Filtration, Coagulation & Sedimentation Practice Questions

Explanation: Coagulation uses coagulants (typically alum or ferric salts) to neutralize the negative charge on colloidal particles, causing them to destabilize and clump together (coagulate). Flocculation then gently mixes the water to allow these particles to grow into larger floc particles that can settle by gravity in the sedimentation basin and be captured by filters.

Explanation: Turbidity measures the cloudiness or haziness of water caused by suspended particles. It is measured in Nephelometric Turbidity Units (NTU). The Surface Water Treatment Rule limits combined filter effluent turbidity to 0.3 NTU or below in 95% of monthly samples and never above 1 NTU. High turbidity can shield pathogens from disinfection and indicates treatment failure.

Explanation: The SWTR requires combined treatment (filtration plus disinfection) to achieve at least 3-log (99.9%) inactivation/removal of Giardia. Conventional filtration receives 2.5-log credit, leaving 0.5-log to be achieved by disinfection. For Cryptosporidium, the LT2 rule requires additional treatment based on source water pathogen levels.

Explanation: Conventional rapid sand filters operate at hydraulic loading rates of approximately 2 to 3 gpm per square foot of filter area. Rates above 5 to 6 gpm/ft² can cause filter breakthrough — passage of turbidity and pathogens through the filter. During backwash, rates of 15 to 20 gpm/ft² are typically used to expand and clean the filter media.

Explanation: Backwash is initiated based on any of these triggers: (1) headloss reaches the design limit (typically 8 to 10 feet for gravity filters); (2) filter effluent turbidity increases (turbidity breakthrough); or (3) maximum run time is reached (typically 24 to 72 hours). Waiting too long risks breakthrough; backwashing too early wastes water and interrupts treatment.

Explanation: The optimal coagulant dose identified in a jar test is the minimum dose that achieves the lowest acceptable settled turbidity — balancing treatment effectiveness with chemical cost. Using more coagulant than necessary adds cost, can overdose (restabilizing particles), and may contribute to residual aluminum in finished water. Jar tests should simulate actual rapid mix and flocculation conditions at the plant.

Explanation: In a conventional sedimentation basin (clarifier), water velocity is reduced dramatically, allowing gravity to settle floc particles to the basin floor as sludge. The settled solids are periodically removed by scrapers or desludging valves. Overflow rate (surface loading rate, expressed as gpm/ft²) is the key design parameter — lower rates allow finer particles to settle. Dissolved air flotation (DAF) is an alternative process that floats particles upward.

Explanation: A bed expansion of 20 to 30% is typically targeted during backwash of a rapid sand filter. Insufficient expansion fails to break up accumulated solids and mud balls; excessive expansion risks washing sand media over the backwash troughs and out of the filter box. Backwash flow rate (typically 15 to 20 gpm/ft²) is adjusted to achieve the correct expansion, and operators visually confirm the bed level.

Explanation: Filter ripening (also called the ripening period or initial effluent turbidity spike) occurs immediately after backwash when clean, freshly expanded media temporarily produces higher effluent turbidity. During ripening, the media pores are too open and clean to effectively capture fine particles. As the filter run progresses, a thin layer of retained particles improves capture efficiency. To manage this, operators may filter-to-waste for 15 to 30 minutes after backwash before returning the filter to service.

Explanation: Enhanced coagulation requires systems to optimize their coagulant dose (typically by lowering the coagulation pH to 5.5 to 6.5 using acid or by increasing coagulant dose) to achieve specified percent TOC removal targets. By removing more natural organic matter (the precursors that react with chlorine to form THMs and HAAs) before disinfection, DBP formation in the distribution system is reduced. TOC removal requirements vary based on source water TOC and alkalinity.