Tiêu đề Module 4_Secondary treat_CEE 381.pdf ✅

5.7 UNITPROCESSESOFSECONDARY TREATMENT Overview The maj or purpose of secondary treatment is to remove the soluble BOD that escapes priman' treatment and to provide further removal of suspended solids. The basic
ingredients needed for conventional aerobic secondary biologic treatment are the availability of many microorganisms, good contact between these organisms and the organic material, the availability of oxygen, and the maintenance of other favorable environmental conditions (for example, favorable temperature and sufflcient time for the organisms to work). A variety of approaches have been used in the past to meet these basiq,ireeds. The most common approaches are (1) trickling filters, (2) activated slqdge, and (3) oxidation ponds (or lagoons). A process that does not fit precisely into either the ftickling filter or the ac- tivated sludge category but does employ principles common to both is the rolaling b io lo gic al c ontacto r (RBC). Tbickling Filters A trickling filter consists of a bed of coarse material, such as stones, slats, or plastic materials (media), over which wastewater is applied. Trickling filters have been a popular biologic treatment process.ls The most widely used design for many years was simply a bed of stones from 1 to 3 m deep through which the wastewater passed. The wastewateris typically distributed over the surface ofthe rocks by a rotating arm (Figure 5-15). Rock filter diameters may range up to 60 m. S€wa8e In Underdrain sFt6m Disinl€ction FIGURE 5-15 T.lckling-lilter plant with enlargement of trickling filter Rohry Distributor Recirculation
As the wastewater trickles through the bed, a microbial growth establishes il- selfon the surface ofthe stone or packing in a fixed film. The wastewater passes orer the stationary microbial population, providing contact between the microorganism_i and the organics. Trickling filters are not prima.rily a filtering or straining process as the name implies. The rocks in a rock filter are 25 to 100 mm in diameter, and hence har.e openings too large to strain out solids. They are a means ofproviding large amouns of surface area where the microorganisms cling and grow in a slime on the rocks as they feed on the organic matter. Excess growtls of microorganisms wash from the rock media and would cause undesirably high levels of suspended solids in the plant effluent if not removed. Thus, the flow from the filter is passed through a sedimentation basin to allow these solids to settle out. This sedimentation basin is referred to as a second.ary clarifer. or fnal clarifer, to differentiate it from the sedimentation basin used for primar.v settling. Although rock trickiing lilters have performed well foryears, they have certain limitations. Under high organic loadings, the slime growths can be so prolific that they plug the void spaces between the rocks, causing flooding and failure of the system. Also, the volurne of void spaces is limited in a rock filter, which restricts the circulation of air and the amount of oxygen available for the microbes. This limitation, in tum, restricts the amount of wastewater that can be processed. To overcome these limitations, other materials have become popular for filling the trickling filter These materials include modules of corrugated plastic sheets (Fig- ure 5-16) and plastic rings. These media offer larger surface areas for slime growths (typically 90 square meters of surface area per cubic meter ofbulk volume, as com- pared to 40 to 60 square meters percubic meter for 75 mm rocks) and greatly increase void ratios for increased air flow. The materials are also much lighter than rock (by a factor of about 30), so that the trickling filters can be much taller without facing structural problems. While rock in filters is usually not more than 3 m deep, synthetic
TABLE 5-9 Comparison of different types of trickling filterso Trickling fi lter classifi cation Design characteristics Low or Intermediate standard rate mte High rate Super mte (stone media) (plastic media) Roughing Hydraulic loading, m/d Organic loading, kg BOD5/d .m3 Recirculalion ratio Filter flies Sloughing Depth, m BODs removal, 7o Effluent quality 1to4 0.08 to 0.32 0 Many Intermittent 1.5 to 3 80 to 85 Well nirrified 15 to 90' 0.32 to 1.0 0to 1 Few Continuous Up to 12 65 to 85 Limited nitrification 60 to l80b Above l 0 I to4 Few Continuous I to6 40 to 65 No nitrification 4to l0 l0to40 0.24 to 0.48 0.32 to 1.0 otol lro3 Varies Few Varies Continuous l.5 10 2.5 I to 2 50 to 70 65 to 80 Some Nitrites nitrification 'Adapted from Joint Cornmjltee ofthe American Society ofCivil EngineeN and the Water Pollution Feder^tin\ WastNater Trcat- nent PLant Desisn(ASCE Manuals and Repods on Enginee.ing Praclice No. 36, WPCFManual of hacdce No. 8). Lancaster, PA: Lancasler Press, Inc., p. 285, 1977. 'r-ol including recirculation. media depths may reach l2 m, thus reducing the overall space requirements for the trickling-filter portion of the treatment plant. Trickling filters are classified according to the applied hydraulic and organic load. The hydraulic load may be expressed as cubic meters of wastewater applied per day per square meter of bulk filter surface area (m3/d ' m3) or, preferably, as the depth of water applied per unit of time (mm./s or m./d). Organic loading is expressed as kilograms of BOD5 per day per cubic meter of bulk filter volume (kgld m3). Common hydraulic and organic loadings for the the various filter classifications are summarized in Table 5-9. An impoftant element in trickling filter design is the provision for retum of a portion of the effluent to flow through the filter. This praclice is called recirculation. The ratio of the returned flow to the incoming flow is called rhe recirculqtion ratio (r). Recirculation is practiced in stone filters for the following reasons: 1. To increase contact efficiency by bringing the waste into contact more than once with active biological material. 2. To dampen variations in loadings over a 24-hour period. The strength oi the re- circulated flow lags behind that of the incoming wastewater. Thus, recirculation dilutes strong influent and supplements weak influent. 3. To raise the DO of the influent. 4. To improve distribution over the surface, thus reducing the tendency to clog and also reduce filter flies. 5, To prevent the biological slimes from drying out and dying during nighnime pe- riods when flows may be too low to keep the filter wet continuously.