Biological treatment in wastewater typically reduces volatile solids by about 50%

What really happens to those solids in wastewater? If you’ve ever peeked behind the curtain of a treatment plant, you’ve likely heard about volatile solids and the role biology plays in trimming them down. Here’s the bottom line, in plain terms: biological treatment can reduce volatile solids by about 50%. It’s a common, practical figure you’ll see echoed in many facilities and training guides from organizations like GWWI and WEF. But like most numbers in water, this one isn’t a hard rule carved in stone — it depends on the day, the plant, and what’s in the incoming wastewater.

What are volatile solids, and why do we care?

Let’s start with the basics. Solids in wastewater are not all the same. Some are inorganic (like sand), while others are organic — the stuff that can be eaten by microbes. When we talk about volatile solids, we mean that portion of the solids that can be vaporized when heated. In practice, volatile solids are a good proxy for the amount of organic matter in the sludge. They’re the fuel for the microbes in biological treatment and a key indicator of how much carbon-based material is left after processing.

Volatile solids are typically measured as a percentage of total solids, often using a method called loss on ignition (LOI): you burn off the organic material and see how much weight disappears. The bigger the volatile solids fraction, the more organic material there is to break down. That makes sense, right? If there’s less fuel, there’s less work for the microbes — and that, in turn, affects how much solids reduction you can achieve in the process.

Biological treatment: how the magic happens

When we say “biological treatment,” we’re talking about processes that rely on microbes to stabilize and break down organics. The two workhorse configurations you’re likely to encounter are activated sludge (an aerobic process) and anaerobic digestion (which happens without free oxygen).

• Activated sludge: In this setup, air or oxygen is pumped into a mixed liquor of water, suspended solids, and a thriving community of microbes. The organisms feed on the organic matter, grow, and become part of the solids that settle out. The result is a significant drop in the concentration of organic material, turning a portion of the volatile solids into new microbial biomass and gases (chiefly carbon dioxide and, in some cases, methane).

• Anaerobic digestion: In this setup, microbes do their work without oxygen. It’s a slower process, but it’s especially effective for handling sludge in larger quantities. The digestion converts organic matter into biogas (a mix of methane and carbon dioxide) and stable, less volatile solids. This pathway can produce energy while reducing the organic load.

So, why is 50% a common figure?

In many municipal systems, the combination of the right microbial crew, adequate retention time, and steady operating conditions yields about a 50% reduction in volatile solids. Think of it as a practical mid-point: the microbes are doing a substantial amount of the heavy lifting, converting a lot of the organic matter into biomass and gases, and leaving behind less organic material in the effluent. It’s not a magic number carved in stone, but it’s a reliable expectation for a well-run biological treatment train under typical conditions.

A quick mental picture

Imagine the sludge as a pot of simmering soup. The microbes are the chefs, and the organic compounds are the ingredients. Over time, the chefs chop, dissolve, and reassemble the ingredients into new things: more biomass, gases released, and a lighter soup. In the end, about half of the original volatile solids have been consumed or transformed. That’s the gist of a 50% reduction in many plants.

What can nudge that number up or down?

While 50% is a solid benchmark, the actual reduction you see can swing with a few factors. Here are some that matter in the real world:

• Process type and design: Activated sludge and anaerobic digestion behave differently. Depending on sludge age, retention time, and reactor design, the extent of reduction varies. In some setups, particularly with long digestion times or high-temperature digestion, you can push the figures higher; in others, it might be closer to the mid-range.

• Temperature and seasonality: Microbes like it warm and steady. In colder climates or during cooler seasons, reaction rates slow down, which can temper volatile solids destruction.

• Influent composition: The recipe of what comes into the plant matters. Wastewater with a lot of readily degradable organics tends to yield quicker reductions, while wastewater rich in more recalcitrant compounds (tougher to break down) may settle toward lower reductions.

• Hydraulic and sludge retention times: Give the microbes enough time with a steady supply of food, and they’ll do more work. Shorter contact times or poor mixing can limit how much solids are reduced.

• Oxygen and mixing: In aerobic systems, proper aeration and mixing help microbes access the organics, but over-aeration or poor mixing can waste energy without significantly boosting reduction. In anaerobic systems, maintaining the right conditions is key for optimal digestion.

• Operational conditions and age of the sludge: The microbial community evolves. The “health” of the biomass—the mix of bacteria, archaea, and other microorganisms—affects how efficiently organics are broken down. Sludge age, part of the plant’s routine, can tip the scales.

The practical implications for treatment plants

Understanding how much volatile solids can be reduced isn’t just an academic exercise. It informs how a plant manages its process, energy balance, and downstream handling.

• Effluent quality: Fewer volatile solids usually mean less organic load in the discharged water, which helps meet regulatory limits and protects receiving waters. A stable reduction also helps with predictable effluent characteristics.

• Sludge handling: The solids that remain after biological treatment are what the plant has to deal with next. If you’ve cut a big chunk of organic matter, the sludge becomes easier to dewater and transport. In digestion-heavy plants, there’s also the benefit of biogas production, which can offset energy costs.

• Biogas and energy: In anaerobic digestion, turning organics into methane can provide heat or electricity for the plant. That’s a nice win, especially when you’re juggling rising energy prices and sustainability goals.

• Compliance and sustainability: Lower organic loads and well-managed solids reduce odors and improve overall process stability. That can translate into smoother operation and better community relations.

A few real-world perspectives

Let me put this in a more human frame. Think of a typical city’s wastewater plant as a busy kitchen. The cooks (microbes) are busy breaking down the ingredients (organic matter). Sometimes you’ve got a recipe with lots of easily digestible stuff; other times, the mix is tougher, with fats, oils, or complex organics slowing everything down. In most kitchens, you’ll see the batch of organic matter drop by about half as the microbes do their work. It’s not flashy, but it’s steady, reliable, and essential for keeping the city’s waters clean.

If you ever tour a plant, you’ll notice the rhythm of the process. The aeration tanks hum with activity, the clarifiers slowly spin, and the digesters—if present—are like quiet energy hubs storing and converting material into usable gas. The numbers you learn in textbooks or training guides aren’t just abstract ideas; they map to real daily operations. A plant aiming to optimize performance might push toward higher reductions with careful tweaks, but the 50% figure remains a practical baseline.

Putting it all together

Here’s the takeaway you can carry into conversations with colleagues, professors, or plant operators: Biological treatment reduces volatile solids by about half in many municipal wastewater systems. That 50% figure is not a universal ceiling; it’s a dependable expectation under typical conditions, reflecting the hardworking microbes at the core of the process. The exact reduction you observe depends on the treatment approach (activated sludge vs anaerobic digestion), the characteristics of the incoming wastewater, and how the plant is operated day to day.

If you’re curious to connect the theory to practice, you can look at the larger picture as well. Volatile solids reduction intersects with energy use, gas production, and solids management. It affects how facilities design their reactors, choose retention times, and plan for dewatering and disposal. And yes, that means every minor adjustment in temperature, mixing, or food supply can ripple through the system in meaningful ways.

A few quick prompts to reflect on

• When you read about a specific plant’s performance, do the numbers for volatile solids align with the 50% benchmark, or do you notice a different trend?

• How does the choice between activated sludge and anaerobic digestion shape the expected reduction and the plant’s energy profile?

• What operational tweaks could nudges the system toward a more consistent 50% reduction, especially during seasonal shifts?

If you want to go deeper, consider looking into how modern plants monitor volatile solids in real time, how digestion gas capture systems are designed, and how operators balance oxygen, nutrients, and retention times to keep performance steady. There’s a lot of practical wisdom in those control strategies, and you’ll start spotting their fingerprints in every treatment line you study.

A final thought

Biological treatment is, at its core, a tale of collaboration — people designing the process, microbes doing the heavy lifting, and the plant operating as a coordinated system. The roughly 50% reduction in volatile solids is a useful compass, helping engineers and operators gauge performance and plan for downstream handling. It’s not a single perfect number, but it’s a reliable, real-world guideline that keeps wastewater treatment grounded in what works day in and day out.

If you’re exploring GWWI WEF Wastewater Treatment fundamentals, this concept often acts as a bridge between theory and practice. You’ll find it echoed in manuals, training briefs, and the conversations you have with plant staff during site visits. And as you move through the materials, you’ll start to see how that 50% target threads through efficiency, sustainability, and the ongoing mission to protect water resources.