Course: BFM 203 Water Treatment and Control

Lecture

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Gravity Filtration

In the gravity filtration process, water level or pressure (head) across the media forces water through the filter media. Dynamic pressure across the filter media is controlled by increasing the vacuum inthe discharge pipe, allowing for a constant filtration rate and level above the media. The filtration rate is typically 2 - 10 gmp/sq. ft. This rate depends on media type; the media types are 1) single media (sand), 2) dual media (sand and anthracite coal), 3) Multi- or mixed-media (sand, anthracite coal, and garnet), and 4) activated carbon which can be used to remove taste, odor, and organic substances.

Media Characteristics. It is important for filtration media to have good hydraulic characteristics. Media should be inert and easy to clean, hard and durable, free of impurities, and insoluable in water. Media is classified according to effective size, uniformity coefficient, specific gravity, and hardness. Media cleaning or replacement may be necessary if mud balls or filter clogging cannot be corrected thorugh the normal backwashing process.

Mechanisms Affecting Filtration. A number of interrelated removal mechanisms within the media itself are necessary for high removal efficiency, such as sedimentation of media, adsorption, absorption, biological action, and straining. Monitoring of filter media condition is achieved by draining the filter basin and performing a physical inspection of each level of media. Look for proper re-stratification of the media after each backwash event.

Operational Criteria. Filtering systems are normally in concrete, aluminum, or steel and usually incorporate at least four filters. Filter production is measured in millions of gallons per day (MGD) and filtration rate is measured in gallons per minute (gpm)/sq. ft. Filtration efficiency is measured by overall plant reduction in turbidity; a 99.5 percent efficiency rate is achievable. The final turbidity and bacterial analysis is more important than filtration efficiency.

Filter Control Systems. Utilities use filter control systems to regulate flow rates through the filter by maintaining adequate head above the media survace and to prevent sudden surges in flow which could discharge solids trapped in filter media. Gravity filtration systems use rate of flow, split flow, declined rate, and self-backwash.

Normal Operation. Regular hands-on operation and inspection allow operators to identifyy adverse filtering conditions before they become severe. Typical functions performed during normal filtering operations are listed below.

- Monitor process performance
- Evalute water quality conditions and make appropriate process changes.
- Check and adjust process equipment.
- Backwash filters.
- Evaluate media conditions.
- Visually inspect facilties.

Abnormal Conditions. A variety of indicators warn operators of abnormal conditions. Clogging refers to a buildup of head loss (pressure drop) across the filter media until it reaches some predetermined design limit. Breakthrough is a condition in which solids are no longer removed by an already loaded filter. Solids pass through to the filter effluent. Air binding is caused by the release of dissolved air in saturated cold water due to a decrease in pressure. Excessive head loss is caused by reduction in the size and number of underdrain openings due to clogging by media, corrosiion, or chemical deposits.

- Mud balls
- Media cracking or shrinkage
- Media boils during backwash
- Excessive media loss or visible disturbance
- Short filter runs
- Filters that will not come clean during backwash
- Algae on walls and media

Corrections. Two primary actions are used to correct abnormal conditions. Backwashing is a process where the flow of water is reversed through the filter media at a rate of 10 - 25 gpm/sq. ft. to remove entrapped solids. Backwashing is performed after a filter clogs, before a breakthrough occurs, or after a predetermined amount of time. A surface wash provides additional scrubbing actions to remove attached floc and other entrapped solids from the filter media. Surface wash systems include baylis, fixed grid, rotary, and air scour systems.

Gravity Filtration Images

The photos in the album below show a series of backwash troughs and washers; a deep bed, high rate filter; the results of a broken underdrain; and a the control panels for a filter system.