Why a Water Resource Recovery Facility may struggle to meet BOD5 and TSS removal when inflow and infiltration dilute the wastewater

Outline you can skim:

• Hook: WRRFs handle a daily, never-ceasing flow—but storms and leaks can dilute what comes in.

• Quick primer: what BOD5 and TSS measure and why plants care.

• The core idea: influent dilution from inflow and infiltration (I&I) as the key culprit.

• How dilution changes the game: fewer organics and solids for microbes to work on, and why that hurts removal efficiency.

• Quick contrast: overloads and old tech can bite too, but they don’t explain the dilution puzzle by themselves.

• How to spot and fix I&I: simple clues, practical steps, and a few real-world tactics.

• A practical takeaway: understanding dilution helps you think about the whole system, not just the plant in a vacuum.

Influent dilution and the real-world challenge of meeting BOD5 and TSS targets

Let me set the stage. A Water Resource Recovery Facility (WRRF) is a busy factory of sorts, where wastewater gets treated so clean water can be released back into the environment. The processes are designed around a certain strength of wastewater—the typical amount of organic material and suspended solids the plant expects to see on a regular basis. BOD5 and TSS are two big deal metrics here. BOD5, or five-day biochemical oxygen demand, tells us how much oxygen the organic stuff in the water will consume as microbes munch through it. TSS, total suspended solids, are the particles that can be caught in a filter, settle in clarifiers, or ride along with the flow.

Why do these numbers matter? If you don’t remove enough BOD5 and TSS, you can starve the receiving water of oxygen or release sludge downstream that isn’t pleasant to deal with. In short, the plant must strip away enough of both to meet regulatory limits and protect the river, lake, or estuary.

Now, about that correct answer you’ll likely encounter in a test setting: Influent concentration dilution from inflow and infiltration (I&I). When heavy rain, melting snow, or groundwater leaks find their way into the sewer system, they dilute the wastewater. Think of it as turning up the faucet with more water but the same amount of soap. The system was designed with a certain concentration in mind. When you add lots of extra water (stormwater or groundwater) without a corresponding increase in wastewater strength, the average concentration of organics and suspended solids drops.

That dilution sounds tiny, but it’s not. In a WRRF, a lot of the treatment machinery—bioreactors, clarifiers, and filtration steps—depends on microbial respiration and settling behaviors that are tuned to a certain substrate load. When the influent is more dilute, there’s less food (organic material) for the microbes to eat, and fewer particles for the clarifiers to grab hold of. The result? The processes run with suboptimal efficiency, and the plant may struggle to meet BOD5 and TSS removal targets.

A closer look at the mechanics: what exactly changes with dilution

BOD5 removal hinges on microbial action. Heterotrophic bacteria, yeast-like organisms, and a diverse crew of microbes grab onto organic molecules and break them down. If the influent has a lower concentration of these organics, the microbes can’t feed at the rate the plant was designed for. The reaction kinetics slow down; the rate of oxygen consumption drops, and the overall “cleaning” pace can lag behind what the regulatory limit expects.

TSS removal depends both on physics and biology. Particles settle in clarifiers, are trapped in biofloc, or are removed by filtration. When you dilute the wastewater, you’re diluting the solids too. Fewer suspended particles mean fewer opportunities for solids to settle out, and the crew has to rely more on the clarifier’s hydraulic residence time to do the heavy lifting. If the flow is also bouncing around due to bursts of rainwater, the settling dynamics can become unsettled, and some solids slip through.

So, yes, dilution from I&I directly targets the kind of removal you’re trying to quantify. It’s not just “too much water in the system.” It’s a shift in the chemical and physical landscape inside the plant, a shift that the design assumptions didn’t account for when the plant was sized and tuned.

Why not the other options you might see on a test?

• Overloaded treatment capacity: This is a real issue when flows spike or the plant runs near or above its design capacity. It can cause poorer removal, but it doesn’t inherently explain dilution. If you had a steady flow with high strength wastewater, you might still meet or fail targets for other reasons—like aging equipment or suboptimal sludge ages. I&I is different: the culprit is the dilution of the feed itself, not just the volume getting processed.

• Inadequate treatment technology: Sure, old or mismatched technology can hamper performance. But if dilution is the primary cause, even capable technology will struggle because the substrate that fuels the biological and physical removal is thinner. In practice, I&I and treatment capability often interact, but the dilution story is the central thread when the plant is seeing unusually low strength influent during wet weather.

• Insufficient microbial populations: Microbial health matters, no doubt. If the microbial community is weak, removal drops. But again, the root here in the dilution scenario is that the microbes aren’t being fed as much as designed, simply because the water coming in is more water than expected. The right microbes still need substrates to work with.

Spotting I&I in the real world

So how would operators and engineers notice that I&I is playing a big role? Here are some practical clues:

• Rain or high groundwater events correlate with lower BOD5 and TSS removal efficiencies. If performance dips right after a storm or snowmelt, that’s a hint.

• Influent flow rates surge without a proportional rise in influent strength. In other words, the plant sees more water, but the organic load doesn’t increase accordingly.

• Inflow points near old or compromised sewer infrastructure: leaking joints, cracked pipes, and illicit connections tend to be the culprits, especially in older neighborhoods or flood-prone areas.

• Manhole and sewer line inspections reveal cracked joints or areas where groundwater might be seeping in.

Reducing I&I: practical steps you’ll see in the field

Addressing I&I doesn’t happen by magic. It requires a mix of detective work, planning, and small but targeted fixes. Here are some common avenues:

• Identify and fix sources of inflow and infiltration. This may mean repairing cracks in pipes, sealing manholes, replacing or lining aging sewer sections, and removing illicit cross-connections (like storm drains dumping into sanitary lines).

• Improve sewers and collection system integrity. A clean, tight sewer network reduces the amount of water that sneaks into the system during rain events and ground thaw.

• Use flow equalization where practical. An equalization basin or buffer can absorb peaks in flow, allowing the plant to process a steadier feed. It doesn’t eliminate I&I, but it helps the plant cope with the variability.

• Enhance monitoring. Installing or upgrading flow meters, level sensors, and early-warning alarms helps operators spot unusual inflow quickly and respond before it cascades into treatment problems.

• Conduct infiltration testing and targeted rehab. Diagnostic approaches—like smoke tests, CCTV surveys of gravity sewers, and manhole inspections—pinpoint the worst offenders. The fixes then follow, prioritized by impact and cost.

• Maintain infrastructure and preventive care. Regular line flushing, valve maintenance, and gasket replacements keep the system tight and less prone to leaks.

A human moment: why this matters beyond the numbers

If you’ve ever stood at the edge of a river or watched a city’s wastewater data dashboard, you know numbers don’t tell the full story. Behind every decimal is a system that people rely on—people who live downstream, communities that want clean water, and workers who keep the plant humming even when the weather acts up.

Understanding that dilution from I&I is a way to look at the whole ecosystem: the pipes, the plant, the regulators, and the public’s role in keeping things tight. It’s not about blaming a single part of the system; it’s about recognizing how rain, groundwater, leaks, and good old-fashioned aging can ripple through a facility’s performance.

A few analogies to keep it relatable

• Think of the plant like a kitchen that’s designed for a certain number of diners. If a big rainstorm brings in extra guests and you’re still cooking for the same group, you don’t have more food—so you’re serving smaller portions. The kitchen can still work, but it’s not operating at the same efficiency, and the leftovers (or, in our case, the wastewater quality) aren’t what you planned.

• Or imagine a river that swells with rain. The water chemistry changes as the blend shifts. The microbes in a WRRF aren’t picky about rain; they just need enough fuel to do their job. When the mix gets watered down, the clean-up act slows down.

Bringing it all home

The takeaway is clear: when a WRRF struggles to meet minimum BOD5 and TSS removal targets, one of the first things to check is whether influent dilution from inflow and infiltration is at play. It’s a straightforward, real-world mechanism that explains why some days feel like you’re working with a “lighter” wastewater load, even if the plant is busy.

That doesn’t mean other factors don’t matter. Overloaded capacity, outdated or mismatched treatment technology, and microbial health still influence performance. But dilution is often the quiet workhorse behind the scenes, shaping how the plant behaves during wet weather, seasonal shifts, and infrastructure aging.

If you’re stepping into wastewater fundamentals, keep this lens in mind: evaluate the flow with an eye on where water is coming from, and ask how the strength of the incoming wastewater might be changing. A small change in concentration can ripple through the system, nudging BOD5 and TSS removal in ways that surprise at first glance.

Final thought: stay curious, stay observant

As you study the core ideas of WRRF operation, remember that practical wisdom lives at the intersection of data and field reality. The dance between water volume, contaminant load, and the biological and physical removal steps is intricate, yes, but you don’t have to solve it with a single magic trick. With good monitoring, a few targeted maintenance moves, and a habit of questioning how rain and leaks influence the numbers, you’ll gain a solid grip on why some days the plant hums smoothly and others—after a storm—requires a little extra attention.

If you’re exploring the fundamentals further, you’ll find that knowledge isn’t just about meeting numbers on a chart. It’s about protecting water resources, supporting healthy communities, and keeping the gears of a city running when the sky opens up or groundwater finds a way in. That broader perspective—that sense of how everything connects—will serve you well as you dive deeper into the world of wastewater treatment.