Odour from Acid Drainage System

[r551n]

I have been charged with solving a odor problem in a laboratory. A variety of plumbing fixtures are upstream of an acid neutralizing tank. The Canadian Plumbing Code mandates that neutralizing tanks must discharge to the sanitary system through a trap:

"Where a fixture or equipment discharges corrosive or acid waste, it shall dischage into a neutralizing or dilution tank that is connected to the sanitary drainage system through: a) a trap or b) an indirect connection"
(National Plumbing Code of Canada (2006), 2.4.4.4)

The plumbing inspector demanded that none of the fixtures upstream of the neutralizing tank be trapped, as that would consitute a double trapped condition. Therefore, some laboratory sinks and some floor drains are untrapped. The problem arises when bulk lab waste is disposed through the laboratory sinks. Offensive odour wafts up through the adjacent floor drains, and generates complaints.

Has this situation been encountered before? I've heard of a rubber "backflow" type insert that can prevent odour, but that seems like a bit of a mickey mouse solution which treats the symptom and not the problem. I believe the problem to be improper venting of the neutralizing tank itself.

Any comments?

 

[Bob NH]

No Traps in Sink Drains?

I suspect that the original installation of floor drains and sinks was not done in the way they are usually done in the US.

Unless you do things differently than in the US, I suspect that the plumbing inspector was wrong in requiring that the sink and floor drains not be trapped.

Drains of all kinds are usually trapped and the discharge of the traps goes to an untrapped main via a tee. Since the flow from any fixture doesn't go through a second trap, there is no "double trap".

The only way there would be a double trap is if there is a "whole house" trap, and in that case there is always a vent between the individual traps and the "whole house" trap.

Does your system have a "whole house" trap in the discharge to the sewer?

 

[Sponsor]

 

[r551n]

Drains of all kinds are usually trapped and the discharge of the traps goes to an untrapped main via a tee. Since the flow from any fixture doesn't go through a second trap, there is no "double trap".

The only way there would be a double trap is if there is a "whole house" trap, and in that case there is always a vent between the individual traps and the "whole house" trap.

Hi Bob, thanks for the response. The plumbing inspector regards the neutralizing tank itself as a trap. As all acid waste fixtures are upstream of (and drain through) the neutralizing tank, the inspector deemed that they are upstream of a trap, and therefore cannot be themselves trapped. I am also of the opinion that the inspector was incorrect, however regardless of previous code interpretations, the system has been installed, and has been buried under slab and framed into walls, and the space is occupied.

The meat of my question is whether there is a minimally invasive solution to this odour issue. We are considering capping all floor drains with cleanout fittings to prevent odour from escaping the system. The floor drains serve as "emergency" fittings only, in case of catastrophic equipment failure.This to me is a mickey mouse solution, but it does avoid the cost of cutting into epoxy flooring, and then cutting through the slab to install traps.

Any further thoughts?

 

[hj]

trap

The basin being trapped is equivalent to a whole house trap. But the only way the individual sinks would be "double trapped", and he is in error about his definition of a double trap, is if the individual sink traps were not vented. A proper drainage installation would insure, and require, that the sink traps be vented so the double trap situation that the code addresses, not what the inspector is thinking, would not be a factor. Send the inspector back to plumbing school if he ever went to one, and if not, give him a scholarship to one. If conditions warranted it, and the traps were each vented, you could put a dozen traps on the line.

 

[Bob NH]

Is the tank under the floor, or above the floor? Are the problem drains on the inlet side of the tank or the outlet side of the tank?

You might be able to put a trap on each of the inlet lines to the neutralizing tank, inside the tank. Gas would be prevented from getting back do those drains. That would involve getting into the tank.

Alternatively, if there is only one inlet to the tank, you could dig up the floor at the tank and install one trap adjacent to the tank.

Then, vent the tank to outside so it is always at atmospheric pressure. That will ensure that the drains will never have back pressure.