With high influent COD and 120% flow, the first step is to reduce sludge wasting so microbes can mature

Let’s picture a wastewater treatment plant facing a steady, stubborn challenge: the plant is getting a heavier load than normal. The influent COD sits around 180 mg/L, and the flow has crept up to 120% of what we usually see. It’s a test of the system’s balance—biomass, aeration, and the flow that keeps everything moving. In this moment, the first operational move matters a lot. The right move isn’t flashy, it’s practical, and it’s about preserving the workforce already on site: the microorganisms in the activated sludge.

First things first: why COD and flow matter to the biomass

COD, or Chemical Oxygen Demand, is like a shorthand for “how hungry is this wastewater?” A higher COD means more organic material for the bugs to eat. When the load goes up, the treatment system needs a robust, mature microbial population that can digest that organic matter efficiently. The solids—tiny living agents in the tank—aren’t just passengers. They’re the ones doing the heavy lifting, breaking down complex molecules into simpler ones that can be discharged safely.

Flow matters, too. A 120% flow rate means more wastewater entering the basin in roughly the same amount of time. More water can pull more solids through the system, strain settling in the clarifier, and push the treated water toward the outlet faster. If the biomass isn’t ready for prime time, the combination of high COD and higher flow creates a mismatch: too much work for the current crowd.

This is where the brains of the operation—your sludge management strategy—needs to stay sharp. The goal isn’t an instant fix; it’s sustaining a biological community that can handle the extra load without losing control of the process.

Why decreasing wasting is the smart first move

In activated sludge systems, wasting (also called waste activated sludge, or WAS) is the process of removing a portion of the settled solids from the reactor to maintain the desired solids concentration and system health. When COD is elevated, and flow is higher than normal, the immediate priority is to keep more biomass in the reactor. Why? Because mature organisms are generally more efficient at breaking down organics than younger ones.

Think of it like a kitchen crew during a rush. If you’re pulling plates away too quickly, you’re curtailing the cooks before they’ve had time to sharpen their knives and learn the prep work. In wastewater terms, excessive wasting reduces the average age and maturity of the microbial population. That can slow down degradation at exactly the moment when the system needs every capable worker on the floor.

So, the first operational strategy is to decrease wasting. By slowing or reducing the WAS removal, you raise the mixed liquor suspended solids (MLSS) and, importantly, the mixed liquor volatile suspended solids (MLVSS). You’re giving the system more mature microbes to deal with the increased organic load. In practical terms, that means your biomass has greater capacity to metabolize the COD, and the higher flow can be accommodated more smoothly because you’ve kept a larger, more effective workforce in the tank.

A concrete path to implementation

Here’s how plant staff typically translate “decrease wasting” into real actions, while keeping a watchful eye on the rest of the process:

• Adjust WAS flow and age. Reduce the rate at which sludge is wasted from the aeration basin or reactor. The exact amount depends on the plant’s baselines, but the objective is clear: keep more biomass in the system to sustain degradation rates.

• Monitor solids concentrations. Keep close tabs on MLSS and MLVSS. You want them to stay within the target range for your process. If solids climb too high, settling in the clarifier can suffer, and the system may become harder to control.

• Balance with oxygen demand. As you retain more biomass, the oxygen demand can rise. Ensure aeration is capable of supplying enough dissolved oxygen (DO) without wasting energy. Fine-bubble diffusers, blower controls, and OTE (oxygen transfer efficiency) metrics help you tune the aeration without overspending on air.

• Watch the food-to-microorganism ratio (F/M). With more biomass present and a high COD coming in, the F/M ratio can drift. A too-low F/M may stall treatment, while a too-high F/M can cause poor floc formation. The goal is a stable, productive balance where microbes have enough substrate to metabolize but not so little that they starve or foaming becomes an issue.

• Maintain settling performance. A higher MLSS can lead to denser sludge. If settling slows, the clarifier might start pushing solids out with the effluent. Keep an eye on sludge blanket levels and use gentle, well-timed sludge wasting to preserve settleability.

• Be prepared for a staged response. Decreasing wasting isn’t a one-and-done action. If the plant observes continued COD loading or signs of stress in the biological system, you can adjust aeration, add a bit more time for the biomass to acclimate, or gently tweak the return sludge flow to keep the reactor well mixed and healthy.

What not to do immediately (and why)

It’s tempting to think bigger aeration, temperature checks, or a dramatic shutdown would fix things fast, but they aren’t the right first moves in this scenario.

• Increase aeration rates: Helpful for DO, sure, but air alone doesn’t fix a biomass shortage. If you flood the system with more oxygen while the population isn’t mature enough to use it efficiently, you’ll waste energy and still struggle with degradation. The biomass needs time to mature, not a bigger oxygen hammer.

• Monitor temperature changes: Temperature does affect microbial kinetics, but in the moment of elevated COD and flow, it’s not the primary lever. Temperature shifts are important for long-term stability, especially in seasonal climates, but they don’t address the immediate need to preserve and grow the microbial community.

• Complete system shutdown: This is a blunt instrument that creates more problems than it solves. Shutting down disrupts ongoing treatment, can cause solids to go anoxic or septic in places, and resets the biological balance you’re trying to maintain. It’s a last resort, not a first response.

• Random “wait and see” without a plan: A steadier, informed approach beats indecision. The aim is to act with intent, not to stall.

A useful mental model: the microbes as the plant’s workforce

If you’ve ever worked in a busy kitchen or a bustling workshop, you know that the right crew matters. You don’t want to fire a bunch of skilled workers during a rush or send them home just when demand surges. In a wastewater plant, the microbial community is your workforce. They need time to mature, enough substrate to eat, and a stable environment to perform.

When influent COD climbs and flow increases, the natural impulse is to preserve as much biomass as possible in the reactor. Reducing wasting buys you time. The microbes have a chance to mature, to become more efficient at metabolizing the organics. It’s a bit like letting the apprentices grow into skilled technicians; the payoff is in better treatment performance when the load is high.

A few practical notes and real-world tangents

• Sludge age matters. Older, well-developed biomass tends to handle higher loading better. That’s why keeping a healthy solids concentration can be more effective than chasing the simplest fix.

• System stability pays off later. A well-managed biomass now reduces the risk of odorous off-gas, foam formation, or poor effluent quality down the line. Stability isn’t flashy, but it’s priceless when the plant is under stress.

• Instrumentation helps, but judgment rules. DO probes, MLSS meters, and settling tests are invaluable. They guide the adjustments, but you still need to interpret the data in the context of the system’s health and the incoming load.

• A touch of engineering humility helps. Every plant has its quirks: different tank configurations, varying return sludge rates, and unique influent characteristics. The first rule is to protect the microbial community; the second is to tune the rest of the system to support that community.

• Clear communication matters. Operators and supervisors should share readings, decisions, and rationales. When the crew understands why wasting is being adjusted, it’s easier to stay focused and avoid overcorrection.

Bringing it home: the core idea in one simple sentence

When the influent COD jumps and flow climbs, the first move is to keep more biomass in the reactor by reducing wasting. It’s all about giving the grown, capable microbes time and space to handle the extra organic load. Only after the biomass is stabilized should you fine-tune aeration, monitor temperature, or consider other adjustments.

If you’re curious about the bigger picture, you can think of a wastewater plant as a living system. It breathes with the flow, it feeds on the organics in the wastewater, and it adapts as conditions change. The first step in any pressure situation is to respect the biology that does the dirty work. In this case, that means letting the organisms mature by slowing the waste—allowing them to flex their hard-earned efficiency when the going gets tougher.

A quick recap for day-to-day decisions

• High COD and high flow? Don’t panic—preserve biomass first.

• Decrease wasting to raise MLSS and empower mature microbes.

• Keep an eye on DO and oxygen transfer as biomass grows.

• Watch settling and sludge age to prevent secondary issues.

• Use data to guide gradual adjustments, not wholesale overhauls.

• Remember: a stable biological core makes everything else easier to manage.

In the end, the smallest, most thoughtful adjustment often yields the biggest payoff. By prioritizing biomass retention when the system is stressed, you’re giving the plant the best chance to ride out the peak load with clean effluent and steady operation. It’s practical, it’s proven, and it echoes the quiet wisdom of good engineering: nurture the living system you depend on, and it will carry you through the rushes.