Increase MLVSS in the aeration tank when influent solids rise to boost biomass and treatment stability.

What to do when influent solids spike in an aeration tank

If you’ve spent any time thinking about how a wastewater treatment plant runs, you know solids come and go. Some days the plant gets a light load, other days, the incoming material is dense with organic matter and solids. When that’s the case, the aeration tank faces a bigger challenge: more organic material to break down, more oxygen demand, and a tighter balance to keep the process humming. Here’s the key idea: to handle a surge in influent solids, you often need to adjust the biomass concentration in the aeration tank. In water-treatment speak, that’s MLVSS — mixed liquor volatile suspended solids.

Let me explain what MLVSS is and why it matters

MLVSS is a measure of the active, living biomass inside the aeration tank. Think of it as the number of hardworking microorganisms (the “workers” in your plant) that are actually digesting organic material. It sits inside the mixed liquor, the blend of wastewater, air, and microorganisms swirling around in the tank. When the influent solids jump, the system needs more of those workers to keep up with the workload. If there aren’t enough active microbes, you can see slower treatment, higher soluble BOD in the effluent, and sometimes bad odors due to overload conditions.

A bigger biomass concentration isn’t just a loud slogan; it’s a practical lever you can pull. With more active microorganisms present, there are more hands-on-deck to break down the extra solids and organics. That’s why the correct answer to the common question about a surge in solids is MLVSS. It represents the biological capacity of the system to process matter in the aeration tank.

How this plays out in the real world

Let’s imagine the aeration tank as a bustling kitchen in a busy restaurant. The incoming flow is the customer rush—the more dishes (solids) they bring, the more cooks (microbes) you need on the line. If you suddenly get a bigger order, you don’t magically manufacture more chefs out of thin air—you either bring in more skilled workers or you keep the ones you have longer, letting them stick around to handle the heavier workload. In a wastewater plant, that “stick around” mindset translates to increasing biomass through tactics that raise the MLVSS in the aeration tank.

There are a few practical reasons why someone would want to boost MLVSS:

• More digestion power: A higher concentration of active microbes means faster breakdown of organics.

• Better resilience to load swings: If the plant sees variable influent quality, a larger active biomass helps absorb those swings without letting effluent quality slip.

• Odor control: Overloaded systems can generate more odors; a robust biomass helps keep odors in check by processing the organic load more efficiently.

But here’s the thing: you don’t increase MLVSS by waving a magic wand. It’s a balance act. Push too hard in one direction, and you can run into other problems, like excessive sludge production, wasted energy, or poor settling characteristics. The trick is to move toward the right level of biomass and keep it there, even as the influent varies.

Ways plant operators increase MLVSS (without turning the plant into a science experiment)

If influent solids surge, the goal is to raise the capacity of the biological reactor without compromising other system parts. Here are common, practical approaches:

• Adjust sludge age (SRT): Let the microbial community stay in the tank longer so it can grow. A longer solids retention time typically means more active biomass in the aeration tank, which translates into higher MLVSS. It’s a careful tweak, because too long an SRT can slow down overall process dynamics or require more robust sludge management.

• Moderate return activated sludge (RAS) rates: By recycling more settled biomass from the secondary clarifier back into the aeration tank, you can boost the amount of active microorganisms in the mixed liquor. This is one of the most direct ways to increase MLVSS without changing the inflow dramatically.

• Manage wasting rate: If you’re trying to accumulate biomass, you’ll want to reduce dumping of sludge. Wasting less sludge means more biomass stays in the system, increasing MLVSS. It’s a balancing act with settling characteristics and effluent quality, though.

• Supportive aeration strategy: While increasing MLVSS is the core move, you also want to ensure the aeration system is effectively delivering oxygen to a larger microbial population. You don’t want to overwork the equipment, but you do want to keep DO levels within the target range so the microbes can thrive.

• Substrate management: Sometimes, variations in the organic load come with changes in specific substrates. Ensuring a steady supply of readily biodegradable material helps the biomass grow and stay active, contributing to a healthier MLVSS level.

• Consider bioaugmentation in some cases: In certain plants, introducing specialized microbial cultures can help boost the active biomass quickly, especially after process upsets. This is more of a targeted tool and should be used with a clear understanding of how it will affect overall system balance.

A note on DO, flow, and sludge age

You’ll hear questions about DO levels, flow rate, or sludge age when discussing aeration tanks. They’re all part of the same conversation, but they play different roles:

• DO levels: Oxygen is essential for the microbes to digest organics. If you’re trying to handle more solids, you want sufficient DO, but simply cranking DO up won’t fix the root issue if biomass is too low. It’s about supplying enough oxygen to the existing biomass so it can do its job.

• Flow rate: The wastewater flow affects hydraulic loading. A big spike in flow can dilute biomass or move solids through the tank faster, complicating settling. It can stress the system, but raising flow alone doesn’t create more microorganisms; it just changes the load dynamics.

• Sludge age (SRT): This is the time the biomass spends in the system. A longer SRT generally increases MLVSS, while a shorter SRT may reduce it. It’s a slow dial, but it has a meaningful impact on how much active biomass you have to treat the load.

So which lever matters most when solids jump? For the question you’ll see on many foundational materials, the answer is MLVSS. You’re basically tuning the biomass dial to match the new load, not just turning up one knob here or there.

Why this matters for plant performance and compliance

When influent solids rise and MLVSS is adequately increased, several downstream benefits tend to appear:

• Improved effluent quality: More complete digestion of organics means lower residual BOD and COD in the effluent. The water leaving the plant looks cleaner and meets permit limits more reliably.

• Stable process dynamics: With a healthy, robust biomass, the system is less twitchy in the face of load changes. That translates to fewer upsets and smoother operation.

• Odor and crust control: A well-balanced microbial community helps minimize odors and reduces the risk of grease or scum buildup that can clog clarifiers or exchangers.

• Energy efficiency: While increasing biomass can require careful energy budgeting, a responsive microbial population often means you don’t have to push aeration as hard to achieve the same treatment performance.

If you’re studying these fundamentals, you’ll hear phrases like “biomass concentration controls treatment efficiency.” The idea is simple in concept, even if it requires some careful management in practice. You’re aiming to keep the biological reactor thriving so the plant can handle whatever the influent throws at it.

A quick, student-friendly recap

• MLVSS stands for mixed liquor volatile suspended solids. It’s the measure of active biomass in the aeration tank.

• When influent solids spike, increasing MLVSS gives you more microorganisms to digest the extra organic load.

• You can boost MLVSS by lengthening sludge age, raising return sludge, easing sludge wasting, and ensuring adequate but not wasteful aeration.

• DO, flow, and sludge age all matter, but biomass concentration is the lever most directly tied to absorbing higher solids.

• The payoff is better effluent quality, more stable operation, and a more resilient treatment process.

A few quick glossary notes you can keep handy

• MLVSS: The volatile, organic portion of suspended solids — a proxy for active microbial biomass in the mixed liquor.

• MLSS: Mixed liquor suspended solids — includes both active biomass and inert solids; it’s a broader measure of solids concentration.

• SRT (sludge age): The average time solids stay in the system. Longer SRT usually means more biomass accumulation.

• DO (dissolved oxygen): The amount of oxygen available for microbial metabolism in the aeration tank.

A conversational send-off

If you’re curious about how these ideas fit into the bigger picture of wastewater treatment, think of the aeration tank as the plant’s living engine. The engine runs best when you have a steady stream of fuel (substrates) and a crew of diligent workers (the microbes) who can work efficiently with the oxygen you supply. When solids surge, it’s not a crisis — it’s a call to adjust the microbial workforce so the system can keep doing its job. And as you study the fundamentals, you’ll notice how these relationships show up again and again, in nitrification, in denitrification, and in the many little balancing acts that keep water clean and communities safe.

If you’re ever unsure what to focus on, remember this guiding thread: boosting the activation of the biological process often boils down to giving it enough active biomass to handle the load. MLVSS is your most direct measure of that capacity.

In short, keep an eye on MLVSS as influent conditions shift. It’s a reliable compass for maintaining performance, protecting effluent quality, and keeping the plant’s rhythm steady even when life (and the daily load) gets a bit heavier.