CLARIFICATION 7,13 _/ I-. F,, ..4 ",, : • *l p-p, ,,t ~ i , - -,'--- = t - l I i ' ~-* -- i i I bl I,' ", "- ,.i ' I • ~, r o ," t i --i r'--'~ ° I i 8 C B B C 8 B C 6 I i -.*', ~ "- ! o .'{! il ~ • P* I. *-t • ,o I I I i I-- • ~° L A t~ ...L.....'.¢ ....t / Plan Viii. .Sl¢lion "AA f~ ..I I ~;' I ' ~"1"~ ~ * L""Il-'"b', '".~, ,,I : I I tI !ii.-. i B C G G C 8 i -4 "r -t-.-~" ' l I , i I * * i | "i,. ,, I : ].__L .... ..., :11 L 1 nalP ~iii C~ ............ .......~.it .¢l.ic,. .......- r~ C C 1 t~ :', ,, , ..... t.'. r. "'I 0~ ! • • i .......... 2:z._, L •., Plan Vitw L~ S¢¢lion-E[ FIGURE 7.7 hopper Typical sludge stnemegnarra rectangular for .snisab ing of the profiles to create an overlapping effect. When the hydraulic cylinder reaches the end of its stroke, the grid system reverses at a speed 2 to 3 times the forward speed. This allows the triangular profiles to wedge their way under the sludge. This oscillating movement of the collector with the concave profile is said by the manufacturer to pro- vide thickening of the sludge. The low clearance of 10 in. (25 cm) on the bottom of the basin allows the system to work well under tube-and-plate settler systems. 7.14 CHAPTER SEVEN I-luHr~l dip ~¢t~irl'r i init iles )ars and wear strip assembly. Guide pipe FIGURE 7.8 Indexing grid sludge removal system. (Courtesy of Parkson Corporation.) Track-Mounted Hydraulic Systems. As shown in Figure 7.9, one type of hydraulic removal system consists of a stainless steel header pipe with orifices sized and spaced for proper sludge removal. The collector pipe is attached to a pneumatically controlled drive assembly that travels on a stainless steel guide rail running the length of the tank. Col- lector pipes are generally a maximum width of 20 to 25 ft (6.1 to 7.6 m), and multiple units must be used to cover the width of wider tanks. The collector pipe is attached to a sludge discharge pipe in the tank wall. The sludge discharge pipe contains a pneumatically actuated sludge valve located below water level. When the sludge valve is open, the water level in the basin creates a driving force to start flow into and through the collector system. The drive assembly is pneumatically powered to travel the length of the tank in both directions. As it moves along, sludge on the basin floor is picked up hydraulically at a travel speed of 1.5 ft/min (0.46 m/min). The number of times the collector traverses the tank must be determined from the volume of sludge produced and the flow capacity of the collector system. This is typically about 90 gpm (5.7 L/s), based on a differential head of 5 ft (1.5 m). The system is fully automated through an electronic control system using a program- mable logic controller to control how often the collector operates and the length of travel. For example, because most of the sludge typically accumulates in the first third of a rec- tangular basin, it is necessary to collect in that area on a frequent basis, with the collec- cO ,.c "0 7.15 r ._J O t- c =.. ~ = E "~ 0 ~ m 0 ¢ 1.11 ~N o~ ~ cN ~'~ • ~. e- N~ oN ¢ "- ~ ," ¢" cO ~ 0 ~'- ~ e-~"~ .-Q ~ --r-- II -- -- - r~ 7.16 I'1 n e- D ~~ ~ • 1~ ffl ,.Q E ~6.N ~03 t- ¢1 r~ C CLARIFICATION 7.17 tor traveling the full length of the basin less frequently. This method of operation avoids collecting a large volume of very low-solids water. Instrument-quality compressed air at 100 psi (689 kPa) is supplied from a compressor system to a local electric/pneumatic interface panel mounted at the basin. Air is provided to the drive assembly and pneumatic sludge valves by means of umbilical hoses from the control panel to the drive assembly and valves. At one end of the basin is an extractor as- sembly consisting of vertical guide rails and a removable winch assembly to lift the col- lector header out of the tank for maintenance. Because these collectors do not require expensive sludge hoppers, they are a low-cost option for retrofitting manually cleaned basins and since they do not require access from the surface, they can also operate effectively beneath plate or tube settler systems. Options to the pneumatically driven collectors include continuous stainless steel tapes or chains, powered by a motor mounted at the top of one end of the basin, that pull the collector pipe back and forth along the bottom-mounted rail. gnitaolF egdirB ciluardyH Systems. Floating suction header solids removal systems were developed to provide a less expensive means of retrofitting existing manually cleaned basins than chain-and-flight units. The units may also be a lower-cost approach for new construction. Figure 7.10 shows a typical siphon desludging unit. The suction unit is mounted on massive floats built of closed-cell Styrofoam encased in fiberglass-reinforced plastic (FRP). These floats are tied together as a rigid structure so that it is freestanding when the basin is drained. A header system supported by the floats draws dense sludge from the bottom of the basin, and by means of low-head siphon discharges it into a longitudinal trough attached to the side of the sedimentation basin. The cross header is broken into several subheaders, each carrying flow overhead to a con- trol siphon freely discharging into the sludge trough. The effect of each lateral section can be observed and modulated. The siphon system is started with a portable low-differ- ential suction source. Experience has shown that, for water treatment sludges, once started, the siphon can be maintained for months. The floating bridge system is towed back and forth, either by a single, center-mounted, stainless steel flexible tow cable or by two cables acting on either end of the rigid float- ing structure. In either case, cables are powered by a geared motor drive and idler sheave arrangement mounted at either end of the basin on top of the wall in an accessible loca- tion. Because the floating system eliminates friction and most wind problems, remarkably little power is required to tow the bridges, even in basins 200 ft (61 m) long or longer. Dense sludge sucked up by the system is siphoned into the longitudinal trough. Valves control the amount of siphonage, with one valve for each siphon section forming the trans- verse header sludge pickup system. Sludge suction pipes are of light-gage, epoxy-coated aluminum, stainless steel, or PVC and are equipped with fluidizing vanes. In operation, the suction system travels up and down the length of the basin at about 6 ft/min (1.8 rn/min) as often as is required to vac- uum the dense sludge layer (compression zone). When there is not enough sludge to re- quire continuous suction up and down the basin, siphon discharge is temporarily arrested by programming the longitudinal trough discharge gate to close. The water level in the trough then rises to equal the basin level. When the next programmed desludging cycle begins, the trough valve opens and the siphon continues from where it stopped, again dis- charging dense sludge into the trough. Both carriage and floating bridge collection mechanisms are constructed with little submerged metal to minimize corrosion problems. However, these units can be used only in temperate climates where ice accumulation is not a problem. Some installations have also experienced drive synchronization problems and incomplete sludge removal. wolfrednU .lortnoC Underflow drawoff must be carefully controlled. If underflow is removed at too low a rate, dense sludge accumulates in the basin, creating a sludge blan- 7.18 CHAPTER SEVEN Drive assembly Mechanical skimming I - mechanism (optional) Scum weir t -- (optional) l~ ~-- Floating bridge Guide wheels Siphon pipes Effluent troughs I t__ Fiberglass floats Collection headers (with patented - - degasing system) - - Individual sludge valves Sludge return Control panel FIGURE 7.10 Floating bridge-type collector. ysetruoC( of ).dlopoeL ket that is too deep. If underflow is removed too quickly for too long an interval, the draw- off "postholes." That is, less viscous liquid breaks through, and dense sludge accumulates in the basin, overloading the desludging equipment. Drawing off underflow at regular intervals is best, either manually, with a programmed blowdown valve, or using a transfer pump. Such a program must be completely adjustable and programmed by the design engineer during early operation to fit plant operation. Man- ual drawoff control by guess and "gut feeling" often leads to operating problems. NOITACIFIRALC 7.19 New facilities should be designed with sludge viewing pits that permit the operator to observe the consistency of the sludge during blowdown. Direct observation provides a means of optimizing withdrawal rates and reduces excessive loading of sludge handling facilities. Circular Basins Circular sedimentation basins became more prevalent in water clarification when periodic manual cleaning of long, rectangular basins became unpopular. The top-drive circu- lar mechanisms used for sludge cleaning have no bearings under water, resulting in lon- gevity with little maintenance. In reasonable sizes--not exceeding 125 ft (38 m) in diameter--the circular center-feed clarifiers perform as well as long, rectangular basins provided there is a reasonably well-balanced radial flow from the center well with sub- stantial water depth maintained at the center. Some circular basins are designed for rim feed with clarified water collected in the center. However, most circular basins used today are the center-feed type. Included in this category are square tanks with center feed that are used for their feature of lower cost by means of common wall construction. A typical circular clarifier is shown in Figure 7.11. A circular clarifier with a center flocculation zone is shown in Figure 7.12. Basin Dimensions. Circular basins, like rectangular basins, are designed based on sur- face overflow rates, and rates used are typically the same as those for rectangular units. Circular basins may be of any diameter but are usually sized based on the commercially EVITARTSULLI NOITCES FIGURE 7.11 clarifier. circular Typical ysetruoC( of Eimco Water ).seigolonhceT 7.20 CHAPTER SEVEN PLAN WEIV ........ , ................. . / ~ /o.o°2~;LoT/~7" NOIT~UC~LF LEW EC~RAEL~" ~ roNc ~B)OCM~E EVITARTSULLI NOITCES FIGURE 7.12 Typical clarifier. flocculator (Courtesy of Eimco Water Technologies.) available standard sludge removal systems. Circular tanks have been built as large as 300 ft (91 m) in diameter but more typically are less than 100 ft (30 m) in diameter. Although settling theory is based on overflow rate, side water depth is an important consideration. Adequate depth mitigates hydraulic instability caused by wind currents, thermal currents, hydraulic scour, and random sludge blanket disturbances. Typical depths range from 10 to 51 ft (3 to 3.6 m). Because sludge is usually scraped to center hoppers, basin bottoms are sloped to the center. Large-diameter basins have two slopes, one steeper near the center to allow adequate depth to move the solids to central hoppers for removal. Inlet Design. Flocculated water is usually introduced to the center of circular or square basins through a center riser into a circular feed well. Some clarifier designs allow the in- troduction of flocculated water into the side of the feed well. The intent of the feed well is to produce a smooth, radial flow outward toward the periphery of the basin. The center feed amounts to a point source, because the feed well seldom represents more than 3% or 4% of basin area. For this reason, a great deal of flow mass is crowded into a small space and does not flow in an exactly radial pattern, leading to hydraulic im- balance and short-circuiting. This problem is accentuated in large-diameter shallow basins at high surface overflow rates in the 800 to 1,500 gpd/ft 2 [33,000 to 61,000 (L/day)/m ]2 range. One questionable feed well design involves using a small-diameter circular skirt of about 1% of the basin area extending only 3 to 4 ft (0.9 to 1.2 m) below the surface. The feed into this well is either from four ports discharging horizontally from a pier riser or from a horizontal pipeline discharging horizontally into the well from just below the sur- face. In the four-port design, variation of flow rate is accommodated with a design exit flow of about 2 ft/s (61 cm/s). This, however, does not ensure equal egress from each port. NOITACIFIRALC 7.21 A more controllable feed design includes using a distribution well inside a large feed well that is about 3% to 4% of the basin area. This distribution well has multiple ports hooded with adjustable biased gates. The gates balance tangential feed discharges by im- posing about a 4-in. (10-cm) head loss through the ports. This type of discharge causes the homogenized mass within the large feed well to rotate around the vertical axis at about 2 ft/s (61 cm/s). The well-distributed, fine-scale turbulence within and below the feed well encourages floc aggregation, and the overall slow rotation ensures that flow from the bot- tom of the skirt into the hindered sludge mass moving radially across the floor has equal displacement vectors. Density and displacement currents for circular basins are much the same as for long, rectangular basins. The vector system is influenced by well-flocculated influent mass sink- ing to the bottom adjacent to the feed well area, typically in the center one-third of the basin (about 10% of the total basin area). The vector system shows displacement radially along the bottom in the blanket zone and upwelling next to the peripheral wall. Clarified water generally flows across the surface toward the effluent. Outlet Design. Clarified water collection must be uniform around the perimeter of the basin. This is accomplished by a circular trough around the perimeter with V-notch weirs or with submerged orifices. Some designs use a double-sided weir trough mounted in- board along at least 15% of the tank radius. This has the advantage of reducing wall flow disturbances and drawing overflow from a more widely distributed region to offset the effects of bottom density currents running up the peripheral wall. Inboard weir troughs also partially break up wind current stirring. Troughs should have small-diameter holes in the bottom to reduce buoyant uplift forces when they are empty. Some designers prefer orifice troughs to overflow weir troughs because less floc breakup occurs. Others point out that the velocity gradient in a weir trough is no greater than in an orifice trough. Weir troughs are far easier to adjust for equal linear overflow, but if not properly adjusted, they have greater variation in flow than improperly adjusted orifices. Submerged orifice troughs reduce passage of floating trash to the filters and permit varia- tion in basin water depths during operation. This capability is useful for balancing differ- ences in plant inflow and discharge rates, such as when multiple filter washing occurs. Regulatory agencies sometimes stipulate that weir rates should not exceed around 20,000 gpd per linear foot (248,000 L/m) of weir. Flood (1961) found that weir overflow rates several times this value could be used if the weirs were well distributed over a sub- stantial portion of the surface. Placing a double-sided weir trough 1 ft (0.3 m) away from the peripheral wall satisfies the regulatory requirements, but still draws overflow from a narrow band of surface immediately in the path of the upwelling peripheral flow. Sludge Removal Sludge is removed from circular basins using circular collection equip- ment powered by a center turntable drive and plows that move sludge into a center sludge hopper. Turntable Drives. The tried and true, relatively trouble-free drive for both bridge- supported and pier-supported circular collectors is the sealed-turntable drive with the gear and pinion running in oil. Properly lubricated and with automatic condensate overflow, these drives operate for years without major repair. Typical turntable drives rotate on re- newable bearing strips, and the gear is split so that the ball bearings and strips can be re- placed without dismantling the remainder of the equipment. These drives are protected by an indicator and overload circuit breaker device actu- ated by the thrust of the primary worm gear driving the pinion and turntable gear. The indicator senses the torque load exerted on the collection arms by the sludge and turns off power if the load exceeds a preset limit. Sludge Hopper and Bottom Slopes. Because a circular sludge hopper surrounding a central pier holds the greatest volume, it is preferable to the older-style offset hopper de- 7.22 CHAPTER SEVEN sign. A pair of heavy stirrups reach down from the arms of the circular scraper to move dense sludge around the hopper to the outlet, to prevent buildup of anchor sludge and grit. The sludge drawoff pipe should never be less than 6 in. (15 cm) in diameter, and it should be designed so that a rotor rodder, or "go-devil," can be placed into the line from outside the basin in case of clogging. In lime softening plants, this line should be given a short purge of clear water after each blowdown cycle to flush out residual slurry. The slope of the basin bottom is important, especially when there is heavy or sticky sludge. Plow blades keep the bottom free of adhesions, literally plowing extremely dense sludge and grit to the center hopper. Otherwise, the thixotropic sludge flows along the bottom to replace the blown-down underflow. As the dense sludge approaches the basin center, the plow blade spacing reduces, and the shorter radius results in reducing tangen- tial blade velocities. In large basins, a second set of arms is typically employed to cover the center half (25% of the basin area) because the blade movement at this point is extremely slow. Deep blades formed into spiral sections bridge the main and auxiliary arms to push the crowded sludge into the hopper. In basins larger than about 80 ft (24 m) in diameter, it is advisable to use a double bottom slope. The double bottom slope is essential for basins more than 125 ft (38 m) in diameter. The two-slope design gives steeper slopes (greater hydraulic gradient) at the sludge hopper without excessive basin depth. The greater center depth dissipates scour- ing currents and is needed because of the concentration of influent energy in this rela- tively small region. Square snisaB Basins larger than 30 ft (9 m) square are typically equipped with circular sludge collec- tion systems with comer sweeps, always in pairs, to clean sludge from comers. Comer sweeps eliminate the need for larger comer fillets, which are generally unacceptable ex- cept in very small basins. Comer sweeps should be avoided in basins larger than 100 ft (30 m) square because of structural problems and wear associated with large cantilevered comer sweep units. Hydraulic problems usually occur with larger square basins. Radial density and dis- placement currents impinge on the peripheral walls at various angles, drift toward the cor- ners, meet, and may cause a rising "comer floc" phenomenon that often contributes tur- bidity to effluent. Where inboard weir troughs are used in square tanks, they are designed to cut across the comer to avoid rising corner floc. Where radial troughs are used (as is typical with upflow basins), they are always arranged to straddle the corner for the same reason. It is best not to run a single peripheral weir around the walls of a square basin because of the comer floc effect. Conventional square basins are not generally recommended. HIGH-RA TE CLARIFICATION High-rate clarification refers to all processes that can be loaded at higher rate than is typ- ically used in designing conventional clarifiers. The principal types of units currently be- ing used are • Tube settlers • Plate settlers • Solids contact units