1. 1 CE 548 Physical Unit Operations

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3. 3 Introduction   Physical unit operations : operations used for the treatment of wastewater in which change is brought about by means of the application of physical forces.   Physical unit operations are a major part of treatment systems.

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5. 5 Screening   First unit operation encountered is wastewater treatment plants.   A screen is a device with generally uniform openings that is used to retain coarse solids.   Screens may be divided into coarse (.25-6”) and fine (<.25”) screens. Microscreens (<50  m) are also available can be used to remove fine solids from treated effluents.   The material that is retained by the screens is know as screenings. Screenings are typically hauled to a landfill or may be buried on site or incinerated depending upon the plant.

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7. 7 Screening  Coarse Screens (Bar Racks): Coarse Screens (Bar Racks): Composed or parallel bars or rods with openings of 0.25-6” and are used to protect pumps, valves from clogging by large objects and rags. Bar racks may be hand cleaned or mechanically cleaned (T5-2 and F5-3). The bars are mounted at some angle to the horizontal and the allowable head loss is 6” or less. Design of Bar Racks: Considerations should include: location ( upstream of grit chamber), approach velocity (1.25-3fps); clear openings between bars or mesh size, headloss through the screens (<6”), screenings and controls. Use 2 units or 1 unit and a by-pass. Head-loss across bar screens can be represented by (equation 5-1):

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9. 9 Screening  Fine Screens Fine Screens. Composed of wires, grating, wire mesh, or perforated plates with openings. The application range from primary treatment to the removal of residual SS solids from biological treatment processes. Fine screens may static, rotary drum or step type. F5-4, p.322 and T5-4, p.323. Design of Fine Screens: Fine screens would be preceded by coarse screens. Use at least 2 sized to handle peak flow. Head-loss across fine screens can be obtained from manufacturer’s rating tables or calculated using Eq. (5-2):

10. 10 Screening  Microscreens Composed of filter fabrics with openings of 10 – 35  m and are fitted on a drum periphery. The principle applications for microscreens are to remove suspended solids from secondary effluent and from stabilization-pond effluent. Typical suspended solids removal range from 10 to 80 percent with an average of 55 percent. Problems encountered with microscreens include incomplete solids removal and inability to handle solids fluctuations. Table 5-6 gives typical design information for microscreens.

11. 11 Screening  Screenings: Screenings are the material retained on bar racks and screens. The quantity of screenings collected depends on the screen openings. Screenings retained on coarse screens: consists of debris such as rocks, branches, leaves, paper, plastics, rags, etc. Typical data on the characteristics and quantity of coarse screenings are shown in Table 5-7. Screenings retained on fine screens: include small rags, plastic material of various types, razor blades, grit, food waste, feces, etc. Fine screenings must be handled and disposed of properly because it contains putrescible matter and substantial grease and scum. Typical data on the characteristics and quantity of fine screenings are shown in Table 5-8.

12. 12 Flow Equalization  Flow equalization Flow equalization is the damping of flow variations to achieve a consistent flowrate. It a method used to:   overcome the operational problems caused by flow rate variations   improve performance of downstream processes   reduce the size and cost of downstream treatment facilities Advantages (benefits):   enhancement of biological treat  shock loading  dilution of inhibition   improved effluent quality  better sedimentation due to constant loading   improved filter performance   Attractive option for upgrading performance of overloaded plants.

13. 13 Flow Equalization  Flow equalization Types:   On-line: achieves both flow and organic damping   Off-line: achieves mainly flow damping   Volume requirements (example 5-2)

14. 14 Grit Removal Grit is sand, gravel, cinders, eggshells, seeds, coffee grounds and other heavy solid material that is much heavier than the organics in wastewater. Specific gravity (sg) of approximately 2.7 but can be as low as 1.3 versus an organic sg of 1. Grit is predominantly inert and relatively dry and highly variable in composition ranging from 13-65% moisture with a volatile content of 1-56%. It has a bulk density of 100lb/ft3. Normally, grit chambers are designed to remove all grit that will be retained on a 0.21-mm-diameter (65 mesh) screen, and some chambers are based on 0.15 mm-diameter (100 mesh). Grit chambers are typically located after the bar racks but before the primary sedimentation tanks.

15. 15 Grit Removal The function of the grit chambers include:   Protect moving mechanical equipment from abrasion and wear.   reduce formation of heavy deposits in pipelines.   reduce the frequency of digester cleaning caused by excessive accumulation of grit. Types of grit chambers: horizontal flow, aerated or vortex. Rectangular Horizontal-flow grit chamber:   Designed to maintain a velocity of 1.0 ft/s and provide sufficient time for grit particles to settle.   Organic particles will be carried out at this velocity.   Designed to remove grit particles that will be retained on a 65- mesh screen (0.21 mm dia).

16. 16 Grit Removal Aerated Grit Chambers:   Because some organic matter is settled with grit, grit washing is usually done in order to clean the grit before disposal   Aerated grit is used in order to eliminate the need for grit washing.   Typical design parameters are given in Table 5 – 17   The velocity governs the size of particle to be removed. o o if velocity is too great, grit will be carried out o o if velocity is too small, organic will be removed with grit o o velocity is controlled by quantity of air.

17. 17 Grit Removal Vortex-type Grit Chambers, Two Types: PISTA unit:   Wastewater enters and exits tangentially   Rotating turbine maintains constant flow velocity and promotes separation of organics from grit.   Grit settles by gravity into the hopper.

18. 18 Grit Removal Teacup unit:   Flow enters tangentially at the top of unit and a free vortex is generated.   Effluent exits the center of the top of the unit from a rotating cylinder.   Grit settles by gravity to the bottom   Organics including those separated by centrifugal forces exit with effluent.

19. 19 Primary Sedimentation   Objective: to remove readily settleable solids and floating material.   Can be used as the principal treatment to provide for the removal of:   settleable solids   free oil and grease and other floating material   a portion of the organic load   When used ahead of biological treatment, their function is to reduce the load on biological units.   Primary sedimentation removes 50 – 70% ss and 25 – 40% BOD.

20. 20 Primary Sedimentation   Design consideration: Detention time: Important since wastewater solids are heterogeneous in nature and susceptible to flocculation.   Normally detention time vary from 1-1/2 to 2-1/2 hours.   Temperature effect is limited, however, at lower temp settling is retarded (Figure 5 – 48). Surface-Loading rates:   Sedimentation tanks are normally designed on the basis of surface-loading (overflow rate) expressed as gal/ft 2.d. (Q/A surface )   overflow rate must be set low enough to ensure satisfactory performance at peak rates

21. 21 Primary Sedimentation   Design consideration: Scour velocity:   In sedimentation basins, horizontal velocities should be kept low so that settling particles are not scoured from the bottom of the basin.   Critical velocity is given by:

22. 22 Primary Sedimentation   Sedimentation tank performance: BOD and TSS removal: Typical performance data for the removal of BOD and TSS are presented on Fig. 5-46. The figure is derived using the following relationship: