equipment for grit removal devices
1971 Edition

This standard covers various grit removal apparatus including aerated grit chambers that suspend organics while allowing grit to settle, detritus tanks for gravity-based separation, vortex grit chambers employing circular flow for grit separation, hydrocyclones utilizing centrifugal forces, and mechanical grit collectors with scrapers or conveyors. These devices are designed to efficiently remove inorganic particles like sand and grit, withstand corrosion and abrasion, and accommodate different sewage flow rates and grit loads.

Recommended materials per the standard include reinforced or plain concrete (IS 456), brick masonry (IS 2212), and stone masonry (IS 1597) for civil works. Mechanical components such as turn tables should be made from high-grade cast iron (IS 210), reduction gears from steel or similar materials (IS 1570, IS 1871), gates from cast iron or steel (IS 3042, IS 226), and scrapers from fabricated steel sheets (IS 513). Shafts are typically cold finished steel (IS 1570), channels are steel (IS 3954), and floats are constructed from plastic materials like polyethylene, PVC, or glass fibre reinforced polyester. These materials ensure durability, corrosion resistance, and mechanical robustness.

The standard recommends designing grit removal units to effectively capture particles with diameters of at least 0.15 mm and specific gravities around 2.3, unless site-specific data suggests otherwise. Flow velocity within grit chambers should not exceed 0.17 m/s for particles approximately 1.5 mm in size with specific gravity 2.3 to ensure proper settling. Additionally, devices must accommodate peak grit volumes, typically estimated at 0.3 cubic meters per 1000 cubic meters of sewage during peak flow conditions, ensuring efficient grit separation tailored to particle characteristics and flow dynamics.

Mechanical grit removal equipment must incorporate automatic motor shutdown mechanisms that activate upon detecting overload conditions caused by sudden grit surges, blockages, or other abnormalities. Larger installations are required to have standby mechanical grit removal units, while smaller plants must provide manual backup devices. Continuous operation devices should prevent grit accumulation using rotating scrapers or ploughs to maintain uninterrupted grit extraction, ensuring operational safety and equipment protection.

Grit washing systems should be engineered to reduce organic matter content to 3% or less by weight prior to disposal. This involves thorough washing through continuous or intermittent agitation methods, sufficient retention times (typically 5 to 15 minutes), and the use of appropriate water flow rates (0.5 to 1.5 cubic meters per hour per square meter of grit bed). Additionally, provisions for water drainage or dewatering, such as perforated screw conveyors, should be incorporated to ensure grit is discharged with minimal water content, facilitating easier handling and compliance with organic matter limits.