Appearance
Preliminary Treatment
First stage of wastewater treatment focused on removing large solids, grit, and FOG to protect downstream equipment and processes.
Overview
Preliminary treatment (also called pretreatment or primary screening) removes materials that could damage equipment, interfere with treatment processes, or create operational problems. This includes large debris, grit, sand, and fats/oils/grease (FOG).
Purpose of Preliminary Treatment
Equipment Protection
- Prevent pump damage and clogging
- Protect fine screens, membranes, and mechanical equipment
- Reduce wear on pipes and valves
Process Optimization
- Minimize grit accumulation in tanks and digesters
- Prevent FOG interference with biological processes
- Improve efficiency of downstream treatment
Operational Benefits
- Reduce maintenance requirements
- Minimize odor issues
- Simplify sludge handling
Process Unit Categories
Screening
Removal of large and fine suspended solids through physical barriers.
Coarse Screen
- Bar spacing: 25-75mm
- Purpose: Remove large debris (rags, sticks, plastics)
- Location: First unit in treatment train
- Removal: 95% of particles >spacing
- Screenings: 5-60 L/1000m³ (depends on sewer type)
When to Use:
- All municipal WWTPs (essential first step)
- Pumping stations before pumps
- Combined sewer systems
Fine Screen
- Opening size: 1-6mm
- Purpose: Remove fine suspended solids and fibers
- TSS removal: 30-85% (depends on opening)
- Can replace: Primary clarifiers in some designs
- Screenings: 15-40 L/1000m³
When to Use:
- Modern plants (increasingly common)
- MBR/MBBR pretreatment
- Primary treatment alternative
- Space-limited installations
Drum Screen
- Configuration: Rotating perforated drum
- Opening size: 0.5-6mm
- Footprint: Very compact (1/3 of static screens)
- TSS removal: 35-90% (depends on opening)
- Self-cleaning: Continuous spray wash
When to Use:
- Space-limited sites
- High or variable flows
- Fibrous industrial wastewaters
- Stable headloss requirements
Step Screen
- Mechanism: Moving steps lift screenings
- Bar spacing: 3-25mm
- Self-cleaning: Mechanical step action
- Reliability: Excellent (no spray wash needed)
- TSS removal: 30-80% (depends on spacing)
When to Use:
- Municipal headworks (reliable performance)
- Combined sewer systems
- Fibrous materials
- When dry screenings are valuable
Grit Removal
Separation of heavy inorganic particles (sand, gravel, grit) from organic solids.
Grit Chamber
- Type: Horizontal flow sedimentation
- Velocity control: 0.15-0.45 m/s (typically 0.30)
- Detention: 45-90 seconds
- Target particle: 0.21mm @ SG 2.65
- Footprint: Large
When to Use:
- All municipal plants (standard practice)
- Combined sewers (high grit loads)
- Simple, proven technology
- Large site area available
Vortex Grit Separator
- Type: Induced vortex in circular basin
- Detention: 20-45 seconds
- Footprint: Very compact (1/5 of horizontal)
- Flow range: Wide (10:1 typical)
- Organic content: Excellent separation (<5%)
When to Use:
- Space-limited installations
- Variable flow (combined sewers)
- Retrofits
- High capital budget
Aerated Grit Chamber
- Type: Air-induced helical flow
- Detention: 2-5 minutes
- Air supply: 0.15-0.45 m³/min/m²
- Organic content: Excellent (3-8%)
- Benefit: Pre-aeration (10-20% BOD removal)
When to Use:
- Plants needing excellent grit/organic separation
- When grit washing is limited
- Pre-aeration benefit desired
- Industrial high-organic wastewaters
FOG Removal
Removal of fats, oils, and grease through density separation.
Grease Trap
- Type: Gravity separation (buoyancy)
- Detention: 10-60 minutes (typically 20-30)
- FOG removal: 85-95% (free FOG)
- Application: Food service, commercial kitchens
- Maintenance: Regular pumping (1-3 months)
When to Use:
- Restaurants, commercial kitchens
- Food processing facilities
- Before discharge to sewer
- Where FOG >50 mg/L
Size Reduction (Alternative to Screening)
Comminutor
- Function: Shreds solids instead of removing them
- Output size: <6-10mm
- Purpose: Pump protection
- Solids removal: 0% (size reduction only)
- Modern use: Declining (screening preferred)
When to Use:
- Pumping stations (supplemental to screening)
- Small plants with limited space
- Industrial specific applications
Not Recommended For:
- Primary treatment replacement
- MBR/membrane pretreatment
- High-performance biological systems
Typical Treatment Train Configurations
Standard Municipal Plant (Separate Sewers)
Influent → Coarse Screen (50mm) → Grit Chamber → Fine Screen (3mm) → Primary Clarifier → ...Modern Compact Plant
Influent → Coarse Screen (50mm) → Fine Screen (1-3mm) → Vortex Grit → Biological Treatment → ...
(no primary clarifier)MBR Plant
Influent → Coarse Screen → Fine Screen (1mm) → Vortex Grit → MBR → ...Combined Sewer System
Influent → Coarse Screen (75mm) → Vortex Grit Separator → Fine Screen (6mm) → Primary → ...Small Pumping Station
Influent → Coarse Screen → [Comminutor] → Pumps → Force Main
(optional)Design Criteria Summary
Screening
| Unit Type | Removal | Headloss | Footprint | Screenings | Capital Cost |
|---|---|---|---|---|---|
| Coarse Screen | Large debris | 150-600mm | Medium | 5-60 L/1000m³ | Low |
| Fine Screen | 30-85% TSS | 200-600mm | Medium | 15-40 L/1000m³ | Medium |
| Drum Screen | 35-90% TSS | 100-300mm | Small | 15-40 L/1000m³ | High |
| Step Screen | 30-80% TSS | 50-500mm | Large | 15-40 L/1000m³ | High |
Grit Removal
| Unit Type | Grit Removal | Organic in Grit | Footprint | Detention | Capital Cost |
|---|---|---|---|---|---|
| Horizontal Chamber | 90-95% | 20-40% (unwashed) | Large | 45-90s | Low |
| Vortex Separator | 95-99% | 5-10% | Very Small | 20-45s | High |
| Aerated Chamber | 95-98% | 3-8% | Medium | 2-5 min | Medium |
FOG Removal
| Application | Trap Size | Detention | Cleaning Frequency |
|---|---|---|---|
| Small restaurant (<50 seats) | 3-6 m³ | 20 min | 4-8 weeks |
| Large restaurant (>150 seats) | 18-36 m³ | 30 min | 2-4 weeks |
| Food processing | 24-72 m³ | 40 min | 1-2 weeks |
UK Design Standards
Screening
| Parameter | Coarse | Fine |
|---|---|---|
| Approach velocity | 0.6-1.0 m/s | 0.8-1.2 m/s |
| Through-screen velocity | <0.9 m/s | <0.6 m/s |
| Headloss (clean) | <150mm | <200mm |
| Headloss (max) | <600mm | <600mm |
Grit Removal
| Parameter | Horizontal | Vortex | Aerated |
|---|---|---|---|
| Velocity/Detention | 0.30 m/s / 60s | - / 30s | - / 3 min |
| Channels | ≥2 | ≥1 | ≥2 |
| Organic content target | <5% (washed) | <5% | <8% |
Performance Expectations
Typical Removal Efficiencies
| Parameter | Preliminary Treatment | Notes |
|---|---|---|
| Large debris (>25mm) | >95% | Coarse screening |
| TSS | 10-60% | Depends on screen opening |
| BOD | 5-25% | Mainly from fine screening |
| COD | 5-20% | Screens + grit |
| Grit (>0.21mm) | 90-98% | Grit chambers/separators |
| FOG (free) | 85-95% | Grease traps |
Waste Streams Generated
| Stream | Quantity | Characteristics | Disposal |
|---|---|---|---|
| Screenings (coarse) | 5-60 L/1000m³ | Wet, high organics (50-70%) | Landfill, incineration |
| Screenings (fine) | 15-40 L/1000m³ | Wet, moderate organics (30-50%) | Landfill, washing + landfill |
| Grit (unwashed) | 5-90 L/1000m³ | Wet, organics 20-40% | Washing required |
| Grit (washed) | 5-90 L/1000m³ | Low organics (<5%) | Landfill, beneficial reuse |
| FOG | Variable | High organics, energy content | Rendering, anaerobic digestion |
Operational Considerations
Automation
- Most modern units are fully automatic
- Level/headloss sensors trigger cleaning cycles
- Remote monitoring increasingly common
- Manual backup always required
Maintenance Requirements
| Unit | Frequency | Key Tasks |
|---|---|---|
| Screens | Daily | Visual check, cleaning verification |
| Weekly | Lubrication, spray wash check | |
| Monthly | Detailed inspection, bar condition | |
| Grit Units | Daily | Visual check, removal operation |
| Weekly | Check velocity/air (if aerated) | |
| Quarterly | Drain and clean, inspect | |
| Grease Traps | Monthly | Measure FOG/solids layers |
| 1-3 months | Pump out and clean |
Common Issues
| Issue | Likely Cause | Prevention |
|---|---|---|
| Screen blinding | Fats, fibers, inadequate cleaning | Increase cleaning, add washing |
| Grit carryover | Velocity too high, short-circuiting | Check flow control, baffles |
| FOG in effluent | Trap full, emulsification | Regular pumping, no detergents |
| Odor | Septicity, poor ventilation | Reduce retention, improve venting |
Integration with Downstream Processes
Primary Treatment
- Preliminary treatment protects primary clarifiers
- Fine screens may replace primary in compact designs
- Grit removal essential to prevent accumulation
Biological Treatment
- TSS removal improves biological performance
- Grit prevents abrasion of diffusers, mixers
- FOG removal prevents biomass inhibition
Membrane Systems (MBR/UF/MF)
- Very fine screening critical (<1-2mm)
- Excellent grit removal essential
- FOG must be minimal (<10-20 mg/L)
Technology Selection Guide
Choose Coarse + Fine Screens When:
- Municipal plant (most common configuration)
- High TSS removal desired
- MBR or membrane downstream
- Screenings disposal is acceptable
Choose Horizontal Grit Chamber When:
- Large site area available
- Simple, proven technology preferred
- Grit washing facilities available
- Low capital budget
Choose Vortex Grit When:
- Space very limited
- Wide flow variations (combined sewers)
- Retrofit applications
- Excellent separation needed
Choose Aerated Grit When:
- Grit washing not available
- Pre-aeration benefit desired
- Industrial high-organic wastewaters
- Moderate footprint acceptable
Future Trends
Emerging Technologies
- Fine screening replacing primary clarifiers (more compact plants)
- Perforated plate screens (1-3mm replacing clarifiers)
- Compact vortex units (replacing large chambers)
- Automated monitoring (headloss, efficiency, cleaning optimization)
Sustainability Improvements
- Energy recovery from screenings (anaerobic digestion, incineration)
- Grit beneficial reuse (construction aggregate)
- FOG to biodiesel (rendering, energy recovery)
Related Topics
- Primary-Treatment - Next treatment stage
- Pumps - Protected by preliminary treatment
- Biological-Treatment - Benefits from upstream solids removal
- Sludge-Treatment - Handles preliminary treatment wastes
References
- Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Reuse, 5th Ed. McGraw-Hill.
- WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed.
- Tchobanoglous, G. et al. (2003). Wastewater Engineering: Treatment and Reuse, 4th Ed. McGraw-Hill.
- MWH (2005). Water Treatment: Principles and Design, 2nd Ed. John Wiley & Sons.
- Water UK (2017). Sewers for Adoption, 8th Ed.
- WRc (1998). Sewage Treatment in Hot Climates. John Wiley & Sons.