The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the ...
Onsite wastewater treatment and disposal system incorporates sand filters with chambers
Suburban and rural communities are continually faced with the challenge of growth and the need for improved solutions for wastewater treatment.
Yet, due to budgets and their decentralized population base, they often do not have the number of treatment options available to them or are simply unable to pursue the same types of treatment, as can the large urban areas. These communities often rely on traditional decentralized technologies to solve their wastewater challenges.
These traditional technologies, often upgraded with new design and technology, provide quality treatment and disposal without the capital cost of sewering and centralized treatment plants.
One such decentralized approach that has stood the test of time is the use of sand filters for onsite wastewater disposal.
This approach is one of the oldest decentralized solutions still in use today. Although the concept has remained the same, the sand filter itself is now often enhanced and its efficiency is improved.
This has been accomplished by adding advanced technology to the mix such as plastic leaching chambers. These onsite systems, if constructed and maintained properly, provide reliable and effective treatment and disposal of wastewater at a relatively low cost.
The sand filter of today might not look exactly like the original wastewater treatment workhorse, but it operates on essentially the same principal.
There are two basic types of sand filters that are commonly used: intermittent sand filters and recirculating sand filters. Within those categories, many different options for design and installation are possible depending on usage, environmental concerns, and local codes.
A common inclusion to these designs today is plastic leaching chambers. Plastic leaching chambers are an enhancement that increases infiltrative surface area, storage capacity and gas exchange. Due to these improvements over traditional materials the efficiency and treatment capability can be improved. Intermittent and recirculating sand filters are similar in construction. They consist of a layer of stone on the bottom to collect the treated effluent, typically followed by 2 ft of specified sand and a distribution system placed over that. Chambers can be installed in two locations: as the distribution system on top of the sand filter and/or as the collection system installed in the bottom of the sand filter.
For effluent disposal, typically a distribution network of piping is installed within a bed of stone.
Very often chambers are installed in lieu of the stone bed. This practice has become common due to the recognition of the many downsides of using stone. These include the presence of fines, which will wash off the stone and move to the infiltrative surface thereby reducing the infiltration rate.
Another downside related to stone is that the heavy construction equipment required to place the stone could damage the filter material causing unequal densities and therefore unequal surface loading. A benefit of chambers in this application is that the spray pattern within the chambers provides a much better, more even distribution pattern than the stone bed.
The chamber itself is a large open cavity. When vented, it provides improved oxygen transfer to the surface of the sand filter. This helps prevent the surface of the sand filter from clogging. The open soil surface provided by the chamber is a much more efficient surface than a surface covered with aggregate. When septic tank effluent is applied to the surface, university studies confirm that the throughput rate is two times larger giving designers a greater factor of safety.
The collection system is also made up of a piping network embedded in stone. This too can be replaced with chambers. The benefit is that these chambers have a much better oxygen transfer rate then the stone system. This keeps the whole sand filter bed aerated and allows for better treatment.
Profiling the Heli Ski Lodge application
The Heli Ski Lodge is located in a remote, pristine area of the western Rocky Mountains of British Columbia.
The owners, Canadian Mountain Holidays, operate 13 lodges in this environmentally sensitive area. With increasing visitors from year-round usage the old traditional septic systems were experiencing failure. The owners needed to find an effective, environmentally sound solution to repair the existing systems in order to continue operations. This was particularly critical because some of the systems discharged directly to glacier-fed pristine streams.
The first two sand filters were installed with a white stone on the surface for the disposal area.
In a short period of time, there has been a noticeable buildup of a black substance (bacteria, or spores and fungi native to the alpine environment) on the white stone. This was aesthetically displeasing and unacceptable to guests coming to visit the pristine environment.
Ron Parker, a project engineer for Eagle Engineering/OSI, reviewed the situation and designed a new sand filter using the latest technology to address these concerns. In addition to designing and building the system, Eagle Engineering provided operation and maintenance training to the maintenance staff.
The new chamber sand filter system is capable of handling a design flow of 8,400 gpd and provides an effluent quality of 10-10 BOD-TSS. Sand filters have been constructed for three separate lodges (in three locations). The three sand filters vary in dimensions to best fit each site and the latest one measures 20 x 126 ft. Equalizer 24 plastic leaching chambers from Infiltrator Systems, Inc., were designed for the collection of the treated effluent. These chambers also address one design flaw of traditional stone and pipe collection systems within the sand filter. The fines and pea gravel were migrating and plugging the collection pipes. Infiltrator Standard H-10 chambers were installed over the specified sand media for dispersal to address the aesthetic problems encountered with the previous designs with the white stone. The H-10’s were covered with 1 ft of stone for insulation required for this harsh environment which is over 6,000 ft in elevation.
The chamber sand filter system installed at the Heli Ski Lodge was chosen for its many improvements in design qualities, simplicity of design and ease of installation. The owners plan to conduct further upgrades to the remaining lodge systems in the future.