Appearance
Rectangular Secondary Clarifier
Rectangular settling tank with traveling bridge or chain-and-flight mechanisms for biological sludge separation and thickening.
Overview
Rectangular secondary clarifiers offer advantages in constrained sites, phased construction, and applications requiring very large surface areas. They provide identical clarification and thickening functions to circular clarifiers but with different flow patterns and mechanical configurations.
When to Use
- Large treatment plants (>50 MLD) where multiple rectangular units are economical
- Constrained or linear sites (along channels or existing structures)
- Phased construction (easier to add identical rectangular units)
- Retrofit applications with existing rectangular tanks
- Where bridge access is required for maintenance
- Very large clarifiers (>50m length) not feasible as circular
Key Characteristics
- Better performance at high length-to-width ratios (3:1 to 5:1)
- More prone to density currents and short-circuiting than circular
- Traveling bridge provides access along entire tank
- Multiple hoppers require coordinated sludge withdrawal
- Can achieve lower SOR than circular due to better flow distribution
- Higher capital cost per unit area than circular
Operating Modes
Basic Mode
- Fixed overflow rate based on geometry
- Simplified flow distribution model
- Basic effluent TSS calculation
- Single-hopper simplified sludge removal
Advanced Mode
- 2D hydraulic model with length and width effects
- Density current simulation
- Multiple-hopper sludge profile
- Plug flow modification to Takács model
- Inlet baffle effectiveness
- Temperature stratification effects
Mathematical Model
Governing Equations - Rectangular Geometry
Tanks-in-Series Approximation:
$$N_{tanks} = \frac{L}{W} \times f_{baffle}$$
Where:
- $N_{tanks}$ = number of ideal CSTRs in series
- $L$ = tank length (m)
- $W$ = tank width (m)
- $f_{baffle}$ = baffle effectiveness factor (0.5-0.8)
Higher $N$ improves performance by reducing short-circuiting.
Rectangular Flow Distribution
Velocity Profile:
$$v(x) = \frac{Q}{W \times H} \times (1 + k_{sc} \times e^{-x/L_c})$$
Where:
- $v(x)$ = horizontal velocity at distance x from inlet (m/h)
- $k_{sc}$ = short-circuit coefficient (0.1-0.3)
- $L_c$ = characteristic length for velocity decay (m)
- $Q$ = total flow (m³/h)
- $H$ = water depth (m)
Density Current Model
Temperature or concentration differences create density currents:
$$v_{density} = k \times \sqrt{g \times H \times \frac{\Delta \rho}{\rho}}$$
Where:
- $v_{density}$ = density current velocity (m/s)
- $g$ = gravity (9.81 m/s²)
- $\Delta \rho / \rho$ = relative density difference
- $k$ = empirical coefficient (0.5-1.0)
Critical for:
- Cold weather (warm influent, cold tank)
- High MLSS concentration gradients
- Thermal stratification
Takács Model (Same as Circular)
$$V_s = V_0 \times (e^{-r_h \times (X - X_{min})} - e^{-r_p \times (X - X_{min})})$$
Parameters remain the same, but rectangular geometry affects the flux analysis:
$$SLR = \frac{Q \times X}{L \times W}$$
Multiple Hopper Sludge Distribution
For $n$ hoppers along tank length:
$$X_{hopper,i} = X_{avg} \times f_i$$
Where:
- $f_i$ = distribution factor for hopper $i$
- Typically: $f_1 = 1.5$, $f_2 = 1.2$, $f_3 = 1.0$, $f_4 = 0.8$ (inlet to outlet)
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Tank length | $L$ | m | 30-100 | 60 | Settling length |
| Tank width | $W$ | m | 6-25 | 12 | Tank width |
| Sidewater depth | $H$ | m | 3.5-5.5 | 4.5 | Effective depth |
| Number of hoppers | $n$ | - | 2-6 | 4 | Sludge collection hoppers |
| Inlet baffle config | - | - | - | Perforated | Inlet energy dissipation |
| L:W ratio | - | - | 3:1-6:1 | 4:1 | Length to width ratio |
| Collector type | - | - | - | Traveling bridge | Mechanical collection |
| Influent MLSS | $X_{in}$ | mg/L | 2000-5000 | 3500 | Mixed liquor concentration |
| SVI | - | mL/g | 80-200 | 120 | Sludge volume index |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Surface area | m² | Total settling area (L×W) |
| SOR | m³/m²·d | Surface overflow rate |
| Weir loading | m³/m·d | Effluent weir overflow rate |
| Detention time | h | Hydraulic retention time |
| Horizontal velocity | m/min | Avg horizontal velocity |
| Effluent TSS | mg/L | Predicted effluent quality |
| Sludge profile | kg/m³ | Solids distribution across hoppers |
Configuration Guide
Step-by-Step Setup
Determine Tank Geometry
Length-to-Width Ratio:
- Optimal: 4:1 to 5:1
- Minimum: 3:1 (risk of short-circuiting)
- Maximum: 6:1 (diminishing returns)
Width Selection:
- Typical: 8-15 m for traveling bridge
- <25m for chain-and-flight
- Wider tanks require heavier bridges
Calculate Surface Area
Same SOR criteria as circular clarifiers:
For SVI = 120 mL/g: SOR_avg = 18 m³/m²·d SOR_peak = 32 m³/m²·d A = Q_peak / SOR_peakSelect Length and Width
Example: A = 600 m² required L:W = 4:1 target A = L × W = 4W × W = 4W² W = √(600/4) = 12.2 m → Use W = 12 m L = 4 × 12 = 48 m → Use L = 50 m A_actual = 50 × 12 = 600 m² ✓Verify Horizontal Velocity
$$v_h = \frac{Q}{W \times H}$$
Limits:
- Average: <0.5 m/min
- Peak: <1.0 m/min
Q_peak = 10,000 m³/d = 6.94 m³/min v_h = 6.94 / (12 × 4.5) = 0.13 m/min ✓Design Inlet Baffle
Options:
- Perforated wall (40-50% open area)
- Multiple ports with diffusers
- Energy dissipation chamber
Velocity through openings:
- Target: 0.3-0.6 m/s (prevents jetting, ensures distribution)
Configure Sludge Hoppers
Number of hoppers: Typically 1 per 10-15m of length
L = 50 m → 4 hoppers @ 12.5m spacing Hopper depth: 0.5-1.0 m below tank floor Hopper slope: 1.5:1 (H:V) minimumDesign Scum Removal
- Surface skimmers (rotating or fixed)
- Scum beach at outlet end
- Manual or automated skimming
Typical Design Values
| Parameter | Small Plant | Medium Plant | Large Plant | Comments |
|---|---|---|---|---|
| Length (m) | 30-40 | 50-70 | 80-100 | Based on site constraints |
| Width (m) | 8-12 | 12-16 | 16-20 | Bridge capacity |
| Depth (m) | 4.0-4.5 | 4.5-5.0 | 4.5-5.5 | Deeper for large plants |
| L:W ratio | 3:1-4:1 | 4:1-5:1 | 5:1-6:1 | Higher for better plug flow |
| SOR (avg, m³/m²·d) | 16-20 | 16-20 | 16-20 | Independent of size |
| Horizontal velocity (m/min) | <0.5 | <0.5 | <0.5 | Critical parameter |
| Weir loading (m³/m·d) | 125-186 | 125-186 | 125-186 | Prefer <150 |
| Number of hoppers | 3-4 | 4-5 | 6-8 | ~1 per 12-15m |
Performance Criteria
Advantages over Circular Clarifiers
| Aspect | Rectangular | Circular | Notes |
|---|---|---|---|
| Phased construction | Excellent | Poor | Add identical units easily |
| Site utilization | Excellent | Good | Linear arrangement |
| Very large areas | Excellent | Poor | Circular >70m diameter impractical |
| Flow distribution | Good (if baffled) | Excellent | Requires good inlet design |
| Short-circuiting | Moderate risk | Low risk | Density currents more problematic |
| Capital cost | Higher | Lower | More complex mechanisms |
| O&M access | Excellent | Good | Bridge provides full access |
Achievable Performance
Same as circular clarifiers - performance depends on SVI, SOR, and SLR, not geometry.
Effluent TSS: 5-15 mg/L with good sludge (SVI 80-120)
RAS concentration: 8,000-12,000 mg/L typical
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Poor effluent (one end) | Inlet jetting/poor distribution | Add/fix perforated baffle, reduce inlet velocity |
| Density currents | Install longitudinal baffles, check temp difference | |
| High effluent TSS (entire tank) | Hydraulic overload | Reduce flow or add capacity |
| Short-circuiting | Improve inlet baffling, check for dead zones | |
| Uneven sludge blanket | Poor sludge collection | Balance hopper withdrawal rates |
| Inlet distribution problem | Fix inlet baffle | |
| Collector mechanism jam | Grit/debris accumulation | Improve primary treatment, clean hoppers |
| Excessive sludge buildup | Increase sludge withdrawal frequency | |
| Scum accumulation | Insufficient skimming | Increase skimmer frequency, check foam source |
| Wind effects (outdoor) | Install wind baffles or covers | |
| Algae growth (outdoor) | Sunlight exposure | Cover tank or add algaecide |
Operation & Maintenance
Daily Checks
- Visual inspection of effluent quality
- Check collector travel and operation
- Monitor blanket levels in each hopper
- Verify RAS/WAS pumping from all hoppers
- Inspect scum removal system
- Observe inlet flow distribution
Weekly Maintenance
- Measure sludge blanket in each hopper (sludge judge)
- SVI test
- Clean scum baffles
- Verify flow distribution at inlet
- Check chain/cable tension (if applicable)
- Inspect weir condition
Monthly Tasks
- Full sludge profile mapping
- Collector mechanism inspection
- Lubricate moving parts
- Check hopper sludge withdrawal balance
- Verify level sensors
- Review effluent TSS trends
Annual Tasks
- Drain and inspect tank interior
- Full inspection of collector mechanism
- Drive system overhaul
- Inlet baffle inspection and cleaning
- Hopper cleaning
- Structural inspection
- Repaint exposed steelwork
Advanced Features
Computational Fluid Dynamics (CFD)
Modern rectangular clarifier design uses CFD to optimize:
- Inlet baffle configuration
- Density current prediction
- Dead zone identification
- Optimal L:W ratio for site conditions
Longitudinal Baffles
Installing curtain baffles along tank length:
- Reduces density currents
- Improves plug flow characteristics
- Typical depth: 0.5-1.0 m from surface
- Spacing: 1/3 to 1/2 of tank width
Traveling Bridge Features
Suction header:
- Continuous sludge removal along length
- Reduces blanket variations
- More uniform RAS concentration
Multiple functions:
- Sludge collection
- Scum skimming
- Inspection access
- Equipment mounting
Energy Dissipation Inlet
Stilling well design:
Volume = 5-10 minutes at peak flow
Inlet velocity < 0.3 m/s
Multiple outlet ports
Submerged dischargeWorked Example
Given
- Peak flow: 18,000 m³/d
- Average flow: 12,000 m³/d
- MLSS: 3,800 mg/L
- SVI: 115 mL/g
- Site constraint: Maximum width 14 m
Solution
Step 1: Select Design SOR
SVI = 115 → SOR_peak = 34 m³/m²·d (conservative for rectangular)Step 2: Calculate Area
A = 18,000 / 34 = 529.4 m²Step 3: Determine Geometry
W_max = 14 m
Target L:W = 4.5:1
A = L × W = 4.5W × W
W = √(529.4/4.5) = 10.8 m → Use W = 12 m
L = A/W = 529.4/12 = 44.1 m → Use L = 45 m
A_actual = 45 × 12 = 540 m² ✓Step 4: Verify Horizontal Velocity
Q_peak = 18,000 m³/d = 12.5 m³/min
H = 4.5 m
v_h = 12.5 / (12 × 4.5) = 0.23 m/min ✓ (<0.5 limit)Step 5: Verify Weir Loading
Weir length = W + 2 × weir on sides = 12 + 2×40 = 92 m
Weir loading = 18,000 / 92 = 195.7 m³/m·d
(Acceptable for peak, avg = 130.4 m³/m·d)Step 6: Configure Hoppers
L = 45 m → 4 hoppers @ 11.25 m spacing
Hopper withdrawal: Balance flow from each to maintain uniform blanketStep 7: Inlet Baffle Design
Perforated wall: 50% open area
Total area needed = Q/(v_max) = (12.5 m³/min)/(0.5 m/s × 60) = 0.42 m²
Baffle area = H × W = 4.5 × 12 = 54 m²
Open area = 27 m² >> 0.42 m² ✓
Velocity through openings ≈ 0.008 m/s (very low, good distribution)Related Units
- Secondary-Clarifier - Circular alternative
- Lamella-Clarifier - High-rate alternative
- Activated-Sludge-Basin - Upstream biological treatment
- RAS-Pumping - Sludge return system
References
- WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 13.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1020-1045.
- ATV-DVWK (2000). Dimensioning of Single-Stage Activated Sludge Plants, ATV-A 131E.
- Ekama, G.A. et al. (1997). Secondary Settling Tanks: Theory, Modelling, Design and Operation.
- Zhou, S., McCorquodale, J.A., and Godo, T. (1994). "Short-Circuiting and Density Interface in Primary Clarifiers." J. Hydraul. Eng. 120(9): 1060-1080.
- Stamou, A.I. (2008). "Improving the Hydraulic Efficiency of Water Process Tanks Using CFD Models." Chem. Eng. Process. 47: 1179-1189.