspired by the success of the virtually no-cost active ventilations systems described by cruisers online, such as the one at http://www.passagemaker.com/articles/technical/systems/foul-odors-run-deep, I am adding an active ventilation system to the holding tank of Johanna Rose all for under $25.  Let's call it the biology experiment.   The cost is cheap, about as much as a few beers out, and I literally get to write about shit.

Active ventilation components

 It turns out that Groco has come out with a similar system called “Sweet Tank,” which uses a small air pump and diffuser to add oxygen thus promoting aerobic bacterial decomposition over the smelly anaerobic bacterial decomposition.


A Simple Active Ventilation System

The following items will comprise the new active ventilation system.  The airline tubing will enter the holding tank through a sealed tee junction in the present 3/4" ventilation line. The tubing will extend to the inside bottom of the tank where it will be connected to an air curtain.  An air check-valve will be used on the air tubing near the air pump tubing connection.  The air pump will run off of the same AC circuit as the battery charger, so while docked at the marina, the holding tank will benefit from the added airflow coming from the active ventilation system.  If needed, future options may go with a DC or solar powered air pump.

Active Ventilation Components

• Air Curtain  
  • get 9”  CD-35667 $4.49
  • http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+3669+19782&pcatid=19782
• Air Check valve
  • single CD-33949  $2.99
  • to prevent back-siphoning of water, place the air pump above the water level or install a check valve.
• Airline tubing
  • 3/16” ID standard
    • 25’ CD-74977 $3.99
• Air Pump
  • http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+3669+12911+12738&pcatid=12738
  • Whisper Model 40, CD-21266  $10.99

Bug nest which came out of blocked vent hose.

Immediate Success

The active ventilation system had an immediate impact on the holding tank ventilation system during the installation process.  In trying to feed the airline tubing through the 3/4" holding tank air vent hose, a blockage was discovered.  The ventilation hose was originally new, free of any blockage, and just installed on the holding tank a little over a year ago. But all that time, the boat was hauled out and on jack stand storage.  While the new holding tank remained unused and empty, apparently bugs found there way into making a nest in the middle of the ventilation hose.  The photo to the right shows what came out of the blocked vent line.  If it was not for the attempt to insert the airline tubing, this blockage would have gone unnoticed.  



Installation

Topside view of holding tank showing air
 line tubing fed through the holding tank vent hose.

The installation this system in a new (unused) holding tank was simple.  The airline tubing was inserted down through the air vent hose into the holding tank.  
Next, the air curtain was connected to the tubing and then mounted to the inside of the holding tank.  The muscle of the active ventilation system is the air diffuser/air curtain component.  3M VHB tape was used to mount the air curtain feet to the tank.  The tank surface was cleaned with alcohol prior to mounting with the 3M tape. 

The airline tubing connected to the air
 curtain prior to the  mounting of  the
 air curtain to the inside of the tank with 3M VHB tape.

The air curtain mounted inside of the holding tank.

A 3-way tee providing access for the airline tubing to the holding
 tank ventilation hose.

The upstream end of the airline tubing exited the holding tank ventilation hose via a 3-way tee and led to the air pump located in the upper head locker. 

A short section of hose was later added (not shown) along with a plastic insert and sealant to provide an airtight cap of the airline tubing to the holding tank ventilation system.

The air pump is installed in a small locker
 in the head next to an AC outlet.  

The heart of the active ventilation system is the air pump.  The air pump is installed in a small upper locker in the head next to an AC outlet.  The pump runs continuously when on shore power.

See addition post regarding head stink due to sea water inlet: Getting fresh water into head