Intermittent Sand Filters

Intermittent Sand Filters provide secondary biological treatment for wastewater through controlled periodic dosing of effluent onto sand media beds. Wastewater is applied in timed intervals (typically 2-6 doses per hour), allowing aerobic biological processes to occur in the sand matrix during rest periods between applications. These systems achieve 85-95% BOD removal and 80-90% TSS removal in municipal applications. The primary trade-off is large land requirements, typically needing 2-4 acres per MGD of capacity, making them suitable mainly for smaller communities with available space and moderate loading rates.

• Secondary Effluent Polishing (5-25 MGD plants): ISFs follow activated sludge or trickling filters to achieve <10 mg/L BOD and TSS for discharge permits. Typically sized at 2-5 gpm/sf loading rates, they're selected over membrane systems for lower O&M costs and simpler operation. Effluent feeds chlorine contact basins.

• Small Community Treatment (0.5-5 MGD): ISFs serve as complete secondary treatment following primary clarification in package plant configurations. Loading rates of 1-3 gpm/sf allow reliable nitrification. Selected for minimal staffing requirements and proven 20+ year service life with basic maintenance.

• Lagoon Upgrade Applications (1-10 MGD): ISFs retrofit aging lagoon systems requiring nutrient removal compliance. Intermittent dosing provides nitrogen removal through nitrification-denitrification cycles. Chosen over constructed wetlands for smaller footprint and consistent year-round performance in northern climates.

• Septage/Sludge Filtrate Treatment: ISFs polish centrate from dewatering operations or septage receiving, handling high-strength streams at 0.5-2 gpm/sf loading rates.

Daily Operations: Operators monitor effluent quality via daily composite sampling and record dosing cycle completion. Flow distribution requires visual inspection for even coverage across filter surface. Influent flow adjustments maintain 2-6 gpm/sf loading rates. Most systems operate automatically with weekly totalizer readings and alarm acknowledgment.

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Maintenance: Monthly media surface raking prevents biomat formation and maintains infiltration rates. Semi-annual underdrain flushing using backwash pumps removes accumulated solids. Media replacement occurs every 8-12 years depending on loading. Standard PPE includes safety glasses and gloves. Basic mechanical skills sufficient for routine maintenance tasks.

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Troubleshooting: Surface ponding indicates media clogging requiring immediate raking or media replacement. Distributor arm binding shows in uneven spray patterns, requiring lubrication or motor replacement. Effluent quality deterioration suggests media exhaustion or hydraulic overloading. Typical service life spans 15-25 years with proper operation and maintenance protocols.

• Filter Media Bed: 24-30 inches of 0.35-0.55 mm effective size sand over 6-12 inches of graded gravel underdrain support. D10 uniformity coefficient <3.5 ensures proper hydraulic conductivity of 1-5 gpm/sf. Media depth determines treatment efficiency and run length.

• Distribution System: Rotating distributors or fixed spray nozzles deliver 2-8 hour dose cycles. Orenco, Consolidated Treatment, and Bio-Microbics offer standard 20-100 gpm capacity units. Dosing frequency controls oxygen transfer and biological activity.

• Underdrain Collection: Perforated PVC laterals in graded gravel (3/8" to 2" stone) collect treated effluent. 6-inch minimum lateral spacing with 0.5% slope prevents clogging. Leopold and Johnson Screens provide municipal-grade underdrain systems.

• Dosing Controls: Timer-based or flow-proportional control systems manage hydraulic loading. Allen-Bradley and Siemens PLCs integrate with SCADA systems for 2-50 MGD facilities.

• Hydraulic Loading Rate: 2-6 gpm/sf for secondary effluent, 1-3 gpm/sf for primary effluent. Higher rates risk breakthrough and reduced treatment efficiency.

• Sand Bed Depth: 24-30 inches effective depth minimum. Deeper beds (36+ inches) provide better nitrification but increase headloss and construction costs.

• Sand Specifications: Effective size 0.35-0.55mm, uniformity coefficient <3.5. Finer sand improves filtration but clogs faster; coarser sand reduces treatment efficiency.

• Underdrain Spacing: 6-12 inches on center for lateral systems. Closer spacing improves backwash efficiency but increases complexity and cost.

• Freeboard: Minimum 24 inches above sand surface for backwash expansion (typically 25-50% bed expansion).

• Backwash Rate: 12-15 gpm/sf for 8-12 minutes. Lower rates cause inadequate cleaning; higher rates waste water and risk sand loss.

• Filter Run Time: 24-72 hours between backwashes depending on influent quality and loading rate. Shorter cycles increase O&M costs.

• Headloss Range: Clean bed 2-4 feet, terminal headloss 8-12 feet maximum. Excessive headloss reduces capacity and increases energy costs.

• Air Scour Rate: 3-5 scfm/sf when provided, typically 2-5 minutes before backwash water application.

• What hydraulic loading rate should be specified for the expected influent quality? Secondary effluent allows 4-6 gpm/sf, while primary effluent requires 1-3 gpm/sf. Oversizing reduces capital efficiency; undersizing causes frequent backwashing, poor effluent quality, and potential regulatory violations. Need influent TSS, BOD, and peak flow data.

• Should air scour be included in the backwash sequence? Air scour reduces backwash water consumption by 30-40% and improves cleaning efficiency, but adds $50,000-100,000 per filter in equipment costs. Essential for primary effluent applications or when backwash water disposal is costly. Evaluate based on influent loading and disposal costs.

• What filter bed configuration optimizes footprint versus performance? Declining rate filters in series provide better effluent quality but require 20-30% more area than constant rate designs. Single-stage systems cost less but may not meet stringent discharge limits. Consider available land, effluent requirements, and expansion needs.

• How many filter cells are needed for reliable operation? Minimum three cells for continuous operation during maintenance. Fewer cells risk treatment interruption; excessive cells increase capital costs unnecessarily. Base on peak flow capacity with one cell out of service.

• Division 40 - Process Integration

• Section 40 32 26 - Intermittent Sand Filtration Systems

• Primary specification section covering complete filter systems including media, underdrains, backwash equipment, and controls for municipal wastewater treatment applications.

• Material/Equipment Verification: Sand gradation analysis and uniformity coefficient verification, Underdrain pipe materials and perforation specifications, Geotextile fabric compatibility with media

• Installation Requirements: Precise grade control for distribution boxes and collection systems, Compaction testing for base materials, Weather protection during media placement

• Field Challenges: Maintaining sand specification during delivery and placement, Preventing contamination of filter media, Coordination with electrical for dosing controls

• Coordination Issues: Site dewatering during construction, Access for maintenance equipment

• Lead times: 8-12 weeks for precast components.

• Orenco Systems - AdvanTex AX-Series modular units for smaller communities (0.01-2 MGD capacity). Strong in packaged treatment applications.

• SeptiTech - Peat/sand filter systems with proprietary media blends, popular in Northeast markets.

• Infiltrator Water Technologies - Precast concrete ISF systems with integrated underdrain networks.

• Norweco - Singulair sand filter units for smaller municipal applications.

• Constructed Wetlands - Lower O&M costs, higher land requirements. 30-40% less expensive than ISF systems.

• Membrane Bioreactors (MBR) - Higher treatment quality, 2-3x capital cost, suitable for tight discharge limits.

• Oxidation Ditches - Proven technology for 1-20 MGD range, similar costs but different land requirements and operational complexity.

Establish relationships with local aggregate suppliers early - sand specification compliance is critical and many suppliers don't understand municipal requirements. Consider oversizing distribution boxes by 25% for easier maintenance access. Budget for annual sand replacement in top 2-3 inches due to biomat formation. Coordinate with local health departments on monitoring requirements - many have specific sampling protocols that affect system design and access requirements.