Granular Activated Carbon (GAC) Media

Granular Activated Carbon (GAC) removes dissolved organic compounds, taste and odor compounds, and chlorine from municipal water through physical adsorption onto its highly porous carbon surface. Water flows through fixed or fluidized GAC beds where contaminants bind to carbon pores ranging from 10-1000 angstroms. Typical municipal installations achieve 90-99% removal of total organic carbon (TOC) and chlorine at empty bed contact times of 10-20 minutes. The primary limitation is eventual carbon saturation requiring thermal regeneration or replacement, with media life typically ranging 1-3 years depending on organic loading and target contaminants.

• Taste and Odor Control (Post-Filtration): GAC contactors follow conventional treatment trains, typically after sand filtration and before clearwell storage. Selected for removing geosmin, 2-MIB, and chlorinous tastes. Upstream receives filtered water at 2-8 NTU; downstream connects to disinfection systems. Contact times of 10-20 minutes at 2-5 gpm/ft².
• TOC/DBP Precursor Removal: Installed post-coagulation/sedimentation, pre-chlorination to reduce disinfection byproduct formation. Removes natural organic matter before final disinfection. Typical loading rates 3-6 gpm/ft² with 15-30 minute contact times.
• Chloramine Removal (Dechlorination): Used in treatment plants switching between chlorine/chloramine disinfection or for process water treatment. Rapid kinetics allow higher loading rates (8-12 gpm/ft²) with shorter contact times (5-10 minutes).
• Pharmaceutical/Micropollutant Removal: Emerging application in advanced treatment trains, often following membrane bioreactors or as polishing step before discharge/reuse applications.

Daily Operations: Monitor differential pressure across beds (typically 2-8 feet head loss), effluent turbidity (<1 NTU), and residual chlorine breakthrough. Adjust flow rates to maintain design contact time. Sample influent/effluent for target contaminants weekly. Check backwash cycle performance and carbon bed level monthly.

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Maintenance: Backwash 1-3 times weekly based on head loss development. Replace GAC media every 6-18 months depending on loading and contaminant type. Requires confined space entry procedures, respiratory protection during carbon handling. Maintenance staff need basic hydraulics knowledge and confined space training. Annual underdrain inspection recommended.

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Troubleshooting: Premature breakthrough indicates media exhaustion or channeling from inadequate backwashing. Rising head loss suggests biological growth or inadequate air scour. Carbon dusting indicates media degradation from excessive

• GAC Media: Coconut shell, coal-based, or wood-based carbon with iodine numbers 900-1200 mg/g. Effective size 0.8-1.2mm, uniformity coefficient <2.0. Selection based on target contaminants - coconut shell for taste/odor, coal-based for TOC removal. Bed depths 3-8 feet typical.
• Underdrain System: Leopold, Johnson Screens, or similar with 0.5-1.0 gpm/ft² backwash capability. Stainless steel construction, 0.014-0.020" slot openings to retain carbon while allowing backwash flow distribution.
• Backwash System: Air scour (3-5 cfm/ft²) followed by water wash (8-15 gpm/ft²). Includes wash water collection troughs, typically at 24-30" spacing. Backwash frequency 1-3 times weekly depending on upstream filtration quality.
• Contactor Vessels: Concrete or steel construction, 10-20 feet diameter for 0.5-10 MGD plants. Include sample ports, level indicators, and access hatches. Designed for 3-6 psig internal pressure with safety factors.

• Flow Loading Rate: 2-10 gpm/ft² (typical 5-8 gpm/ft²) for potable water; 1-5 gpm/ft² for wastewater applications. Higher rates reduce contact time and breakthrough efficiency.
• Empty Bed Contact Time (EBCT): 10-30 minutes for taste/odor control; 15-45 minutes for organics removal; 20-60 minutes for advanced treatment applications. Longer contact times improve removal efficiency but increase capital costs.
• Bed Depth: 4-12 feet typical, with 6-8 feet most common for municipal applications. Minimum 4 feet to prevent channeling; maximum limited by head loss and structural considerations.
• Backwash Rate: 12-20 gpm/ft² for 8-15 minutes, typically every 2-7 days depending on influent quality. Requires 25-40% bed expansion.
• Head Loss: Clean bed 2-8 feet; dirty bed up to 15 feet maximum before backwash required. Design for 10-12 feet total available head.
• Media Specifications: Coconut shell preferred for potable water (iodine number >900); coal-based acceptable for wastewater. Effective size 0.8-1.2mm; uniformity coefficient <2.0; minimum hardness number 80.

• What removal efficiency is required and for which contaminants? TOC removal 20-50%, taste/odor threshold reduction, or specific organics like PFAS (<70 ng/L). Wrong decision leads to regulatory non-compliance or oversized systems. Need influent water quality data, target effluent limits, and contaminant-specific isotherm data.
• Should design use single-stage or multi-stage configuration? Single stage for <5 MGD with moderate loading; parallel trains for >10 MGD; series operation for >80% TOC removal. Wrong choice affects operational flexibility and replacement costs. Requires peak/average flow analysis and redundancy requirements.
• What media replacement frequency is acceptable? 12-18 months typical for potable water; 6-12 months for wastewater. Impacts annual O&M costs ($150-300/1000 lbs media). Need pilot testing data, budget constraints, and labor availability assessment.
• How will spent carbon be handled? Thermal reactivation (85-90% recovery) vs. disposal ($200-400/ton). Affects long-term economics and sustainability goals. Requires volume projections, local disposal costs, and reactivation facility access evaluation.

• Division 40-48: 40 31 33 - Granular Media Filtration Equipment
• Primary section covering GAC contactors, media specifications, and associated equipment. May reference 40 20 00 for process water treatment systems integration.

• Material/Equipment Verification: COA for iodine number, ash content, hardness number; NSF 61 certification for potable water contact; Particle size distribution verification
• Installation Requirements: Proper underdrain design prevents media migration; Backwash system sizing critical for bed expansion; Carbon dust removal during startup essential
• Field Challenges: Media fines create turbidity issues initially; Proper bed depth uniformity during loading; Adequate freeboard for backwash expansion
• Coordination Issues: 4-6 week lead times for virgin carbon; Reactivation facility coordination for spent media

• Calgon Carbon (Chemviron) - Filtrasorb series (F300, F400) with proven municipal track record including Chicago and Denver systems
• Evoqua Water Technologies - AquaCarb series for municipal applications, extensive installations across California
• Cabot Norit - GAC 1240 and ROW 0.8 Supra widely used in municipal plants
• Haycarb - CTC series gaining market share in smaller municipal facilities

• Powdered Activated Carbon (PAC) - Lower capital cost, more flexible dosing, preferred for seasonal taste/odor issues. Operating costs 20-30% higher than GAC.
• Ion Exchange - Superior for specific contaminants like nitrates or PFAS, 40-60% higher capital cost.
• Advanced Oxidation (UV/H2O2) - Effective for emerging contaminants, 2-3x operating costs but no media replacement required.

Establish reactivation contracts early - thermal reactivation typically costs 60-70% of virgin carbon while maintaining 90-95% adsorption capacity. Work directly with carbon suppliers rather than equipment vendors for better pricing and technical support. Consider bulk storage silos for facilities using >50,000 lbs annually to reduce handling costs and ensure consistent supply during peak demand periods.