FiltrationIn the conventional water treatment process, filtration usually follows coagulation, flocculation, and sedimentation (see Fig. 1.1). At present, filtration is not always used in small water systems. However, recent regulatory requirements under EPAs Interim Enhanced Surface Water Treatment Rule (IESWTR) may make water filtering necessary at most water supply systems.Water filtration is a physical process of separating suspended and colloidal particles from water by passing water through a granular material. The process of filtration involves straining, settling, and adsorption. As floc passes into the filter, the spaces between the filter grains become clogged, reducing this opening and increasing removal.Fig. 1.1 Filtration

Some material is removed merely because it settles on a media grain. One of the most important processes is adsorption of the floc onto the surface of individual filter grains. This helps collect the floc and reduces the size of the openings between the filter media grains.In addition to removing silt and sediment, floc, algae, insect larvae, and any other large elements, filtration also contributes to the removal of bacteria and protozoans such as Giardia lamblia And cryptosporidium. Some filtration processes are also used for iron and manganese removal.

Types of Filter TechnologiesThe Surface Water Treatment Rule (SWTR) specifies four filtration technologies, although it also allows the use of alternate filtration technologies (e.g., cartridge filters). These include slow sand filtration or rapid sand filtration, pressure filtration, diatomaceous earth filtration, and direct filtration. Of these, all but rapid sand filtration are commonly employed in small water systems that use filtration. Each type of filtration system has advantages and disadvantages. Regardless of the type of filter, filtration involves the processes of straining (where particles are captured in the small spaces between filter media grains), sedimentation (where the particles land on top of the grains and stay there), and adsorption (where a chemical attraction occurs between the particles and the surface of the media grains).Slow Sand FiltersThe first slow sand filter was installed in London in 1829 and was used widely throughout Europe, though not in the U.S. By 1900, rapid sand filtration began taking over as the dominant filtration technology, and a few slow sand filters are in operation today. However, with the advent of the Safe Drinking Water Act (SDWA) and its regulations (especially the Surface Water Treatment Rule) and the recognition of the problems associated with Giardia lamblia and cryptosporidium in surface water, the water industry is reexamining slow sand filters. This is because low technology requirements may prevent many state water systems from using this type of equipment.On the plus side, slow sand filtration is well suited for small water systems. It is a proven, effective filtration process with relatively low construction costs and low operating costs (it does not require constant operator attention). It is quite effective for water systems as large as 5000 people; beyond that, surface area requirements and manual labor required to recondition the filters make rapid sand filters more effective. The filtration rate is generally in the range of 45 to 150 gal/d/ft2.Components making up a slow sand filter include the following:1. A covered structure to hold the filter media2. An underdrain system3. Graded rock that is placed around and just above the underdrain4. The filter media, consisting of 30 to 55 in. of sand with a grain size of 0.25 to 0.35 mm5. Inlet and outlet piping to convey the water to and from the filter, and the means drain filtered water to wasteFlooding the area above the top of the sand layer with water to a depth of 3 to 5 ft and allowing it to trickle down through the sand operates slow sand filters. An overflow device prevents excessive water depth. The filter must have provisions for filling it from the bottom up. It must also be equipped with a loss-of-head gauge, a rate-of-flow control device (e.g., an orifice or butterfly valve), a weir or effluent pipe that assures that the water level cannot drop below the sand surface, and filtered waste sample taps.When the filter is first placed in service, the head loss through the media caused by the resistance of the sand is about 0.2 ft (i.e., a layer of water 0.2 ft deep on top of the filter will provide enough pressure to push the water downward through the filter). As the filter operates, the media becomes clogged with the material being filtered out of the water, and the head loss increases. When it reaches about 4 to 5 ft, the filter needs to be cleaned.For efficient operation of a slow sand filter, the water being filtered should have a turbidity average less than 5 turbidity units (TU), with a maximum of 30 TU. Slow sand filters are not backwashed the way conventional filtration units are. The 1 to 2 in. of material must be removed on a periodic basis to keep the filter operating.Rapid Sand FiltersThe rapid sand filter, which is similar in some ways to slow sand filter, is one of the most widely used filtration units. The major difference is in the principle of operation the speed or rate at which water passes through the media. In operation, water passes downward through a sand bed that removes the suspended particles. The suspended particles consist of the coagulated matter remaining in the water after sedimentation, as well as a small amount of uncoagulated suspended matter.Some significant differences exist in construction, control, and operation between slow sand filters and rapid sand filters. Because of the construction and operation of the rapid sand filtration with its higher filtration, the land area needed to filter the same quantity of water is reduced.The rapid sand filter structure and equipment includes the following:1. Structure to house media2. Filter media3. Gravel media support layer4. Underdrain system5. Valves and piping system6. Filter backwash system7. Waste disposal systemUsually 2 to 3 ft deep, the filter media is supported by approximately 1 ft of gravel. The media may be fine sand or a combination of sand, anthracite coal, and coal (dual-multimedia filter).Water is applied to a rapid sand filter at a rate of 1.5 to gal/min/ft2 of filter media surface. When the rate is between 4 and 6 gal/min/ft2, the filter is referred to as a high-rate filter; when the rate is over gal/min/ft2, the filter is called ultra-high-rate. These rates compare to the slow sand filtration rate of 45 to 150 gal/d/ft2. High-rate and ultra-high-rate filters must meet additional conditions to assure proper operation.Generally, raw water turbidity is not that high. However, even if raw water turbidity values exceed 1000 TU, properly operated rapid sand filters can produce filtered water with a turbidity or well under 0.5 TU. The time the filter is in operation between cleanings (filter runs) usually lasts from 12 to 72 h, depending on the quality of the raw water; the end of the run is indicated by the head loss approaching 6 to 8 ft. Filter breakthrough (when filtered material is pulled through the filter into the effluent) can occur if the head loss becomes too great. Operation with head loss too high can also cause air binding (which blocks part of the filter with air bubbles), increasing the flow rate through the remaining filter area.Rapid sand filters have the advantage of lower land requirement, and have other advantages as well. For example, rapid sand filters cost less, are less labor-intensive to clean, and offer higher efficiency with highly turbid waters. On the downside, operation and maintenance costs of rapid sand filters are much higher because of the increased complexity of the filter controls and backwashing system.In backwashing a rapid sand filter, cleaning the filter is accomplished by passing treated water backwards (upwards) through the filter media and agitating the top of the media. The need for backwashing is determined by a combination of filter run time (i.e., the length of time since the last backwashing), effluent turbidity, and head loss through the filter. Depending on the raw water quality, the run time varies from one filtration plant to another (and may even vary from one filter to another in the same plant).Note: Backwashing usually requires 3 to 7% of the water produced by the plant.Pressure Filter SystemsWhen raw water is pumped or piped from the source to a gravity filter, the head (pressure) is lost as the water enters the floc basin. When this occurs, pumping the water from the plant clearwell to the reservoir is usually necessary. One way to reduce pumping is to place the plant components into pressure vessels, maintaining the head. This type of arrangement is called a pressure filter system. Pressure filters are also quite popular for iron and manganese removal and for filtration of water from wells. They may be placed directly in the pipeline from the well or pump with little head loss. Most pressure filters operate at a rate of about 3 gal/min/ft2.Operationally the same as and consisting of components similar to those of a rapid sand filter, the main difference between a rapid sand filtration system and a pressure filtration system is that the entire pressure filter is contained within a pressure vessel. These units are often highly automated and are usually purchased as self-contained units with all necessary piping, controls, and equipment contained in a single unit. They are backwashed in much the same manner as the rapid sand filter.The major advantage of the pressure filter is its low initial cost. They are usually prefabricated, with standardized designs. A major disadvantage is that the operator is unable to observe the filter in the pressure filter and determine the condition of the media. Unless the unit has an automatic shutdown feature on high effluent turbidity, driving filtered material through the filter is possible.Diatomaceous Earth FiltersDiatomaceous earth is a white material made from the skeletal remains of diatoms. The skeletons are microscopic, and in most cases, porous. There are different grades of diatomaceous earth, and the grade is selected based on filtration requirements.These diatoms are mixed in water slurry and fed onto a fine screen called a septum, usually of stainless steel, nylon, or plastic. The slurry is fed at a rate of 0.2 lb/ft2 of filter area. The diatoms collect in a pre-coat over the septum, forming an extremely fine screen. Diatoms are fed continuously with the raw water, causing the buildup of a filter cake approximately 1/8 to 1/5 in. thick. The openings are so small that the fine particles that cause turbidity are trapped on the screen. Coating the septum with diatoms gives it the ability to filter out very small microscopic material. The fine screen and the buildup of filtered particles cause a high head loss through the filter. When the head loss reaches a maximum level (30 psi on a pressure-type filter or 15 inHg on a vacuum-type filter), the filter cake must be removed by backwashing.The slurry of diatoms is fed with raw water during filtration in a process called body feed. The body feed prevents premature clogging of the septum cake. These diatoms are caught on the septum, increasing the head loss and preventing the cake from clogging too rapidly by the particles being filtered. While the body feed increases head loss, head loss increases are more gradual than if body feed were not use.Although diatomaceous earth filters are relatively low in cost to construct, they have high operating costs and can give frequent operating problems if not properly operated and maintained. They can be used to filter raw surface waters or surface-influenced groundwaters, with low turbidity (900 gal/d/ft3 and a continuous sloughing frequency. It does not normally include recirculation and typically has a 40 to 65% BOD removal rate and 40 to 65% TSS removal rate.General Process DescriptionThe trickling filter process involves spraying wastewater over a solid media such as rock, plastic, or redwood slats (or laths). As the wastewater trickles over the surface of the media, a growth of microorganisms (bacteria, protozoa, fungi, algae, helminthes or worms, and larvae) develops. This growth is visible as a shiny slime very similar to the slime found on rocks in a stream. As the wastewater passes over this slime, the slime adsorbs the organic (food) matter. This organic matter is used for food by the microorganisms. At the same time, air moving through the open spaces in the filter transfers oxygen to the wastewater. This oxygen is then transferred to the slime to keep the outer layer aerobic. As the microorganisms use the food and oxygen, they produce more organisms, carbon dioxide, sulfates, nitrates, and other stable by-products; these materials are then discarded from the slime back into the wastewater flow and are carried out of the filter. The process is shown in the following equation:Organics + Organisms + O2 = More Organisms + CO2 + Solid WastesThe growth of the microorganisms and the buildup of solid wastes in the slime make it thicker and heavier. When this slime becomes too thick, the wastewater flow breaks off parts of the slime. These must be removed in the final settling tank.In some trickling filters, a portion of the filter effluent is returned to the head of the trickling filter to level out variations in flow and improves operations (recirculation).Overview and Brief Summary of Trickling Filter ProcessThe following list provides an overview of the trickling filter process:1. A trickling filter consists of a bed of coarse media, usually rocks or plastic, covered with microorganisms.2. The wastewater is applied to the media at a controlled rate, using a rotating distributor arm or fixed nozzles. Organic material is removed by contact with the microorganisms as the wastewater trickles down through the media openings. The treated wastewater is collected by an underdrain system.3. The trickling filter is usually built into a tank that contains the media. The filter may be square, rectangular, or circular.4. The trickling filter does not provide any actual filtration. The filter media provides a large amount of surface area that the microorganisms can cling to and grow in a slime that forms on the media as they feed on the organic material in the wastewater.5. The slime growth on the trickling filter media periodically sloughs off and is settled and removed in a secondary clarifier that follows the filter.6. Key factors in trickling filter operation include the following concepts:A. Hydraulic loading rateB. Organic loading rateC. RecirculationD. Total FlowIf the recirculated flow rate is given, total flow is:Total Flow (MGD) = Influent Flow (MGD) + Recirculation Flow (MGD)Total Flow (gal/d) = Total Flow (MGD) x 1,000,000 gal/MGNote: The total flow to the tricking filter includes the influent flow and the recirculated flow. This can be determined using the recirculation ratio:Total Flow (MGD) = Influent Flow + (Recirculation Rate + 1.0)

Process Calculations1. Calculating the Rate of FiltrationIn waterworks operation (and to an increasing degree in wastewater treatment), the rate of flow through filters is an important operational parameter. While flow rate can be controlled by various means or may proceed at a variable declining rate, the important point is that with flow suspended matter continuously builds up within the filter bed, affecting the rate of filtration.Problem:A filter box is 20 30 ft (also the sand area). If the influent value is shut, the water drops 3 in./min. What is the rate of filtration in MGD?Solution:Given:Filter box = 20 x 30 ftWater drops = 3 in./minRequired: Find the volume of water passing through the filterEquations:v = A x HA = W x LStep 1: Calculate the area; convert 3 in. to feet, and divide 3 by 12 to find feet:A = 20 ft x 30 ft = 600 ft23.0 /12 = 0.25 ftv = 600 ft2 x 0.25 ft= 150 ft3 of H2O passing through the filter in 1 minuteStep 2: Convert ft3 to gallon150 ft3/min x 7.48 gal/ft3 = 1122 gal/minStep 3: The problem asks for the rate of filtration in MGD. To find MGD, multiply the number of gallons per minute by the number of minutes per day.1122 gal/min x 1440 min/day = 1.62 MGD

2. Filter BackwashIn filter backwashing, one of the most important operational parameters to be determined is the amount of water in gallons required for each backwash. This amount depends on the design of the filter and the quality of the water being filtered. The actual washing typically lasts 5 to 10 min and uses amounts of 1 to 5% of the flow produced.Problem:A filter has the following dimensions:L = 30 ftW = 20 ftDepth of filter media = 24 in.Assuming a backwash rate of 15 gal/ft2/min per minute is recommended, and 10 minutes of backwash is required, calculate the amount of water in gallons required for each backwash.Solution:Given:L = 30 ftW = 20 ftDepth of filter media = 24 in.Rate = 15 gal/ft2/minBackwash time = 10 minRequired: Find the amount of water in gallons requiredStep 1: Calculate the area of the filter:30 ft x 20 ft = 600 ft2Step 2: Calculate the gallons of H2O used per square foot of filter:15 gal/ft2/min x 10 min = 90,000 gal required for backwash

3. Trickling Filter Total FlowProblem:The trickling filter is currently operating with a recirculation rate of 1.5. What is the total flow applied to the filter when the influent flow rate is 3.65 MGD?Solution:Total Flow (MGD) = 3.65 MG x (1.5 + 1.0)= 9.13 MGD

4. Trickling Filter Hydraulic LoadingProblem:A trickling filter 90-ft in diameter is operated with a primary effluent of 0.488 MGD and a recirculated effluent flow rate of 0.566 MGD. Calculate the hydraulic loading rate on the filter in units gallons per day per square foot.Solution:The primary effluent and recirculated trickling filter effluent are applied together across the surface of the filter, therefore:0.488 MGD + 0.566 MGD = 1.054 MGD= 1,054,000 gal/dCircular surface area = 0.785 x (Diameter)2= 0.785 x (90 ft)2= 6359 ft2(1,054,000 gal/d) / 6359 ft2 = 165.7 gal/d/ft2

5. Trickling Filter Organic LoadingAs mentioned earlier, trickling filters are sometimes classified by the organic loading rate applied. The organic loading rate is expressed as a certain amount of BOD applied to a certain volume of media.Problem:A trickling filter, 50 ft in diameter, receives a primary effluent flow rate of 0.445 MGD. Calculate the organic loading rate in units of pounds of BOD applied per day per 900 ft3 of media volume. The primary effluent BOD concentration is 85 mg/L. The media depth is 9 ft.Solution:0.445 MGD x 85 mg/L x 8.34 lb/gal = 315.5 BOD applied/dSurface Area = 0.785 x (Diameter)2= 0.785 x (50 ft)2= 1962.5 ft2A x D = v1962.5 ft2 x 9 ft2 = 17,662.5 (Trickling filter volume)To determine the pounds of BOD/1000 ft3 in a volume of thousands of cubic feet, we must set up the equation as shown below:Regrouping the numbers and the units together:= 17.9 lb BOD/d/1000ft3

Coarse Solids ReductionAs an alternative to coarse bar screens or fine screens, comminutors and macerators can be used to intercept coarse solids and grind or shred them in the screen channel. High-speed grinders are used in conjunction with mechanically cleaned screens to grind and shreds screening that are remove from wastewater. The solids are cut up into a smaller, more uniform size for return to the flow stream for subsequent removal by downstream treatment operations and processes. Comminutors, macerators and grinders can theoretically can eliminate the messy and offensive task off screening handling and disposal. The use of comminutors and macerators is particularly advantageous in a pumping station to protect the pumps against clogging by rags and large objects and to eliminate the need to handle and dispose of screenings. They are particularly useful in cold climates where collected in screening are subject to freezing.There is a wide divergence of views, however, on the suitability of using devices that grind and shred screenings at wastewater-treatment plants. One school of thought maintains that once coarse solids have been removed from wastewater, they should not be returned, regardless of the form. The other school thought maintains that once cut up, the solids are more easily handled in the downstream process. Shredded solids often present downstream problems, particularly with rags and plastic bags, as they tend to form rope like strands. Rag and plastic strands can have a number of adverse impacts, such as clogging pump impellers, sludge pipelines, and heat exchangers, and accumulating on air diffusers and clarifier mechanisms. Plastics and other nonbiodegradable material may also adversely affect the equality of biosolids that are to be beneficially reused.Approaches to using comminutors, macerators, and grinders are applicable in many retrofit situation. Example of retrofit applications include plants where a spare channel has been provided for the future installation of a duplicate unit or in very deep influent pumping stations where the removal of screenings may be too difficult or costly to achieve. Alternative approaches may also be possible, such as using chopper pumps at pumping station or installing grinders ahead of sludge pumps.

Comminutors

Comminutors are used most commonly in small wastewater-treatment plants, less than 0.2m3/s (5Mgal/d). Comminutors are installed in a wastewater flow channel to screen and shred material to sizes 6 to 20mm (0.25 to 0.77 in) without removing the shredded solids from the flow stream. A typical comminutor uses a stationary horizontal screen to intercept the flow and a rotating oscillating arm that contains cutting teeth to mesh with the screen. The cutting teeth and the shear bars cut coarse material. The small sheared particles pass through the screen and into the downstream channel. Comminutors may create a string of material, namely, rags that can collect on downstream treatment equipment. Because of operating problems and high maintenance with comminutors, newer installations generally use a screen or a macerator described below.

Macerators

Macerators are slow-speed grinder that typically consist of two sets of counter rotating assemblies with blades. The assemblies are mounted vertically in the flow channel. The blades or teeth on the rotating assemblies have a close tolerance that effectively chops material as it passes through the unit. The chopping action reduces the potential for producing ropes of rags or plastic that can collect on downstream equipment. Macerators can be used in pipeline installations to shred solids, particularly ahead of wastewater and sludge pumps, or in channels at wastewater-treatment plants. Sizes for pipelines applications range from 100 to 400mm (40 to 16 in) in diameter. Another type of macerator used in channel application is a moving linked screen that allow wastewater to pass through the screen while diverting screening to a grinder located at one side of the channel. Standard size of this device are available for use in large channels ranging from widths of 750mm to1800mm (30 to 72in) and depths of 750 to 2500mm (30 to 100in). The headloss is lower than that of the units counter rotating blades.

Grinders

High-speed grinders, typically referred to as hammermills, receive screened materials from bar screens. The materials are pulverized by a high-speed rotating assembly that cuts the materials passing through the unit. The cutting or knife blades force screenings trough stationary grid or louver that encloses the rotating assembly. Washwater is typically used to keep the unit clean and to help transport material back to the wastewater stream. Discharge from the grinder can be located either upstream or downstream of the bar screen.

Design Consideration

Comminuting and macerating devices may be preceded by grit chambers to prolong the life of the equipment and to reduce the wear on the cutting surfaces. Comminutors should be constructed with a bypass arrangement so that a manual bar screen is used in case flowrates exceeds the capacity of the comminutor or when there is a power or mechanical failure. Stop gates provisions for dewatering the channel should also be included to facilitate maintenance. Headloss through a comminutor usually ranges from 0.1 to 0.3m (4 to 12 in), and can approach 0.9m (3ft) in large units at maximum flowrates.In cases where a comminutor or macerator precedes grit chambers, the cutting teeth are subject to high wear and require frequent sharpening or replacement. Units that use cutting mechanism ahead of the screen grid should be provided with rock traps in the channel upstream of the comminutor to collect material that could jam the cutting blade.Because these units are complete in themselves, no detailed design is necessary. Manufacturers data and rating tables for these units should be consulted for recommended channel dimension, capacity ranges, headloss, upstream and downstream submergence, and power requirements. Because manufacturers capacity ratings are usually based on clean water, the ratings should be decreased by approximately 80 percent to account for partial clogging of the screen.