What is a typical volatile solids percentage in lagoon sludge?

It varies widely between lagoons and there is no figure that can be assumed responsibly. It depends on the catchment, the age of the accumulation, how much grit and road material reaches the works, and any industrial contribution. Because it varies this much and because it determines what treatment can achieve, it should be measured from a sample rather than estimated.

Does reducing volatile solids reduce sludge depth?

Yes, but by less than the volatile solids percentage alone would suggest, because the inert fraction remains in place. A reduction in volatile solids reduces the total mass and volume of the blanket in proportion to how much of that blanket was volatile to begin with.

Volatile solids vs total solids

The difference between total and volatile solids in lagoon sludge, why only the volatile fraction is available to biological treatment, and how the split is determined.

The short answer

Total solids is all the solid material in a sludge sample, measured by drying it. Volatile solids is the portion of that material which burns off when the dried sample is ignited, and it approximates the organic, biodegradable content. The remainder is the fixed or inert fraction: sand, grit, silt, mineral precipitates and plastics. The distinction is decisive for lagoon treatment because bioaugmentation acts on the volatile fraction only. A lagoon with a high inert fraction has correspondingly less material available to biological treatment, which is why the split has to be established from a sample before a treatment programme is sized or quoted.

How the two are measured

Total solids is determined by evaporating the water from a known mass of sludge and weighing what remains. Volatile solids is then determined by igniting that dried residue: the mass lost on ignition is reported as volatile solids and taken to represent the organic content, while the ash that remains is the fixed or inert fraction. The two are usually reported as percentages, and volatile solids is often expressed as a percentage of total solids.

Why it decides whether treatment is worth doing

Bioaugmentation works by enzymatic hydrolysis of organic material. Proteases, lipases, amylases and cellulases break down protein, fat, starch and cellulose. None of them act on sand. So the volatile fraction sets a ceiling on what any biological programme can possibly achieve, before dose, format, strain selection or season are considered at all.

What the published reductions refer to

When this category quotes a percentage reduction, it is important to know which quantity is being reduced. Published volatile solids reductions run from roughly 33 percent in controlled enzyme studies to 60 percent in vendor case studies. Those are reductions of the volatile fraction, not of total sludge volume. A lagoon in which volatile solids are half the total, treated to the middle of that published range, would see a materially smaller change in total depth than the headline percentage implies.

33%

Volatile suspended solids reduction reported in a controlled enzyme study.

Controlled study

30–50%

Total suspended solids reduction from protease, cellulase and lipase mixtures.

Peer-reviewed

60%

The upper figure quoted in the category, which comes from vendor case studies.

Vendor-published

This is not an argument against biological treatment. It is an argument for stating clearly which number is being reduced, and for measuring the lagoon before quoting anything at all.

Sources

US Environmental Protection Agency (2001). Method 1684: Total, Fixed, and Volatile Solids in Water, Solids, and Biosolids (EPA-821-R-01-015). US EPA Office of Water.RegulatorThe reference method behind this whole distinction: ignition at 550°C is what separates the volatile organic fraction from the fixed mineral fraction. Ask which method a laboratory used.
Nelson, K. L., Jiménez Cisneros, B., Tchobanoglous, G. & Darby, J. L. (2004). Sludge accumulation, characteristics, and pathogen inactivation in four primary waste stabilization ponds in central Mexico. Water Research 38(1): 111–127. DOI: 10.1016/j.watres.2003.09.013Peer-reviewedCharacterised the composition of real pond sludge, which is what makes the volatile-versus-fixed split a practical question rather than an academic one.

Common questions

What is a typical volatile solids percentage in lagoon sludge?: It varies widely between lagoons and there is no figure that can be assumed responsibly. It depends on the catchment, the age of the accumulation, how much grit and road material reaches the works, and any industrial contribution. Because it varies this much and because it determines what treatment can achieve, it should be measured from a sample rather than estimated.
Does reducing volatile solids reduce sludge depth?: Yes, but by less than the volatile solids percentage alone would suggest, because the inert fraction remains in place. A reduction in volatile solids reduces the total mass and volume of the blanket in proportion to how much of that blanket was volatile to begin with.

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