Secondary Treatment Process Units

Biological and physical separation processes for organic matter removal, nitrification, and biomass separation.

Overview

Secondary treatment employs biological processes to convert dissolved and colloidal organic matter into settleable biomass, followed by clarification to produce low-TSS, low-BOD effluent. Modern secondary treatment also achieves nitrification and, in advanced configurations, denitrification and biological phosphorus removal.

Treatment Objectives

Conventional Secondary Treatment:

BOD removal: >85% (influent 200-300 mg/L → effluent <20 mg/L)
TSS removal: >85% (effluent <20 mg/L)
Ammonia oxidation (nitrification): Optional or seasonal

Advanced Secondary Treatment (BNR):

Complete nitrification: NH₄-N <1-3 mg/L
Denitrification: TN removal 70-85%
Biological P removal: TP <1 mg/L (with chemical assist)

Process Categories

Clarification (4 Units)

Secondary clarifiers perform dual functions - biological solids separation and biomass thickening:

Unit	Configuration	Capacity	Application	Footprint
Secondary-Clarifier	Circular	15-30 m³/m²·d SOR	Standard AS, all sizes	Medium
Rectangular-Clarifier	Rectangular	15-30 m³/m²·d SOR	Large plants, phased construction	Medium
Lamella-Clarifier	Inclined plates	60-120 m³/m²·d SOR	Space-constrained sites	Small (1/3 of conventional)
RAS-Pumping	Pump station	Variable	All AS systems	Minimal

Key Design Parameter: Surface Overflow Rate (SOR)

$$SOR = \frac{Q}{A}$$

Typical values:

Conventional clarifiers: 15-30 m³/m²·d (average), 30-50 m³/m²·d (peak)
Lamella clarifiers: 60-120 m³/m²·d (projected area)

Performance Correlation:

$$TSS_{eff} = f(SOR, SVI, MLSS)$$

For SVI <120, expect <15 mg/L TSS effluent at design SOR.

Typical Process Trains

Conventional Activated Sludge

Influent → Primary Clarifier → Aeration Basin → Secondary Clarifier → Effluent
                    ↓                                      ↓
                 Primary                              RAS (return)
                 Sludge                              WAS (waste)

Key Parameters:

F/M: 0.3-0.6 kg BOD/kg MLSS·d
MLSS: 1,500-3,000 mg/L
SRT: 5-10 days
HRT: 4-8 hours

Extended Aeration (Package Plants)

Influent → Aeration Basin (long HRT) → Secondary Clarifier → Effluent
                               ↓                ↓
                            No primary      RAS/WAS

Key Parameters:

F/M: 0.05-0.15 kg BOD/kg MLSS·d
MLSS: 3,000-5,000 mg/L
SRT: 20-30 days
HRT: 18-36 hours

BNR (Nutrient Removal)

Modified Ludzack-Ettinger (MLE) for Nitrogen:

Influent → Anoxic Zone → Aerobic Zone → Secondary Clarifier → Effluent
              ↑                             ↓
            RAS + Nitrate recycle         RAS/WAS

A²O Process (Nitrogen + Phosphorus):

Influent → Anaerobic → Anoxic → Aerobic → Secondary Clarifier → Effluent
              ↑          ↑                       ↓
            RAS     Nitrate recycle            RAS/WAS

Key Parameters:

Total HRT: 10-16 hours
SRT: 10-20 days
MLSS: 3,000-4,500 mg/L
Anaerobic: 0.5-1.5 h, Anoxic: 2-4 h, Aerobic: 6-10 h

Design Philosophy

Secondary Clarifier Sizing

The clarifier performs two critical functions:

1. Clarification (SOR limit):

Driven by hydraulic loading
Affected by SVI (settling characteristics)
Design for peak wet weather flow

2. Thickening (Solids flux limit):

Driven by solids loading
Takács settling model + state point analysis
Design for peak solids load

Governing Criteria:

$$A_{required} = \max\left(\frac{Q_{peak}}{SOR_{allow}}, \frac{Q \times X}{G_L}\right)$$

Where:

$G_L$ = limiting solids flux (kg/m²·d) from Takács model
Typical: 100-150 kg/m²·d for good settling sludge

RAS/WAS Control

RAS (Return Activated Sludge):

Returns biomass to maintain MLSS
Ratio R = Q_RAS / Q_inf = 0.5-1.5
Controls MLSS concentration

$$X_{MLSS} = \frac{X_{RAS}}{1 + 1/R}$$

WAS (Waste Activated Sludge):

Controls sludge age (SRT)
Determines F/M ratio and treatment performance
Affects nutrient removal capability

$$SRT = \frac{V \times X_{MLSS}}{Q_{WAS} \times X_{WAS}}$$

Sludge Settling Characteristics

Sludge Volume Index (SVI):

$$SVI = \frac{\text{Settled volume (mL/L)} \times 1000}{\text{MLSS (mg/L)}}$$

SVI (mL/g)	Settling Quality	RAS Concentration	Clarifier Impact
50-80	Excellent	>12,000 mg/L	Low SOR needed
80-120	Good	8,000-12,000 mg/L	Standard design
120-150	Moderate	6,000-8,000 mg/L	Reduced loading
150-200	Poor (bulking)	4,000-6,000 mg/L	Very low SOR
>200	Failing	<4,000 mg/L	Process unstable

Performance Monitoring

Daily Operating Parameters

Parameter	Measurement	Target	Action if Outside Range
MLSS	Aeration basin sample	2,500-4,000 mg/L	Adjust RAS/WAS
SVI	30-min settling test	80-120 mL/g	Investigate bulking if >150
RAS concentration	RAS line sample	8,000-12,000 mg/L	Adjust RAS rate
Effluent TSS	Clarifier effluent	<15 mg/L	Check SOR, SVI, blanket
DO (aeration basin)	Probe	1.5-2.5 mg/L	Adjust aeration
Blanket depth	Sludge judge	1.0-1.5 m	Adjust RAS rate

Weekly/Monthly Parameters

SRT calculation (measure MLSS, WAS flow, RAS concentration)
F/M ratio calculation
Sludge production rate
Microscopic examination (filament identification)
Effluent NH₄-N, NO₃-N (if nitrifying)
Secondary clarifier solids flux analysis

Troubleshooting Guide

Issue	Symptoms	Likely Cause	Solution
High effluent TSS	Cloudy effluent, >20 mg/L	Hydraulic overload, high SVI	Reduce flow, address bulking
Rising sludge	Floating sludge clumps	Denitrification in clarifier	Increase RAS, reduce nitrate
Bulking sludge	SVI >150, poor settling	Filamentous bacteria	Adjust F/M, DO, nutrients
Foam/scum	Brown foam on surface	Nocardia, grease	Chlorination, skim, reduce SRT
Low MLSS	Decreasing biomass	Excessive WAS, toxicity	Reduce WAS, check influent
High MLSS	Increasing biomass	Insufficient WAS	Increase WAS to target SRT
Poor nitrification	High NH₄-N effluent	Low SRT, low DO, low temp	Increase SRT, increase DO

UK Regulatory Context

Urban Wastewater Treatment Directive (UWWTD)

Secondary Treatment Standards:

Population Equivalent	BOD (mg/L)	COD (mg/L)	TSS (mg/L)
2,000-10,000 PE	25 (70-90% removal)	125 (75% removal)	35 (90% removal)
>10,000 PE	25 (70-90% removal)	125 (75% removal)	35 (90% removal)
Sensitive areas (>10,000 PE)	Add N/P limits	Add N/P limits	35 (90% removal)

Sensitive Areas (Nutrient Limits):

Total Nitrogen: 10-15 mg/L (70-80% removal)
Total Phosphorus: 1-2 mg/L (80% removal)

Discharge Consents

Most UK WWTPs have tighter limits than UWWTD minima:

BOD: 10-15 mg/L (common)
Ammonia: 3-5 mg/L (95%ile)
TSS: 20-25 mg/L
Phosphorus: 0.5-1.0 mg/L (chemical dosing often required)

Biological-Treatment - Upstream aeration basin design
Primary-Treatment - Upstream primary clarification
Tertiary-Treatment - Downstream polishing
Sludge-Treatment - WAS processing
Process-Control - Advanced control strategies

References

WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 10-13.
Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th Ed.
Ekama, G.A. et al. (1997). Secondary Settling Tanks: Theory, Modelling, Design and Operation. IAWQ STR No. 6.
Takács, I., Patry, G.G., and Nolasco, D. (1991). "A Dynamic Model of the Clarification-Thickening Process." Water Research 25(10): 1263-1271.
ATV-DVWK (2000). Dimensioning of Single-Stage Activated Sludge Plants, ATV-A 131E.
UK Environment Agency (2013). Waste Water Treatment Works: Treatment Monitoring and Compliance.
CIWEM (2015). Activated Sludge Process Control Manual.

Units

Activated Sludge

Anaerobic

Biofilm Systems

Zones

Preliminary Treatment