Tab. 8-I Trickling filter fsbrical classification of trickling filters applications designs are classified ow. or by hydraulic or organic loading rates ipe of pocking Rock Rock Rock looding 40-200 have been classified anic looding 0.07-0.22 0.24-0.48 0.4-2.4 standard-rate tortulofion ratio intermediate-rate Continuous Continuous and high-rate. Plastic 0.9-6 mo removal efficiency, packing is used -90 40-70 品 ent quality No nitrification No nitrification 10-20 however packing has also been used at lower organic loadings, near the high end of those used for intermediate-rate rock filters. Much higher organic loadings have been used for rock or plastic packing designs roughing"applications where only partial BOD removal occurs Low-Rate Filters. A low-rate filter is a relatively simple, highly dependable device that produces an effluent of consistent quality with an influent of varying strength The filters may be circular rectangular in shape. Generally, feed flow from a dosing tank is maintained by suction level controlled pumps or a dosing siphon. Dosing tanks are small, usually with only a 2-min detention time based or twice the average design flow, so that intermittent dosing is minimized. Even so, at small plants, low nighttime flows may result in intermittent dosing and recirculation may be necessary to keep the packing moist. If the interval between dosing is longer than 1 or 2 h, the efficiency of the process deteriorates because the character of the biological slime is altered by a lack of moisture In most low-rate filters, only the top 0.6 to 1. 2 m of the filter packing will have appreciable biological slime. As a result, the lower portions of the filter may be populated by autotrophic nitrifying bacteria, hich oxidize ammonia nitrogen to nitrite and nitrate forms. If the nitrifying population is sufficiently well established, and if climatic conditions and wastewater characteristics are favorable, a well-operated low rate filter can provide good BOD removal and a highly nitrified effluent With a favorable hydraulic gradient, the ability to use gravity flow is a distinct advantage. If the site is too flat to permit gravity flow, pumping will be required. Odors are a common problem, especially if the wastewater is stale or septic, or if the weather is warm. Filters should not be located where the odors would create a nuisance. Filter flies(Psychoda) may breed in the filters unless effective control measures are used8-4 Trickling filter designs are classified by hydraulic or organic loading rates. Rock filter designs have been classified as low- or standard-rate, intermediate-rate, and high-rate. Plastic packing is used typically for high-rate designs; however, plastic packing has also been used at lower organic loadings, near the high end of those used for intermediate-rate rock filters. Much higher organic loadings have been used for rock or plastic packing designs in "roughing" applications where only partial BOD removal occurs. Low-Rate Filters. A low-rate filter is a relatively simple, highly dependable device that produces an effluent of consistent quality with an influent of varying strength. The filters may be circular or rectangular in shape. Generally, feed flow from a dosing tank is maintained by suction level controlled pumps or a dosing siphon. Dosing tanks are small, usually with only a 2-min detention time based on twice the average design flow, so that intermittent dosing is minimized. Even so, at small plants, low nighttime flows may result in intermittent dosing and recirculation may be necessary to keep the packing moist. If the interval between dosing is longer than 1 or 2 h, the efficiency of the process deteriorates because the character of the biological slime is altered by a lack of moisture. In most low-rate filters, only the top 0.6 to 1.2 m of the filter packing will have appreciable biological slime. As a result, the lower portions of the filter may be populated by autotrophic nitrifying bacteria, which oxidize ammonia nitrogen to nitrite and nitrate forms. If the nitrifying population is sufficiently well established, and if climatic conditions and wastewater characteristics are favorable, a well-operated low rate filter can provide good BOD removal and a highly nitrified effluent. With a favorable hydraulic gradient, the ability to use gravity flow is a distinct advantage. If the site is too flat to permit gravity flow, pumping will be required. Odors are a common problem, especially if the wastewater is stale or septic, or if the weather is warm. Filters should not be located where the odors would create a nuisance. Filter flies (Psychoda) may breed in the filters unless effective control measures are used. Tab. 8-1