Trickling filter

Secondary treatment Activated sludge Trickling filter- Trickling filter-... 

Where there are large volumes of contaminated water under a small site, it is sometimes most convenient to treat the contaminant in a biological reactor at the surface. Considerable research has gone into reactor optimization for different situations and a variety of stirred reactors, fluidized-bed reactors, and trickling filters have been developed. Such reactors are usually much more efficient than in situ treatments, although correspondingly more expensive. 

Air. Biofilters are an effective way of dealing with air from industrial processes that use halogenated solvents such chloromethane, dichioromethane, chloroethane, 1,2-dichloroethane and vinyl chloride, that support aerobic growth (26). Both compost-based dry systems and trickling filter wet systems are in use. Similar filters could be incorporated into pump-and-treat operations. 

Fig. 17. Pretreatment of organic wastewater on trickling filters. Industry types for which coordinates have been plotted are 1, kraft pulp and paper 2, mixed iadustry 3, wet com milling 4, dairy 5, tannery 6, meat packiag 7, food 8, pharmaceutical 9, refinery and 10, textile.

Anaerobic Filter. The anaerobic filter is similar to a trickling filter ia that a biofilm is generated on media. The bed is fully submerged and can be operated either upflow or downflow. For very high strength wastewaters, a recycle can be employed. 

Trickling Filters. The so-called trickling filter is not a filter but a bed of stones or other coarse material (packing) over which the sewage flows. In terms of the total number of installations, it is the most widely used biological treatment process. However, the greatest total volume of waste is treated by the activity-sludge process (12). 

The importance of the final settling, or humus, tank can be seen by an examination of what occurs in the trickling filter itself. A new filter is broken in by applying settled sewage as in the normal operation. After a period of time the microbial, or zoogleal, mass forms on the filter medium and stabilizes the waste. Waste material is first adsorbed, and then assimilated by the microorganisms. 

A portion of the effluent is recirculated, ia order to smooth out flow, keep the food concentration constant, lower film thickness and control psychoda flies, and reseed the appHed sewage with acclimatized organisms. The psychoda, or filter fly is a very small iasect that breeds ia thick trickling-filter slimes. It does not bite, but can be a nuisance. Its radius of flight is small, but it can be carried great distances by the wiad. The fly can be controlled ia the development phase by occasional flooding of the filter or chlorination of the appHed sewage. 

The floe returned to the aeration tank (10—50%) has the same function as trickling-filter slime. However, the floe concentration can be varied as operational needs dictate. Usually the so-called mixed Hquor—suspended soHds (MLSS) are 600—4000 mg/L (2.3—15 g/gal). 

The four most common aerobic biological treatment processes are stabilization ponds, aerated lagoons, trickling filters, and activated sludge (see... 

Slurry Reactors with Mechanical Agitation The catalyst may be retained in the vessel or it may flow out with the fluid and be separated from the fluid downstream. In comparison with trickle beds, high heat transfer is feasible, and the residence time can be made veiy great. Pressure drop is due to sparger friction and hydrostatic head. Filtering cost is a major item. 

Aerobic systems including trickling filters and rotating biological contactors (RBC) are operated in a nonflooded mode to ensure adequate oxygen supply. Other aerobic, anoxic, and anaerobic systems employ flooded reactors. The most common systems are packed beds (anaerobic trickling filter) and fluidized or expanded bed systems. 

Plastic packing of very low pressure drop (just greater than wood slats), transfer Coefficients about same as 2-inch Raschig rings. Most useful applications in gas cooling systems or biological trickling filters. 

High-rate filtration Oxidation, see Activated sludge. High-rate filtration. Trickling filter Reduction, see Anaerobic digestion Phenol removal... 

As a result of several studies, the following conclusions regarding viruses in sewage warrant consideration (1) primary sewage treatment has little effect on enteric viruses (2) secondary treatment with trickling filters removes only about 40 percent of the enteroviruses (3) secondary treatment by activated sludge treatment effectively removes 90 percent to 98 percent of the viruses and (4) chlorination of treated sewage effluents may reduce, but may not eliminate, the number of viruses present. 

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