COD Fractionation

Understanding chemical oxygen demand components

Overview

COD (Chemical Oxygen Demand) fractionation divides total COD into components based on biodegradability and physical state. Proper fractionation is essential for accurate biokinetic modeling.

COD Fractions

Biodegradability Classification

Total COD (CODt)
├── Biodegradable COD (bCOD)
│   ├── Readily biodegradable (rbCOD or SS)
│   │   ├── Fermentable (SF)
│   │   └── VFA (SA)
│   └── Slowly biodegradable (sbCOD or XS)
│       ├── Particulate (XS)
│       └── Colloidal
└── Non-biodegradable COD (nbCOD)
    ├── Soluble inert (SI)
    └── Particulate inert (XI)

Physical State Classification

Fraction	Size	Settleable
Soluble	<0.1 μm	No
Colloidal	0.1-1 μm	Partial
Particulate	>1 μm	Yes

Standard Fractions

ASM1/ASM3 Components

Symbol	Description	Typical %
SS	Readily biodegradable	10-20%
SI	Soluble inert	5-10%
XS	Slowly biodegradable	40-50%
XI	Particulate inert	15-25%
XBH	Active heterotrophs	5-10%

ASM2d Components

Symbol	Description	Notes
SF	Fermentable COD	Portion of SS
SA	VFA/Acetate	Portion of SS
XS	Slowly biodegradable	Same as ASM1
SI	Soluble inert	Same as ASM1
XI	Particulate inert	Same as ASM1

Determination Methods

Laboratory Methods

Method	Fraction Determined
Filtration (0.45 μm)	Soluble vs particulate
Flocculation	Colloidal separation
BOD/COD ratio	Biodegradable fraction
Respirometry	SS from OUR test

Respirometric Test

OUR
 ↑
 │    ┌──── Exogenous OUR (SS)
 │────┤
 │    └──── Endogenous OUR
 └────────────────────────→ Time

SS = ∫(OURexo - OURendo) dt × (1 - YH) / YH

Default Values

When no data available:

Wastewater Type	SS%	XS%	SI%	XI%
Domestic (raw)	15	50	5	15
Domestic (settled)	25	35	8	12
Industrial	Variable - measure

COD Balance

Through Treatment

$$COD_{in} = COD_{out} + COD_{oxidized} + COD_{sludge}$$

Component Tracking

Stream	SS	XS	SI	XI	XBH
Influent	75	250	25	100	25
Primary eff	60	100	25	40	10
Secondary eff	5	10	25	5	2
WAS	0	80	0	200	500

ErgoPlanner Configuration

Influent Composition

1. Select Influent node
2. Open Composition tab
3. Enter COD fractions:

{
  "COD": {
    "total": 500,
    "fractions": {
      "SS": 75,
      "SF": 50,
      "SA": 25,
      "XS": 250,
      "SI": 25,
      "XI": 100,
      "XBH": 25
    }
  }
}

Automatic Calculation

If only total COD provided:

• Uses default ratios
• Based on wastewater type
• Can override individual fractions

Impact on Modeling

Readily Biodegradable (SS)

• Drives immediate oxygen demand
• Required for EBPR (as VFA)
• Fast kinetics (hours)

Slowly Biodegradable (XS)

• Requires hydrolysis first
• Main source of sludge production
• Slower kinetics (days)

Soluble Inert (SI)

• Passes through treatment unchanged
• Sets minimum effluent COD
• Cannot be removed biologically

Particulate Inert (XI)

• Accumulates in sludge
• Affects SRT calculations
• Removed by settling

Typical Ranges

Municipal Wastewater

Parameter	Raw	Primary Eff	Secondary Eff
Total COD	400-600	250-350	30-60
SS	50-100	60-100	5-10
XS	200-300	100-150	5-15
SI	20-40	20-40	20-40
XI	80-120	30-50	2-5

BOD/COD Relationship

$$bCOD = \frac{BOD_5}{0.68}$$

Typical BOD5/COD ratios:

• Raw domestic: 0.4-0.6
• Industrial: 0.2-0.8
• Food processing: 0.6-0.8

Troubleshooting

Issue	Cause	Solution
High effluent COD	High SI	Check for industrial input
Low sludge production	Low XS	Verify fractionation
Poor nitrification	Low SS for decay	Check rbCOD
EBPR failure	Low VFA (SA)	Add primary fermenter

Best Practices

1. Measure when possible - Especially for industrial
2. Start with defaults - Adjust based on results
3. Validate with mass balance - Check closure
4. Consider seasonal variation - Temperature effects
5. Document assumptions - For reproducibility

Related Topics

• Nitrogen Balance
• Phosphorus Balance
• ASM1
• Steady-State Simulation

References

1. Henze, M. "Wastewater Characterization"
2. Ekama, G. "COD Fractionation Methods"
3. Roeleveld, P. "Dutch Guidelines for COD"