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Re: How to Eliminate Water Hammer on Basement Sink Waste Line

From: adammiller@enter.net
Category: Plumbing
Remote Name: 71.225.51.174
Date: 17 Jan 2006
Time: 09:05 PM

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Question: I have a laundry sink in the basement. The waste line runs through a pump which pumps the waste through a check valve, up into the main waste line leading to the septic tank. When water runs down the drain, the pump turns on. This causes a strong shock which can be heard throughout the house. This probably wouldn't be much of a problem except that the water softener dumps into the sink during the recharge cycle. So every day when the softener cycles, the waste line hammers away. (Note that the sink is only used for softener discharge and occasional hand washing; the washing machine is upstairs.) There are some additional considerations. Can the water softener discharge easily be plumbed into the waste line so it doesn't pass through the laundry sink pump? I've heard softener discharge can be detrimental to septic fields and shouldn't be run into the septic at all. Should the softener discharge be dealt with in some other manner?

Builders Websource Answer®: Water hammer is a phenomenon normally associated with closed water supply lines, not water discharge lines. However, at its most basic level, water hammer is the result of a shock wave introduced into the water supply line due to an instantaneous (step function) change in flow usually associated with a solenoid-activated valve (such as a washing machine) or a sudden pump surge. In the case of a discharge or waste line, the same basic phenomenon applies -- that is, the vertical column of water trapped by the check valve will experience a sudden shock when the pump first activates. Depending on the type of discharge pipe (plastic or cast iron), you may hear the impulse response from the pump -- or it could be a pipe banging due to poor strapping. Cast iron tends to dampen the sound due its mass, whereas plastic waste pipe is generally noisier and more resonant.

As to your point about the water softener discharge into your septic system, this is a more interesting question. First, you should be aware that many states and/or local building departments specifically prohibit the discharge of water softener brine into a septic system. Whether these bans are reasonable is a matter of vigorous and inconclusive scientific and commercial debate. For example, specific studies commissioned by the Water Quality Association reveal that there is no material impact on septic performance due to discharge of brine into the system. However, keep in mind that the industry that makes water softening products has a conflict of interest in that any ban on this practice has a direct financial impact on their business.

By contrast, other scientific analysis coupled with empirical data gathered from observations and inspections of real septic systems suggest that while the studies noted above may hold true under "ideal" conditions (meaning a perfectly maintained water software and septic system of sufficient capacity), real-world results show a correlation to leach field clogging and brine discharge. Specifically, there is indication that the cellulose fiber in toilet paper may not fully biodegrade, which tends to clog the percolation field. This is often a problem with many scientific studies; that is, these studies are conducted in ideal laboratory settings and fail to account for real-time maintenance issues that could materially distort or change the results of a lab study. A relatively unbiased treatment of this is available in an article Water Softener Use Raises Questions for Septic System Owners (from Pipeline, a newsletter of the National Small Flows Clearinghouse, Winter, 2001). Since there are so many variables such as the size of your septic system, the number of stages, the soil, the age, the amount of daily flow, and the level of maintenance, it's impossible to know for sure whether your particular situation is acceptable. Only carefully controlled studies that monitor each of these parameters over time would yield more conclusive results. However, prevailing building and plumbing codes always take precedence, regardless of what the current scientific community concludes.

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