- After the coagulation, flocculation, and sedimentation processes are complete, the raw water is much clearer, but is not yet potable. Some residual iron, manganese, clay, inert solids, bacteria, and other constituents are still present in the water. Even in a high quality unfiltered treated water having a total suspended solids concentration of 0.1 mg/l, approximately 200 million particles are present per liter. Therefore, filtration of the water is essential in further reducing the concentration of these constituents.
- Traditionally, filtration has been designed and used to remove suspended material, such as colloidal river silt, iron, and manganese from drinking water supplies. It has been desirable to remove these constituents because, if they are found in finished water, they can harbor bacteria and interfere with the disinfection process. More recently, however, filtration facilities are being designed to remove aquatic organisms, primarily Giardia cysts. These cysts, along with other constituents such as nematodes, algae, diatoms, mites, and flagellated protozoans, are being found in rural sources of drinking water which can consistently meet State and Federal turbidity standards. Since the presence of Giardia cysts in water supplies can result in waterborne outbreaks of Giardiasis, filtration is necessary to completely remove these organisms.
- Filtration is the process - of passing a properly treated water through a bed of some type of media for the purpose of reducing the particle concentration present in the water.
- Typical bacteria and Giardia cysts that can be removed in filtration are about 10 to 20 times smaller than the space between the filter media grains. If these contaminants to be removed are not pulled together in the coagulation process, most of them will easily pass through the filter media. However, if effective coagulation and flocculation are practiced, these materials will combine to form floc which will usually range in size from slightly less than 0.1 mm to 2 mm.
- The two main processes occurring in filtration are straining and adsorption. adsorption is much more prevalent than straining, but both depend on proper pretreatment prior to the water contacting the filter bed.
1. General Information
- Purpose of Filtration:
to remove particulate impurities and floc from water. This includes suspended particles (silt, clay), colloids, bacteria, plankton, and floc...Filters can remove up to 99.5% of suspended solids.
today the focus is removal of cryptosporidium (4-6mm) and giardia (8-18 mm) sized particles. Disinfection is ineffective against these parasites.
Effectiveness is measured by turbidity and particle count.
- Types of Filtration Processes
- conventional - coagulation, flocculation and sedimentation.
- direct filtration - coagulation, flocculation but no sedimentation.
- in-line filtration - pressure application
- Mechanism of Filtration (Physical and Chemical Process)
Removal of turbidity based on:
- chemical characteristics of the source water.
- nature of the suspension (physical and chemical)
- type and degree of pretreatment: coagulation, flocculation, and sedimentation.
- Filter type and operation - flow rates important.
- Particulate RemovalMechanisms
- Adsorption
- Sedimentation on media
- Straining (however, most particles removed are smaller than the media pore sizes).
- Absorption - carbon fibers
- Biological Action - BAF's
- Types of Filters
- Slow sand filters - take UP a lot of land, manually cleaned, low filtration rates (0.015 to 0.15 g.p.m./sq ft).
- Rapid sand filters - higher flow rates and automated backwashes.
- gravity filtration - 2 to 10 g.p.m./sq ft filtration rate.
- pressure filtration - not recommended due to breakthrough.
- diatomaceous earth filtration (also called pre-coat filtration). - used in food and beverage industry.
The filtration rate used depends on the type of media being used.
- Types of Filter Media
- single media - sand only. Filtration rate < 2.0 g.p.m./sq ft. (~ 3.0 MGD)
- dual media - sand and anthracite coal. Higher filtration rates and longer filter runs. Filtration rate <4.0 g.p.m./sq ft.
- multi-media - sand, anthracite coal an Garnet. May become more popular in efforts to remove cryptosporidium and giardia sized particles.
- Activated Carbon- used for taste and odor removal and on rivers susceptible to oil spills.
Gravel is not a media. Its purpose is to support the media. Leopold's IMS plate can be used instead of gravel on new plastic underdrain tiles.
- Filter Media Characteristics
- good hydraulic characteristics - permeable (water flows through it easily).
- inert - does not react with water and treatment chemicals.
- hard and durable - don't want it to crumble (long useful life 20-30 years)
- free of impurities - non-toxic.
- insoluble in water - don't want it to dissolve.
- Classification of Filter Media
- Effective Size: diameter size in @ of 10% by weight as determined by sieve analysis
- Uniformity Coefficient: ratio of particle diameters as determined by sieve analysis
- uniformity coefficient = Dia 60 (60% by weight,mm)
Dia 10 (10% by weight), mm The lower the uniformity coefficient, the more uniform the size of the media. The more uniform the media size, the slower the headloss buildup of a filter. Therefore, a low uniformity coefficient is desirable. Uniformity coefficients less than 1.5 are commonly used. PWD specs 1.4 for new dual media filters.
- specificgravity: sand=2.6, coal 1.5-1.8, garnet 3.1-4.3 gravel 2-6
- hardness: using the MOH scale (sand ~ 7, coal Ps 3, garnet z, 6.5 - 7.5)
Filter media selection based upon:
- time required to reach turbidity breakthrough.
- time required to reach terminal headloss.
- new attention being given to removal of crypto-sized particles
If headloss is a problem, you can go to a larger size sand.
If turbidity breakthrough is a problem, you can go to a smaller media size.
If both are a problem a deeper filter bed may be required but this may not be physically possible with existing filter structures.
II. Filter Operation
- Filtration Process
- Typical filter run times are 24-48 hours. Dual media filters have much longer filter runs than sand filters. Using alum also increases filter run time. Excessive filter runs should be avoided to prevent particle breakthrough.
- Polymer can be used as a filter aid. Non-ionic polymer is recommended for filter application. We have been using cationic polymer as filter aid and coagulant aid.
- polymer filter aid is very effective in improving filter effluent turbidity in the event of a coagulant feed problem or algae bloom It is also effective when high filtration rates are required.
- Text books state that polymer can improve filter run time but our experience has been that it drastically reduces filter run times.
- Prolonged use of polymer can lead to "gumming-up" of a filter with mudballs. Polymer as a filter aid should only be used when necessary to maintain water quality.
- Filter Performance Criteria
- Goal is to produce water with turbidity less than 0.1 NTU at an times.
- Peak turbidity after backwash should be limited to 0.3 NTU for no more than 15 minutes. If this cannot be achieved, consider filter to waste or backwash water treatment to reduce peak turbidity. We are just starting to aggressively monitor continuous on-line turbidity to monitor peaks.
- State regulations state combined filter effluent must be less than 2.0 NTU at all times and 95% of the monthly samples must be less than O.5 NTU. These will most likely be drastically lowered in light of the crypto outbreaks and the Partnership for Safe Water Guidelines.
- Process Controls
- Monitor Filter Applied and Filter Effluent turbidity. Filter Applied turbidity ranges 0,5-2.0 NTU and Filter Effluent turbidity ranges 0.05-0.20 NTU. Trend is toward continuous on-line turbidity monitors. Recommendation is for each individual filter.
- Monitor headloss: as filtration rate increases the headloss increases.
- Monitor filter run time: as filtration rate increase the run time decreases. Algae causes short Filter Runs.
- Monitor Particle Counts: to be implemented in near future. Results vary from instrument to instrument.
- Factors that can Deteriorate Filter Performance
- Do not make drastic sudden filtration rate changes. Guideline is no more than 0.3 MGD per hour (0.1 MGD per 30 minutes). Sudden increases in filtration rate can cause particle and turbidity breakthrough to occur.
- Do not exceed maximum filtration rate for the filter. Our filters are rated at a max. rate of 3.0 MGD. Belmont WTP filters are 4.0 MGD
- Do not start filters that have been taken off line without first backwashing them Never restart a "dirty" filter.
- Monitor filtration rates of individual filters closely. Defective Rate of Flow valves can cause turbidity excursions in individual filters.
Set the desired filtration rate for the filters. This can be done by setting an individual rate for each filter or in groups of filters (another of our plants has 3 filter groups). As the headloss in the filter increases, the rate of flow valve opens more to maintain the set point flow rate. We want to backwash the filter before turbidity breakthrough or particle breakthrough occurs. Therefore the filter is backwashed when a terminal headloss (5 feet) is reached. Filters can also be washed upon reaching a terminal run time. Each plant has set its own limit (40-60 hours). This will be redefined once particle counters are placed on-line. Until recently, the focus has been on turbidity. This is now shifting to particle counts, especially in cryptosporidium and giardia sized particle range.
III. Filter Backwashing
- The goal is to fluidize the filter bed to remove captured solids. The backwash rates vary with water temperature. Backwash rates are lower in cold water and higher in warm water. The PWD plants target a bed expansion of 30%. Cold water is more denser than warm water, thus it has more lifting capability and lower backwash rates can be used to fluidize the media. Dual media filters have lower backwash rates than sand filters, due to the lighter weight of the media.
- Backwash rates vary from approximately 11,000 gpm to 24,000 gpm.
- Backwash times are normally 6-7 minutes, except when step backwash procedure is used (10.5-15 minutes).
- If the backwash time is too short, the filter is not property cleaned. Mudballs and cracking of the media surface can be to occur, resulting in higher effluent turbidity and particle counts.
- If the backwash time is too long, you waste backwash water. Typical backwashing consumes 2-4% of the plants water production. Long backwash time alone will not clean a filter; you need rate also.
- If the backwash rate is too low, you don't fluidize the filter media enough to properly clean the filter. Mudballs and cracking of the media surface can begin to occur, resulting in higher effluent turbidity and particle counts.
Process Controls for Backwash rates and times:
Surface Wash is critical to properly cleaning a filter. Surface wash agitates the filter surface before and during the backwash to break up the matted solids on the top of the media.
- Our water plants use rotary sweeps ( 6 per filter) which spray water jets over the media to break up the accumulated solids on the surface of the filters. Problems with surface wash sweeps include:
- sweeps won't turn (bad bearings, not enough flow or pressure).
- surface wash valves hang up resulting in reduced flow rates and pressures.
Ineffective surface wash leads to improperly cleaned filters which results in higher effluent turbidity and particle counts.
- influent valve closes. the filter continues to filter water.
- When float switch senses low level the filter effluent valve closes, thus trapping water in the filter bed.
- The drain valve then opens and drains the filter down to the trough level.
- The surface wash sweeps are activated 2 minutes before backwashing starts when the surface wash valve opens and the surface wash pumps are on. Surface wash time is on a timer.
- The backwash valve opens to backwash the filters (also on timer).
- Surface wash valve closes before end of max. rate wash.
- Backwash valve closes
- Drain valve closes
- Influent Valve opens to refill the filter with water.
- Float switch now senses high level and begins to open filter effluent valve, returning the filter to service.
- The rate of flow valve ramps open over a specified period of time.
At my plant, the filter then sits in a 30 minute wait state. Then, the filter slowly ramps open to it's setpoint rate over another 30 minutes. This procedure is designed to allow the filter to "prime" itself to reduce peak turbidity after backwash. Rate of Flow valves can cause problems if the overshoot set point or malfunction.
The Backwash water can come directly from the Backwash pumps or from the washwater tank. The source of the backwash water and surface wash water is the filter effluent conduit or finished water clearwells.
IV. Filter Inspections
- State law requires a turbidimeter on each filter or an annual filter inspection of each filter. Filter inspections are important to find problems with filters such as missing media, media in underdrains, missing or clogged nozzles on surface wash sweeps, sweeps that don't turn, passing valves, condition of media surface, etc. Inspection forms have been developed at all three plants. (At my treatment plant, I have computerized the filter inspection forms & drawings..as data is entered, it feeds a summary sheet that compiles the year end totals and percentages for submission to management downtown & the DEP) Comprehensive inspections generate maintenance work orders which keep filters operating properly. (I also manage the Preventive Maintainence program @belmont.treatment.plant)
V. Continuous On-Line Instrumentation
- The Partnership for Safe Water recommends, at a minimum a turbidimeter on each filter and particle counting on a regular basis for each filter. This is definitely the trend for the future. The PVM plants are just beginning to implement on-line turbidimeters and particle counters. The data from these instruments can provide information such as:
- algae blooms or difficult to treat water quality (add polymer)
- when to switch coagulants
- check surface wash sweeps
- when to adjust backwash rates or time adjust filtration rates, hydraulic flow rates, set maximum filter run times, etc.
- Particle Counters measure the number of particles in various size ranges. They can be used at various stages of the treatment process to determine log-removals. We will also incorporate Particle Counter data into the annual Filter Inspections starting Fiscal Year 1997.
VI. The Do's and Don'ts of Filtration
- Do:
- perform thorough filter inspections and correct problems as needed.
- monitor peak turbidities, particle counts, backwash water turbidity, media % expansions, run times, headloss, filtration rates, surface wash pressure and flow rates, etc.!
- adjust backwash rates & times as needed to properly clean the filters. - observe backwash for signs of air and other problems. - continue to optimize backwash procedures by implementing step backwash procedures or backwash water treatment (polymer or coagulant).
- record keeping is very important for monitoring filter performance. If numbers are out of range, they should trigger a maintenance work order or corrective action. Be specific on work order requests since problems may occur on off-hours.
- Don't:
- exceed filtration rate maximum for the filters (~ 3.0 MGD)
- make sudden and significant filtration rate changes. At my plant, no more than 0.1 MGD per filter in 30 minutes.
- re-start "dirty or off-line filters without backwashing first.
- keep poor performing filters, based on high effluent turbidity or particle counts, in service. Take bad filters out of service.
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