The filter is usually 1 to 2.5 m deep, but filters packed with lighter plastic filling can be up to 12 m deep. The ideal filter material is low cost and durable, has a high surface to volume ratio, is light, and allows air to circulate. Whenever it is available, crushed rock or gravel is the cheapest option. The particles should be uniform and 95% of them should have a diameter between 7 and 10 cm. A material with a specific surface area between 45 and 60 m2/m3 for rocks and 90 to 150 m2/m3 for plastic packing is normally used.
Larger pores (as in plastic packing) are less prone to clogging and provide for good air circulation. Primary treatment is also essential to prevent clogging and to ensure efficient treatment. Adequate air flow airflow is important to ensure sufficient treatment performance and prevent odours. The underdrains should provide a passageway for air at the maximum filling rate. A perforated slab supports the bottom of the filter, allowing the effluent and excess sludge to be collected. The trickling filter is usually designed with a recirculation pattern for the effluent to improve wetting and flushing of the filter material.
With time, the biomass will grow thick and the attached layer will be deprived of oxygen; it will enter an endogenousstate, will lose its ability to stay attached and will slough off. High-rate loading conditions will also cause sloughing. The collected effluent should be clarified in a settling tank to remove any biomass that may have dislodged from the filter. The hydraulic and nutrient loading rate (i.e., how much wastewater can be applied to the filter) is determined based on the characteristics of the wastewater, the type of filter media, the ambient temperature, and the discharge requirements.
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===Appropriateness===
This technology can only be used following primary clarification since high solids loading will cause the filter to clog. A low-energy (gravity) trickling system can be designed, but in general, a continuous supply of power and wastewater is required. Compared to other technologies (e.g., [[Waste Stabilization Pond|Waste Stabilization Ponds]], T.5), trickling filters are compact, although they are still best suited for peri-urban or large, rural settlements. Trickling filters can be built in almost all environments, but special adaptations for cold climates are required.
===Health Aspects/Acceptance===
