Key Points

1. A trickling filter is an attached growth, aerobic biochemical operation consisting of five major components: (1) the media bed, (2) the containment structure, (3) the wastewater application (or dosing) system. (4) the underdrain, and (5) the ventilation system.

2. Oxygen is provided either by natural draft or forced draft ventilation. Natural draft ventilation occurs because of differences in the density of air inside and outside the trickling filter. Forced draft ventilation requires a motive force for air, such as a fan.

3. Trickling filter systems often include a clarifier to separate produced biomass, although it may not be needed in nitrification applications because of the low growth yield.

4. The liquid-flow pattern through a trickling can be characterized as plug-flow with dispersion. Consequently, a spatial distribution of microorganisms will develop with heterotrophic bacteria in the upper portion of the trickling filter and, if the organic loading is sufficiently low, autotrophic nitrifying bacteria in the lower portion.

5. Trickling filters are classified by their treatment objective: partial removal of organic matter (referred to as a roughing trickling filter), relatively complete removal of organic matter (carbon oxidation), combined carbon oxidation and nitrification, and separate stage nitrification. These applications are defined by the total organic loading (TOL) applied to the trickling filter and the characteristics of the wastewater.

6. Due to significant differences in performance and operational characteristics, trickling filter processes are also classified by media type.

7. Adequate pretreatment must be provided before wastewater is applied to a trickling filter. Most installations include primary clarifiers for this purpose.

8. Coupled trickling filter/activated sludge (TF/AS) systems consist of a trickling filter, a suspended growth bioreactor, and a clarifier. The trickling filter effluent passes directly into the suspended growth bioreactor without clarification. As a consequence, the biologies of the two biochemical operations interact directly.

9. Trickling filters can be used to treat a wide range of wastewaters. They are often used to treat high-strength, readily biodegradable wastewaters where they provide preliminary or roughing treatment prior to a suspended growth system. They can also be used for municipal wastewaters, although in some cases they must be coupled with a suspended growth bioreactor to produce the high-quality effluent required. Combined carbon oxidation and nitrification and separate stage nitrification can also be readily accomplished.

10. Trickling filter process loadings can be expressed as cither the total organic loading (TOL) or the surface organic loading (SOL). The TOL is expressed per unit of media volume (e.g., kg COD or BODs/(m'• day)), while the SOL is expressed per unit of media surface area (e.g., kg COD or BOD,/(nr • day)). Similar loadings can be defined for ammonia-N and total Kjeldahl nitrogen (TKN) for nitrification applications.

11. The total hydraulic loading (THL) is the applied flow rate per unit of cross-sectional area, and typically has units of m/hr. Application of a minimum THL is required to achieve effective use of all of the media. In some cases this requires recirculation of treated effluent. However, the recirculation rate is not a fundamental design parameter. Increased recirculation flow beyond the amount required to achieve the minimum THL will not improve performance.

12. For a constant TOL, trickling filter performance is affected by media depth only over a relatively narrow range. Performance improves as the depth is increased up to about 3 to 4 meters, but little improvement occurs as the depth is increased further.

13. Trickling filter performance is affected by the temperature of the wastewater flowing over the media. Because trickling filters are effective heat transfer devices, steps must be taken during winter operation to mitigate the cooling effects of the ambient air temperatures.

14. Trickling filters can use either rotary or fixed nozzle distributors. Rotary distributors possess significant performance and operational advantages, and they are used more frequently.

15. Natural draft ventilation systems are designed to provide sufficient vent and underdrain areas to allow adequate convective air movement for oxygen transfer. Forced draft ventilation systems utilize fans and duct systems to evenly distribute the needed air.

16. Trickling filter effluents may contain fine, colloidal suspended solids that settle poorly in conventional clarifiers. Coupled TF/AS systems provide greater control over these suspended solids by flocculating them for removal in the clarifier.

17. Process loading factors, such as the TOL and JS A, can be used to size trickling filters. Process performance data from either comparable full-scale applications or a pilot plant is used to select the appropriate loading factor.

18. The Velz/Germainigix equation is an empirical model that has been used frequently to size trickling filters. Because of its frequent use, a significant database exists to allow selection of appropriate model coefficients

19. The model of Logan et al.*"11 is a more fundamental trickling filter model. Experience with it is currently limited, and this must be considered when using it to size full-scale applications.

20. A trade-off is inherent in the design of a coupled TF/AS system. Use of a large trickling filter allows use of a small suspended growth bioreactor, and vice versa. Consequently, the trickling filter TOL and suspended growth bioreactor SRT must be selected together.

21. Oxygen requirements in coupled TF/AS systems can be determined by either fundamental process calculations or empirical correlations. In either case, the trickling filter reduces the oxygen requirement in the suspended growth bioreactor.

22. Improved trickling filter performance can be obtained by proper control of the biofilm thickness through periodic flushing. Flushing can be accomplished in numerous ways.

23. Significant operating economies can be achieved when a trickling filter system is designed with the flexibility to be operated in more than one mode.

24. Nuisance organisms that can proliferate in trickling filters include flies, snails, and worms. Problems caused by the growth of these organisms range from simple nuisance to reduced performance. Techniques available to control their growth include control of the THL, flushing, chlorination, and flooding.

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