Tertiary Filtration

Granular and cloth media filtration systems for suspended solids removal, turbidity reduction, and effluent polishing following secondary biological treatment.

Overview

Filtration is the most common tertiary treatment process, removing residual suspended solids and reducing turbidity to meet discharge standards or prepare effluent for disinfection and reuse. Filters operate through physical straining, sedimentation within pores, and biological activity (in some configurations), producing high-quality effluent at relatively low cost and energy consumption.

Available Filter Types

Rapid-Sand-Filter

Workhorse of tertiary filtration

• Media: Sand (0.5-1.0 mm effective size)
• Depth: 0.6-1.2 m
• Filtration rate: 5-15 m/h (120-360 m³/m²·day)
• Effluent TSS: 2-5 mg/L
• Backwash frequency: 24-72 hours
• Backwash water: 2-4% of production

Key features:

• Iwasaki depth filtration model (exponential removal)
• Carmen-Kozeny headloss prediction
• Reliable, proven technology
• Biological nitrification possible at low rates (5-8 m/h)
• Giardia/Cryptosporidium removal: 2-3 log (with proper operation)

Best for:

• Standard tertiary polishing
• Upstream of UV or chlorine disinfection
• Water reuse (non-potable to potable)
• Nitrification polishing (biological mode)

Disk-Filter

Compact cloth media filtration

• Media: Woven cloth (10-20 μm openings) on rotating disks
• Filtration rate: 10-25 m/h
• Effluent TSS: 3-8 mg/L
• Backwash: Continuous or intermittent spray wash
• Footprint: 50-70% less than sand filters

Key features:

• Very compact (vertical disk configuration)
• Simple operation (automated backwash)
• Low backwash water (1-3%)
• Suitable for variable flow (adjustable speed)
• Lower capital cost than sand filters (in many cases)

Best for:

• Space-constrained sites
• Simple O&M requirements
• Variable flow applications
• Peak wet weather treatment

GAC-Filter

Dual-function: Filtration + Adsorption

• Media: Granular activated carbon (0.6-2.0 mm)
• Depth: 1.0-2.0 m (deeper than sand for adsorption capacity)
• Filtration rate: 5-10 m/h (lower than sand due to adsorption contact time)
• Effluent TSS: 2-4 mg/L
• Effluent organics: 50-90% TOC removal (fresh carbon)
• Carbon life: 1-3 years (exhaustion depends on organic loading)

Key features:

• Freundlich isotherm adsorption model
• Removes dissolved organics + TSS
• Micropollutant removal (pharmaceuticals, pesticides)
• Color and taste/odor removal
• Requires thermal or chemical regeneration (or replacement)

Best for:

• Water reuse requiring organics removal
• Color removal (humic/fulvic acids)
• Micropollutant removal
• Combined filtration + adsorption (avoid separate units)

Anthracite-Filter

High-rate coarse media

• Media: Anthracite coal (1.0-2.0 mm, coarser than sand)
• Depth: 0.6-1.0 m
• Filtration rate: 10-20 m/h (higher than sand)
• Effluent TSS: 3-6 mg/L (slightly higher than sand)
• Lower headloss: Coarser media allows higher rates

Key features:

• Higher filtration rates (shorter HRT, smaller tanks)
• Lower pressure drop (energy savings)
• Longer filter runs (less frequent backwash)
• Lighter media (easier handling)
• Often used in dual-media configuration (see below)

Best for:

• High-rate applications (flow surges, peak wet weather)
• Lower effluent quality acceptable (3-6 mg/L TSS)
• Space-constrained sites (smaller tanks at higher rates)

Dual-Media-Filter

Optimized stratification

• Upper layer: Anthracite (1.0-2.0 mm, 0.4-0.6 m depth)
• Lower layer: Sand (0.5-0.7 mm, 0.3-0.5 m depth)
• Total depth: 0.9-1.2 m
• Filtration rate: 10-20 m/h
• Effluent TSS: 2-5 mg/L (superior to single-media at same rate)

Key features:

• Coarse-to-fine stratification (optimized solids capture)
• Higher rates than sand alone with comparable quality
• Reduced headloss buildup (solids distributed through depth)
• Longer filter runs than single-media
• More complex backwash (ensure proper restratification)

Best for:

• Balance of high rate and high quality
• Replacing aging sand filters (retrofit)
• Water reuse applications requiring reliability
• Flow equalization challenges (variable flow rates)

Pile-Cloth-Filter

Continuous backwash fabric filtration

• Media: Fabric (polyester or polypropylene, 10-15 μm)
• Configuration: Vertical tubes (piles) submerged in tank
• Filtration: Outside-in through fabric
• Backwash: Continuous internal suction (1-3% flow)
• Effluent TSS: 3-8 mg/L

Key features:

• Continuous operation (no offline cycles)
• Very compact footprint
• Automated operation (minimal manual intervention)
• Low chemical consumption (no chemical backwash typically)
• Fabric replacement: 5-10 years

Best for:

• Continuous flow applications (no flow equalization needed)
• Very limited space
• Simple operation (unstaffed plants)
• Replacement for aging sand filters (retrofit in existing basins)

Technology Comparison

Filter Type	Rate (m/h)	Effluent TSS (mg/L)	Headloss (m)	Backwash (% water)	Footprint	Capital Cost	O&M Complexity
Rapid Sand	5-15	2-5	0.5-2.5	2-4%	Baseline (100%)	$$	Medium
Disk	10-25	3-8	0.3-1.0	1-3%	30-50%	$	Low
GAC	5-10	2-4	0.8-3.0	2-4%	120-150%	$$$$	High (regeneration)
Anthracite	10-20	3-6	0.4-2.0	2-4%	60-80%	$$	Medium
Dual Media	10-20	2-5	0.5-2.5	3-5%	70-90%	$$$	Medium-High
Pile Cloth	5-15	3-8	0.2-0.8	2-4%	40-60%	$$$	Low

Capital Cost Legend:

• $ = $50-100 per m³/d capacity
• $$ = $100-200 per m³/d capacity
• $$$ = $200-400 per m³/d capacity
• $$$$ = $400-800 per m³/d capacity (GAC includes carbon cost)

Selection Guide

By Effluent Requirement

TSS ❤️ mg/L (stringent):

1. Rapid Sand Filter (proven, reliable)
2. GAC Filter (dual benefit if organics also concern)
3. Dual Media Filter (high-rate alternative to sand)

TSS <5 mg/L (standard tertiary):

1. Rapid Sand Filter (most common)
2. Dual Media Filter (if high rate needed)
3. Anthracite Filter (if moderate quality acceptable)

TSS <10 mg/L (less stringent):

1. Disk Filter (simplest, lowest cost)
2. Pile Cloth Filter (continuous operation)
3. Anthracite Filter (high rate)

Organics Removal (color, TOC, micropollutants):

• Only option: GAC Filter
• Alternatives: GAC after sand filter (separate units)

By Site Constraints

Limited Space (small footprint):

1. Disk Filter (most compact)
2. Pile Cloth Filter (vertical configuration)
3. Anthracite or Dual Media at high rate (smaller than sand)

Limited Operational Staff:

1. Disk Filter (automated, simple)
2. Pile Cloth Filter (continuous, minimal intervention)
3. Rapid Sand Filter with automation (proven reliability)

Variable Flow (no flow equalization):

1. Pile Cloth Filter (continuous, handles surges)
2. Disk Filter (variable speed control)
3. Declining rate sand filters (self-regulating)

Peak Wet Weather Treatment:

1. Disk Filter (rapid deployment, simple operation)
2. Anthracite Filter (high rate, handles surges)
3. Dual Media Filter (high rate with better quality)

By Application

Standard Tertiary Treatment (discharge compliance):

Secondary Effluent (TSS 10-20 mg/L) → Rapid Sand Filter → TSS <5 mg/L
- Proven, reliable
- Moderate cost
- Well-understood O&M

Upstream of UV Disinfection (turbidity critical):

Secondary Effluent → Dual Media Filter → TSS <3 mg/L, Turbidity <0.5 NTU → UV
- Lower turbidity than disk or anthracite
- Consistent quality for UV performance

Water Reuse (non-potable irrigation):

Secondary Effluent → Disk Filter → TSS <8 mg/L → Storage
- Simple, low-cost
- Adequate quality for irrigation
- Minimal O&M

Water Reuse (potable, upstream of RO/NF):

Secondary Effluent → Dual Media Filter or MF → TSS <2 mg/L → Membrane System
- High-quality pretreatment reduces membrane fouling
- MF alternative if virus barrier also required

Micropollutant Removal (pharmaceuticals, pesticides):

Secondary Effluent → GAC Filter → Reduced organics, TSS <4 mg/L → Discharge
- Adsorption removes trace organics
- Filtration removes TSS
- Carbon regeneration or replacement required

Retrofit (limited space, replace aging filters):

Option 1: Remove sand, install Pile Cloth Filter in existing basin
Option 2: Remove sand, install Disk Filters in existing basin
Option 3: Upgrade to Dual Media (better performance, same basin)

Common Design Features

Backwash Systems

Water Backwash (all filters except pile cloth):

• Fluidize media at 1.2-1.5× minimum fluidization velocity
• Duration: 8-15 minutes
• Frequency: 24-72 hours (headloss or time trigger)
• Sand: 50-70 m/h, Anthracite: 40-60 m/h

Air Scour (optional but recommended for sand/anthracite/dual media):

• Air rate: 0.6-1.0 m/min (36-60 m/h)
• Duration: 2-5 minutes before water backwash
• Improves cleaning efficiency (breaks up mudballs)
• Reduces water backwash volume

Surface Wash (optional):

• Rotating arms with high-pressure jets
• Concurrent with backwash
• Improves cleaning of surface layer

Cloth Filters (disk, pile cloth):

• Continuous or intermittent spray wash
• Low-pressure backwash (no fluidization)
• Water consumption: 1-3% of production

Underdrain Systems

Purpose:

• Uniform backwash distribution (±10% flow variation)
• Support media (prevent loss during backwash)
• Collect filtered water

Types:

1. Nozzle floor: Plastic filter nozzles, eliminates gravel support
2. Pipe laterals: Perforated pipes in gravel layers (traditional)
3. Block floor: Proprietary plastic blocks with integral channels

Filter Media Specifications

Sand:

• Effective size (d₁₀): 0.5-0.7 mm
• Uniformity coefficient (UC = d₆₀/d₁₀): <1.7 (well-graded)
• Silica content: >95%
• Acid solubility: <5%

Anthracite:

• Effective size: 1.0-2.0 mm
• Uniformity coefficient: <1.7
• Hardness: >2.5 Mohs
• Specific gravity: 1.4-1.7 (lighter than sand)

GAC:

• Iodine number: >900 mg/g (adsorption capacity)
• Effective size: 0.6-2.0 mm
• Hardness: >95% (abrasion resistance)
• Ash content: <10%

Performance Monitoring

Key Performance Indicators (KPIs)

Effluent Quality:

• TSS: Daily composite sample, target <5 mg/L
• Turbidity: Online monitoring, target <1 NTU
• Pathogen removal: Periodic testing (Giardia/Crypto if required)

Operational Efficiency:

• Filter run time: Trend analysis (declining run time = fouling or influent quality degradation)
• Headloss development: Plot headloss vs. time (linear = healthy, exponential = mudballing or channeling)
• Backwash frequency: Minimize while maintaining effluent quality
• Backwash water percentage: Target <5% of production

Unit Filter Run Volume (UFRV):

$$UFRV = \frac{Volume_filtered}{Area} = Rate \times Runtime$$

• Typical: 500-2,000 m³/m² per run
• Higher UFRV = better performance (less frequent backwash)
• Track over time (declining UFRV indicates problem)

Troubleshooting Common Issues

Problem	Likely Cause	Solution
High effluent TSS	Filtration rate too high	Reduce rate, add filter cells
	Media too coarse	Replace with finer media
	Filter run too long (breakthrough)	Reduce run time, backwash sooner
	Channeling/short-circuiting	Repair underdrain, check media surface
Rapid headloss buildup	High influent TSS	Improve secondary treatment
	Mudballing	Increase backwash intensity, air scour
	Biological growth	Chlorinate filter, increase backwash frequency
Short filter runs (<24h)	Very high influent TSS	Pre-treatment or coagulation
	Backwash ineffective	Increase BW rate/duration, check distribution
	Terminal headloss too low	Increase headloss setpoint
Media loss during backwash	Backwash rate too high	Reduce to proper fluidization velocity
	Inadequate freeboard	Increase freeboard or add wash troughs
Poor backwash (media not clean)	Insufficient fluidization	Increase backwash rate
	No air scour	Install air scour system
	Water temperature low (high viscosity)	Adjust for temperature, longer backwash

Advanced Features

Declining Rate Filtration

Filters operate at variable rate based on headloss:

• No rate control valves (gravity flow)
• Clean filters accept higher flow, dirty filters slow down naturally
• Equalizes run time across all cells
• Reduces operational complexity

Biological Nitrification Filters

Operate filters at low rate with long contact time:

• Filtration rate: 2-5 m/h (very low)
• DO requirement: >2 mg/L in influent
• Nitrification: NH₄-N removal from 3-5 mg/L to <1 mg/L
• Periodic backwash removes excess biofilm
• Dual-purpose: TSS removal + biological treatment

Filter-to-Waste

Initial filtrate after backwash contains:

• Fines from media degradation
• Residual backwash water (higher turbidity)
• Solution: Waste first 5-10 minutes of filtrate to drain before returning to service

Coagulation Pretreatment

Add coagulant (ferric chloride, alum) upstream of filter:

• Dose: 5-20 mg/L (as Fe or Al)
• Improves TSS removal (to <2 mg/L)
• Improves pathogen removal (2.5-3.5 log Giardia)
• Reduces turbidity (to <0.3 NTU, excellent for UV)
• Trade-off: Shorter filter runs, higher backwash frequency

Process Unit Links

• Rapid-Sand-Filter - Standard granular media filtration
• Disk-Filter - Cloth media on rotating disks
• GAC-Filter - Activated carbon filtration + adsorption
• Anthracite-Filter - High-rate coarse media
• Dual-Media-Filter - Stratified anthracite + sand
• Pile-Cloth-Filter - Continuous backwash fabric media

Related Processes

• Secondary-Clarifier - Upstream clarification
• UV-Disinfection - Downstream disinfection (requires low turbidity)
• Chlorination - Alternative disinfection (less sensitive to turbidity)
• Microfiltration - Membrane alternative (tighter barrier, higher cost)
• Chemical-Dosing - Coagulation pretreatment

References

1. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 16: Filtration.
2. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1242-1290.
3. AWWA (2011). Water Quality and Treatment, 6th Ed., Ch. 8: Granular Media Filtration.
4. Kawamura, S. (2000). Integrated Design and Operation of Water Treatment Facilities, 2nd Ed., Wiley.
5. Crittenden, J.C. et al. (2012). MWH's Water Treatment, 3rd Ed., Ch. 10: Granular Filtration.
6. Tobiason, J.E. et al. (2011). "Characterization and Performance of Filter Media for Drinking Water Treatment." J. AWWA 103(12): 61-74.
7. US EPA (2006). Membrane Filtration Guidance Manual, EPA 815-R-06-009 (comparison to membrane alternatives).