What are Sand Filters, Common Sand Filter Designs, Many Factors Affect Sand Filter Performance, How Well Do Sand Filters Perform?,
Sand Filters Require Maintenance, How Much Do Sand Filters Cost?, Information
Thanks to a technology that was developed more than 100 years ago, homes, schools, businesses, and small communities in rural areas have an alternative system for wastewater treatment.
Sand filters treat wastewater using naturally occurring physical, biological, and chemical processes. They are one of the best options for additional onsite treatment where septic tank/soil absorption systems have failed or are restricted due to high groundwater, shallow bedrock, poor soils, or other site conditions. They also can be a good choice for homes, businesses, institutions, and small residential developments and communities in areas where centralized treatment is unavailable or too expensive.
Sand filters usually are used as the second step in wastewater treatment after solids in raw wastewater have been separated out in a septic tank, aerobic unit, or other sedimentation process. Wastewater treated by sand filtration is usually colorless and odorless. When discharged to soil, sand filter effluent receives further treatment in a soil absorption field, even at some sites where conventional septic tank/soil absorption systems, by themselves, cannot be used.
Over the years, sand filters have proven to be a reliable technology when they are properly designed, constructed, and maintained. Their performance is relatively consistent and they have low operation and maintenance requirements. In addition, overall treatment costs should compare favorably with other alternative systems.
What Are Sand Filters?
Sand filters are constructed beds of sand or other suitable granular material usually two to three feet deep. The filter materials (called media) are contained in a liner made of concrete, plastic, or other impermeable material. Depending on the design, the filter may be situated above ground, partially above ground, or below ground, and the filter surface may be single pass or covered. If covered, it should be vented to maintain aerobic conditions.
Partially treated wastewater is applied to the filter surface in intermittent doses and receives treatment as it slowly trickles through the media. The wastewater then collects in an underdrain and flows to further treatment and/or disposal. The pipes, pumps, and valves can be purchased commercially in a kit.
Sand filter units are constructed or assembled onsite by a contractor. Most materials are available locally, sometimes with the exception of filter media. If the appropriate media cannot be obtained nearby, it must be shipped in, which can greatly increase the filter's cost.
Suitable filter media can be purchased from aggregate companies or other suppliers. The media must be as clean and uniform in size as possible to allow the wastewater to flow correctly through it. If not, smaller grains will settle in the spaces between the larger grains, leaving no place for the wastewater to flow.
There are several different sand filter types and designs. Some are better suited for small communities, clusters of homes, large businesses, or institutions, while others are more appropriate for individual homes and small businesses.
Sand Filter Basics
There are a few basic operating and design principles common to every type of sand filter system. First, to prevent the filter from clogging, the wastewater must be pretreated to remove solids and scum. Pretreatment usually takes place in a septic tank, Imhoff tank, or aerobic unit. Screens or filters are sometimes used in the pretreatment tank as an additional step to ensure that no solids carry over to the filter in times of heavy water use.
After the solids have been removed, a pump equipped with an adjustable timing mechanism or a siphon doses the wastewater to the filter in timed intervals or when the tank becomes full. Applications are spaced intermittently to allow the filter media to drain between doses. This helps ensure that oxygen is introduced into the filter with every dose of wastewater. Oxygen is critical to the biological and chemical treatment processes that take place inside the filter.
It also is important that wastewater be applied evenly across the filter surface. This is accomplished either by flooding the surface completely with a thin layer of wastewater, or spraying the wastewater evenly over the filter surface.
How Treatment Occurs
As the wastewater percolates slowly through the filter media, natural physical, biological, and chemical processes combine to provide treatment. Most treatment occurs in the first 6 to 12 inches of the filter surface.
Some of the organic matter breaks down in the filter. Particles stick to grain surfaces or get caught in crevices or voids on grains or in spaces between grains. In addition, negatively charged grain surfaces can attract positively charged waste particles and bond with them through a process called adsorption. Chemical bonding also takes place as certain particles in the wastewater come in contact with and react with the media.
Sand filters accomplish much of their treatment through biological processes. Sand filters are home to a variety of organisms, many of which contribute to treatment by consuming organic matter in the wastewater. Bacteria are the most abundant organisms in the filters, and they do most of the work. There are other organisms, such as protozoa and worms, which also contribute to treatment. After the filter has had a chance to mature-usually after approximately two weeks use-a miniature ecological system develops as the organisms multiply and rely on each other to survive.
The most significant part of the filter ecosystem is a thick layer called the biomat, which eventually forms near the surface of the filter. This layer contains bacteria which consume particles in the wastewater. In turn, protozoa feed on the bacteria and help prevent the biomat from becoming so dense that it clogs the filter. This balance between the various life forms and the physical and chemical processes that take place in the sand filter results in extremely efficient wastewater treatment requiring minimal operation and maintenance. Eventually, the biomat becomes clogged, and the top layer of sand needs to be raked or removed as part of regular filter maintenance.
Why Consider Sand Filters?
Common Sand Filter Designs
There are many possible sand filter system designs. Three of the most common types of sand filters, buried, single pass, and recirculating, are intermittent designs. Intermittent sand filters receive and treat wastewater in doses. This publication highlights single pass and recirculating sand filters as appropriate technologies for Indiana.
Single Pass Sand Filters
Single pass sand filters-often called intermittent sand filters-are a practical option for treating wastewater from small communities, residential developments, recreational areas, shopping centers, and institutions. They are used most often for sources generating up to 120,000 gallons of wastewater per day.
Single pass sand filters are at least partially above ground. The entire filter unit is contained in an impermeable liner. Underdrain pipes and a graded layer of washed gravel or crushed rock are placed at the bottom of the filter bed-with the finer gravel on top of the coarser gravel to keep the media grains from washing into the underdrains. The filter media is then placed on top of the layer of fine gravel. As with all sand filters, the depth of the media depends on the size of the grains and other factors, but normally ranges from 24 to 36 inches.
Another graded layer of gravel is placed on top of the media bed and surrounds a network of distribution pipes. However, the order is reversed this time and the finer gravel is placed under the coarser gravel closest to the media bed. A geotextile fabric is placed on top of the entire filter bed.
Hydraulic loading rates for single pass filters typically range from two to five gallons per square foot per day and could be higher in some cases. Single pass filters receiving larger flows usually have two or more beds that can be operated in parallel or in series, allowing a portion of the filter to be rested at any time. Many single pass sand filters have removable covers to insulate them from extreme cold weather, minimize maintenance, and reduce the potential for odorse. To ensure safety, sand filters serving individual residences should not remain exposed.
Because some odor is released when septic tank effluent is dosed to the filter surface, single pass sand filters should be sited downwind and at least 100 feet from residences and businesses. Where this is impractical, a recirculating sand filter can be used instead.
Regular maintenance of the septic tank is essential to prevent solids from clogging the filter. Periodic maintenance is also important for screens, pumps, siphons, timers, and components of soil absorption fields.
Recirculating Sand Filters
In the late 1960s, two engineers with the Illinois Department of Health, Michael Hines and R.E. (Tony) Favreau, set out to solve a common problem facing their area. Although sand filters were the best pretreatment option for many local sites with poor soils or other limiting conditions, odor associated with them made them unsuitable for more developed areas. In response to this challenge, they developed the recirculating sand filter, which has since been widely adapted and used ever since for homes, schools, businesses, recreational areas, and small communities.
Recirculating sand filters reduce odors by ensuring an adequate supply of oxygen to the wastewater. Wastewater flows by gravity from a septic tank to a recirculation tank, which is equipped with a pump, a timing mechanism, and float valves. The wastewater is pumped to the filter when the wastewater reaches a certain level in the tank or in timed doses.
After receiving treatment in the sand filter, the wastewater collects in underdrains and a portion of it is directed back to the recirculation tank, where it mixes with the septic tank effluent and is recirculated to the sand filter. The remaining sand filter effluent goes directly to further treatment or disposal.
From three to five part of sand filter effluent is recirculated for each part that passes to disposal. How this amount is controlled varies with individual system designs. Weirs, moveable gates, and other devices can be used to direct part of the flow from the sand filter underdrains back to the recirculation tank. The net result is that the wastewater applied to the sand filter contains less organic matter and more oxygen than normal septic tank effluent, reducing odors. The final sand filter effluent also is of higher quality and typically ranges from 2 to 5 mg/L BOD, and from 3 to 5 mg/L TSS.
Recirculating sand filter media ranges from an effective size of 0.8 mm to 3 mm, which is somewhat coarser than sand filter media used in single pass filters and, therefore is less prone to clogging. Hydraulic loading rates typically range from three to five gallons per square foot per day, meaning that less land area is needed to treat the same amount of wastewater than with other sand filter designs. Energy and routine maintenance requirements are more than for single pass sand filters but less than is required for extended aeration package plants.
A drawback to recirculating sand filters is they are more sensitive to cold temperatures and more prone to freezing than systems that are regularly dosed with warm septic tank effluent. This problem sometimes can be offset by adjusting the dosing frequency and the recirculation ratio or by covering the sand filter bed.
Many Factors Affect Sand Filter Performance
Pretreatment is very important to sand filter performance. Most of the solids in the wastewater must be removed to prevent them from clogging the filter. Septic tanks, aerobic units, screens, and other pretreatment units must be designed and operated properly and receive regular maintenance. Septic tanks are the most common and usually the least expensive pretreatment method for sand filters. It is very important that all tanks be watertight. The system hydraulics have been engineered for a specific flow rate. Tanks allow groundwater infiltration often lead to rapid system failure. Tank integrity should be tested and/or certified before operation begins.
Composition, size, uniformity, and depth of the media all affect sand filter performance. If sand is not available locally, other materials, such as crushed glass, anthracite, garnet, mineral tailings, or bottom ash, have been used for filter media. Characteristics of the media's composition, such as its solubility, acidity, and hardness, must be considered in the filter design. It also is extremely important that the media be washed. Media should be inspected for cleanliness by a qualified individual before it is used in the filter.
The size and uniformity of the grains also affect performance, filter depth, and the amount of wastewater that can be treated at one time. The media grains are sorted and measured through a series of mechanical sieves. The grains must be relatively uniform in size to prevent clogging. "Effective size" and "uniformity coefficient" are measurements used to express these characteristics. Effective sizes for sand filter media range from 0.3 mm to 3 min in diameter. Each sand filter type has its own media size range requirements. A uniformity coefficient of four or less is recommended for all filter media.
In addition, the media should be neither too coarse nor too fine. Coarse media allows wastewater to pass too quickly through the filter without receiving adequate treatment, while very fine media slows down treatment too much, is prone to clogging, and can keep oxygen from reaching certain parts of the filter.
Filter depth depends on the type of filter and media size, but normally ranges from 24 to 36 inches. Most treatment occurs in the first 6 to 12 inches, but a few additional inches improves overall performance, pathogen removal, and allows for maintenance.
Organic Loading Rate
Organic loading rate depends on the strength of the wastewater. Strong wastewater containing high levels of organic material can reduce the filter's performance over time and increase the need for maintenance.
Hydraulic Loading Rate
Hydraulic loading rate is the amount of wastewater applied to the filter in one day. Sand filters are less effective at removing certain pathogens and other wastes from wastewater at high hydraulic loading rates. The appropriate rate is determined based on the dosing pattern, the size of the media, and the organic loading rate.
Dosing Method and Frequency
Careful dosing and even distribution of the wastewater across the filter surface are needed to ensure consistent treatment. Uneven distribution can cause one part of the filter to become overloaded, and wastewater can be flushed through the filter before receiving adequate treatment. Too frequent dosing causes similar problems. Doses should be spaced to allow the filter adequate time to drain and reaerate.
Climate and Temperature
All wastewater treatment methods that rely on natural processes are affected by temperature. Treatment slows down in cold temperatures, so organic loading rates must be lower to maintain adequate treatment. Freezing can be a problem with sand filters . Adjusting hydraulic loading rates and dosing frequencies can improve filter performance in low temperatures.
Typical Design Values for Sand Filters
|Design Factor||Single pass||Recirculating|
|Pretreatment||Settling or other removal of solids||Settling or other removal of solids|
|Material||Washed, durable granular material||Washed, durable granular material|
|Effective size||0.3-1 mm||0.8-3 mm|
|Depth||24-36 inches||24-36 inches|
|Hydraulic Loading||2-5 gpd/ft2||3-5 gpd/ft2 (based on forward flow only)|
|Organic Loading||<5 x 10-3 lbs. BOD5/day/ft2||<5 x 10-3 lbs. BOD5/day/ft2|
|Media Temperature||>5o C||>5o C|
|Dosing Frequency||>2 per day||5-10 min. everu 30 min.|
|Recirculation Ratio||NA||3:1 to 5:1|
How Well Do Sand Filters Perform?
Depending on the system design, sand filters are capable of reducing five-day biochemical oxygen demand (BOD) and total suspended solids (TSS) in wastewater to 10 milligrams per liter or less. Both BOD5 and TSS are indicators used by regulatory agencies to assess treatment and its potential impact on the environment. BOD5 is a measure of the amount of oxygen microorganisms need to consume and break down organic matter. TSS is a measure of the amount of waste particles suspended in the wastewater.
Sand filters remove many pathogens, such as viruses and harmful bacteria. However, further treatment, such as passage through a soil absorption field, is necessary before the effluent can safely be returned to the environment. One disadvantage of sand filters is that they are not very effective at removing phosphorus from wastewater.
Sand filters are not permitted in all areas. Check with your local health department for permit requirements in your area.
Sand Filters Require Maintenance
Like all on-site systems, sand filters require regular maintenance. Routine maintenance of single pass and recirculating sand filter beds includes periodic leveling and raking the surface, and raking or removing the surface layer when it begins to clog. How often clogging occurs depends on organic loading rates and the filter media size. For example, sand filters receiving septic tank effluent may need more frequent attention than those receiving aerobic unit effluent, because the organic strength of the septic tank effluent is higher. Because recirculating sand filters use coarser media and receive lower organic loading rates, they tend to clog less frequently.
Sometimes simply raking the filter surface will not suffice and the top one-inch layer of media must be removed. Most sand filters are designed to be deep enough to allow several layers to be removed before the media needs to be replenished or replaced. Filter beds that are exposed to sunlight should be weeded regularly and may develop algal mats that need to be removed.
Pretreatment tanks need to be inspected and pumped, and electrical components, such as pumps and timers need to be checked and serviced according to manufacturer recommendations. Pumps often are designed to last from 10 to 25 years, but eventually need to be replaced. Pipes, valves, and other system components need to be checked regularly, and screens and filters need to be cleaned.
Many larger sand filters are operated in sections to allow portions of the filter bed to rest by switching the sections to be dosed. It also is sometimes necessary to regulate hydraulic loading rates to prevent the filter from being overloaded or to prevent the filter surface from freezing.
Another cold-weather operating strategy entails raking the filter bed in a pattern of ridges and furrows and flooding the surface until an ice sheet forms. The filter can then be loaded below the insulating sheet of ice.
General maintenance requirements for both single pass and recirculating sand filters are summarized in the table below.
Sand Filter Maintenance
|Pretreatment||Depends on process (septic tank, aerobic unit, etc.)|
|Pumps and controls||Check every 6 months.|
|Timer sequence||Check and adjust every 6 months.|
|Appurtenances||Check every 6 months.|
|Raking||Check every 6 months. If drainage time between doses has increased significantly, rake top 3 in. (for surface filters only).|
|Replacement||Skim media when heavy incrustations occur. Add new media when depth falls below 24 in. Rest when ponded continuously. Replace top 2-3 in. media when surface ponds more than 12 in. deep. Rest while alternate unit in operation (60 days).|
|Other||Weed as required, Maintain distribution device as required, Protect against ice sheeting on the surface of the filter, Check high water alarm (for single pass sand filters only).|
How Much Do Sand Filters Cost?
Exact costs for sand filter construction, operation, and maintenance depend on the filter design and local costs for labor and materials. Costs for pretreatment and additional treatment and disposal also need to be factored in when evaluating the overall system costs.
Construction of the sand filter units themselves usually is economical because the filters can be constructed or assembled onsite using local labor and materials. The two most significant factors that affect the cost of sand filter treatment are land and media costs. In areas where media is expensive or needs to be hauled a long distance, costs are much higher.
Operating costs include electricity used by the pump, and the cost for inspections and maintenance.
Initial costs for sand filters are sometimes higher than those for extended aeration package plants and other treatment options. However, in the long-run sand filters' low energy costs, lower operating costs, and dependable high performance often make them an attractive choice.
Note: At the time of publication, sand filters are permitted on an experimental basis in Indiana. They have been successful in neighboring states for both new construction and on-site system repair. Sand filters appear to be a good option for many sites and soils in Indiana. Experience and a method to ensure maintenance are needed prior to widespread use. (November, 1997)
Health Departments: If you would like more information about sand filters or are interested in utilizing one, contact your local health department or the Indiana State Department of Health at (317) 233-7177 for assistance. (Local health department phone numbers are usually listed in the government section of local phone directories.)
National Small Flows Clearinghouse (NSFC): The National Small Flows Clearinghouse (NSFC), which specializes in on-site technology, operation, maintenance, regulations, management, finance, and education, has a variety of free and low-cost products available. NSFC can be reached at (800) 624-8301.
Extension Service: Extension service offices can provide assistance and information about many of the wastewater treatment issues discussed. To locate the extension office in your area, call Purdue University at (888) 398-4636, the U.S. Department of Agriculture at (202) 720-3377, or NSFC.
Pipeline, Summer, 1997. Vol. 8, No. 3.
*Catherine Taylor and Joseph Yahner, Agronomy (765) 496-3454, Don Jones, Agricultural Engineering (765) 494-1178, Purdue University and Alan Dunn, Indiana State Department of Health (317) 233-7177. The On-Site Project is in cooperation with the Indiana State Department of Health.
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