Design Of Large Treatment Plants

Although the design details for large sewage treatment plants are beyond the scope of this text, some of the major design elements are presented in the following sections for general information. Also, state and federal regulatory agencies have recommended standards and guidelines and should be consulted.64 Treatment processes and bases of design are summarized in Figure 3.15 and Table 3.21, while process efficiencies are given in Table 3.22. Typical flow diagrams are shown in Figures 3.12 and 3.13.

Larger sewage treatment plants should be designed for a population at least 10 years in the future, although 15 to 25 years is preferred, and a per capita flow of not less than 100 gpd. Where available, actual flow studies, population trends, zoning, and growth potential should be used. Plants should be accessible from highways but as far as practical from habitation and sources of water supply, and protected from a 100-year flood. The required degree of treatment should be based on the water quality standards and objectives established for the receiving waters and other factors, as noted previously.

FIGURE 3.15 Conventional sewage treatment unit processes. Tertiary treatment may include denitrification, phosphorus removal; coagulation, sedimentation, and/or filtration; adsorption, ion exchange, electrodialysis, reverse osmosis, or any combination of processes depending on the end use of the renovated wastewater.

FIGURE 3.15 Conventional sewage treatment unit processes. Tertiary treatment may include denitrification, phosphorus removal; coagulation, sedimentation, and/or filtration; adsorption, ion exchange, electrodialysis, reverse osmosis, or any combination of processes depending on the end use of the renovated wastewater.

TABLE 3.21 Conventional Sewage Treatment Plant Design Factors

Preliminary Treatment

Coagulation and Sedimentation Treatment

Racks

Area: 200% plus sanitary sewer; 300% plus combined sewer. Bar space: 1-1.75 in., dual channels.

Screens

Net submerged area: 2 ft2/mgd for sanitary sewer: 3 ft2/mgd for combined sewer. Slot opening 0.125 in. min. Dual units, preceded by racks.

Grit Chamber

Sewage velocity: 1 fps mean. 0.5 fps. minimum. Detention: 45-60 sec. floor 1 ft below outlet. Minimum of 2 channels.

Skimming Tank

Air or mechanical agitation with or without chemicals. Detention: 20min for grease removal. 5-15min for aeration. 30 min for flocculation.

Comminutors

Duplicate or bypass, downstream from grit chamber.

Sedimentation

Surface settling rates at peak flows: primary and intermediate set—tanks 1.500 gpd/ft2; final set—tanks 1.200 gpd/ft2 after trickling filters or rotating biological contactors and for activated sludge for conventional, step aeration, contact stabilization, and carbonaceous staged of separate-stage nitrification: following extended aeration 1.000 gpd/ft2: for physicochemical treatment using lime: 1.400 gpd/ft2.

Weir rates: 10.000 gpd per linear foot for average flows to 1.0 mgd and up to

15.000 for larger flows

Sludge hopper: 1 horizontal to 1.7 vertical.

Sludge pipe: 6in./min.

Chemical Precipitation

Rapid mix. coagulation, sedimentation. Ferric chloride, ferrous sulfate, ferric sulfate, alum. lime, or a polymer.

Imhoff Tank

Detention period: 2-2.5 hours. Gas vent: 20% total area of tank minimum. Bottom slope: 1.5 vertical to 1 horizontal. Sludge compartment: 3-4 ft3 per capita 18 in. below slot: 6—10 ft3 per capita secondary treatment. Bottom slope: 1 to 1 or 2. Slot and overlap: 8 in. Sludge pipe: 8 in. minimum under 6 ft head. Velocity: 1 fpm. Surface settling rate: 600 gpd/ft2

(continues)

TABLE 3.21 (continued)

Preliminary Treatment

Coagulation and Sedimentation Treatment

Flow Basis

100 gal per capita plus industrial wastes. Usual to assume total flow reaches small plants in 16 hr.

Flow Equalization

Based on 24-hr plot to smooth out hydraulic and organic loading.

Chemical Treatment

For odor control, oxidation, corrosion control, neutralization.

Tube and Inclined Plate Settlers

PVC or metal tubes, at 45 1 60from horizontal. 2 in. x two 6-in., 4 ft long. May be installed in existing basin.

Biological Treatment

Sludge Treatment

Intermittent Sand

Filter rate: 50.000-100.000 (gpad/'with plain settling and 400.000 gpad with trickling filter or activated sludge. Sand: 24 in. all passing 0.25-in. sieve. Effective size 0.35-0.6mm. Uniform coefficient <3.5.

Digester"

Capacity: with plain sedimentation 2-3 ft3 per capita heated or 4-6ft3 unheated. With standard trickling filter 3-4ft3 heated and 6-8 ft3 unheated; 4-5 ft3 heated and 8-10 ft3 unheated with a high-rate filter. With activated sludge 4-6ft3 per capita heated and 8—12ft3 unheated. Bottom slope: 1 on 4. gravity.

Contact Bed

Filter rate: 75.000-100.000 gpad/ft.

Sludge Drying Bed

Open: 1 ft2 per capita with plain sedimentation. 1.5 ft2 with trickling filter.

Trickling Filter

Standard rate: 400-600lb BOD/acre-ft/day; or 2-4 mgadc, 6 ft deep. High rate: 3000+ lb BOD/acre-ft/day. or 30 mgad for 6 ft deep. Minimum filter depth 5 ft. maximum 10 ft. 1-1/34 ft2 with activated sludge and 2 ft2 with chemical coagulation. Glass covered: reduce area by 25%.

Vacuum Filtration

Pounds per square foot per hour dry solids. Primary 6 to 10. trickling filter 1.5-2.0. activated sludge 1-2.

Activated Sludge

Normally 2 hr retention in primary and final sedimentation and 6-8 hr aeration.

Centrifuging

Flow rate based on gallons per minute per horsepower.

Rapid Filtration—Tertiary Treatment

gpm/ft2.rf

Land Treatment See text.

Stabilization Pond—Facultative

15-35 lb BOD/acre-ft/day, 3-5 ft liquid depth, center inlet; variable withdrawal depth. 3-ft freeboard, detention 90-180 days; multiple units; winter flow retention. Use up to 501b BOD loading in mild climate and 15-20 lb in cold areas.

Rotating Biological Contactors See text.

Wet Combustion

Sludge thickener: loading of 10 lb/day/ft2.

Land Disposal

Stabilized sludge only. See text. Incineration

Tons per hr depending on moisture and solids content. Temperature 1,250-1,400'F. Pyrolysis temperature higher.

Gas Production

A properly operated heated digester should produce about 1 ft3 of gas per capita per day from a secondary treatment plant and about 0.8 ft3 from a primary plant. The fuel value of the gas (methane) is about 640 Btu/ft3.

Disinfection

Chlorine, ozone: see text.

"Anaerobic sludge digestion will require approximately 65 days at 55 F. 56 days at 60 F, 42 days at 71 F. 27 days at 86 F, 24 days at 95 F, 20 days at 113 . The optimum temperature is 86 -95 F. Mixing of sludge can reduce digestion time up to 50%. In large plants, sludge is usually digested in two stages. Temperature of 140 F causes caking on pipes.

^Gallons per acre per day = gpad.

^Million gallons per acre per day = mgad.

dFor multimedia, see state standards.

Note: Surface setting rate = gpd/ft- =

180: tank depth in ft detention, hr

TABLE 3.22 Sewage Treatment Plant Unit Combinations and Efficiencies: Approximate Total Percent Reduction

Treatment Plant Suspended Biochemical

Solids Oxygen Demand

TABLE 3.22 Sewage Treatment Plant Unit Combinations and Efficiencies: Approximate Total Percent Reduction

Treatment Plant Suspended Biochemical

Solids Oxygen Demand

Sedimentation plus sand filter

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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