The Lowdown on Laundry Lint

Would putting a filter at the wastewater outlet on a washing machine extend the life of an onsite system?

Question

There are several laundry machine lint filters on the market. What are the benefits as you see them? If the septic tank has an effluent filter in the outlet tee, would this negate the benefit of blocking laundry lint from entering the septic tank? I haven’t seen a lot of talk on this subject from anyone except the filter sales literature. For a heavily used septic system, is it reasonable to divert the laundry water to a few chambers, with an overflow back to a tee in front of or even after the septic tank? Let’s assume surface distribution of laundry water is not an option.

Answer

You have raised some interesting questions about lint filters and the discharge of laundry water. I will repeat each of your questions as I answer them.

What are the benefits of laundry machine lint filters as you see them?

When clothes are washed, some fine material is removed from the fabric. This material, called lint, ends up in the wash and rinse water. The wash and rinse water goes into the house sewer and ends up in the sewage tank.

The fine lint particles would easily be carried by the flowing wastewater. How much of the lint gets into the soil treatment system is another question. This would depend on the liquid volume of the septic tank, whether or not the tank had compartments, and the volume of wastewater discharged by a cycle of the automatic washer. In a large tank where the liquid moves through very slowly, I would suspect at least some of the lint would settle to the bottom and become a part of the sludge layer.

I am not aware of any research studies reporting on the amount of lint entering a sewage tank and the amount leaving the tank. I also am not aware of any studies confirming lint is a major problem in clogging the distribution media in a soil treatment system. Logic does seem to say it would be wise to keep as much solid matter as possible out of the soil treatment system. A good filter on the wastewater discharge line from the automatic washer should catch most of the fine lint particles.

If the septic tank has an effluent filter in the outlet, would this negate the benefit of blocking laundry lint from entering the septic tank?

No, the outlet filter on the septic tank is designed to keep particles from flowing out of the tank. The size of the particles screened would depend on the openings in the outlet filter. If the lint particles are already screened out, this should make the septic tank outlet filter more effective.

As you know, there are many different types of septic tank outlet filters available on the market. An outlet filter must be serviced periodically to clean the filter to be sure it is not plugged. Filters should be selected with ease of maintenance as one of the considerations.

As an aside, when an outlet filter is placed in a sewage tank, the installer should offer the homeowner a service contract. Servicing an outlet filter of a sewage tank is not a very desirable job for the average homeowner. Yet the outlet filter does need periodic service or there will be a sewage backup. The need for periodic service of the outlet filter should be emphasized to the homeowner at the time the outlet filter is installed and the service contract is offered.

For a heavily used septic system, is it reasonable to divert the laundry water to a few chambers, with an overflow back to a tee in front of or even after the septic tank?

There are a number of implications here and I will answer as best I can. First of all, a heavily used septic system should be sized adequately for the average daily sewage flow. So there should be little difference in the flow pattern between systems of various sizes if they are all sized properly.

In some state codes, septic tanks are required to have chambers. Usually two chambers is the accepted number. Discharging the laundry water to the second chamber of a two-chamber septic tank would increase the turbulence in that chamber. As a result, more particles would be in the effluent flowing out of the tank. Without a lint filter, the rapid flow of discharge from the automatic washer would promote the movement of lint particles out of the septic tank. The laundry wastes should not be discharged closer to the tank outlet, in my opinion. The reason is all of the wastewater should flow through the sewage tank as slowly as possible.

Many tanks have an opening in the compartment wall through which the sewage flows from one compartment to the next. There is a much better way to provide more effective treatment and a slower flow through the sewage tank. The compartment should have an outlet tee on the compartment wall. As the sewage flows into the first compartment, the liquid level rises and begins to flow into the second compartment.

The liquid level in the second compartment should be at least two inches lower than the liquid level in the first compartment. The liquid level in the second compartment must rise before any effluent flows out of the sewage tank. This process is called flow attenuation. Attenuation is the slowing down of a rapid inflow into a slow outlet flow. Sewage tanks having a compartment wall with an outlet tee and a lower elevation in the second compartment will have a much slower flow at the final tank outlet.

The slowest possible flow through the sewage tank will result in the best possible effluent quality. And good effluent quality is essential for an extended life of the soil treatment system.



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