Volatile Fatty Acids

Volatile fatty acids are short-chain organic acids produced by fermentation. In wastewater engineering they are usually discussed as VFA and often reported as \text{mg/L as acetic acid}.

VFA matters because it is a high-value carbon source for biological nutrient removal. Enhanced biological phosphorus removal depends strongly on VFA in the anaerobic zone, and denitrification can use VFA as an electron donor. Total COD may not show whether this fast carbon is actually present.

Engineering Meaning

Important VFA species include:

The reporting basis matters. A lab may report individual acids, total VFA as acetic acid, or an estimated COD-equivalent value. These are not interchangeable unless the conversion basis is stated.

COD Equivalent

For acetic-acid-equivalent reporting, a useful COD screen is:

where VFA_{HAc} is in \text{mg/L as acetic acid}.

If:

then:

This is a screening conversion. A mixture rich in propionate or butyrate has a different COD equivalence than pure acetate.

VFA to Alkalinity

In anaerobic or fermenting systems, VFA is often reviewed with alkalinity:

For:

the ratio is:

This ratio is a stability screen, not a universal limit. The acceptable range depends on process type, buffering, loading and trend history.

EBPR Carbon Check

For enhanced biological phosphorus removal, a VFA carbon-to-phosphorus screen can be written:

If:

then:

This ratio helps evaluate whether PAO selection is carbon limited, but nitrate intrusion, zone sequence, SRT and sludge wasting still control performance.

Denitrification Support

For nitrate removal:

If:

then:

With COD_{VFA}=128\ \text{mg/L}, the screened carbon margin is:

The margin does not prove denitrification because dissolved oxygen, recycle flow, biomass condition and competing uptake can consume the same carbon.

Measurement Boundary

VFA can change quickly after sampling. Fermentation can continue, biological uptake can continue, pH can shift acid/base form and sample storage can change the measured result. Reports should state method, preservation, holding time, filtered or unfiltered basis, individual acids if available and whether the value is as acetic acid or as COD.

Process Use and Risk

VFA can be beneficial in the right process zone and harmful in the wrong place. In an EBPR anaerobic selector it can improve PAO selection. In an overloaded collection system, sludge blanket, wet well or sidestream, the same fermentation signal may indicate odor, corrosion, septicity, pH depression or uncontrolled recycle load.

The engineering question is therefore location specific. A high VFA value upstream of a designed selector can be useful; a high VFA value in a final effluent, stagnant tank or return stream may show loss of control.

Validation Evidence

Useful VFA evidence includes rbCOD, total COD, soluble COD, BOD, TOC, pH, alkalinity, ORP, temperature, fermentation tank condition, primary sludge handling, nitrate in anaerobic zones, orthophosphate release, phosphorus uptake, denitrification trend, odor complaints and historical diurnal pattern.

Common mistakes include treating VFA as total COD, comparing VFA as acetic acid with COD without conversion, using one grab sample for a fermenting process, ignoring sample holding time, assuming high VFA is always beneficial and checking VFA without nutrient loads. A strong review states the reporting basis, carbon equivalence, process zone, trend history and validation evidence.