Screening • First unit operation generally encountered in wastewater treatment plants • Device with openings, generally of uniform size that is used to retain solids found in influent wastewater • Principal role is to remove coarse material from flow that could: 1. 2. 3. 4 Damage subsequent process equipment Reduce overall treatment process reliability and effectiveness Contaminate waterways Classification of screens Screens Bar or coarse screen 6-75 mm Microscreens 1𝜇𝜇𝜇𝜇 – 0.3 mm Used principally for removing residual solids from effluent Table 13-3 Mackenzie 5 Fine screen 1.5 mm – 6 mm Very fine screen 0.25-1.5 mm Fine screen are sometimes used in place of or following coarse screen where greater removal of solids are required: 1. Protect equipments which may be more sensitive to solids such as membrane bio-rectors 2. Eliminate materials that may inhibit the beneficial reuse of biosolids Screening elements may consist of parallel bars, rods or wires, wire mesh or perforated plates Opening may be of any shape generally circular or rectangular Screen composed of parallel bars or rods are called a bar rack or coarse screens. The material removed are called Screenings 6 Screenings retained on coarse screens Rocks, branches, leaves, paper, tree root, plastics, bottles cans and rags The quantity and characteristics of screenings collected for disposal vary depending on the type of bar screen and size of bar screen openings. • Manually cleaned coarse screens • Mechanically cleaned bar screens Manually used • Ahead of pumps in small wastewater pumping stations • Often they are used as standby screening in bypass channels during high flow periods • When mechanically cleaned screens are being repaired • In the event of power failure 7 8 9 Design of coarse screens Design consideration 1. Location 2. Approach velocity 3. Clear openings between bars or mesh size (Table 13-3 Mackenzie) 4. Headloss through screens 5. Screenings handling , processing and disposal For most mechanically cleaned coarse screen two or more units should be installed so that unit maybe taken out of service for maintenance . Each channel should have adequate capacity to pass the peak flowrate Slide gates should be provided ahead of and behind of each screen so that the unit can be dewatered for maintenance and repair (Figure 13-6 Mackenzie) The channel invert should be 75-150 mm below the invert of incoming sewer. 10 Velocity Screen channel should be designed to prevent the settling and accumulation of grit nd othe heavy materials An approach velocity of at least 0.4 m/s is recommended To prevent the pass through of debris at peak flowrates, velocity through bar screens should not exceed 0.9 m/s Headloss • Headloss through mechanically cleaned screen is typically 150 mm . • The racking mechanisms are operated normally based on differential headloss through screen or by a time clock On for 5 minutes Off for 25 minutes (with higher flowrates increase the frequency of cleaning) 11 Hydraulic losses through bar screens ℎ𝐿𝐿 = 1 𝐶𝐶 𝑉𝑉 2 𝑡𝑡ℎ𝑟𝑟𝑟𝑟 − 𝑉𝑉 2 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎ℎ 2𝑔𝑔 hL = headloss, m C= an empirical discharge coefficient to account for turbulence and eddy losses Typically 0.7 for clean screen and 0.6 for clogged screen V thru= velocity through bar screens, m/s V approach= approach velocity in upstream channel, m/s g = acceleration due to gravity 9.81 m/s2 The approximate number of bars 𝑁𝑁𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ 𝑜𝑜𝑜𝑜 𝑐𝑐ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 − 𝑏𝑏𝑏𝑏𝑏𝑏 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑏𝑏𝑏𝑏𝑏𝑏 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ + 𝑏𝑏𝑏𝑏𝑏𝑏 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 Number of bar spaces= Nbars + 1 Example 13-4 Mackenzie 12 Fine screens Used; • preliminary treatment after coarse screens • Primary treatment as substitute for primary clarifier At small WWTP (up to 0.13m3/s) Example; 0.25mm drum rotary screens Achieves BOD removal of 5-20% and TSS of 5-30% • Removal of solids from primary effluent that could cause clogging problems in trickling filters or membrane bioreactors Tables 13-6, 13-7 Band screen, bar screen, drum screen and step screen 13 Design consideration • Minimum two parallel each of capability of handling peakflow • Velocities approach velocity in the range of 0.6 m/s – 1.2 m/s another criteria; greater than 0.3 m/s at low flow and less than 1.4 m/s at peak flow • channel dimensions are designed to achieve depth of 7.5 to 9m Headloss The clear water headloss may be obtained from manufacturers rating tables or calculated using the equation 14 ℎ𝐿𝐿 = 1 2𝑔𝑔 𝑄𝑄 2 𝐶𝐶𝐶𝐶 Range from 0.8-1.4 m hL = headloss, m C= coefficient of discharge (typical value of 0.6 for clean screen, g = acceleration of gravity, m/s2 Q = discharge through screen m3/s A= effective open area of submerged screen, m2 15 Screenings handling and disposal In mechanically cleaned screens, screenings are typically discharged into a travelling belt or pneumatic ejectors Screenings can be discharged directly into a screenings grinders, or containers for disposal Disposal • Hauling and co-disposal with solid wastes • Monofilling at site (small installations only) • Incineration alone or with sludge and grit • Discharge to grinders where it is returned to wastewater treatment plant. 16 Grit Removal Grit in wastewater consists of sand, gravel , cinders or other heavy solid materials that have specific gravities substantially greater than the of the organic solids in wastewater. Grit removal is necessary to 1) Reduce formation of heavy deposits in aeration tanks, aerobic digesters, pipelines, channels. 2) Reduce the frequency of digester cleaning caused by excessive accumulation of grit 3) Protect moving mechanical equipment from abrasion. 17 The removal of grit is essential for equipment with closely mechanical metal surfaces, such as grinders, fine screens, centrifuges. Wastewater Grit Characteristics Grit composition. Grit consists of sand, gravel, cinders and other heavy materials. It also includes organic matters such as eggshells, seeds and coffee grounds. As grit travels through the collections system, grit particles come in contact with organic matter and surface active agents that can adhere to the grit particles. The presence of these constituents can alter the size and settling characteristics of the grit size and settling characteristics of the grit particle. Generally, what is removed from the wastewater flow as grit is predominantly inert. However, the composition can be highly variable, with moisture content ranging form 13% to 65%, and volatile content from 1% to 56%, with a wide range of specific gravity. 18 Grit particle size. Based on limited available information, the actual size distribution of collected grit exhibits wide variation due to differences in collection system characteristics, as well violations in grit removal efficiency. Generally most collected grit particles are retained on a 0.15 mm (100 mesh ) sieve, reaching nearly 100 percent retention is some instances. However the particle size can vary significantly. Grit quantities. The quantities of grit will vary greatly from one location to another, depending on the type of sewer system, the condition of the collection system, the types of industrial wastes, the number of household with food waste grinders and amount of infiltration in areas with sandy soils. 19 Traditionally , grit removal have been designed based on the assumptions that, grit consists of inorganic, settleable solids ranging in size from 0.050-1.0 mm with settling characteristics similar to clean silica sand with a specific gravity of 2.65 and a particle size predominantly larger than 0.2 mm. As a consequence of these assumptions, many existing grit removal systems fall short of performance expectations resulting in excessive maintenance. 20 Grit settling characteristics. The settling characteristics of grit vary considerably form the point of entry in the collection system and its location in the treatment plant. In wastewater surface active agents (SAAs) adhere to grit particles, when sufficient amount of SAAs have accumulated, the coated grit particle buoyancy increases. Accordingly, because of SAAs, grit particles reaching the treatment plant under normal conditions are often lighter than anticipated during design. Assuming, the settling velocity of clean sand increases as particle size increases, however the settling velocity of wastewater grit is independent of particle size because of the buoyant effect of SAAs. 21 Consequently, wastewater grit passes through traditionally designed grit removal processes to the primary settling tanks where they settle out with the primary sludge or ar passed to the aeration tanks. When exposed to biological activity, the SAAs decompose and the remaining high density grit particles settle rapidly. As a result, there is often excessive grit deposition in biological reactors, such as aeration tanks, that leads to frequent and costly cleaning. Typically, two thirds of grit that enters a WWTP is larger than the deposit limit size of clean sand 22 Sand Equivalent Size (SES) SES relates the settling velocity of wastewater grit to that of clean sand. Wastewater grit particle coated with SAAs with the same physical size as a clean particle of sand will have a lower settling velocity than the clean sand particle. The SES of a wastewater grit particle is the equivalent size to that of a clean san particle size that has the same settling velocity. 23 24 Settling velocity of wastewater grit When possible, a grit study should be conducted to determine the SES of the incoming wastewater grit and the design SES to achieve the desired level of grit removal. If a grit study is not viable, regional data can be used as a substitute. When data are not available, design can be based on a qualitative evaluation. 25 The physical size and the SES of wastewater grit begins to diverge at 0.106 mm with the difference increasing as the physical size increases. For example, for a physical particle size of 0.210 mm, the SES could vary from 0.106 mm to 0.210 mm. Consequently, to remove particles with a physical size of 0.210 mm, an SES as low as 0.106 mm may have to be used as basis for design 26 A design based on an SES of 0.106 mm will result in 90 percent grit removal for most collections systems. However, for those collection systems where very fine silt is expected, the grit removal efficiency might be significantly lower than 50-65 percent. If the goal is to achieve 90% grit removal, a design SES as low as 0.075 mm may be required. Once the design SES ha been identified, the surface loading rate of targeted grit particle can be established using the settling characteristics of clean sand. For an SES of 0.106 mm, the surface loading rate is 0.49 m/min. For an SES of 0.075 mm, the required surface loading rate is 0.24 m/min, which doubles the size of the required grit removal system. 27 Grit separators for wastewater The separation of grit from wastewater is usually accomplished in separate grit chambers designed to physically separate heavy grit particles form lighter organic solids. Grit chambers are most often located after the bar screens and before the primary sedimentation tanks. There are generally three types of grit separation devices: • Horizontal flow grit chambers, with either rectangular or square configuration. • Aerated grit chamber • Vortex grit chamber 28 Grit separators for wastewater The behavior of settling particles in a grit chamber is commonly describes as type I sedimentation (discrete settling) To separate the inert grit material from organic particles, grit removal devices depend on the difference in specific gravity between organic and inorganic solids. In standard gravity separation all particle are assumed to settle in accordance with Newton’s equation. 29 Grit separators for wastewater And by Camp’s equation, to be scoured at a velocity The dimensionless constant (β) range from 0.04 to 0.06. The Darcy-Weisbach friction factor is taken to be in the range 0.02 to 0.03 In horizontal flow grit chamber, to assure removal of the grit and scour of organic matter, three conditions must be met: 1. The overflow rate of the chamber must be equal to the settling velocity of the inert grit particle 2. The horizontal velocity must be less than the scour velocity of the inert particles. 3. The horizontal velocity must be greater than the scour velocity of the organic particles 30 1. Horizontal flow grit chambers Rectangular and square horizontal flow grit chambers have been used for many years. Their use, however, in new installations has limited in favor of aerated and vortex type grit chambers. 1.1 Rectangular horizontal flow grit chambers. These units are designed to maintain a velocity of as close to 0.3 m/s as practical and to provide sufficient for grit particles to settle to the bottom of the chamber. The design velocity will carry most organic particles through the chamber but will permit the heavier grit to settle out. The basis of design of rectangular horizontal-flow grit chambers is that under the most adverse conditions, the lightest particle of depositable grit will reach the bed of the channel prior to its outlet end 31 1.1 Rectangular horizontal flow grit chambers. Typically, grit chambers were designed to remove all grit particles that will be retained on a 0.21 mm diameter screen (70 mesh screen). Although, design for removal of grit particles retained on screen of 0.15 mm diameter (100 mesh screen). The settling velocity to be used should be based on the SES of the wastewater grit using the settling rates from Figure 5-34 b (Metcalf and Eddy, 2014- shown on slide 15 of this document). Grit removal is accomplished usually by a conveyor with scrapers. 32 1.2 Square horizontal flow grit chambers Square horizontal flow grit chambers have also been used for over 60 years. Influent to the units is distributed over the cross section of the tank by a series of vanes or gates, and the distributed wastewater flows in straight lines across the tank and overlfows a weir in a free discharge. These types of grit chambers are designed based on overflow rates that are dependent of particle size and the temperature of the wastewater. Typically, these units have been designed to remove 95% of the 0.15 mm diameter particles at peak flow. 33 1.2 Square horizontal flow grit chambers Typical set of design curves is shown on the Figure below (Figure 5-36 Metcalf and Eddy, 2014). The SES should be used for determining the settling area required 34 1.2 Square horizontal flow grit chambers In square grit chamber, the solids are removed by a rotating raking mechanism to a sump at the side of the tank. Settled grit may be moved up by an inclined reciprocating rake mechanism or an inclined screw conveyor. Grit may also be pumped form the tank through a cyclone degritter to concentrate the grit. 35 2. Aerated grit chambers In aerated grit chambers, air is introduced along one side of a rectangular tank to create a spiral flow pattern perpendicular to the flow through the tank. Wastewater will move through the tank in a spiral path and will make two t three passes across the bottom of the tank at maximum and more passes at average flow. The heavier grit particles settle to the bottom of the tank. Lighter, principally organic particles remain in suspension and pass through the tank. The velocity of roll or agitation governs the size of particles of a given specific gravity that will be removed. 36 2. Aerated grit chambers 37 2. Aerated grit chambers 38 2. Aerated grit chambers 39 2. Aerated grit chambers 40 If the velocity is too high, grit will be carried out of the chamber, if it is too low, organic material will be removed with the grit. The quantity of air is easily adjusted. With proper adjustment, almost 100 percent of depositable grit will be removed. Advantages and disadvantages of aerated grit chambers shown in Table 13-9 (Mackenzie Davis-Text book) Aerated grit chamber/ design consideration Aerated grit chambers have been designed to remove 0.21 mm diameter particles or larger, with 2-5 minutes detention periods at the peak hourly rate of flow. 41 A grit hopper about 0.9 m deep with steeply sloping sides is located along one side of the tank under the air diffusers. The air diffusers are located about 0.45 to 0.6m avove the normal plane of the bottom. 42 43 Vortex type grit chambers These devices uses a vortex flow pattern: Mechanically induced vortex: Wastewater is directed to the vortex unit by an inlet channel that is designed to guide the wastewater flow into the vortex unit while the grit is directed downward. The toroidal motion is enhanced by a rotating turbine impeller within the unit that cause the grit to move towards the center where it passes under a separating plate into a grit slurry hopper while the lighter organic materials are maintained in suspension and guided to the surface where they leave the unit. The grit hopper contents are kept fluidized by anxial flow propeller. Grit may be removed by air lift pump. 44 Hydraulically induced vortex: The vortex is generated by the flow entering the unit without any mechanical devices. Wastewater is directed to the unit by a channel and enter tangentially into the cylindrical unit, causing the contents to rotate slowly about the vertical axis. The heavy grit moves down a spiral path to the center where it passes under a center cone to the grit slurry hopper. Multi-tray grit separator. This type consists of multi trays which maximizes surface area and minimizing settling distances. This allows for a very compact installation with low head loss. 45 Flow is directed into the multiple tray unit by an influent distributor header which distributes influent tangentially into the multiple tray system. The tangentially feed established a hydraulically forced vortex flow pattern where grit settles by gravity along the sloped surface of each tray and through a center opening which allows collected grit from each tray to settle into a single sump located below the center of unit. 46 Primary Sedimentation 47 Primary sedimentation The objective of treatment by sedimentation is to remove settleable solids and floating material found in wastewater and thus support solids content. Primary treatment is typically the first step in the further processing of wastewater following the removal of coarse solids and grit. Efficiently designed and operated, from 50 to 70 percent of the suspended solids and from 25 to 40 percent of the BOD can be removed by primary sedimentation tanks. The prime purpose of the sedimentation is to remove substantial portion of the organic solids. 48 Description Almost all treatment plants that have primary sedimentation, use mechanically cleaned sedimentation tanks of standardized circular or rectangular design. The selection of the type of sedimentation unit for a given application is governed by the size of the installations, by local site conditions, and by the experience and judgment of the engineer. Two or more tanks should be provided so that the process may remain in operation while one tank is out of service for maintenance and repair work. At large plants, the number of tanks is determined largely by size limitations. 49 Rectangular tanks Rectangular sedimentation tanks may use either chain and flight solids collector, 50 Engineering services Rectangular tanks 51 Rectangular tanks Rectangular sedimentation tanks may use as well traveling bridge sludge collector 52 Multiple rectangular tanks require less land area than multiple circular tanks and find application where site space is at a premium. Rectangular tanks also lend themselves to nesting with aeration tanks in activated sludge plants, thus permitting common wall construction and reducing construction costs. They are also used generally where tank roofs or covers are required. While construction cost maybe less, the chain and flight collector systems used in rectangular tanks generally require more maintenance than the rotating sludge collector mechanism used with circular settling tanks. Important issues with rectangular tanks include 1. Sludge removal 2. Flow distribution 3. Scum removal 53 54 Flow distribution Because flow distribution in rectangular tanks is critical, on e of the following inlet design is used: (1) full width inlet channel with inlet weirs, (2) inlet channel with submerged orifices, (3) inlet channels with slotted baffles Figure 14-15 55 Circular tanks Circular sedimentation tanks are customarily arranged in groups of two or four. The flow is divided among the tanks by a flow split structure, commonly located between the tanks. Solids are usually withdrawn by sludge pumps for discharge to the solids processing and disposal units. 56 Photos adapted from OVIVO Worldwide experts in wastewater treatment 57 Flow pattern. In circular tanks the flow pattern is radial- as opposed to horizontal flow in rectangular tanks. To achieve a radial flow pattern, the wastewater to be settled generally introduced in the center. The wastewater enter a circular well designed to distribute the flow equally in all directions. The center well has a diameter typically between 15 to 25 percent of the tank diameter and ranges from 1 to 2.5 m in depth. 58 Sedimentation tank weirs/ rectangular sedimentation tanks. Settled effluent from sedimentation tanks is discharged over weirs into an effluent launder which in turn discharges in a conduit leading to the biological treatment process. Small rectangular settling tanks are often fitted with a single transverse weir on the discharge end wall. However, larger rectangular settling tanks require additional weir length to minimize head loss and short circuiting. Head loss and short circuiting can be minimized by adding a series of launders with double weirs that are aligned either longitudinally (parallel to the length of the settling tank) or transversely (perpendicular to the length of the settling tank, or both longitudinally and transversely. 59 Sedimentation tank weirs/ circular sedimentation tanks. Because most circular settling tanks are designed with center feed, treated effluent is discharged over a series of V-notch weirs located peripherally at the tank wall. For large circular tanks, inboard launders with double weirs my be provided to increase the number of V-notches and reduce the head loss at high flow. The v-notches are spaced equally along the tank circumference and provide a simple means to detect short circuiting when the weir is not level. The V-notches are normally 90 degrees Vs and have a total depth of 75 mm. surface baffles are placed in front of the peripheral weir to retain surface scum. 60 Sedimentation tank performance BOD and TSS removal Typical performance data for the removal of BOD and TSS in primary sedimentation tanks, as a function of the detention time and constituent concentration, are presented in the figure below (adapted from Metcalf and Eddy, 2014). 61 The curves shown are derived from observations of the performance of actual sedimentation tanks. The curvilinear relationship in the figure can be modeled using the following relationship. Where: R=expected removal efficiency t= nominal detention time, T a,b= empirical constants • Typical values for the empirical constants at 20°C are as follows 62 Item b a BOD 0.020 0.018 TSS 0.014 0.0075 The efficiency of sedimentation basins with respect to the removal of BOD and TSS is influnced by short circuiting and hydraulic stability: In an ideal sedimentation basin, a given block of entering water should remain the basin for the full detention time. Short circuiting, which can lead to reduced treatment performance, can result from temperature differences, wind driven circulation patterns and the presence of dead zones resulting form poor design, inadequate mixing and dispersion. 63 64 Temperature effects. Temperature effects can be significant in sedimentation basins. It has been shown that a 1°C differential between the incoming wastewater and the wastewater in the sedimentation tank will cause a density current to form. The impact of the temperature effects on performance will depend on the material being removed and its characteristics. Temperature effects can be more pronounced in secondary settling tank where a less dense sludge is processed Wind effects: Wind blowing across the top of opened sedimentation basins can cause circulation cells to form. When circulation cells form, the effective volumetric capacity of the basin is reduced. As with temperature effects, the impact of the reduced volume on performance will depend on the material being removed and its characteristics. 65 Design consideration If all solids in wastewater were discrete particles of uniform size, uniform density and uniform shape, the removal efficiency of these solids would be dependent on the surface area of the tank and time of detention. The depth of the tank would have little influence, provided that horizontal velocities would be maintained below the scouring velocity. However, the solids in wastewater are not of such regular characteristics but are heterogeneous in nature, and the conditions under which they are present range from total dispersion to complete flocculation. Design parameters for sedimentation are considered below. Typical design information and dimension for rectangular and circular sedimentation tanks used for primary treatment are presented in tables below. 66 Typical design information for primary sedimentation tanks Primary sedimentation tanks followed by secondary treatment Item Detention time Overflow rate Average flowrate Peak hourly flowrate Weir loading rate Sludge hoppers Unit h m3/m2.d m3/m2.d m3/m.d Range 1.5-2.5 Typical 2.0 30-50 40 80-120 100 125-500 250 1.7 vertical to 1 horizontal Typical design information for rectangular and circular tanks is stated in Table 14-1 Text book (Mackenzie Davis) 67 Detention time: The bulk of the finely divided solids reaching primary sedimentation tanks is incompletely flocculated but is susceptible to flocculation. Flocculation is aided by eddying motion of the fluid with the tanks and proceeds through the coalescence of fine particles, at a rate that is a function of their concentration and of the natural ability of the particles to coalesce upon collision. As a general rule, coalescence of a suspension of solids becomes more complete as time elapses, thus, detention time is a consideration in the design of sedimentation tanks. The mechanics of flocculation are such, that as the time of sedimentation increases, less and less coalescence of remaining particles occurs. Normally, primary sedimentation tanks are designed to provide 1.5 to 2.5 hours of detention based on the average rate of wastewater flow . 68 In cold climates , increases in water viscosity at lower temperatures retard particle settling in clarifiers and reduce performance at wastewater temperature below 20°C. The curve shown below, present the increase in detention time necessary to equal the detention time at 20°C (WPCF, 1985). For wastewater having a temperature of 10°C for example, the detention period is 1.38 times that required at 20°C to achieve the same efficiency. Thus, in cold climates, safety factors should be considered in clarifiers design to ensure adequate performance. 69 Surface loading rates: Sedimentation tanks are normally designed on the basis of a surface loading rate (commonly termed overflow rate) expressed as cubic meters per square meter of surface area per day, m3/m2.d. When the area of the tank has been established, the detention period in the tank is governed by water depth. Overflow rates in current use result in detention periods of 2-2.5 h based on average design flow. The effect of the surface loading rate and detention time on suspended solid removal varies widely depending on the characteristics of the wastewater, proportion of settleable solids and concentration of solids. It should be emphasized that overflow rate must be set low enough to ensure satisfactory performance at peak rates of flow, which may vary from over 3 times the average flow in small plants to 2 times the average flow in large plants. 70 Weir loading rates In general weir loading rates have little effect on the efficiency of primary sedimentation tanks, and should not be considered when reviewing the appropriateness of clarifier. For general information purposes only, typical weir loading rtes are given in Table 14-1. Baffles should be placed ahead of effluent weirs in primary settling tanks to reduce hydraulic short circuiting. The baffles also prevent the discharge of scum into the effluent. Scour velocity To avoid the resuspention (scouring) of settled particles, horizontal velocities through the tank should be kept sufficiently low. The critical velocity can be obtained by the following equations: 71 Where: VH = Horizontal velocity that will just produce scour (m/s) K= constant that depends on type of material being scoured (unitess) S= specific gravity of particles g= acceleration due to gravity 9.81 m/s2 D= diameter of particles, L F= darcy-Weisbach friction factor , unitless. Typical values of k are 0.04 for sand and 0.06 for more sticky matter. The term f (Darcy-Weisbach friction factor) depends on the characteristics of the surface over which flow is taking place and the Re number. Typical values of f are 0.02 to 0.03. 72

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