Tertiary Treatment

Advanced wastewater treatment processes for TSS reduction, disinfection, and dissolved solids removal to meet stringent discharge standards or enable water reuse.

Overview

Tertiary treatment (also called advanced treatment or effluent polishing) follows secondary biological treatment to further improve effluent quality. These processes remove residual suspended solids, pathogens, nutrients, dissolved organics, and in some cases dissolved salts, enabling safe discharge to sensitive receiving waters or water reuse applications.

Process Categories

Filtration

Physical separation processes removing suspended solids and reducing turbidity:

• Rapid-Sand-Filter - Granular sand (0.5-1.0 mm) at 5-15 m/h, TSS to <5 mg/L
• Disk-Filter - Cloth media on rotating disks, 10-20 μm filtration, low footprint
• GAC-Filter - Activated carbon for TSS + dissolved organics, adsorption + filtration
• Anthracite-Filter - Larger media (1.0-2.0 mm) for higher rates, 10-20 m/h
• Dual-Media-Filter - Anthracite + sand stratification, optimized backwash
• Pile-Cloth-Filter - Fabric media on vertical tubes, continuous backwash, compact

Comparison:

Filter Type	Rate (m/h)	Effluent TSS (mg/L)	Backwash (%)	Application
Rapid Sand	5-15	2-5	2-4%	Standard tertiary, reliable
Disk	10-25	3-8	1-3%	Space-constrained, simple O&M
GAC	5-10	2-4	2-4%	Organics + TSS, micropollutants
Anthracite	10-20	3-6	2-4%	Higher rates, coarser filtration
Dual Media	10-20	2-5	3-5%	Optimized performance
Pile Cloth	5-15	3-8	2-4%	Very compact, continuous operation

Disinfection

Pathogen inactivation for public health protection:

• UV-Disinfection - UV-C (254 nm) dose 40-100 mJ/cm², no chemicals, Cryptosporidium barrier
• Ozone-Disinfection - O₃ dose 5-15 mg/L, CT 0.5-4 mg·min/L, oxidation + disinfection
• Chlorination - Cl₂ or NaOCl, CT 15-450 mg·min/L, persistent residual, requires dechlorination
• Chlorine-Dioxide - ClO₂, pH-independent, no THMs, chlorite byproduct limits dose
• Peracetic-Acid - CH₃COOOH, 2-8 mg/L, no DBPs, biodegradable to acetic acid

Comparison:

Disinfectant	Log Reduction (Bacteria)	Cryptosporidium	Residual	DBP Formation	Energy/Chemical Cost
UV	4-6 log	4+ log (best)	No	None	Low energy, no chemicals
Ozone	4-6 log	2-3 log	No (decays)	Minimal (bromate)	High energy, on-site generation
Chlorine	4-6 log	Poor (<1 log)	Yes (requires dechlorination)	Yes (THMs, HAAs)	Low cost, simple
Chlorine Dioxide	4-6 log	1-2 log	No	No THMs (chlorite/chlorate)	Moderate, on-site generation
Peracetic Acid	3-5 log	2-3 log	No	None	High cost, simple dosing

Membranes

Pressure-driven separation for particle, pathogen, and dissolved solids removal:

• Microfiltration - 0.1-10 μm pores, 0.1-0.5 bar, bacteria/protozoa barrier, RO pretreatment
• Ultrafiltration - 0.01-0.1 μm (10-300 kDa), 0.5-2 bar, virus barrier (4-6 log LRV)
• Nanofiltration - 200-1000 Da, 5-20 bar, hardness/color removal, partial desalination
• Reverse-Osmosis - <100 Da, 10-70 bar, 95-99.5% salt rejection, desalination/reuse

Comparison:

Membrane	Pore Size	TMP (bar)	Rejection	Recovery (%)	Energy (kWh/m³)	Application
MF	0.2-0.4 μm	0.1-0.5	Particles, bacteria, protozoa	90-98	0.02-0.05	Tertiary polishing, RO pretreat
UF	0.02-0.1 μm	0.5-2.0	+ Viruses, macromolecules	90-98	0.05-0.10	Drinking water, virus barrier
NF	0.5-2 nm	5-20	+ Divalent ions, color, organics	70-85	0.4-0.8	Softening, micropollutants
RO	<0.001 μm	10-70	+ Monovalent ions, TDS (95-99%)	40-85	0.5-5.0	Desalination, water reuse

Selection Guide

By Effluent Requirement

TSS Removal (<5 mg/L):

• Primary: Rapid Sand Filter, Disk Filter, Dual Media Filter
• Alternative: MF (if virus barrier also needed)

Pathogen Reduction (4-log bacteria):

• Primary: UV, Chlorination (with CT control)
• Alternative: Ozone, PAA, UF

Virus Barrier (4-log LRV):

• Required: UF or UV
• Alternative: Ozone (2-3 log) + UV (additive effect)

Cryptosporidium Barrier:

• Best: UV (4+ log)
• Acceptable: Ozone (2-3 log), UF/MF (physical barrier)
• Ineffective: Chlorine (very high CT required)

Color Removal (>90%):

• Primary: GAC, NF
• Alternative: Ozone (partial)

Hardness Removal (>85%):

• Primary: NF (selective for Ca²⁺, Mg²⁺)
• Alternative: RO (total removal, higher cost)

TDS Removal (>95%):

• Required: RO
• Not suitable: All other processes (no TDS removal)

Micropollutants (pharmaceuticals, pesticides):

• Best: RO (>95% for most), GAC (adsorption, requires regeneration)
• Moderate: NF (60-95%, MW-dependent), Ozone (oxidation, variable)
• Minimal: UV, MF, UF (no removal)

By Application

Discharge to Sensitive River (Urban):

Secondary Effluent → Rapid Sand Filter → UV Disinfection → Discharge
- TSS <5 mg/L (protect UV)
- 4-log pathogen reduction
- No chlorine residual (fish protection)

Discharge with Chlorination (Rural, large river):

Secondary Effluent → Disk Filter → Chlorination → Dechlorination → Discharge
- Simple operation
- Persistent residual through contact tank
- Dechlorination to <0.01 mg/L

Water Reuse (Non-Potable: Irrigation, Cooling Towers):

Secondary Effluent → Dual Media Filter → UV or Chlorination → Storage
- TSS <5 mg/L (prevent nozzle clogging)
- 4-log pathogen reduction
- Chlorine residual optional (biofilm control in distribution)

Water Reuse (Indirect Potable Reuse, IPR):

Secondary Effluent → MF/UF → RO → UV/Chlorine → Aquifer Recharge or Blending
- Multi-barrier: MF/UF (virus) + RO (TDS, organics) + UV (final disinfection)
- Permeate TDS <200 mg/L
- 12+ log virus reduction (combined LRV)

Water Reuse (Direct Potable Reuse, DPR):

Secondary Effluent → MF/UF → RO → UV + AOP (H₂O₂) → GAC → Chlorination → Distribution
- Advanced multi-barrier treatment train
- Cryptosporidium, virus, chemical barriers
- Extensive monitoring and fail-safes

Brackish Water Desalination:

Brackish Source → Multimedia Filter → Cartridge Filter (5 μm) → RO → Remineralization → Chlorination
- RO for TDS removal (1,500-10,000 mg/L → <250 mg/L)
- Pretreatment critical (SDI <3)
- Post-treatment for water stability

Seawater Desalination:

Seawater Intake → Dissolved Air Flotation → UF → RO (two-stage) → Remineralization → Chlorination
- Rigorous pretreatment (SDI <2)
- High-pressure RO (55-70 bar)
- Energy recovery essential (ERD, 95% efficiency)

By Constraints

Limited Space:

1. Disk Filter (most compact filtration)
2. MF/UF (vertical configuration, small footprint)
3. UV (inline, no contact tank)

Low Energy Budget:

1. Rapid Sand Filter + Chlorination (no energy for filtration, low pumping)
2. UV (0.02-0.05 kWh/m³)
3. MF/UF (0.02-0.10 kWh/m³)

Low Operating Complexity:

1. Disk Filter + UV (minimal chemical handling)
2. Pile Cloth Filter + UV (continuous operation, automated)
3. Chlorination (simple, but requires dechlorination)

No Chemicals Desired:

1. UV (no chemicals, no DBPs)
2. MF/UF (physical barrier, backwash only)

High Pathogen Removal (>5-log):

1. UF + UV (additive: 6 log virus + 4 log UV = 10 log)
2. MF + Ozone + UV (multi-barrier)

Design Considerations

Pretreatment Requirements

For Filtration:

• Secondary clarification: TSS <30 mg/L (prevent rapid clogging)
• Flow equalization: Minimize rate variation (extend filter runs)

For Disinfection:

• Turbidity <5 NTU for UV (light transmission, <2 NTU preferred)
• TSS <10 mg/L for chlorine (reduce demand, improve CT)
• Filtration typically required upstream

For Membranes:

• MF/UF: Secondary treatment, minimize TSS and BOD (reduce fouling)
• NF/RO: UF or tight filtration (SDI ❤️), dechlorination (prevent membrane oxidation)

Process Sequencing

Typical sequence (most to least common):

1. Filtration → Disinfection (80% of plants)

   • Filter reduces turbidity for UV or chlorine
   • Disinfection as final barrier

2. Membrane → Disinfection (water reuse, high-quality)

   • MF/UF provides pathogen barrier + turbidity reduction
   • UV or chlorine as final safety barrier

3. Disinfection → Filtration (rare, specific cases)

   • Pre-chlorination for biofilm control in filter
   • Filter removes chlorine demand substances
   • Re-disinfection after filtration

4. Filtration → Membrane → Disinfection (advanced reuse)

   • Multimedia filter: Reduce fouling of downstream membrane
   • MF/UF or RO: Primary barrier
   • UV: Final disinfection

Redundancy and Reliability

Critical systems (no bypass allowed):

• Multiple parallel trains (N+1 or 2N capacity)
• Automated backup (e.g., two UV banks, one duty + one standby)
• Online monitoring with fail-safe shutdown

Example:

Flow = 10,000 m³/d
Design: 4 filter cells × 2,500 m³/d capacity
Operational: 3 cells online, 1 offline for backwash or maintenance
Allows failure of 1 cell and continued operation at 100% capacity

Treatment Performance Summary

Process	TSS Removal	Pathogen Reduction	TDS Removal	Organics Removal	Energy (kWh/m³)	Capital Cost ($/m³/d capacity)
Rapid Sand Filter	>95% (to <5 mg/L)	1-2 log (protozoa)	None	None	0.01-0.02	$50-150
Disk Filter	90-95% (to <8 mg/L)	1-2 log	None	None	0.01-0.02	$80-200
GAC Filter	>95% (to <5 mg/L)	1-2 log	None	50-90% (adsorption)	0.01-0.02	$150-300
UV Disinfection	Minimal	4-6 log (all)	None	Minimal (AOP if H₂O₂ added)	0.02-0.05	$100-250
Ozone	Minimal	4-6 log (bacteria, viruses)	None	30-70% (oxidation)	0.10-0.20	$300-600
Chlorination	Minimal	4-6 log (bacteria, viruses)	None	None	0.01-0.02	$50-150
Microfiltration (MF)	>99% (to <1 mg/L)	6+ log (bacteria, protozoa)	None	None	0.02-0.05	$400-800
Ultrafiltration (UF)	>99% (to <1 mg/L)	6+ log (all, including viruses)	None	20-50% (MW >10 kDa)	0.05-0.10	$500-1,000
Nanofiltration (NF)	>99%	6+ log	40-80% (selective)	60-95%	0.40-0.80	$800-1,500
Reverse Osmosis (RO)	>99.9%	6+ log	95-99.5%	95-99%	0.5-5.0	$1,000-3,000

Notes:

• Capital costs are approximate installed cost per m³/d of capacity (2024)
• Energy values include process only, not upstream pumping
• Operating costs vary widely based on local chemical/energy prices

Process Unit Links

Filtration

• Rapid-Sand-Filter
• Disk-Filter
• GAC-Filter
• Anthracite-Filter
• Dual-Media-Filter
• Pile-Cloth-Filter

Disinfection

• UV-Disinfection
• Ozone-Disinfection
• Chlorination
• Chlorine-Dioxide
• Peracetic-Acid

Membranes

• Microfiltration
• Ultrafiltration
• Nanofiltration
• Reverse-Osmosis

Related Processes

• Secondary-Clarifier - Upstream clarification
• Chemical-Dosing - Coagulation, antiscalant, disinfectant feed
• Nutrient-Removal - Nitrogen and phosphorus removal
• Sludge-Handling - Backwash waste, concentrate disposal

References

1. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Vol. 3: Tertiary Treatment.
2. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th Ed., McGraw-Hill, Ch. 10-11.
3. AWWA (2011). Water Quality and Treatment: A Handbook on Drinking Water, 6th Ed., McGraw-Hill.
4. Crittenden, J.C. et al. (2012). MWH's Water Treatment: Principles and Design, 3rd Ed., Wiley.
5. US EPA (2012). Guidelines for Water Reuse, EPA/600/R-12/618.
6. WHO (2017). Potable Reuse: Guidance for Producing Safe Drinking-Water, World Health Organization.
7. Tchobanoglous, G. et al. (2015). Decentralized Wastewater Management, Water Environment Federation.