This is the third in a series of articles reproduced from the National Small Flows Clearinghouse publication, Pipeline.
Morgantown, W.V., April 11, 2002 -- As the name suggests, a gravelless system is an onsite system that does not use gravel in its drainfield trenches or beds. Instead, these systems may use alternative materials in place of gravel, such as rubber, sand, fiber membrane, plastic, glass, or expanded clay, shale or polystyrene foam chips.
The alternative media in gravelless systems can function similarly to gravel. It can support the sidewalls of the drainfield trenches and prop up the perforated drainfield pipes so they don't lie directly on the soil and clog. When soil is saturated from the weather or surge wastewater loadings occur, the effluent can be stored in the media until the soil absorbs it. Like gravel, the alternative media also can help to distribute wastewater along the length of the trenches.
Gravelless Pipe Systems
In some gravelless drainfields, wrapped or slitted corrugated pipe is used instead of an alternative media material.
One gravelless drainfield design consists of large corrugated plastic pipes, with inside diameters of eight to 10 inches, covered with a geotextile fabric or a spun bonded nylon filter fabric. The effluent infiltrates the areas of the soil that come in contact with the fabric.
Gravelless pipe systems should include a cleanout to allow the pipe to be inspected and for any sludge that may accumulate in the pipe to be flushed out.
Chamber systems, which are discussed in more detail in next week's installment, are another popular type of gravelless system.
When Is a Gravelless System a Good Choice?
Like conventional drainfields, gravelless systems can be designed to work on a variety of homesites and under most conditions. Some studies suggest that they may perform better than gravel-lined drainfields. These systems also require the same maintenance as traditional drainfields.
Homeowners should first check with their local health department about local and state regulatory requirements. Also, most communities require a professional site evaluation be performed to assess the appropriateness of the site for any onsite system.
Why Gravelless May Be Better In Some Cases
Cost-Although gravel is a common natural resource and is usually fairly inexpensive, high-quality gravel is not readily available in every community. In certain parts of the country, the cost of transporting a heavy shipment of gravel can raise the cost of onsite system construction considerably. In these areas, using a lightweight or locally available alternative media may be preferable. However, in many areas, gravel-lined systems are still the most economical systems to construct.
Easier to handle-Because some alternative media materials are lightweight, heavy equipment may not be needed to haul the media to the installation site. This can help minimize the disruption to property that machinery can cause. Also, the lighter weight media is easier to handle, which can reduce labor costs and allow the systems to be constructed in areas inaccessible to heavy machinery. The pipe in gravelless pipe systems is light2weight and flexible, allowing it to easily conform to sloped or curved trenches.
Easier on Soils-Gravelless systems may be a good choice in areas that have soils susceptible to smearing or other structural damage. Soil structure can be impacted by the weight of gravel itself or by construction and heavy machinery on the site.
Constructing a typical gravel-filled drainfield requires several tons of gravel and numerous trips up and down the sides of trenches with a front loader or gravel truck. This can cause the soil around the drainfield to compact, reducing its permeability.
Avoids Dust and Fines-Another disadvantage of using improperly washed gravel as media is the possibility that dust and other fine materials from the gravel will find its way into the system as the gravel is dumped and shoveled during installation. The dust and debris can clog the soil. Problems with fines can be minimized or avoided completely by using such alternative media as plastic.
Simple to Maintain-In general, gravelless drainfields are as easy to maintain as gravel-lined drainfields. Many are proprietary designs, and therefore, homeowners should follow manufacturer recommendations concerning operation and maintenance.
Proper onsite system maintenance begins inside the home or business. Solids and greases, such as food scraps, coffee grounds and leftover cooking oils should be disposed of in the garbage rather than down the drain. Cigarette butts and items other than toilet paper should not be flushed down the toilet. Allowing extra solids and greases into the system can over-burden the septic tank. If the septic tank becomes too full, solids sometimes will be discharged into the drainfield. For this reason, onsite systems should be inspected regularly or pumped every three to five years.
In addition, system owners should take care to fix leaky faucets and running toilets immediately to conserve water and to avoid overloading the septic tank and the drainfield. It is important to maintain a grass cover over the drainfield to keep the soil adequately aerated. Also, homeowners should prevent anyone from driving or placing heavy materials on the drainfield.
The National small Flows Clearinghouse offers a separate Pipeline issue called Maintaining Your Septic System: A Guide for Homeowners, Item #SFPLNL03 for those interested in more information on this subject. The document can be obtained by calling 800-624-8301 and requesting it.
Sizing Gravelless Drainfields
The term infiltrative surface refers to places in a drainfield where effluent is absorbed into the soil. Some researchers believe that gravel and other media may mask as much as 50 to 75% of the potential infiltrative surface in a drainfield. In other words, by lying against the soil, the media may create areas where the effluent does not pass through into the soil and the biomat does not form.
Some gravelless system proponents claim that because less masking occurs with certain gravelless systems, these systems can be sized significantly smaller than conventional drainfields. While some state regulations do allow reductions in gravelless drainfield size, the U. S. Environmental Protection Agency and others advise caution before doing so due to a lack of sufficient scientific evidence backing up this practice and the increase of organic loading that may occur as a result. Instead, drainfield size should always be based upon the amount of wastewater flow the household generates and the amount of effluent the soil can effectively receive, called its hydraulic capacity.
Adding secondary treatment or a gravelless or chamber drainfield system does not change the overall hydraulic capacity of the lot. It may be possible to reduce the overall area of the trenches required with a system, but not the overall length of the system across the slope of a lot.
Pipeline magazine is published by the National Small Flows Clearinghouse, West Virginia University, Tel: 800-624-8301.
Coming up next: More on Site Evaluations
