7: Filtration
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Learning Objectives
After reading this chapter you should be able to identify and explain the following:
- Treatment technologies
- Filter media configurations and types
- Filter operation and backwashing
- Filtration math
Filtration is the final and most important removal requirement required by the Surface Water Treatment Rule (SWTR). Water passes through material such as sand, gravel, and anthracite coal to remove floc and disease-causing microorganisms from the finished water. Physical removal of colloids is also achieved during sedimentation but this filtration is the final step. This is the process where suspended colloidal particles are removed from the water. Along with removing possible pathogenic material in the water, removal of turbidity is also achieved which could hide pathogenic organisms and add color to the finished water.
The SWTR sets forth guidelines for all public water agencies that use surface water as a source. Surface water was covered in chapter #1 and is any water open to atmosphere that is susceptible to runoff. The minimum requirements for treatment is disinfection, but most water sources do not meet these very stringent guidelines.
The effectiveness of filtration is based on several important factors. Proper filtration occurs based on incoming source water quality. For example a storm event near the source water could case higher incoming turbidity than the treatment plant is used to handling. Operational changes such as washing filters may be required. The physical and chemical characteristics of suspended material also come into play during treatment. Too much or too little chemical can lead to ineffective filtration. To follow the storm event example an operator may need to make changes to coagulant and polymer doses to account for increased turbidity and particulate entering the treatment plant. Finally the type of filtration used by a treatment facility is also very important. This decision is mostly out of operators hands as engineers and water quality experts will decide what the most effective treatment process is for the source water before building a treatment facility.
Pictured below is a filter in normal operation.
Treatment Technologies
There are four approved treatment technologies in the United States. The most widely used treatment technologies include conventional treatment, direct filtration, diatomaceous earth treatment, and slow sand filtration. Below are the descriptions:
Slow sand filtration facilities are becoming less common because of the large amount of time it takes to treat water and the large amount of space the facilities require. The filtration rates for slow sand filtration are .05 to .10 GPM/sq ft. Particles are adsorbed in a chemical layer known as a schmutzdeke. After an amount of time the biological layer must be manually removed by an operator or maintenance staff. The slow time and intensive labor make this treatment method the least ideal especially in areas with larger populations.
Diatomaceous Earth filtration is accomplished through pressure filtration. It can also be referred to as precoat filtration. The filter media in this case is added as slurry to the treatment vessel. Within the vessel lays a pipe known as a septum. The slurry attaches itself to the septum and water is run through the vessel where pathogenic and suspended material is captured and strained out of the finished water. This kind of treatment process is very common for swimming pool treatment and beverage companies. This type of treatment method is generally not used by larger municipalities because of the large amount of disposal of sludge and the continuous purchasing of filter media.
Gravity filtration is comprised of the final two approved Water Treatment technologies. Direct Filtration and conventional treatment are the most widely used treatment technologies in the United States. It is described as gravity filtration because the head pressure of the water forces the water to travel through filtered media in order to remove impurities from the drinking water. Direct Filtration differs from conventional treatment because the sedimentation process is skipped. Areas with source water higher in quality may opt for direct filtration to reduce costs and the amount of land space for sedimentation basins can be substantial. The average filtration rate for gravity filtration beds is 3.0 GPM/Sq. ft.- 6.0 GPM/ sq. ft.
Alternative treatment plant methods can be approved on a case by case basis. Newer technologies such as membrane filtration and reverse osmosis have been utilized as the technology has improved and the costs associated with running these particular operations have decreased. Santa Clarita Valley (SCV) Water utilizes an alternative technology known as Upflow Clarification. SCV Water is able to use this technology because of the very low turbidity levels in the water provided by Castaic Lake. It is a more condensed version of conventional treatment and requires much less space due to the lack of sedimentation basins.
Filter Media
The type of media used in gravity filters is sand, anthracite coal and garnet. Garnet is a reddish colored mineral sand comprised of silicates (calcium, iron, manganese, and magnesium) and its density is greater than sand. Gravel is also used as a filter under layer below the filter media being used. It has to be heavier than the filter media so it is able to settle back under media after a completed filter wash cycle.
When choosing filter media it is important to select media that has good hydraulic characteristics, is durable, has no impurities, is insoluble in water, will not dissolve, and does not react with constituents in the water supply. Media is classified by four parameters including its effective size, uniform efficiency, specific gravity, and the hardness of the media. The effective size is the sieve opening in the media that allows water to pass through while collecting the impurities in water. 90 percent of the particles must be bigger than the opening to filter out particles.
When deciding what kind of media to choose for the filer it is important to consider the amount of time it takes for filter turbidity to break through. For example, you operate a treatment plant that requires individual filters say below 0.3 NTU (or Nephelometric Turbidity Units). Once an individual filter goes above this limit it must be washed. Secondly, head loss must be a consideration. Head loss occurs after material builds up over time during the filtration process. The head loss will cause longer filtration times and cause the filter level to rise. Once a filter reaches terminal head loss it must be back washed. The media is not always the cause of head loss and turbidity breakthrough. As discussed earlier, operators must ensure they are dosing chemicals properly as improper dosage can cause the aforementioned head loss and breakthrough.
The uniform coefficient is the ratio between the different sizes of media comprised in the filter bed. The lower the uniform coefficient means the media is closer to the same size than if it were higher. The lower the efficiency number adds to the cost of the media.
Filter production is the amount of water that a given filter can produce in a day. This flow is usually accounted for in Million Gallons per Day (MGD). Individual filtration rates are calculated by dividing the flow rate of the filter in gallons per minute by the surface area of a filter. The filtration rate for gravity filters can be between 2gpm-10gpm/ square foot. This topic will be discussed in more detail during the math portion of this chapter.
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Filter Media Types |
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Gravel Image by Martin Olsson is licensed under CC BY-SA 3.0 |
Sand Image by Yug is in the public domain |
Garnet Image by Siim is licensed under CC BY-SA 4.0 |
Anthracite Image is in the public domain |
There are several different types of media configurations. Filtration plants can utilize monomedia, duel media, and multimedia configurations. A monomedia plant has only one type of media which could be coarse sand or anthracite coal. Single media filters may have to be washed more frequently as they tend to be smaller and have more frequent head loss issues. Dual media filters consist of a lower sand level and an upper anthracite layer with larger diameter pores that allow deeper solids penetration. Finally, multimedia filters are used in pressure vessel treatment applications. Multimedia filters have sand, anthracite, and an upper garnet layer. The drawback to pressure vessels is the inability to view filter media within the pressure vessel.
Filter Operation and Backwashing Filters
Throughout the chapter, the subject of “washing” has come up. Filter run times are dependent on three factors but first it is important to know how filter efficiency is measured. The efficiency of a filter is measured by the filter effluent turbidity. And the overall plant efficiency measures the combined effluent turbidity of all the filters. Improper coagulation and filtration could lead to turbidity spikes and in the worst-case scenario could lead to a public health crisis.
Pictured below is a photo of a typical filter deck at a water treatment plant.
The first factor that an operator uses to determine whether a filter wash is necessary is the individual filter turbidity. Each treatment plant will have its own operating conditions and permits to follow. If an individual filter fails to meet the turbidity goals or limits, the filter must be put in a backwash cycle. Filters that continue to have decreased run times may need a filter profile ran to figure out why the filters are not meeting standards.
High head loss is the second factor that will lead to a filter wash. After the filter is used for a certain period of time it becomes clogged with the solids the filter is removing. This condition will also lead to increased turbidity as the solids that should be getting captured “breakthrough” the effluent into the treated water supply. Finally, a filter wash will be performed after a certain period of time no matter what the operating conditions are. This is the ideal scenario for washing a filter.
The backwash procedure is the reverse flow of water through the filter. This process removes solids from the filter after breakthrough or the filter run time is hit. Operators must operate the backwash rates at an optimal range because inadequate rates will not properly remove the solids from the filter and excessive rates can cause mud balls and mounds to form within the filter.
Washed water is able to be recycled by sending the waste stream to collection basins. The water can then be returned to the head works of the plant and be mixed with raw water to be treated again. The filter backwash rule limits the amount of water that can be returned into the head works of the plant at a given time. Returning too much recycled water could increase the chances of allowing microorganisms such as Cryptosporidium into the treatment plant.
The Filter to the left is in the process of a filter back wash. The water is moving in the opposite direction and overflowing over the weir and heads to the waste basin where it is collected and eventually returned into the head works of the plant to be treated again. Operator Actions
As a Water Treatment operator, you will be expected to have knowledge of how to properly use equipment related to filtration. While running a treatment plant you will routinely:
- Monitor filter performance
- Check turbidity levels with online analyzers and grab samples
- Adjust chemical flow rates
- Backwash filters
- Visually inspect filters
Filters run differently under changing water conditions. The plant will not always run the same as temperature differences and storm events will make operators examine important operational considerations from time to time. It is important to look at the weather and understand how it might affect the treatment plant. A severe rainstorm near your source water could increase turbidity levels coming into the plant. Under these conditions, operators may have to wash filters more frequently and make adjustments to chemical doses.
There are other abnormal conditions to consider when monitoring filter performance. Monitor filter washes to look for mud balls, excessive boiling in certain spots, and media being displaced and sent to waste basins. These conditions would indicate that the backwash flow is too high. Also identify shorter filter run times, filters that may not be coming clean, and algae growth. These factors may be due to improper chemical dosing and a backwash flow that is too low.
Filtration Math
Filtration Rate
Calculating the filtration rate of the filters in your plant is an important function. Operating plans and permits limit the amount of water a filter can produce so it is important to have an understanding of filter rates also known as loading rates. Filtration rates will also give an operator a basic understanding of the treatment plants daily average production.
The filtration rate formula is a velocity equation. These formulas are easily confused with flow problems so make sure you pay attention to the units you add into the formula and pay attention to what the problem is asking. The formula for filtration rate is:
Filtration Rate = Flow Rate ÷ Area
Filtration rate equations will use GPM and the area of a given filter. The area of the filter is length x width. A problem may give you the depth of a filter to confuse you. Pay very close attention to the wording in the problem.
Example 1
What is the filtration rate of a treatment plant that has 3 filters that are 20 ft. wide and 20 ft. in length in a plant that produces 1 MGD?
Filtration Rate = (20 ft x 20 ft x 3) ÷ 1 MGD
1,000,000 gal/1 Day x 1 Day/24 hours x 24 hours/60 min = 694 GPM1
*Note: Quick shortcut for future equations, there are 1,440 minutes in a day.
Filtration Rate = 1,200 ft 2 ÷ 694 GPM
Filtration Rate = 1.73 gpm/ft 2
Backwash Rate
As discussed in the chapter, after a filter reaches its capacity due to head loss, turbidity break through, or maximum amount of hours run, the filter must be washed. The filter wash cycle water velocity will be much greater than the amount of water that flows through the filter during normal operation. Many math equations will have the operator solve for rate of rise which is expressed as in/min. The backwash is the flow of water in the opposite direction where water is moving up instead of down.
Example 2
The maximum backwash rate for a filter is 5,000 GPM. The filter is 20 ft. wide and 20 ft in length. What is the rate of rise in the filter?
Rise Rate = Flow Rate ÷ Area
Rise Rate = 5,000 GPM ÷ 400 ft 2
Rise Rate = 12.5 GPM/ sq./ft
12.5 gal/min(ft)/1 x 12 inches/1 ft x 1 ft³/7.48 gal = 20 inches rise/min
Percent Back Wash
Water treatment plants are very efficient at recycling waste stream water. The water sent to waste basins and lagoons is able to be recycled but only a certain amount at a time. The percent backwash math problems compare the total plant production with the amount of finished water used to backwash a filter.
Example 3
A treatment plant treats 2 MGD. It has 2 filters that are washed each day and each uses 10,000 gallons during the wash. What is the percent backwash water?
2 filters x 10,000 gallons = 20,000 gallons
20,000 gal/2,000,000 gal = 0.01
100 × 0.01 = 1%
1% of the water the plant uses is for backwash water.
Chapter Review
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Solids removed from a filter are most commonly removed by what method?
- Adsorption
- Straining
- Deactivation
- Flocculation
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What is a typical filtration rate for slow sand filters?
- 2.0-6.0 GPM/sq. ft
- 6.0-10.0 GPM/sq. ft
- 1.0-2.0 GPM/sq. ft
- 0.5-0.10 GPM/sq. ft
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In a typical conventional treatment plant, the finished water turbidity for an individual filter should be less than ___________.
- 1.0 NTUs
- 0.3 NTUs
- 5.0 NTUs
- 3.0 NTUs
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A filter running under normal conditions will see head loss in a filter ___________.
- Remain constant
- Increase slowly
- Rapidly increase
- Decrease slowly
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A filter must be washed if this condition is met:
- Head loss
- Turbidity breakthrough
- Maximum filter run time
- All of the above
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Filter performance is measured by the removal of ___________.
- Oxygen
- Head loss
- Turbidity
- Chlorine
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What is the biologically active layer of a slow sand filter called?
- Mixed media
- Duel media
- Sludge layer
- Schmutzdecke
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The pressure drop in a filter is called ___________.
- Turbidity breakthrough
- Head Loss
- Filtration
- Backwash
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What is the most common reason for putting a filter into the wash cycle?
- Head loss
- Filter run time
- Turbidity breakthrough
- Water level decrease
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Formation of mud balls and excessive boiling during a wash is an indicator of ___________.
- Proper backwash rate
- Too low backwash rate
- Excessive backwash rate
- Improper chemical dose
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Important processes which occur during filtration are ___________.
- Sedimentation
- Adsorption
- Straining
- All of the above
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Typical filtration rates for a conventional treatment plant are ___________.
- 0.2-0.6 GPM/sq.ft
- 2.0-10.0 GPM/sq.ft
- 10.0-20.0 GPM/sq.ft
- 200-400 GPM/sq.ft
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There are four filters at a water treatment plant. The filters measure 20 feet wide by 30 feet in length. What is the filtration rate if the plant processes 8.0 MGD?
- 1.51 GPM/sq.ft
- 2.31 GPM/sq.ft
- 2.61 GPM/sq.ft
- 2.91 GPM/sq.ft
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A water treatment plant treats 6.0 MGD with four filters. The filters use 60,000 gallons per wash. What is the percent backwash at the plant?
- 10%
- 8%
- 6%
- 4%
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A treatment plant filter washes at a rate of 10,000 GPM. The filter measures 18ft. wide by 24ft. long. What is the rate of rise expressed in inches per minute?
- 17 inch/min
- 27 inch/min
- 37 inch/min
- 47 inch/min