Plumb­ing in a cou­ple of well-placed hold­ing tank vents will help keep nasty odors at bay.

Cruising World - - Contents - Monthly Main­te­nance by Steve D’an­to­nio

Re­cently, I re­ceived an email from a reader shar­ing the tale of her odor­if­er­ous hold­ing-tank plight. She ex­plained the tank had been added five years ago, and of late it had be­gun to smell. She won­dered if adding ven­ti­la­tion might help. She was on to some­thing.

Hold­ing tanks are a nec­es­sary (but im­por­tant) evil, and with the range of no-dis­charge zones in­creas­ing with each pass­ing year, the need to have one that func­tions prop­erly, and doesn’t stink, is ever more crit­i­cal.

The bac­te­ria that grow within hold­ing tanks and san­i­ta­tion sys­tems pro­duce, among other things, hy­dro­gen sul­fide gas. We’ve all smelled it; it’s that un­mis­tak­able, pun­gent rot­ten-egg odor. These un­de­sir­able anaer­o­bic bac­te­ria thrive in an oxy­gen­de­pleted en­vi­ron­ment, which is pre­cisely what ex­ists within most hold­ing tanks. On the other hand, aer­o­bic, or good, bac­te­ria thrive in an oxy­gen-rich en­vi­ron­ment (it’s “good” be­cause it di­gests ef­flu­ent and doesn’t pro­duce foul smells). The ques­tion is, how does one pro­mote good bac­te­ria while dis­cour­ag­ing their bad brethren? The an­swer is sim­ple: Add air to the hold­ing tank.

Turn­ing a hold­ing tank from an oxy­gen-de­pleted to an oxy­gen-rich en­vi­ron­ment is straight­for­ward enough. Plumb larger and, ide­ally, more vents into the tank, cre­at­ing cross ven­ti­la­tion. This can be ac­com­plished in one of two ways. The first in­volves the in­stal­la­tion of two 1½-inch-di­am­e­ter vents to the tank, prefer­ably at op­po­site ends, each of which is then plumbed to dif­fer­ent sides of the ves­sel. (There is nearly al­ways a slight dif­fer­ence in pres­sure be­tween port and star­board sides, es­pe­cially when un­der sail.)

With this ar­range­ment, con­stant air­flow through the tank dis­cour­ages bad bac­te­ria, and di­lutes odors over­all. This plumb­ing is eas­i­est to in­stall when the ves­sel is be­ing built or re­fit be­cause af­ter­mar­ket in­stal­la­tions can be chal­leng­ing. If 1½-inch hose won’t fit, go as large as pos­si­ble; ¾ inch or 1 inch is bet­ter than the com­monly used ½ inch or 5/8 inch. If even that’s im­prac­ti­cal, the sec­ond ap­proach in­volves the in­stal­la­tion of an ac­tive ven­ti­la­tion sys­tem, one of which, called the Sweet Tank, is of­fered by ma­rine plumb­ing man­u­fac­turer Groco. It uti­lizes a small air pump not un­like the bub­blers used in fish tanks to con­stantly in­tro­duce a fresh sup­ply of air into the tank’s liq­uid. It doesn’t use much power; how­ever, even if run only when the en­gine is on, or while dock­side and plugged in, it will help.

Char­coal hold­ing-tank vent fil­ters, while sea­son­ally ef­fec­tive at neu­tral­iz­ing odors leav­ing the tank via the vent, tend to im­pede cross ven­ti­la­tion. If the fil­ter gets wet from an over­flow­ing tank, or from wa­ter be­ing pushed back through the vent, it be­comes in­ef­fec­tive, and it can swell, block­ing air­flow.

Yet an­other ad­van­tage to the larger vent(s) is that it is nearly im­pos­si­ble for them to clog, and thereby avoid tank pres­sur­iza­tion, or a tank im­plo­sion, dur­ing fill­ing or pump-outs, re­spec­tively. These make im­plo­sion pre­ven­ters un­nec­es­sary.

When I first be­gan pre­scrib­ing this ap­proach to boat­builders, dur­ing new-build or re­fit projects, I re­ceived some quizzi­cal com­ments. “You want a 1½-inch hold­ing-tank vent?!” they would ask. “No,” I would re­ply. “I want two of them.” Most were re­luc­tant; some got it right away. The bot­tom line, how­ever, is that it does work. Fresh air is all that’s needed to change the chem­istry in­side a hold­ing tank. Even a whiff of non­aer­ated, and thus anaer­o­bic, bac­te­ria-laden ef­flu­ent gas can make an en­tire cabin un­in­hab­it­able. Thus, an added ben­e­fit of the su­per­ven­ti­lated hold­ing tank is a less smelly tank in­te­rior, which in turn is less likely to ex­ploit weak­nesses in the sys­tem else­where. In other words, if the gas in the tank doesn’t smell so bad, it’s less likely to no­tice­ably per­me­ate hoses and other fit­tings.

From left: Im­plo­sion pre­ven­ters stop a clogged vent from dam­ag­ing a hold­ing tank. Vent fil­ters will neu­tral­ize odors leav­ing the tank, but don’t keep them from de­vel­op­ing. Plumb­ing large twin cross vents makes it less likely anaer­o­bic bac­te­ria will be able to ex­ist.

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