Volatile acidity in wastewater is expressed as milligrams of equivalent acetic acid.

Volatile acidity: the quiet signal behind the digester’s rhythm

If you’ve spent time around a wastewater treatment plant, you’ve seen the dance of pumps, meters, and glowing dashboards. The water itself is only part of the story. What you can’t always see is the chemistry bubbling beneath the surface—the acids quietly shaping how microbes eat, sleep, and run the show. One of the most telling signals is volatile acidity, or VA. It’s a value that helps operators gauge whether fermentation is on track, whether the digestion process is stable, and whether the system is about to tilt toward trouble. The key thing to know? VA results are typically expressed as milligrams of equivalent acetic acid per liter. Let me explain why that matters.

What is volatile acidity (VA)?

Think of VA as the snapshot of short-chain fatty acids in the wastewater or sludge that can vaporize under the right conditions. The main players are acetic, propionic, and butyric acids, with acetic acid usually reigning in terms of concentration. Why call it “volatile”? Because these acids are volatile enough to separate from the liquid phase during digestion, and their presence strongly influences the pH and microbial activity inside anaerobic digesters or other treatment stages.

In practice, VA is a practical surrogate for the acidity coming from fermentation. It tells you how much acid is piling up as microorganisms break down organic matter and produce fatty acids. A rising VA can signal an upshift in fermentation or a lag in the system’s ability to convert acids into methane and carbon dioxide. Either way, VA is a leading indicator of digester health.

Why express VA as milligrams of equivalent acetic acid?

Here’s the thing: there are several acids that can contribute to the “volatile acidity” mix. If you reported VA as “mg/L of acetic acid,” you’d have a single, consistent reference point across plants, tests, and studies. Acetic acid is a common, well-understood standard in water and wastewater work. By converting all the contributing acids into an acetic acid equivalent, you get a consistent yardstick for comparison. That makes it easier to compare data from different facilities, track changes over time, and communicate conditions clearly to the entire operations team.

In short, mg/L as acetic acid provides a common language. It’s not about pretending every acid is truly acetic acid; it’s about shaping a uniform metric so trends and responses are easy to read, no matter where the sample came from.

What VA tells you about process performance

• Digester stability: In anaerobic digestion, microbes chow down on organics and produce acids before they finish the job with methane. If VA climbs faster than alkalinity can balance it, the pH can swing downward. That’s a red flag for digester stability and a hint that methanogens may be feeling the pressure.

• Balance with alkalinity: VA doesn’t operate in a vacuum. Alkalinity acts like a cushion, absorbing acid and keeping pH steadier. Operators watch VA alongside alkalinity to gauge whether the system has enough buffering capacity to handle feed changes, shocks, or fluctuations in temperature.

• Organic loading and pollution signals: A sudden uptick in VA can indicate stronger fermentation activity, shifted feed composition, or organic pollution entering the system. It’s not a verdict on content by itself, but it’s a clear clue that warrants a closer look at upstream inputs and process controls.

• Odors and gas production: Higher VA often correlates with more fatty acids present in the system. That can influence odor profiles and the composition of biogas, which in turn affects digester design, gas collection, and safety considerations.

How VA is measured and reported

Most labs report VA as milligrams per liter (mg/L) of acetic acid equivalent. The measurement is rooted in standard methods that convert the concentration of volatile fatty acids into a single reference value. In practice, technicians may perform titration or more advanced analyses to gauge the total acidity, then express it as mg/L CH3COOH (chemical shorthand for acetic acid).

• Quick summary of the approach: a sample is treated to release volatile acids; the acidity is measured, and the result is converted to an acetic acid equivalent. That conversion lets you compare a sample today with one from yesterday—or with samples from other plants—without getting bogged down in a mix of different acids.

• What to watch on the report: you’ll see VA expressed as mg/L as CH3COOH, sometimes alongside total alkalinity, pH, and total dissolved solids (TDS) or other related metrics. The context matters: VA by itself isn’t a verdict; it becomes meaningful when paired with buffering capacity and process conditions.

Interpreting VA numbers in real life

Let’s ground this in a simple mental model. Imagine the digester as a bathtub. The acid part of the bathwater is VA. If you keep adding more water (organic load) and the tub doesn’t have a good drain (sufficient alkalinity and buffering), the level of VA can rise, and the pH can start to fall. If the pH slides too far, some microbes slow down or shut off, and the system can wobble.

• Low VA, steady pH, smooth operation: When VA stays low and the buffer is strong, the digester hums along. Microbial communities work efficiently, gas production is steady, and odors stay under control.

• Moderate VA with healthy buffering: A moderate VA rise isn’t a crisis if alkalinity keeps the pH within a comfortable range. Operators may adjust feed rates, temperature, or recirculation to maintain balance.

• High VA or rising VA with shrinking alkalinity: This is a warning sign. If alkalinity is lagging and VA climbs, the digester can become unstable. Actions might include adjusting the feed mix, adding buffering agents, or tweaking gas withdrawal to protect system health.

A simple framework for quick assessment

• Compare VA to alkalinity (VA/TA ratio): If VA climbs while alkalinity stays flat or drops, you’ve got a tighter margin for error. A rising ratio can signal a digestion upset forming on the horizon.

• Watch trends, not single numbers: One-off spikes happen. The real story shows up in a pattern over days or weeks. Consistent increases often prompt a process review.

• Consider the bigger picture: VA is part of a family of signals—pH, alkalinity, ammonia, and gas production all work together. A holistic view usually gives the clearest guidance.

Real-world tangents that matter

• Upstream inputs: The quality and nature of the feed entering the plant influence VA. Fats, oils, and greases, or sudden shifts in industrial discharges, can alter fermentation dynamics. Keeping an eye on feed composition helps explain VA movements.

• Temperature effects: Warmer conditions speed up microbial activity, which can change VA dynamics. In cold weather, digestion shifts may quiet down, sometimes masking underlying issues until temperatures swing again.

• Odor and compliance considerations: An uptick in VA can accompany stronger odors, especially if gas production shifts or system control lags. Operators balance odor control with process efficiency, often with practical tweaks rather than sweeping changes.

• Practical tools in the field: Many plants rely on routine lab checks and online sensors to track VA alongside pH and alkalinity. Simple titration kits can yield quick mg/L as acetic acid readings, while online analyzers may offer continuous VA estimates based on surrogate signals.

Common pitfalls to avoid

• Treating VA in isolation: VA tells you about acidity, but the story is richer when you look at alkalinity and pH. A high VA with strong buffering may still be fine, while low VA with low alkalinity can spell trouble.

• Ignoring seasonal shifts: Temperature, influent composition, and digestion mode can all swing VA levels. Regular trend analysis helps separate ordinary variation from meaningful change.

• Overcorrecting without understanding the cause: Adding buffering materials is helpful, but it’s better to know whether a feed change, a mechanical issue, or a bioprocess upset is the root cause before acting.

A practical takeaway for readers

Volatile acidity is one of those quiet indicators that pays off when you pay attention. The way VA is expressed—as milligrams of equivalent acetic acid—gives everyone a clear, comparable metric to read a plant’s health across time and between facilities. It’s not about chasing a perfect number; it’s about understanding how close the system is to tipping and what levers you can pull to keep the process steady.

If you’re exploring topics under the GWWI WEF umbrella, you’ll notice that VA sits at the intersection of chemistry and process control. It’s the kind of metric that rewards a practical mindset: monitor it alongside alkalinity, pH, and gas production, track trends, and keep the feed and environment balanced. When you do, you’ll often see smoother operations, better digester stability, and more predictable biogas outcomes.

A closing thought—think of VA as the plant’s early warning system

In the grand scheme of wastewater treatment, VA is like a compass for fermentation health. It doesn’t tell you every detail, but it points you toward the right direction. By expressing VA as mg/L of acetic acid, the industry hands you a consistent measure that helps teams communicate, analyze, and act with confidence. That shared language makes it easier to compare notes, spot patterns, and keep the treatment train running smoothly.

If you’re curious about how different treatment stages respond to acidity changes, or you want to see more real-world examples of how VA readings correlate with digester performance, there are plenty of resources and case studies out there. The underlying idea remains straightforward: a clear, comparable VA reading helps engineers and operators keep the system balanced, productive, and safe for the long haul.