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A Trickling Filter is a fixed bed, biological filter that operates under (mostly) aerobic conditions. Pre-settled wastewater is ‘trickled’ or sprayed over the filter. As the water migrates through the pores of the filter, organics are degraded by the biomass covering the filter material.
The Trickling Filter is filled with a high specific surfaceareamaterial such as rocks, gravel, shredded PVC bottles, or special pre-formed filtermaterial. Amaterial with a specific surface area between 30 and 900m2/m3 is desirable. Pre-treatment is essential to prevent clogging and to ensure efficient treatment. The pre-treated wastewater is
‘trickled’ over the surface of the filter. Organisms that grow in a thin biofilmover the surface of themedia oxidize the organic load in the wastewater to carbon dioxide and water while generating new biomass.
The incoming wastewater is sprayed over the filter with the use of a rotating sprinkler. In this way, the filter media goes through cycles of being dosed and exposed to air. However, oxygen is depleted within the biomass and the inner layers may be anoxic or anaerobic.
The filter is usually 1 to 3m deep but filters packed with lighter plastic filling can be up to 12m deep. The ideal filter material has a high surface to volume ratio, is light, durable and allows air to circulate. Whenever it is available, crushed rock or gravel is the cheapest option. The particles should be uniform such that 95% of the particles have a diameter between 7 and 10cm.
Both ends of the filter are ventilated to allow oxygen to travel the length of the filter. A perforated slab that allows the effluent and excess sludge to be collected supports the bottom of the filter.
With time, the biomass will grow thick and the attached layer will be deprived of oxygen; it will enter an endogenous state, 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.
This technology can only be used following primary clarification since high solids loading will cause the filter to clog. A skilled operator is required to monitor and repair the filter and the pump in case of problems. 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. WSPs), trickling filters are compact, although they are still are best suited for peri-urban or large, rural settlements.
Trickling Filters can be built in almost all environments, although special adaptations for cold climates are required.
The odour and fly problems require that the filter be built away from homes and businesses. There must be appropriate measures taken for pre-treatment, effluent discharge and solids treatment, all of which can still pose health risks.
The sludge that accumulates on the filter must be periodically washed away to prevent clogging. High hydraulic loading rates can be used to flush the filter.
The packing must be kept moist. This may be problematic at night when the water flow is reduced or when there are power failures.[1]
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