Boosting Biofilter (Part 4): How BOD Strangles Your MBBR

In Part 3, we cracked open the concept of Biological Oxygen Demand (BOD) and why it’s the invisible hand brake for your system’s performance.

 

Now, we’re taking that theory and applying it directly to the workhorse of modern aquaculture: the Moving Bed Biofilm Reactor (MBBR).

If you are running an MBBR, you might think "more bacteria is better." You’d be wrong. When it comes to biofilm, thin is in. If you ignore BOD, you aren't building a biofilter; you’re building a sludge factory.

Here is exactly how high BOD destroys the efficiency of your MBBR media.

The War for Real Estate: Heterotrophs vs. Nitrifiers

Your biofilter media is a battlefield. On one side, you have the autotrophs (the nitrifying bacteria you want MORE of). These guys are slow growers, but they convert ammonia and nitrite.

On the other side, you have heterotrophs. These bacteria eat organic carbon (BOD). They reproduce at lightning speeds compared to nitrifiers. 

You are going to have both, its inevitable. But some are beneficial, while others are just plain bothersome. 

When your BOD levels are high, you are essentially feeding the hetertrophs more, they are gorging themselves.

The heterotrophs explode in population, overgrowing the nitrifiers and burying them under layers of slimy biofilm. This leads to a massive increase in biofilm thickness.

The Oxygen Diffusion Problem

Why does thickness matter? Because oxygen has to physically travel (diffuse) through the biofilm layers to reach the bacteria.

In a high-BOD environment with thick, gooey biofilms:

1. Surface Hogging: The aggressive heterotrophs on the outer layer of the biofilm consume the oxygen first as they break down the organics.
2. Starvation: By the time the water penetrates to the inner layers—where your nitrifying bacteria are trying to hide—the oxygen is gone.
3. Anaerobic Zones: The deep layers of the biofilm die off or become anaerobic.

This is why you can have high Dissolved Oxygen (DO) in the water column but zero nitrification happening on the media. The oxygen simply can’t get through the layer of "fat" to do the work.

And while nitrifiers only have 1 job (to eat ammonia and nitrite) heterotrophs can do almost anything. 

That realestate is valuable on those little wagon wheels, so you want as many bacteria eating ammonia and nitrite as possible. Otherwise, what's the point right? 

The "Sinking Media" Phenomenon

Have you ever noticed your floating media starting to sit lower in the water, or even sinking to the bottom of the reactor? (it usually comes along with media that looks really dark)

 

That isn't a manufacturing defect; it’s a management defect.

When biofilm gets too thick due to high BOD, the sheer weight of the biomass makes the media heavy. Heavy media doesn't tumble correctly. If it doesn't tumble, it doesn't shear off the old dead bacteria. This creates a feedback loop of sludge buildup, leading to dead zones in your filter and reduced biofilter performance. 

Thin Biofilms: The Lean, Mean Nitrifying Machines

You want your biofilm to be a thin, active layer—not a shag carpet.

A thinner biofilm controlled by low BOD allows oxygen to penetrate all the way to the substrate. This means:

• Higher Efficiency: You process more ammonia and nitrite per square meter of surface area.
• Oxygen Economy: You aren't wasting expensive oxygen feeding heterotrophs that are just generating sludge.
• Better Flow: The media stays buoyant and moves freely, ensuring consistent contact with the water.

The "Scrubbing" Effect: MBBRs as Polishers

When an MBBR is running optimally with controlled BOD, it does more than just convert chemicals; it physically polishes the water.

MBBRs have an incredible ability to absorb fine particulates. A healthy, sticky (but not thick) biofilm captures fines, effectively reducing your Total Suspended Solids (TSS) from the inlet to the outlet. We often see drops in turbidity by more than 5-10 NTU across a well-tuned MBBR.

This "scrubbing" effect results in overall clearer water, which makes UV sterilization more effective and reduces the load on your mechanical filtration. But this only works if the biofilm is healthy. If it’s choked with sludge, it stops scrubbing and starts shedding waste back into the system.

The "Everywhere" Problem

High BOD doesn't respect the boundaries of your biofilter. If there is excess organic carbon in the water, biofilms will form everywhere—inside your pipes, on the tank walls, and in your pumps.

No one has time to scrub tanks and flush pipes constantly.

And if you did have spare time to scrub tanks and pipes constantly, ill guarantee it nots your favorite job in the world.

By controlling BOD at the source (feed management,mechanical filtration, OZONE), you starve out the nuisance biofilm in the rest of the system.

The Bottom Line

If you want a reliable, predictable biofilter, you have to stop looking at just Ammonia and Nitrite and start respecting BOD.

By monitoring your MBBR and keeping BOD low, you ensure your biofilm stays thin, oxygen-rich, and highly active. This creates a system with a higher carrying capacity that creates clearer water and fewer headaches.

Hot Tip:

Nitrite oxidising bacteria are more sensitive to changes in their environment, and abrasion within a biofilter. 

If when tuning, restarting or maintaining your biofilter, you notice a nitrite spike, this is an indication that too much has changed too quickly (with water chemistry, temperature or BOD), or the environment is physically antagonistic to the Nitrite oxidising bacteria (i.e, to much air and mixing).

If you notice a Nitrite spike, chances are you have either shocked the bacteria due to a change in water chemistry,or you are over aerating your MBBR leading to scrubbing the Nitrite oxidising bacteria off the media. 

In most cases, reduce your aeration slightly and you'll notice the nitrite spike subsides.