Which of the following is false about rapid gravity type filters used for water purification?

Explanation:

Rapid Gravity Type Filters

• Used for large-scale municipal water treatment to remove suspended solids after coagulation and flocculation.

• Water passes rapidly through a sand bed under gravity (rate: 4,000–12,000 liters/m²/hr).

• Coagulation is essential before filtration to form flocs for efficient removal.

• Requires periodic backwashing (usually every 1–2 days) to clean clogged filter media.

• Needs skilled supervision for operation, backwash management, and chemical handling.

• Initial and operational costs are high compared to slow sand filters.

• Removes turbidity, color, and microorganisms effectively when used with disinfection.

• Typically consists of a filter bed (sand), underdrain system, and wash water troughs.

Additional InformationSlow Sand Filters:

• Operate at a slow filtration rate (100–200 liters/m²/hr), much slower than rapid filters.

• Mainly used in small-scale or rural water treatment plants with low turbidity water.

• Filtration is done through a fine sand bed, forming a biologically active layer called schmutzdecke on top.

• No need for coagulation or chemical treatment before filtration.

• Require large land area due to slow filtration rate and longer filter runs.

• Cleaning is done by scraping the top sand layer, not backwashing.

• Low operational cost and does not require highly skilled supervision.

• Effective in removing bacteria, turbidity, and pathogens through biological and physical processes.