Choose your Service Provider Carefully
It is always a challenge to find a quality service provider within any trade. Our membership is committed to a standard of work and ethics that sets the bar for the rest of the industry in the State of Washington. We encourage you to do business with WOSSA members who support this industry ethic and standard of quality. Hiring a WOSSA Member ensures that you are working with qualified, knowledgeable and professional service representatives who know how to perform quality work, while protecting our environment.

You can find member companies in your area by going to the “Find a Service Provider” buttons on this page. Select the type of service that you need (designer, pumper or operations and maintenance or installer) and scroll down to the appropriate County where the septic system is located, for a list of WOSSA members within your service area.

Your Local County Health Department

Your local county health office can also provide you with a list of licensed service providers.
WA County Environmental Health Department Links

Find Your LHJ

Onsite Sewage System Information Categories

  • Definitions
  • Onsite Sewage System Inspections
  • Choosing a Service Provider
  • Find your Local Health Department
  • Find More Information on the Rules for Septic Systems
  • Items to avoid in an Onsite Sewage System
  • Pressure Distribution Systems
  • Sand Filters
  • Mounds


Onsite Sewage System

We used to call them "Septic Systems" but now they are most commonly called "Onsite systems" because they "treat" and "dispose" of your household wastewater on your own property.

Septic Tank
There are lots of different configurations that have been out there over the years. Older systems may have a large single compartment tank that will vary in size. Newer tanks commonly have two compartments. The main function is to separate the solids from the liquids, and initially lower the waste strength before it goes to the drainfield. Be careful of what you put into the tank. Keep in mind that the other primary function of this tank is to be like a "garbage can". The city comes by once a week to pick up the trash.....your tank will need to be pumped out based on your use, family size and how much "trash" you send out to it.

The technical term is "subsurface absorption system" but, this is the final treatment component before the "effluent" is released back into the environment. If your helping your septic tank do its job, then you are protecting the drainfield from getting too much water, too strong a waste strength, chemicals, and any solids.....if not, eventually it will plug something up. Drainfield replacements are expensive, messy and a problem if you have expensive and mature landscaping. It is better (and less expensive) to get your tank checked and pumped than to figure out you have a clogged drainfield when the system backs up into the house.....lastly, make sure you don't drive, run livestock, build a deck, garage, driveway, park your RV or Boat or construct your family swing-set over your drainfield. It seems obvious, but we see it all the time.

This is the term used to describe the blackwater (toilets) and greywater (everything else) that comes from your house into your "onsite system".

A few hours ago it was the "influent", but describes the water that goes out of your tank into the next system component, which may be your drainfield, but could also be the "pump" tank if you have a pressure distribution system. This also recognizes that the wastewater strength changes from when it enters the tank to when it leaves.

Scum Layer
This is the material that accumulates at the top of your tank. It consists mostly of fats, oil and grease from cooking and bathing along with paper products and other things that float when you flush.

As things digest, settle down(and out) of your wastewater, this material accumulates on the bottom of your tank. When the combined volume of Scum and Sludge measure about 25% to 30%, its time to get pumped. Your individual usage may cause that to happen in two years or eight. It's all up to you!

Other Stuff
That's the basics of system components but, depending on what type of system you have, you may have other things like "pump tanks", "pumps", "floats", "Pump Control/Alarm boxes", "inspection ports", "risers", "mounds", "sand filters" or "proprietary technology" (see below).

Onsite Sewage System Inspections

Many counties or lenders have a requirement for an inspection of the onsite system at the point of sale. As a consumer, you are responsible for accepting the final "report of system status" in some form or other. Some counties and lenders do not have strict requirements on inspection of the system and merely require that the tank be "pumped".

Generally, this would be like buying a car with the hood welded shut. We recommend that a full system inspection including, all components that can be tested for performance, are tested. The purchase of a home is, for most people the single greatest investment that they will make in their lifetimes. Make sure that you are getting enough information from the inspection of this component of your purchase to make a good decision. The more complicated the technology, the better record keeping of maintenance by the previous owner should be readily available. If its not, then some red flags should be going up indicating a need for a more thorough inspection at point of sale.

For the homeowner/consumer, click on the following link for Basic On-Site Information.

This publication will give you general information of gravity system components, how it works and what you can do to keep it healthy and working well. The information on use, care and maintenance is a good guideline even if your system has other components.

Click Here

We are committed to continuous improvement and best business practice.

Check out some of the information below to educate yourself a bit more on other applications and technologies.

Items to Avoid in an Onsite Sewage System

The following guidance is a collaborative effort of wastewater professionals within the National Onsite Wastewater Recycling Association (NOWRA). The purpose is to identify common issues that can cause problems with the operations of newer onsite treatment and traditional septic systems. Many operational problems exist today because owners are either unaware of the results of daily practices to these systems. NOWRA’s goal is to ensure that owners are educated and informed about the safe practices for their treatment systems, in order to avoid costly repairs and to protect groundwater quality. The items listed below are known to have caused failures of onsite treatment systems and must be considered if waste generated by/from a particular site will contain them in excessive quantities. Since excessive is a subjective word, it is highly recommended by NOWRA that you share concerns with your Wastewater Professional to come up with a treatment strategy for your particular needs.

Inert Materials:
Plastic, Rubber, Scouring Pads, Dental Floss, Kitty Litter, Cigarette Filters, Bandages, Hair, Mop Strings, Lint, Rags, Cloth and Towels do not degrade in an on-site treatment system.

Inert Materials will build up solids, and lead to system malfunction, clogging or increased pump out frequency.

Paper Products:
Disposable Diapers, Paper Towels, Baby Wipes, Facial Tissues, Baby Wipes, Lotions, Scented or Quilted Toilet tissue, Moist Toilet Paper, do not dissolve readily in an onsite treatment system. Excessive Amounts of toilet tissue will also not decompose. All can lead to system malfunction, back-up or increased pump out frequency.

Food Wastes:
Do not put Animal Fats & Bones, Grease, Coffee Grounds, Citrus & Melon Rinds, Corn Cobs, Egg Shells, down the sink. Garbage disposal use should be limited to waste that cannot be scooped out and thrown in the trash. Spoiled Dairy Products and Yeasts from home Brewery or Baking may cause excessive growth of microbes that do not degrade sewage.

Household Products:
Do not flush Baby Wipes, Lotions, Scented or Quilted Toilet tissue, Female Sanitary Products, Cotton Balls or Swabs, or Condoms. Anti-microbial Soaps and Automatic Disinfection Tablets (blue, clear or otherwise) may kill the organisms needed to consume waste.

Normal use of over the counter medications do not affect the performance of onsite systems. Do not flush expired Medicines/Antibiotics into an onsite treatment system. Some prescription medications are known to cause biological disrupt. Among these diseases or conditions are those suffering from bulimia, severe infections (including AIDS), chronic diarrhea, intestinal/colon by- pass, or other gastrointestinal conditions and cancer. Oral or intravenous chemotherapy is known to cause serve disruption to the treatment process and will require more frequent pump out intervals or the use of biologically based additives.

Commercial Additives:
Both the U.S. Environmental Protection Agency and the Small Flows Clearing House have reported that there is no evidence to support the use of additives with normally functioning Onsite Treatment Systems. Some Septic Tank additives have been shown to do more harm than good. A normally functioning system should not require additives.

Chemicals & Toxins:
These materials kill the microbes necessary for the biological treatment to occur. Paint, Paint Thinner, Solvents, Volatile Substances, Drain Cleaners, Automotive Fluids, Fuels, Pesticides, Herbicides, Fertilizers, Metals, Disinfectants, Sanitizers, Bleach, Mop Water, Floor Stripping Wastes, Excessive use of Household chemicals, and Backwash from Water Softener regeneration.

Laundry Practices:
On-site systems must process the water as it enters the system. Laundry should be spread out over the week, not all run at one time. Excessive use of Detergents, especially those containing bleach, can affect system performance. Liquid detergents are recommended over powders. Fabric Softener sheets are recommended over liquid softeners. Bleach should be used sparingly and at half the rate indicated on the container.

Clear Water Waste:
From A/C Discharge lines, Floor Drains, Gutters, Whole House Water Treatment Systems and Sump Pumps can increase the flow to your treatment system. These flows can at least disrupt, if not destroy your treatment process.

Pressure Distribution Systems
A pressure distribution system is Pressure distribution applies effluent uniformly over the entire absorption area such that each square foot of bottom area receives approximately the same amount per dose at a rate less than the saturated hydraulic conductivity of the soil. This process promotes soil treatment performance by maintaining (vertical) unsaturated flow at all times and also reduces the degree of clogging in finer textured soils. Pressure distribution closely approaches uniform distribution.

Please click on the following link to get detailed information on how your "pd" system works and how to take care of it.

Click Here

Sand Filter Systems
Septic tanks with gravity flow drainfields have been used for many years in areas not served by public sewers. Unfortunately, not all soil and site conditions are well-suited for conventional septic systems. To protect public health and water quality, alternative systems are often used in areas where conventional systems cannot guarantee safe sewage treatment. The intermittent sand filter is one such alternative.

To get more detailed information on sand filter systems, please click on the following link.

Click Here

Mound Systems
On some properties, the amount or quality of available soils for final treatment and dispersal is limited in some way. Designers use alternative treatment technologies to achieve treatment. The mound system is one such alternative. It provides:

Dosing and resting cycles.

Uniform distribution of effluent.

Known level of sewage treatment in the sand fill before disposal.

Greater distance (vertical separation) for effluent to travel before reaching groundwater.

To get more information on care and use of a mound system, click on the following link.

Click Here

WOSSA Consumer - Information

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​Washington On-Site Sewage Association