May 20, 2020

What is a Mound Septic System? All Your Questions Answered

what is a mound septic system
Not every home is a good fit for a traditional septic system. In areas with adverse or overly wet soil conditions, you may need a mound septic system to manage your wastewater effectively.

What is a Mound Septic System?

A mound septic system is an above-ground waste management system used in places where the soil cannot properly absorb wastewater. It includes a mound-shaped drainfield that is raised above the natural soil surface.

Unlike a traditional septic system, a mound-style drainfield is not dependent on the quality and depth of the local soil to function. Instead, the drainfield is built above ground with engineered layers and a dosing chamber that keeps effluent from overwhelming the treatment area.

Mound septic systems are common in areas where the soil is extremely permeable or impermeable, in areas with a shallow soil cover over porous bedrock, on glacial till, or on terrain that features a high water table.

How Does a Mound Septic System Work?

A mound septic system uses a septic tank, a dosing chamber, and a mound-shaped drainfield built in layers to treat waste from your home or workplace.

First, the waste is sent to the septic tank. The solid portion of the waste sinks to the bottom of the tank. Meanwhile, the liquids are sent to a second tank called a dosing chamber.

The dosing chamber then distributes the wastewater to the mound in doses. The effluent is partially treated as it moves through a sand layer in the mound, then fully treated and disposed of in the soil beneath the mound.

The absorption mound is built in layers that are determined by the depth of the natural soil. Based on Washington State recommended standards, the layers should be built from bottom to top as follows:

  1. At least 24 inches of undisturbed, unsaturated soil above the original ground surface.
  2. At least 12 inches of sand or other filter media on top of the soil.
  3. A layer of gravel containing the dosing chamber’s distribution pipes on top of the sand.
  4. Construction fabric and additional soil on top of the gravel to help keep the pipes from freezing.
  5. A top layer of soil that can be planted with grass or selected other plants to control erosion.

An advantage of the mound system is that it does not allow all the effluent to enter the drainfield at once. The metered wastewater distribution system allows the mound to clean effluent more effectively and keep the system from failing.

Why Should I Use a Mound Septic System?

If your home or workplace is in an area with aberrant soil conditions, a mound septic system may be the best option for you.

The mound septic system allows you to use land that would otherwise be unusable, such as terrain with a high seasonal water table or shallow bedrock. A traditional septic system requires a certain quality and depth of natural soil, while a mound septic system can function virtually anywhere.

The mound system can help you avoid a septic system overload by distributing wastewater to the drainfield in manageable doses. The dosing chamber’s metered distribution system allows the mound to clean effluent more effectively.

Do Mound Septic Systems Smell?

You should not smell odors from your mound septic system most of the time. However, there may be occasional wafting odors from any type of septic system, including a mound system.

Unpleasant smells are typically either the result of a high gas pressure level or recent flooding. Heavy rainfall and surface water can sometimes penetrate the topsoil of your mound system. If your septic system is clogged with too much water, it may prevent wastewater from being treated and cause odors that can back up into your house and yard.

Odors can also come from a low bacteria population in the waste treatment area. Bacteria are vital for digesting and cleansing waste products. If there are not enough bacteria, the partially digested waste will produce gas with distinct odors.

Tips for Reducing Odors from a Mound Septic System

If you are experiencing unpleasant smells from your mound septic system, try the following tips to reduce odors:

  • Reduce your daily water usage. You may want to cut your shower time or distribute laundry loads throughout the week to avoid a heavy load on the septic system.
  • Check your septic system for leaks. While leaks are rare in a well-maintained mound septic system, they can occasionally happen.
  • Switch household cleaning products. Choose gentle products that do not unnecessarily harm bacteria, which are vital to a functional septic system. Every day, there are more and more “green” cleaning products on the market that do not compromise cleaning performance for environmental friendliness.

If changing your household habits does not fix the smell, you can also call a septic service provider to do an inspection and find the problem.

Can I Walk on a Mound Septic System?

We do not recommend walking on your mound septic system except when necessary to care for low-maintenance plant cover. Walking compacts the soil, which interferes with drainfield function.

Mound septic systems can be more sensitive to pressure than traditional septic systems. If you put too much pressure on the soil above your mound septic system, it can damage your septic system’s ability to evaporate effluent.

What Can I Plant Above a Mound Septic System?

You should plant only low-maintenance plants with short roots above a mound septic system. We recommend drought-tolerant grasses, herbaceous plants, or wildflowers.

Landscaping not only makes your septic system more attractive, but it also prevents erosion and helps keep your mound system functioning properly. You can read more about landscaping over a septic drainfield on our blog.

How Much Does a Mound Septic System Cost?

Mound septic systems are typically more expensive than traditional septic systems.

As with other septic systems, you will also need to pay for annual maintenance inspections and septic pumping every few years. You can plan to spend at least a few hundred and up to a thousand dollars on septic maintenance each year.

Septic Installation and Maintenance in Western Washington

Ace Acme provides professional septic services in Snohomish, Skagit, Island, and North King counties. If you are looking to install a septic system, get your septic tank pumped, or hook up to a municipal sewer system, contact us today.

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