Aerated Grit Chamber

Aerated grit chambers remove inorganic solids (sand, gravel, cinders) from wastewater while maintaining organic matter in suspension through controlled air injection. Spiral flow patterns created by diffused aeration along one wall allow heavier grit particles to settle while keeping lighter organic solids suspended and moving toward the outlet. Typical installations achieve 95% removal of particles larger than 0.2mm at detention times of 2-5 minutes. The primary trade-off is higher energy consumption compared to conventional grit chambers, requiring 8-12 cfm per foot of tank length, but this investment significantly reduces downstream equipment wear and improves overall plant performance.

• Primary Treatment Headworks (0.5-20 MGD): Aerated grit chambers serve as the primary grit removal step following screening, typically handling 2-8 fps velocities. Selected for superior organics separation compared to horizontal chambers - achieving 95%+ grit removal while maintaining <3% organics content. Downstream connects to primary clarifiers

• Combined Sewer Overflow (CSO) Treatment: Essential for high-flow events (10-50 MGD peak), removing heavy grit that would damage downstream pumps and processes. The 15-20 minute detention time allows settling during variable hydraulic loading while air scour prevents septicity

• Retrofit Applications: Replaces aging horizontal chambers in 2-15 MGD plants where footprint constraints exist. Provides 40-60% smaller footprint while improving performance. Upstream of existing primary treatment, often requires new grit handling systems

• High-Strength Industrial Discharge Plants: Municipal plants receiving significant industrial flows (food processing, manufacturing) use aerated chambers for enhanced heavy particle removal and grease separation through controlled air injection patterns

Daily Operations: Operators monitor air flow rates (typically 0.8-1.5 cfm/ft²) and adjust based on influent characteristics and grit accumulation patterns. Visual inspection of spiral flow patterns ensures proper mixing without short-circuiting. Collection system operation monitored for consistent grit removal rates and proper discharge consistency.

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Maintenance: Monthly lubrication of collector drives and quarterly inspection of diffuser condition. Semi-annual blower maintenance including filter changes and belt adjustments. Annual tank dewatering for concrete inspection and diffuser replacement. Requires confined space entry procedures and gas monitoring equipment. Maintenance staff need mechanical aptitude for drive systems and basic pneumatic knowledge.

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Troubleshooting: Poor grit removal indicates insufficient air flow or worn diffusers - check for 50% reduction in bubble density. Excessive organics in grit suggests over-aeration (reduce to 0.5 cfm/ft²) or collection timing issues. Collector jamming from debris requires immediate shutdown to prevent drive damage. Typical diffuser life: 3-5 years; collector chains: 8-12 years with proper maintenance.

• Aeration System: Coarse bubble diffusers (typically 6-12" spacing) along one wall create spiral flow pattern. Blowers sized for 0.5-2.0 cfm/ft² surface area. Stainless steel headers with PVC or ceramic diffusers rated for continuous operation

• Grit Collection Mechanism: Chain-and-flight collectors or air-lift pumps remove settled grit from hoppers. Collectors sized for 1-3 fpm chain speed, handling 0.1-2.0 ft³/MG grit volumes. Materials include stainless steel flights with UHMW wear strips

• Tank Structure: Reinforced concrete basins, typically 8-15 feet deep with 4:1 length-to-width ratios. Detention times of 2-5 minutes at peak flow. Includes walkways, handrails, and access hatches per OSHA requirements

• Grit Washing Equipment: Screw classifiers or cyclone separators achieve <3% organics content in removed grit. Sized for 15-50% solids concentration output, with wash water recycle systems

• Hydraulic Loading Rate: 2,000-5,000 gpd/sf (typical 3,000 gpd/sf) based on surface area at peak hourly flow

• Detention Time: 2-5 minutes at peak flow (3 minutes typical for combined systems, 2-3 minutes for separate systems)

• Air Supply Rate: 0.2-0.5 cfm/ft of tank length, with 4-8 scfm/1000 gal influent flow typical

• Tank Dimensions: Length: 30-100 feet (L:W ratio of 3:1 to 5:1), Depth: 7-16 feet (12-14 feet typical), Width: 10-25 feet

• Grit Removal Efficiency: 95% removal of particles ≥65 mesh (0.21mm), 100% removal of particles ≥35 mesh (0.50mm)

• Organic Content: <3% in removed grit (5-15% without washing)

• Cross-Collector Speed: 1-2 fpm longitudinal velocity

• Air Diffuser Placement: 18-24 inches above tank bottom, one side wall mounting

• Peak Flow Multiplier: 2.5-4.0 times average daily flow for sizing

• What peak hourly flow rate and peaking factor should drive sizing? Need 10+ years of flow data to establish 2.5-4.0x peaking factors. Undersizing by 20% results in grit carryover to downstream processes, causing excessive wear on pumps and blowers. Oversizing increases construction costs by $50-100K per chamber

• Should you include grit washing systems for <5 MGD plants? Plants >2 MGD typically justify washing systems ($75-150K) to achieve <3% organics versus 10-15% without washing. Consider landfill disposal costs and odor complaints when determining threshold

• What air supply redundancy is required? Single blower acceptable for <1 MGD plants with manual grit removal backup. Plants >1 MGD require N+1 blower redundancy. Loss of aeration causes 50-70% reduction in grit removal efficiency within 30 minutes

• How many parallel chambers are needed for reliability? Single chamber acceptable only for <0.5 MGD with manual bypass capability. Plants >2 MGD require minimum two chambers for maintenance flexibility and process reliability

• Division 40 - Process Integration

• Section 40 05 00 - Common Work Results for Process Integration

• Section 46 21 00 - Grit Removal Equipment

• Primary specification under 46 21 00 covers aeration systems, collection mechanisms, and grit handling equipment

• Material/Equipment Verification: Verify 316SS construction for all wetted metallic components, Confirm concrete coating specifications (epoxy, polyurea), Validate blower capacity at actual site elevation

• Installation Requirements: Crane access for basin installation (12-16 week lead time typical), Electrical coordination for VFD controls and air piping, Bypass pumping capabilities during installation

• Field Challenges: Achieving proper air flow distribution across basin width, Concrete tolerances affecting equipment fit, Integration with existing hydraulic profile

• Coordination Issues: HVAC systems for blower rooms, Grit handling/disposal equipment sequencing

• Lakeside Equipment: RAPTOR series with spiral flow pattern technology

• Smith & Loveless: PISTA grit chambers with fine bubble aeration

• Headworks International: Bio-Grit systems with integrated screening

• Evoqua (formerly Envirex): Aerated grit chambers with cross-collector mechanisms

• All maintain extensive municipal references from 0.5-50 MGD installations

• Vortex Grit Chambers: 15-20% lower capital cost, better for peak flow variations, preferred for facilities >10 MGD

• Horizontal Flow Grit Chambers: Simplest operation, lowest maintenance, suitable for small plants <2 MGD but require more space

• Detritor/Classifier Systems: Higher removal efficiency (95% vs 85%), integrated washing, but 30-40% higher capital cost. Best for plants with high grit loading or stringent effluent requirements

Manufacturer Relationships: Establish preferred vendor agreements with 2-3 manufacturers for standardized spare parts inventory across multiple facilities. Cost Savings: Specify common blower sizes (15-25 HP range) to leverage bulk purchasing. Consider concrete suppliers experienced with water treatment - poor concrete work causes 60% of installation delays. Many operators prefer manual grit removal systems over automated for facilities under 5 MGD due to maintenance complexity.