3.1 Water Filtration Technologies & Media

3.1 Water Filtration Technologies & Media

Why This Topic Matters for the Exam

On the Water Treatment Operator certification exam, filtration is heavily tested because it is the primary physical barrier against pathogens like Cryptosporidium and Giardia, which are highly resistant to chemical disinfection. Operators must understand how different media configurations, filter hydraulics, and operational problems affect water quality. You will be expected to diagnose filter failures, calculate backwash rates, and select appropriate media configurations based on raw water characteristics.

Filtration Process and Media Configurations

Filtration is the physical and chemical process of separating suspended solids from water by passage through a porous medium. In a conventional water treatment plant, filtration follows coagulation, flocculation, and sedimentation.

Filters are categorized by their hydraulic loading rates and media configurations:

1. Slow sand filtration: These filters operate at low loading rates (0.015 to 0.15 gpm/sq ft) and rely on a biological layer called the schmutzdecke at the sand surface to remove pathogens and organic matter. They do not require chemical pretreatment and are cleaned by scraping the top layer of sand.
2. Rapid sand filtration: Operating at rates of 2 to 4 gpm/sq ft (or up to 10 gpm/sq ft for high-rate filters), these require chemical coagulation. They are cleaned via hydraulic backwashing.
3. Dual-media filtration: Uses a layer of anthracite coal (lower specific gravity, larger size) on top of a layer of silica sand (higher specific gravity, smaller size). This allows deeper penetration of particles, increasing run times.
4. Mixed-media filtration: Typically consists of three layers: anthracite coal, silica sand, and garnet (densest and finest media) supported by a layer of gravel. This configuration provides a graded pore structure that tapers from coarse at the top to fine at the bottom, maximizing solids storage capacity.

Membrane Filtration Technologies

For advanced removal, plants utilize membrane filtration. Membranes act as physical sieves with precise pore sizes, classified by their nominal pore diameters:

• Microfiltration (MF): Pores around 0.1 microns. Removes suspended solids, algae, protozoa (Giardia and Cryptosporidium), but not bacteria or viruses.
• Ultrafiltration (UF): Pores around 0.01 microns. Removes all pathogens, including viruses and large organic molecules.
• Nanofiltration (NF): Pores around 0.001 microns. Removes divalent ions (hardness), viruses, and dissolved organic carbon. Often called membrane softening.
• Reverse osmosis (RO): Pores less than 0.001 microns. Removes monovalent ions (desalination) and almost all dissolved constituents.

The Backwash Cycle

Over time, filtered particles clog the media, increasing head loss (frictional resistance to flow) and leading to turbidity breakthrough (where suspended solids pass through the filter). To restore capacity, operators perform a backwash cycle, reversing the flow of water upward through the filter bed.

The key steps in a backwash cycle are:

1. Filter drawdown: The influent valve is closed, and water drains down to just above the media surface.
2. Auxiliary scour: Air scour or surface wash systems inject air or high-pressure water to break up compacted surface layers and mudballs.
3. Upflow backwash: Clean water is pumped upward through the underdrain system. The flow rate must be high enough to achieve a media expansion of 15% to 30%, which allows particles to fluidize and release trapped debris.
4. Backwash waste washwater discharge: Dirty water is directed to a waste washwater basin.
5. Filter-to-waste: Once backwashing is complete, the filter is placed back in service but the initial filtrate is discharged to waste (usually for 15 to 30 minutes) until turbidity drops below regulatory limits (typically 0.1 NTU). This period is known as the ripening phase.

Operational Problems and Solutions

Operators must identify and correct several common filtration issues:

• Mudballs: Agglomerations of floc, sand, and organic matter that form due to inadequate backwashing or poor surface wash. They sink to the bottom, causing short-circuiting. Correct by adjusting chemical doses or increasing backwash duration and air scouring.
• Air binding: Release of dissolved gases from water within the filter bed, caused by negative head pressure or sudden temperature increases. This blocks water flow and creates physical channels. Correct by backwashing more frequently or keeping water levels higher above the media.
• Filter cracking: Fissures that develop in the media bed when the media is compacted too tightly under high head loss, allowing untreated water to bypass filtration. Correct by backwashing before head loss becomes excessive.
• Sand boiling: Localized high-velocity upward water channels during backwashing, causing media to shift and mix. Often caused by clogged underdrain nozzles. Correct by inspecting and repairing underdrain systems.
• Turbidity breakthrough: The sudden passage of suspended solids through the media bed, indicating that the filter's capacity is exhausted. Operators must monitor effluent turbidity continuously to initiate backwash before breakthrough occurs.

Realistic Exam Scenarios

1. Scenario 1: An operator notices a rapid increase in filter effluent turbidity within 2 hours of putting a filter back in service. Head loss is minimal. The cause is likely an insufficient filter ripening period (not filtering to waste long enough) or an overdose of coagulant chemicals, which prevents proper floc shear resistance.
2. Scenario 2: During a backwash cycle, the operator notices that the media expansion is only 5%. This low expansion rate is insufficient to fluidize the bed and will lead to the formation of mudballs and subsequent short-circuiting. The operator must increase the backwash pump rate to achieve the target 15-30% expansion.