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On-site wastewater treatment systems

Trickling filter

Soil absorption field

Septic tank Clarifier/Dosing tank

Figure 1: Trickling filters are a simple technology for treating wastewater. Trickling filter Bruce Lesikar and Russell Persyn Extension Agricultural Engineering Specialist, Extension Assistant-Water Conservation The Texas A&M University System

trickling filter is a bed of gravel or plastic media over which allows biological materials to pretreated wastewater is sprayed. In trickling filter systems, mi- settle out of the water. It also croorganisms attach themselves to the media in the bed and form houses a pump to dose water over A the top of the filter. a biological film over it. As the wastewater trickles through the media, ✓ the microorganisms consume and remove contaminants from the water. A trickling filter, which is a tank of media such as gravel or plastic Trickling filters were a common Each trickling filter system has material. Wastewater is distrib- technology for treating municipal several components: uted over the top of the media wastewater before cities began using and flows downward across the ✓ A septic tank, which removes the activated sludge aeration systems. media surface in a thin film. It settleable and floatable solids Now, homes and businesses use then exits the bottom of the tank from the wastewater. trickling filters in on-site wastewater and flows into the clarifier/dosing treatment systems. ✓ A clarifier/dosing tank, which is a tank. concrete or fiberglass tank that ✓ A land application system, which The microorganisms remove allow the biological material to have distributes the treated water under nutrients and dissolved materials from good aeration. The large openings the ground surface. the wastewater, storing them as food. also enable the biological material to As the biological material grows, it flow to the bottom of the filter after it Although trickling filters are a becomes too large to remain attached falls off the media so that it can exit simple technology for improving to the media and breaks away. It is into the clarifier/dosing tank. wastewater quality, few manufactur- carried with the water back into the The pump should be elevated ers sell them already built. Most clarifier/dosing tank, where it accu- above the bottom of the clarifier/ trickling filters are professionally mulates in the bottom of the tank, dosing tank to ensure that clear water designed and built by an installer. forming a sludge blanket. In some can circulate to the trickling filter. The systems, a sludge pump sends this According to Texas regulations, pump requires little horsepower material to the septic tank, where it wastewater from trickling filter because it lifts the water only from can decompose further. systems cannot be applied to the the clarifier/dosing tank to the top of ground surface. Texas allows only the trickling filter, about 10 feet. systems certified as class I aerobic Design treatment units or sand filters to apply The flow rate for the pump can be When choosing an appropriate wastewater onto the ground surface, fairly low, about 3 gallons per minute, trickling filter system for a site, you unless the system is specially de- depending on the dosing rate and the must consider several components: signed by a professional engineer for surface area of the filter. A valve on the area and volume of the filter surface application. Wastewater the pipe entering the top of the trick- surface; the type of media; the size of distributed by such systems must be ling filter allows the pump flow to be the pump; and the requirements for tested periodically to make sure it adjusted. operating the trickling filter. meets the quality requirements for The wastewater must be distrib- surface application. Trickling filters can handle from uted evenly over the media so that it 25 to 100 gallons of wastewater per can flow in a thin film down through Treatment square foot of filter surface per day. the media. The water can be sprayed They are usually designed to treat 50 over the top of the media or channeled Wastewater dosed to a trickling gallons per square foot per day. filter must be pretreated, such as by a through a pipe and dropped onto a septic tank. Solids and greases must The amount of biological splash plate, which is a plastic or be removed before the wastewater is material that a treatment system can fiberglass plate lying on top of the sprayed over the trickling filter. If handle per day is called the organic media. these materials are not removed, they loading rate. For trickling filters, it is Dosing to the trickling filter can measured in pounds of BOD per day can cover the thin layer of microor- 5 be continuous, or controlled with a ganisms growing on the media and per cubic foot. The organic loading timer. If the flow is continuous, the kill them. rate for a trickling filter is generally rate should be fairly low, about 3 from 0.005 to 0.025 pounds of BOD 5 gallons per minute, to allow the A trickling filter can reduce: per day per cubic foot of media. ✓ biological material that falls off the Biochemical oxygen demand The depth of the bed of media for media to settle in the clarifier/dosing (BOD ), a measurement of the 5 trickling filters can vary. The deeper a tank. If the flow is timer-controlled, amount of the dissolved oxygen trickling filter’s media, the more the system should be dosed often that microorganisms need to enough to prevent the biological BOD5 it can handle per day. Commu- decompose organic matter. High nity-scale trickling filters range from material from drying out. BOD normally indicates poor 5 3 to 8 feet deep. A home-scale water quality; a low BOD The pump should be connected to 5 trickling filter can be 2 to 3 feet deep. an on-off float in case the flow of generally indicates good water The depth chosen depends on the quality. Removing dissolved water is interrupted. Without an on- amount and strength of wastewater off float, the pump will run with no solids from the wastewater the system is expected to handle per lowers the BOD . water in the tank if flow is disrupted 5 day. ✓ from the home (such as when the Pathogens, or disease-causing The media in the trickling filter family goes on vacation) and the organisms. should be a porous material such as water evaporates from the trickling ✓ Fecal coliforms, or bacteria from rock or plastic. It should have a large filter, thus reducing the volume of human or animal wastes. surface area with large openings to water in the clarifier/dosing tank. The last step in the process is to apply the wastewater to the soil. For Splash plate gravity-flow systems, the wastewater flows by gravity through an outlet from the clarifier/dosing tank and into trenches in the drain field. If a pressurized land application Filter media system is needed, the wastewater flows from the clarifier/dosing tank into a pump tank, which collects the wastewater and then doses it to the drain field through a low-pressure dosing, subsurface drip or spray distribution system. Drain line How to keep it working To perform well, trickling filter systems require proper operation and maintenance. Please review the materials from the manufacturer or Figure 2: A trickling filter is a tank of media such as gravel or plastic material. designer to make sure you comply with their guidelines. You may have to have the filter of incoming wastewater by Trickling filter systems contain media removed and washed to managing the quantity of waste several components—a septic tank, reduce the amount of biological entering the system, such as by clarifier/dosing tank, trickling filter material on the media. Make sure discontinuing use of a garbage and land application field—working the outlet is large enough so that disposal or sending less grease together to improve the quality of the the wastewater can exit the filter. down the drain. effluent. For specific operation and ✓ Water not being dosed to the ✓ Biological growth being killed maintenance guidance on septic tanks trickling filter: Could be caused on the filter: Could be caused by and land application fields, see by failure of the pump, the on-off greases or solids entering the Extension publications on those float or the control panel, or filter, coating the biological topics. They can be ordered from the disruption of electrical power. growth and killing it; or, the Extension service and are available on wastewater could contain high Check these components to make the World Wide Web at amounts of cleaners, disinfectants sure they are functioning. http://agpublications.tamu.edu. or pesticides. ✓ Effluent water containing a Here are some common problems Check the septic tank to make high BOD concentration: with trickling filters, their possible 5 sure solids and grease are being Could be caused by the dosing causes and recommendations for retained. Evaluate your habits in rate to the filter being too low; or, remedies: the home to make sure you are the incoming water could be too not using too many cleaners or ✓ Standing water in the filter: strong. Could be caused by a plugged disinfectants, or continuous filter exit to the clarifier/dosing Raise the dosing rate by running disinfectants in the toilet bowl. tank or by a buildup of biological the pump longer or adjusting the Also, make sure you are not material in the filter. flow valve at the discharge to the disposing of solvents or pesti- filter surface. Lower the strength cides in the toilet. The On-Site Wastewater Treatment Systems series of publications is a result of collaborative efforts of various agencies, organizations and funding sources. We would like to acknowledge the following collaborators: Texas State Soil and Water Conservation Board USEPA 319(h) Program Texas On-Site Wastewater Treatment Research Council Texas Agricultural Extension Service Texas Natural Resource Conservation Commission Texas Agricultural Experiment Station USDA Water Quality Demonstration Projects Texas On-Site Wastewater Association Consortium of Institutes for Decentralized Wastewater Treatment USDA Natural Resources Conservation Service