Introduction
Effective wastewater treatment equipment design is crucial for environmental protection, regulatory compliance, and operational efficiency in industrial and remote settings. This guide provides best practices for designing systems to collect, treat, reuse, and dispose of wastewater and oily waste sludges, ensuring minimal environmental impact and adherence to global standards. Covering sewers, lift stations, septic tanks, reuse systems, and disposal methods like evaporation ponds and injection wells, this article offers actionable insights for engineers and facility managers worldwide.
All wastewater treatment equipment designs should comply with local environmental regulations and secure necessary approvals to protect public health and optimize resource use.
1. Collection Equipment: Sewers and Lift Stations
Efficient collection systems are the backbone of wastewater and oily waste sludge management, ensuring safe transport to treatment or disposal facilities.
Sewers
- Design Standards: Sewers must meet sanitary and industrial drainage standards, maintaining minimum separation distances from water lines (e.g., 0.3 m from domestic lines, 3 m from pressurized lines) to prevent contamination.
- Materials and Features: Use nonmetallic materials like fiberglass-reinforced plastic for manholes to resist corrosion. Avoid potable water connections and design channels to prevent solid buildup, incorporating fillets and gates for unused sections.
- Oily Wastewater: Install sand separators upstream of lift stations to remove solids, enhancing system longevity.
- Remote Areas: For sources >150 m from sewers, vacuum truck hauling may be permitted with regulatory approval, ideal for temporary or remote wastewater management.
Lift Stations
- Design Requirements: Ensure flood resistance, accessibility, and separation from potable water sources for lift stations in wastewater treatment equipment design.
- Pumps: Equip with at least two pumps, each exceeding peak flow capacity, and include emergency storage to handle failures.
- Ventilation and Odor Control: Provide 12-30 air changes/hour. Use activated carbon for flows <150 m³/day or fan-forced systems for larger flows in residential areas (<60 m from work) to manage odors.
- Alarms: Install alarms for power or pump failures, telemetered for unattended stations, ensuring operational reliability.
- Force Mains: Design to handle peak flows and pressures, adhering to industry standards for wastewater collection.
2. Treatment Equipment: Preliminary to Tertiary
Treatment systems are critical for removing contaminants to meet discharge or reuse standards, tailored to sanitary or oily wastewater.
Preliminary Treatment
- Bar Screens and Comminutors: Essential for sanitary wastewater treatment equipment, use manually cleaned screens (25-40 mm spacing) or mechanical screens (down to 2 mm for membrane bioreactors). Maintain channel velocities of 0.3-1.0 m/sec to prevent settling.
- Grit/Sand Removal: Use aerated or vortex chambers for sanitary wastewater and sand separators for oily wastewater to protect downstream equipment.
Primary Treatment
- Oily Wastewater: Employ API or corrugated plate interceptor (CPI) separators, dissolved/induced air flotation (DAF/IAF), or settling ponds. Equip with interface sensors and air gaps to manage oily waste sludge effectively. Dispose of sludges in approved facilities.
- Equalization: Include tanks with 1-day surge capacity for DAF/IAF systems to stabilize flow and composition.
Secondary and Tertiary Treatment
- Secondary Treatment: Utilize activated sludge, extended aeration, or rotating biological contactors for sanitary wastewater. Oily wastewater may require biological or chemical oxidation processes.
- Tertiary Treatment: For wastewater reuse systems, implement filtration (sand or membrane) and disinfection (chlorination with 1.0 mg/L residual after 30 minutes, 0.5 mg/L at the furthest point) to meet stringent quality standards.
- Sludge Handling: Stabilize oily sludges with lime and dispose in approved facilities. Sanitary sludges can be composted or landfilled with regulatory approval.
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3. Septic Tank Design for Small-Scale Treatment
Septic tanks are ideal for small-scale wastewater treatment equipment design, particularly in remote locations.
- Design Standards: Tanks require two or more compartments, with the first comprising 50-66% of volume (minimum 1.9 m³) and the second ≥1 m³.
- Materials: Use watertight materials like concrete, fiberglass, or fiberglass-reinforced plastic. Precast tanks need 75 mm wall thickness and corrosion-resistant coatings (e.g., bituminous or epoxy-based).
- Inlet/Outlet: Position inlet invert ≥50 mm above liquid level, outlet 50 mm lower. Use vented inlet tees (0.3 m below liquid) and outlet devices (40% liquid depth, 35% for cylindrical tanks).
- Access: Provide manholes (≥0.5 m diameter) per compartment. If >0.5 m below grade, extend to 0.2 m below finished grade for maintenance access.
- Installation: Backfill in thin, tamped layers; use water to settle if tank is filled. Connect to distribution boxes with PVC or reinforced thermosetting resin (RTR) piping.
- Testing: Fill tanks with water overnight to check for leaks, repairing as needed to ensure system integrity.
4. Wastewater Reuse Systems: Unrestricted Irrigation
Wastewater reuse systems for irrigation reduce freshwater demand but require rigorous treatment and design.
- Treatment Requirements: Treat wastewater through oxidation, coagulation, flocculation, clarification, filtration (or MBR), and disinfection to achieve ≤2.2 Coliform/100 mL (median) and ≤23/100 mL in any 30-day period, with no detectable pathogens in 40 L samples.
- Equipment: Install turbidity meters at secondary effluent discharge and chlorine analyzers at final effluent with continuous recording. Use backflow preventers and 0.15 m air gaps for systems combined with raw water.
- Piping and Valves: Employ color-coded piping and secure valves to prevent unauthorized access. Tag components to warn against drinking or bathing.
- Storage: Provide effluent storage tanks with 1.5 days’ capacity to manage demand fluctuations.
- Safety Measures: Install warning signs with pictorial and written notices. Prohibit irrigation within 150 m of domestic wells and ensure no crop contact 30 days before harvest.
5. Wastewater Disposal Systems: Ponds and Injection Wells
Wastewater disposal systems must prevent environmental contamination and comply with regulations.
- Evaporation Ponds:
- Design for 2 m depth, above the water table or with a pressure line. Size at 100 m²/person (housing) or 25 m²/person (workplace), reducible for <1-year use with fencing.
- Use 30-mil high-density polyethylene liners over 0.3 m compacted clay or 0.5 m compacted clay (permeability ≤1×10⁻⁵ cm/sec). Install one upgradient and two downgradient groundwater monitoring wells.
- Maintain 0.75 m minimum depth, 0.6 m freeboard, with compacted embankments (1:3 slope) and wave protection.
- Percolation Ponds:
- Similar to evaporation ponds but with a 10 cm gravel and 20 cm clean stone bottom. Size at 10 m²/30 employees, 1.5 m above groundwater.
- Injection Wells:
- Require detailed engineering studies and regulatory approval. Suitable for high-TDS wastewater (>30,000 mg/L) with treatment to prevent groundwater contamination.
- General Requirements: Fence ponds, locate ≥150 m from residences/work sites, and close with lime and backfill. Use carbon vent canisters for odor control.
6. Installation and Testing
- Installation:
- Follow manufacturer’s instructions for all wastewater treatment equipment. Grade sites to prevent surface water drainage into units.
- Backfill septic tanks and trenches in thin, tamped layers. Use non-woven geotextile (90-110 g/m²) in trenches and evapotranspiration fields.
- Absorption trenches use 20-65 mm clean stone (0.3 m deep); evapotranspiration fields use 20-60 mm stone and 0.4 m fine sand (95% silica) with vegetation.
- Testing:
- Conduct manufacturer-recommended tests, including oxygen transfer rates for aeration processes.
- Test sewers for water tightness and septic tanks for leaks by filling with water overnight.
Conclusion
Designing wastewater treatment equipment for collection, treatment, reuse, and disposal requires integrating robust systems tailored to site-specific needs. By following these best practices, facilities can achieve environmental compliance, protect public health, and optimize resource use. Consult local regulations and secure approvals to ensure compliance with environmental standards. For more insights on wastewater treatment equipment design, explore international engineering guidelines or contact regulatory bodies.