Water Treatment Plant Design (McGraw-Hill Handbooks)

972 Pages·2004·6.66 MB·English

CLARIFICATION 7,13
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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
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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-
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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
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- mechanism (optional)
Scum weir t
-- (optional) l~ ~-- Floating bridge
Guide wheels
Siphon pipes
Effluent troughs
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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
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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