What is Trickling Filters in Sewage Treatment Plant?

Trickling filters are an established technology for secondary treatment in sewage treatment plants. As attached growth processes, they facilitate microorganism communities that aerobically degrade organic matter within wastewater, trickling over their surface area. While overshadowed by activated sludge systems in recent decades, trickling filters offer distinct advantages, including low operating costs, energy efficiency, and simplicity. As circular or rectangular beds filled with non-submerged plastic or stone filter media, trickling filters provide a robust, reliable secondary treatment solution still prevalent in many wastewater treatment plants today.

Principles of Trickling Filter Treatment

In the trickling filter process, pre-settled wastewater is continuously distributed and sprayed over the top surface area of the filter bed media. As the water trickles downwards, microbes in the biologically active slime layer (biomass) attached to the media surfaces absorb and metabolise organic compounds. Air circulates through the porous bed, providing oxygen to sustain aerobic conditions and allow the microorganisms to thrive. Sloughed-off biomass and treated humus are collected in an underdrain system below the media for discharge to a clarification step, separating it from the liquid effluent stream.

Classic Stone and Plastic Media Designs

The original trickling filter media consisted of coked coal, rocks or specially moulded ceramic materials in deep beds up to 10 meters, providing immense surface areas for biofilm growth. Rotating distribution arms sprayed settled sewage over the bed's top surface area. Advances shifted to lighter-weight plastic media in variously shaped modular pieces. Common random plastic media designs include cross-flow with high void ratios, vertical-flow for high-strength wastes, and composite-loop variations. Different surface properties and void configurations optimise attached biomass thickness while preventing clogging.

Advanced Structured Sheet Media 

More recently, structured sheet media composed of continuous crimped synthetic sheets provides exceptional treatment performance in a compact footprint. The sheets are vertically oriented and properly spaced, yielding higher active surface areas per unit volume than stone or random media. Specific surface areas up to 240 m2/m3 boost organic loading capabilities. The sheets utilisespecialised surface properties tailoring microbial communities' attachment. Air is uniformly distributed through the bed's open bottom. Covered ammonia removal trickling filters even achieve partial nitrification for nutrient treatment. Structured media minimises hydraulic headloss and improves flushing for self-cleaning compared to traditional media.

Process Considerations and Enhancements

While effective for removing carbonaceous BOD, trickling filters achieve limited nutrient removal on their own. Combined activated sludge-trickling filter designs utilise the filters for roughing, followed by liquid-stream aeration basins that facilitate nitrification. Separating treatment allows controlling concentration independently in each stage. Trickling filter recirculation permits increasing hydraulic loading for enhanced performance. Plastic media accommodates higher recirculation ratios than stone media. Dual-stage trickling filter setups provide buffering capacity during peak flows. Various attached-growth enhancements like anoxic garland reactors for denitrification can supplement trickling filters as well.

Applications and Design Considerations

Due to their passive operation, trickling filters are well-suited for smaller treatment plants and decentralised wastewater systems. They require less technical oversight than activated sludge. However, clogging risks, flies/odours, and sensitivity to temperature fluctuations necessitate monitoring. Factors like hydraulic and organic loading, recirculation, ventilation, and dosing rates govern trickling filter sizing and performance. Energy efficiency stems from avoiding the aeration requirements of activated sludge, although odour control and recirculation pumping consume power. Trickling filters retain advantages mainly where land area and capital costs are limited. 

Conclusion

As a proven, reliable wastewater treatment technology, trickling filters remain relevant in modern sewage treatment plant designs worldwide. Their passive attached growth operation using plastic media provides economical organics removal even for relatively small or remote facilities. Compact structured sheet media innovations enable unprecedented high-rate treatment capacity in a small footprint. When integrated intelligently with other activated sludge or nutrient removal operations, trickling filters shine as exceptionally robust, energy-efficient secondary treatment solutions. While advanced processes garner more attention, the trickling filter remains an important part of the wastewater engineer's toolkit for meeting any discharge requirement or reuse quality target.

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