Steady-State Simulation

Time-independent equilibrium calculations

Overview

Steady-state simulation calculates the equilibrium conditions of your wastewater treatment plant. It determines the operating point where all inputs, outputs, and internal states are constant over time.

When to Use

Ideal For

• Plant sizing and design verification
• Capacity analysis
• Mass balance calculations
• Equipment selection
• Permit compliance checking
• Baseline performance estimation

Not Suitable For

• Diurnal variation effects
• Startup/shutdown procedures
• Controller tuning
• Upset response analysis

Running Steady-State

Quick Start

1. Build your process flow diagram
2. Configure all unit properties
3. Click Run Simulation (or press F5)
4. Select Steady-State
5. Click Start

Settings

Setting	Default	Description
Tolerance	0.01	Convergence criterion
Max Iterations	100	Maximum solver iterations
Relaxation	Auto	Under-relaxation factor
Initial Guess	Default	Starting point for iteration

Convergence

How It Works

The solver iteratively adjusts stream compositions until mass balance errors are below the tolerance:

Iteration 1:  Error = 15.2%
Iteration 5:  Error = 3.8%
Iteration 10: Error = 0.95%
Iteration 15: Error = 0.008% ✓ Converged

Convergence Indicators

Status	Meaning
Converged	Solution found, error < tolerance
Converging	Error decreasing, continue
Oscillating	Error fluctuating, may need adjustment
Diverging	Error increasing, check configuration

Improving Convergence

1. Simplify first: Start with fewer units
2. Check connections: Ensure all streams connected
3. Verify influent: Realistic compositions
4. Reduce tolerance: Try 0.05 initially
5. Initial conditions: Provide better guesses

Mass Balance

Verification

The solver ensures:

$$\sum \dot{m}{in} = \sum \dot{m} + \sum \dot{m}_{reaction}$$

For each component:

• COD: Influent = Effluent + Oxidized + Sludge
• N: Influent = Effluent + Denitrified + Sludge
• P: Influent = Effluent + Sludge

Checking Balance

View mass balance table:

1. Go to View > Mass Balance
2. Check "Balance" column
3. Values should be < 1%

Biokinetic Calculations

Basic Mode

Fixed efficiency calculations:

TSS_out = TSS_in × (1 - Removal_efficiency)

Advanced Mode

Full biokinetic model:

ASM1 Heterotrophic Growth: $$\mu_H = \mu_{max,H} \cdot \frac{S_S}{K_S + S_S} \cdot \frac{S_O}{K_{O,H} + S_O}$$

Steady-State Biomass: $$X_{BH} = \frac{Y_H \cdot (S_{S,in} - S_{S})}{1 + b_H \cdot SRT}$$

Results

Stream Results

Each stream displays:

• Flow rate (m3/d)
• All component concentrations
• Mass flow rates

Unit Results

Each unit shows:

• Removal efficiencies
• Operating conditions
• Capacity utilization
• Energy consumption

Export Options

Format	Use
CSV	Spreadsheet analysis
Excel	Formatted report
PDF	Documentation
JSON	Programmatic access

Typical Applications

Design Verification

1. Size units for design flow
2. Run steady-state simulation
3. Check effluent quality vs. limits
4. Verify capacity margins

Scenario Comparison

1. Create base case
2. Duplicate project
3. Modify parameters
4. Compare results side-by-side

Sensitivity Analysis

1. Identify key parameter
2. Vary over range (e.g., ±20%)
3. Run multiple steady-states
4. Plot response curve

Example: MLE Plant

Configuration

Unit	Size
Anoxic Zone	2,000 m3
Aerobic Zone	6,000 m3
Secondary Clarifier	1,200 m2
RAS	100%
IRR	200%

Influent

Parameter	Value
Flow	20,000 m3/d
COD	400 mg/L
TKN	40 mg/L
TP	8 mg/L

Results

Parameter	Effluent	Limit
TSS	8 mg/L	30 mg/L
BOD	5 mg/L	20 mg/L
NH4-N	1.2 mg/L	5 mg/L
TN	8 mg/L	10 mg/L

Troubleshooting

Common Issues

Issue	Cause	Solution
Not converging	Large recycle loop	Add initial guess
Negative concentration	Invalid parameters	Check kinetics
Mass imbalance	Disconnected stream	Verify connections
High TSS in effluent	Clarifier underloaded	Check SOR
Poor N removal	Low SRT	Increase sludge age

Debugging Steps

1. Check all streams connected
2. Verify influent complete
3. Run with Basic mode first
4. Enable detailed logging
5. Check unit by unit

Best Practices

1. Start simple: Basic mode, simple configuration
2. Build up: Add complexity gradually
3. Verify each step: Check mass balance frequently
4. Document assumptions: Note key parameters
5. Save checkpoints: Before major changes

Related Topics

• Dynamic Simulation
• Calculation Modes
• Mass Balance
• Running Simulations

References

1. Henze, M. "Activated Sludge Models ASM1, ASM2, ASM2d and ASM3"
2. Grady, C.P.L. "Biological Wastewater Treatment" (3rd Edition)
3. Tchobanoglous, G. "Wastewater Engineering" (5th Edition)