Is The Backwash Turnover Rate Different Form The Filter Rate

Typical construction
Types of Media
Mixed media filter beds
Capping of sand filters
Gravity filters
Pressure filters
Upflow filters
Automatic gravity filters
Continuous cleaning filters
Filter washing-Gravity filters
In-line description
Precoat filtration

Filtration is used in addition to regular coagulation and sedimentation for removal of solids from surface h2o or wastewater. This prepares the water for utilize as drink, boiler, or cooling make-upwardly. Wastewater filtration helps users meet more stringent effluent discharge permit requirements.

Filtration, usually considered a uncomplicated mechanical process, actually involves the mechanisms of adsorption (concrete and chemical), straining, sedimentation, interception, diffusion, and inertial compaction.

Filtration does not remove dissolved solids, but may be used together with a softening procedure, which does reduce the concentration of dissolved solids. For example, anthracite filtration is used to remove residual precipitated hardness salts remaining after description in precipitation softening.

In well-nigh h2o clarification or softening processes where coagulation and precipitation occur, at least a portion of the clarified water is filtered. Clarifier effluents of 2-10 NTU may be improved to 0.ane-1.0 NTU by conventional sand filtration. Filtration ensures acceptable suspended solids concentrations in the finished h2o fifty-fifty when upsets occur in the description processes.

TYPICAL CONSTRUCTION

Conventional gravity and pressure rapid filters operate downflow. The filter medium is ordinarily a 15-30 in. deep bed of sand or anthracite. Single or multiple grades of sand or anthracite may be used.

A large particle bed supports the filter media to prevent fine sand or anthracite from escaping into the underdrain system. The support bed also serves to distribute backwash h2o. Typical support beds consist of 1 8-ane in. gravel or anthracite in graded layers to a depth of 12-sixteen in.

TYPES OF MEDIA

Quartz sand, silica sand, anthracite coal, garnet, magnetite, and other materials may be used equally filtration media. Silica sand and anthracite are the most normally used types. When silica is not suitable (e.g., in filters following a hot process softener where the treated h2o is intended for boiler feed), anthracite is ordinarily used.

The size and shape of the filter media touch the efficiency of the solids removal. Sharp, angular media course large voids and remove less fine textile than rounded media of equivalent size. The media must exist coarse plenty to allow solids to penetrate the bed for 2-iv in. Although near suspended solids are trapped at the surface or in the commencement i-two in. of bed depth, some penetration is essential to prevent a rapid increment in pressure driblet.

Sand and anthracite for filters are rated by effective particle size and uniformity. The constructive size is such that approximately 10% of the total grains past weight are smaller and xc% are larger. Therefore, the effective size is the minimum size of most of the particles. Uniformity is measured by comparison of effective size to the size at which 60% of the grains by weight are smaller and twoscore% are larger. This latter size, divided by the effective size, is called the uniformity coefficient-the smaller the uniformity coefficient, the more compatible the media particle sizes.

Effectively sands result in shallower zones for the memory of suspended affair. The about desirable media size depends on the suspended solids characteristics besides every bit the effluent quality requirements and the specific filter design. In general, rapid sand filters use sand with an effective size of 0.35-0.sixty mm (0.014-0.024 in.) and a maximum uniformity coefficient of 1.seven. Coarse media, often 0.6-1.0 mm (0.024-0.04 in.), are used for closely controlled coagulation and sedimentation.

MIXED MEDIA FILTER BEDS

The terms "multilayer," "in-depth," and "mixed media" apply to a blazon of filter bed which is graded by size and density. Coarse, less dense particles are at the pinnacle of the filter bed, and fine, more dense particles are at the bottom. Downflow filtration allows deep, uniform penetration past particulate matter and permits loftier filtration rates and long service runs. Because small particles at the bottom are also more than dense (less infinite between particles), they remain at the bottom. Even later on high-rate backwashing, the layers remain in their proper location in the mixed media filter bed.

Table 6-i lists iv media that are used in multilayer filtration. Several other mixed media combinations have also been tested and used effectively. The employ of too many different media layers tin can cause severe backwashing difficulties. For example, if all four materials listed in Table 6-one were used in the aforementioned filter, a wash charge per unit loftier enough to expand the magnetite layer might wash the anthracite from the filter. High wash h2o requirements would besides result.

Table six-1. Media used in multilayer filtration.
Media	Effective size, mm (in.)	Specific gravity
Anthracite	0.7-1.vii (0.03-0.07)	1.4
Sand	0.3-0.7 (0.01-0.03)	2.6
Garnet	0.iv-0.6 (0.016-0.024)	iii.8
Magnetite	0.three-0.5 (0.01-0.02)	4.9

Anthracite/sand filter beds normally provide all of the advantages of single-media filtration but require less backwash water than sand or anthracite solitary. Similar claims have been made for anthracite/sand/garnet mixed units. The major advantages of dual-media filtration are higher rates and longer runs. Anthracite/sand/garnet beds have operated at normal rates of approximately 5 gpm/ft² and peak rates as loftier as 8 gpm/ft² without loss of effluent quality.

CAPPING OF SAND FILTERS

Rapid sand filters can be converted for mixed media operation to increment capacity by 100%. The cost of this conversion is much lower than that of installing additional rapid sand filters.

Capping involves the replacement of a portion of the sand with anthracite. In this conversion, a two-6 in. layer of 0.4-0.half dozen mm (0.016-0.024 in.) sand is removed from the surface of a bed and replaced with 4-eight in. of 0.nine mm (0.035 in.) anthracite. If an increase in capacity is desired, a larger amount of sand is replaced. Airplane pilot tests should be run to ensure that a reduction in the depth of the finer sand does not reduce the quality of the effluent.

GRAVITY FILTERS

Gravity filters (run across Effigy six-ane) are open up vessels that depend on system gravity caput for functioning. Apart from the filter media, the essential components of a gravity filter include the following:

The filter shell, which is either concrete or steel and can be square, rectangular, or circular. Rectangular reinforced physical units are nigh widely used.
The support bed, which prevents loss of fine sand or anthracite through the underdrain system. The support bed, usually one-2 ft deep, also distributes backwash water.
An underdrain arrangement, which ensures compatible collection of filtered water and compatible distribution of aftermath h2o. The system may consist of a header and laterals, with perforations or strainers spaced suitably. False tank bottoms with accordingly spaced strainers are also used for underdrain systems.
Wash h2o troughs, large enough to collect backwash water without flooding. The troughs are spaced so that the horizontal travel of backwash h2o does not exceed 3-three ft. In conventional sand bed units, wash troughs are placed approximately two ft above the filter surface. Sufficient freeboard must exist provided to prevent loss of a portion of the filter media during performance at maximum backwash rates.
Control devices that maximize filter performance efficiency. Menses rate controllers, operated by venturi tubes in the effluent line, automatically maintain uniform delivery of filtered water. Backwash period rate controllers are also used. Menstruum rate and head loss gauges are essential for efficient performance.

PRESSURE FILTERS

Pressure filters are typically used with hot procedure softeners to permit loftier-temperature operation and to forbid estrus loss. The use of pressure filters eliminates the need for repumping of filtered h2o. Force per unit area filters are similar to gravity filters in that they include filter media, supporting bed, underdrain organization, and command device; however, the filter vanquish has no wash water troughs.

Pressure filters, designed vertically or horizon-tally, take cylindrical steel shells and dished heads. Vertical pressure filters (see Figure half-dozen-two) range in bore from i to x ft with capacities as great as 300 gpm at filtration rates of 3 gpm/ft². Horizontal pressure filters, ordinarily 8 ft in bore, are 10-25 ft long with capacities from 200 to 600 gpm. These filters are separated into compartments to allow private backwashing. Aftermath water may exist returned to the clarifier or softener for recovery.

Pressure filters are usually operated at a service flow charge per unit of three gpm/ft². Dual or multimedia filters are designed for six-8 gpm/ft². At ambience temperature, the recommended filter backwash rate is six-viii gpm/ft² for anthracite and 13-15 gpm/ft² for sand. Anthracite filters associated with hot process softeners require a backwash rate of 12-15 gpm/ft² considering the water is less dense at elevated operating temperatures. Common cold water should non be used to backwash a hot process filter. This would cause expansion and wrinkle of the system metallurgy, which would lead to metal fatigue. Likewise, the oxygen-laden cold water would accelerate corrosion.

UPFLOW FILTERS

Upflow units comprise a single filter medium–usually graded sand. The finest sand is at the top of the bed with the coarsest sand below. Gravel is retained by grids in a fixed position at the bottom of the unit. The function of the gravel is to ensure proper water distribution during the service cycle. Another grid above the graded sand prevents fluidization of the media. Air injection during cleaning (non considered aftermath because the direction of period is the same as when in-service) assists in the removal of solids and the reclassification of the filter media. During operation, the larger, coarse solids are removed at the lesser of the bed, while smaller solids particles are allowed to penetrate farther into the media. Typical service flow rates are v-ten gpm/ft². An case of this unit is shown in Figure 6-3.

Automatic GRAVITY FILTERS

Several manufacturers take developed gravity filters that are backwashed automatically at a preset head loss. Head loss (h2o level higher up the media) actuates a backwash siphon and draws wash water from storage upwards through the bed and out through the siphon pipage to waste. A depression level in the backwash storage section breaks the siphon, and the filter returns to service.

Automated gravity filters are bachelor in diameters of up to 15 ft. When equipped with a high-rate, multilayer media, a single big-diameter unit can filter equally much every bit 1,000 gpm. An example is shown in Figure 6-4.

CONTINUOUS CLEANING FILTERS

Continuous cleaning filter systems eliminate off-line backwash periods by backwashing sections of the filter or portions of the filter media continuously, on-line. Various designs have been introduced. An example is shown in Effigy 6-5.

FILTER WASHING-GRAVITY FILTERS

Periodic washing of filters is necessary for the removal of accumulated solids. Inadequate cleaning permits the formation of permanent clumps, gradually decreasing filter capacity. If fouling is astringent, the media must be cleaned chemically or replaced.

For cleaning of rapid downflow filters, make clean water is forced back up and through the media. In conventional gravity units, the backwash water lifts solids from the bed into wash troughs and carries them to waste. Either of two backwash techniques can be used, depending on the design of the media support structure and the accessory equipment available:

High-rate aftermath, which expands the media past at to the lowest degree 10%. Backwash rates of 12-15 gpm/ft² or college are common for sand, and rates for anthracite may range from 8 to 12 gpm/ft².
Low-rate aftermath, with no visible bed expansion, combined with air scouring.

Where only water is used for aftermath, the aftermath may be preceded by surface washing. In surface washing, strong jets of high-pressure water from fixed or revolving nozzles assist in breaking the filter surface crust. After the surface wash (when there is provision for surface washing), the unit of measurement is backwashed for approximately five-10 min. Post-obit backwash, a small-scale amount of rinse h2o is filtered to waste product, and the filter is returned to service.

High-charge per unit backwash can cause the germination of mud balls inside the filter bed. A high backwash rate and resulting bed expansion can produce random currents in which sure zones of the expanded bed move upward or downwardly. Encrusted solids from the surface can exist carried down to class mud assurance. Efficient surface washing helps prevent this condition.

Air scouring with low-rate backwashing tin can break upwardly the surface crust without producing random currents, if the underdrain system is de-signed to distribute air uniformly. Solids removed from the media collect in the layer of h2o between the media surface and launder channels. Later on the air is stopped, this dirty water is nor-mally flushed out by increased backwash h2o flow charge per unit or by surface draining. Launder water consumption is approximately the same whether water-only or air/water backwashing is employed.

IN-LINE CLARIFICATION

In-line description is the removal of suspended solids through the addition of in-line coagulant followed by rapid filtration. This process is likewise referred to as in-line filtration, or contact filtration. The process removes suspended solids without the use of sedimentation basins. Coagulation may be achieved in in-line clarification past either of two methods:

an inorganic aluminum or iron salt used alone or with a loftier molecular weight polymeric coagulant
a strongly cationic organic polyelectrolyte

Because metal hydroxides form precipitates, but dual-media filters should be used with inorganic coagulant programs. Floc particles must be handled in filters with fibroid-to-fine graded media to prevent rapid blinding of the filter and eliminate backwashing difficulties. Where a high molecular weight polymeric coagulant is used, feed rates of less than 0.1 ppm maximize solids removal by increasing floc size and promoting particle absorption within the filter. This filtration technique readily yields effluent turbidities of less than 0.five NTU. Figure 6-6.

The second method of coagulant pretreatment involves the utilise of a single chemical, a strongly charged cationic polyelectrolyte. This treatment forms no precipitation floc particles, and commonly no floc germination is visible in the filter influent. Solids are removed within the bed by adsorption and by flocculation of colloidal affair straight onto the surface of the sand or anthracite media. The procedure may be visualized as seeding of the filter bed surfaces with positive cationic charges to produce a strong pull on the negatively charged particles. Because gelled hydroxide precipitates are not present in this process, single- media or upflow filters are suitable for poly-electrolyte clarification.

In-line description provides an excellent manner to better the efficiency of solids removal from turbid surface waters. Effluent turbidity levels of less than 1 NTU are common with this method.

PRECOAT FILTRATION

Precoat filtration is used to remove very modest particulate affair, oil particles, and even bacteria from water. This method is applied just for relatively pocket-sized quantities of water which incorporate depression concentrations of contaminants.

Precoat filtration may be used following conventional description processes to produce water of very low suspended solids content for specific application requirements. For example, precoat filters are oft used to remove oil from contaminated condensate.

In precoat filtration, the precoat media, typically diatomaceous world, acts as the filter media and forms a cake on a permeable base of operations or septum. The base must prevent passage of the precoat media without restricting the catamenia of filtered h2o and must be capable of withstanding high pressure level differentials. Filter cloths, porous rock tubes, porous paper, wire screens, and wire-wound tubes are used equally base materials.

The supporting base of operations fabric is get-go precoated with a slurry of precoat media. Additional slurry (trunk feed) is usually added during the filter run. When the accumulation of matter removed past filtration generates a high pressure drop across the filter, the filter blanket is sloughed off by backwashing. The filter bed is then precoated and returned to service. Chemical coagulants are not normally needed but take been used where an ultrapure effluent is required.

Effigy 6-1. Typical gravity filter unit. (Courtesy Roberts Filter Manufacturing Co.)

Figure vi-2. Vertical-type pressure sand filter. (Courtesy the Permutit Company, Inc.)

Figure 6-3. Upflow in-line filter. (Courtesy of L'Eau Claire Systems, Inc.)

Effigy half-dozen-4. Monovalve gravity filter. (Courtesy of Graver Div., Ecodyne Corporation.)

Effigy half dozen-5. DynaSand continuous cleaning filter. (Courtesy of Parkson Corp.)

Effigy 6-6. Principles of diatomite filtration. (Courtesy of Johns-Manville Corp.)