Industry Solutions

Why Ammonia Removal Drops After an MBBR Capacity Increase: What to Check First

Ammonia rising after an MBBR capacity increase? Use this troubleshooting workflow to separate load, oxygen, alkalinity, mixing and media limitations.

Increasing the capacity of an existing MBBR does not automatically increase its ammonia-removal capacity at the same rate.

A plant may add production, accept more wastewater, increase carrier volume, or raise the biological load and then find that effluent ammonia begins to rise even though the reactor still appears to be operating normally.

That does not necessarily mean the new media is ineffective.

Nitrification depends on several conditions working together: the ammonia load reaching the reactor, oxygen availability, alkalinity and pH, temperature, carrier movement, hydraulic conditions, active biofilm and the time allowed for nitrifying biomass to respond to the new duty.

The right troubleshooting question is therefore:

What changed when the capacity increased, and which condition is now limiting nitrification?

This guide provides a practical diagnostic sequence for existing MBBR systems that show weaker ammonia removal after a production or treatment-capacity increase.

First Confirm What "Capacity Increase" Actually Means

Capacity can increase in several different ways.

The plant may have:

increased average wastewater flow;

increased peak flow;

increased production hours;

added a new production line;

increased influent ammonia concentration;

increased BOD or COD at the same flow;

added more MBBR media;

increased the target loading on the existing reactor;

changed upstream pretreatment;

tightened the effluent ammonia requirement.

These changes do not place the same demand on the biological stage.

For example, a higher hydraulic flow with similar wastewater strength may create a different limitation from a higher ammonia concentration at the same flow.

Before adjusting the reactor, document the change in both hydraulic load and pollutant load.

1. Recalculate the Actual Ammonia Load

Start with mass load, not concentration alone.

A higher influent ammonia concentration clearly increases the nitrification duty, but a large increase in flow can also increase the daily ammonia mass load even if the concentration remains almost unchanged.

Collect:

current average flow;

current peak flow;

influent ammonia or TAN;

previous stable flow;

previous stable ammonia concentration;

current operating hours;

current effluent ammonia;

required effluent target.

Compare the present ammonia load with the period when the MBBR was operating reliably.

This helps answer a basic question:

Did the nitrification duty increase faster than the available biological and oxygen-transfer capacity?

If the new operating condition has never previously been demonstrated, treat it as a new process duty rather than assuming the former performance should continue automatically.

2. Check Whether BOD or COD Also Increased

Nitrifiers do not operate in isolation.

If the capacity increase also raises biodegradable organic loading, heterotrophic bacteria may consume more oxygen and occupy more of the available biofilm environment.

Review:

influent BOD;

influent COD;

soluble COD where available;

pretreatment performance;

production changes;

solids or grease carryover;

current versus previous organic load.

A plant may describe the problem as "higher ammonia after expansion" when the underlying change is actually a combined increase in carbon and ammonia loading.

If oxygen delivery was already close to its practical limit, the additional organic load may reduce the oxygen available for nitrification.

3. Measure Dissolved Oxygen Under the New Load

A blower running normally does not prove that oxygen is adequate.

Check dissolved oxygen in the reactor under actual operating conditions, especially during:

peak production;

peak ammonia load;

the period following major feed or wastewater discharge;

the warmest or coldest operating conditions where relevant;

periods when several process stages compete for air.

Also review:

blower operating point;

available spare blower capacity;

discharge pressure;

airflow where available;

diffuser condition;

diffuser fouling;

air distribution;

pipe pressure losses;

water depth;

carrier movement.

The important question is not simply:

"Is the blower on?"

It is:

"Can the existing aeration system supply the increased oxygen demand and still maintain effective carrier movement?"

If oxygen is limiting, adding more carrier may increase nominal biofilm area without increasing the amount of ammonia that the reactor can actually oxidize.

4. Check Alkalinity and pH Before Changing the Media

Nitrification consumes alkalinity.

When ammonia loading increases, alkalinity demand also increases.

A system that previously maintained acceptable pH may begin to experience lower residual alkalinity or greater pH variation after the capacity increase.

Review:

influent alkalinity;

reactor alkalinity where monitored;

effluent alkalinity;

pH trend;

chemical dosing;

make-up water changes;

upstream process changes.

Do not wait for a large pH drop before checking alkalinity.

A falling alkalinity reserve can be an earlier warning that the new nitrification duty is consuming more buffering capacity than the previous operating condition.

If alkalinity becomes limiting, changing carrier type or adding more media will not address the underlying chemistry.

5. Review Wastewater Temperature

Nitrifying microorganisms respond more slowly at lower wastewater temperatures.

A capacity increase that appears acceptable during warm operation may become more difficult when temperature falls.

Compare:

current wastewater temperature;

temperature during the previous stable period;

seasonal minimum temperature;

ammonia trend by season;

any change in tank exposure or process heating.

Do not evaluate a retrofit only from warm-weather performance if the plant must meet the same ammonia target in colder conditions.

For capacity upgrades, the relevant question is whether the biological system can meet the required duty under the project's actual operating-temperature range.

6. Check Carrier Movement and Mixing

In a moving-bed reactor, available surface area is only useful if the media can move through the reactor and contact wastewater and oxygen effectively.

Inspect:

carrier circulation;

stagnant zones;

accumulation near retention screens;

media remaining on the tank floor;

strongly uneven air distribution;

excessive media crowding;

localized solids accumulation;

changes after additional media was installed.

Adding media can change mixing behavior.

If the existing air grid was designed around a lower fill condition, the reactor may require a new review of mixing and air distribution after media volume is increased.

Poor movement can create a situation where the theoretical biofilm area increases while the effective use of that area does not improve proportionally.

7. Confirm the Retention Screen Is Not Creating a Hydraulic Problem

More media and more flow both increase the importance of the outlet retention system.

Check:

screen opening;

total open area;

fouling;

media accumulation;

water-level difference across the screen;

cleaning frequency;

current peak flow;

any bypass or overflow behavior.

A restricted retention screen can change reactor hydraulics and media distribution.

If the screen becomes the limiting hydraulic element, the reactor may show symptoms that appear biological even though the immediate constraint is flow through the outlet structure.

8. Check Whether Upstream Pretreatment Changed

The MBBR only treats what reaches it.

After a production expansion, the biological stage may receive a different wastewater profile even when the main flow figure appears reasonable.

Check upstream units such as:

screening;

equalization;

primary clarification;

DAF;

oil and grease removal;

chemical treatment;

solids separation.

Ask whether the expansion changed:

TSS;

grease;

biodegradable COD;

intermittent shock loads;

cleaning chemicals;

production chemicals;

salinity;

toxic or inhibitory compounds.

A capacity increase can therefore create an MBBR problem indirectly by changing the quality of wastewater entering the biological stage.

9. Distinguish a Loading Problem From a Biofilm-Recovery Problem

If additional media was installed recently, the new carrier does not immediately contribute the same active nitrifying biomass as established media.

The system may need time for biofilm development and adaptation.

Review:

date new media was installed;

whether established media remained in operation;

ammonia and nitrite trends after installation;

carrier appearance and movement;

operating load during the start-up period;

any seeding or commissioning procedure used;

recent cleaning or process disturbance.

Do not judge a newly expanded MBBR only from the first short operating period.

At the same time, do not assume that time alone will solve a system that is actually limited by oxygen, alkalinity, hydraulics or loading.

10. Read Ammonia and Nitrite Trends Together

Ammonia alone does not show the complete nitrification response.

Where nitrite is monitored, the relationship between ammonia and nitrite can help organize the investigation.

Ammonia Rises but Nitrite Does Not Change Much

Investigate:

increased ammonia load;

oxygen limitation;

poor contact or mixing;

reduced active ammonia-oxidizing biomass;

recent disturbance;

pH, alkalinity or temperature changes.

Ammonia Begins to Improve but Nitrite Rises

This can occur when the first nitrification step responds faster than the second step.

It may be seen during:

start-up;

recovery;

load increase;

commissioning of additional biofilm area.

The trend should be evaluated together with operating conditions rather than treated as a separate problem automatically.

Both Ammonia and Nitrite Deteriorate

Check broader system conditions:

oxygen;

alkalinity and pH;

temperature;

hydraulic flow;

upstream wastewater change;

chemical inhibition;

biofilm disturbance.

The timing of the change is often as important as the absolute concentration.

A Practical Troubleshooting Table

Observation After Capacity Increase	First Checks	Possible Limitation
Ammonia rises immediately with higher production	Flow, ammonia load, BOD/COD load, DO	Biological or oxygen demand increased
DO falls during peak load	Blower, airflow, diffuser condition, organic load	Oxygen transfer
pH becomes less stable	Alkalinity, ammonia load, dosing	Buffering/alkalinity
Media movement becomes uneven	Air distribution, fill condition, screen accumulation	Mixing
Water level rises across retention screen	Screen fouling, open area, peak flow	Hydraulic restriction
Performance declines in colder weather	Wastewater temperature, load, operating margin	Reduced nitrification rate
New media installed but ammonia remains high	Start-up time, DO, alkalinity, mixing, active biofilm	Commissioning or another process limit
Organic load increased with ammonia load	BOD/COD, DO, pretreatment	Competition for oxygen/biofilm environment
Sudden decline after production change	Chemicals, pH, salinity, toxicity, pretreatment	Wastewater-characteristic change

Use an Ordered MBBR Ammonia Troubleshooting Workflow

Step 1 — Confirm the New Duty

Calculate the change in:

flow;

ammonia load;

organic load;

treatment target.

Step 2 — Compare With the Previous Stable Condition

Identify what is different from the operating period when ammonia removal was reliable.

Step 3 — Check Oxygen and Mixing

Measure DO under load and inspect air distribution and carrier movement.

Step 4 — Check Alkalinity, pH and Temperature

Confirm that the biological environment still supports the new nitrification duty.

Step 5 — Check Hydraulics and Retention

Look for screen restriction, uneven flow and media accumulation.

Step 6 — Check Upstream Wastewater Changes

Review pretreatment, solids, grease, chemicals and shock loads.

Step 7 — Check Biofilm and Commissioning Status

Determine whether established active biomass is available for the new duty.

Step 8 — Decide Whether More Capacity Is Actually Required

Only after the operating limits are identified should the project evaluate:

additional media;

aeration upgrade;

diffuser modification;

retention-screen modification;

additional reactor volume;

improved pretreatment;

process staging;

or another biological retrofit.

When Can Additional Media Help?

More carrier may be worth evaluating when:

ammonia load has genuinely increased;

available biofilm area is limiting;

reactor volume can accept additional media;

aeration can support the increased oxygen demand;

media can still move effectively;

the retention system can handle the selected fill condition and flow;

alkalinity, temperature and wastewater characteristics are suitable.

Additional media should be part of a system-level capacity review, not an automatic response to rising ammonia.

When Is More Media Unlikely to Solve the Problem?

Adding more carriers alone is unlikely to correct the problem when:

DO is inadequate under peak load;

blower or diffuser capacity is limiting;

alkalinity is insufficient;

carrier movement is already poor;

the retention screen is hydraulically restricted;

temperature has reduced nitrification activity;

upstream solids or organic loading increased substantially;

inhibitory wastewater characteristics changed;

recently installed media has not yet developed active biofilm;

the real limitation is outside the MBBR reactor.

What Data Should You Send for an Ammonia-Removal Review?

For a preliminary review, provide as much of the following as available:

current and previous flow;

influent and effluent ammonia or TAN;

nitrite trend where available;

influent BOD/COD;

wastewater temperature;

alkalinity;

reactor dimensions and water depth;

existing media type and quantity;

any recently added media quantity;

operating DO;

blower information;

diffuser type and layout;

carrier movement observations;

retention-screen details;

upstream pretreatment;

date and nature of the capacity increase;

current treatment target;

recent operating data or trend charts.

This information helps separate a true biological-capacity limitation from an oxygen, chemistry, hydraulic or commissioning problem.

Technical Basis

Nitrification is carried out by slow-growing aerobic microorganisms and is strongly influenced by operating conditions. U.S. EPA nutrient-removal guidance identifies dissolved oxygen, temperature, pH, alkalinity and inhibitory conditions as important factors affecting nitrification. EPA guidance also describes alkaliEPA guidancetion during ammonia oxidation and the sensitivity of nitrification performance to temperature.

These principles are especially important during MBBR retrofit work because increasing load or media volume changes the biological duty without automatically increasing oxygen transfer, buffering capacity, mixing or hydraulic capacity.

Ammonia Rising After an MBBR Capacity Increase?

Do not start by assuming that more media is the answer.

Send HydroNovus the current and previous flow, ammonia load, BOD/COD, temperature, pH, alkalinity, reactor dimensions, installed media, DO, blower and diffuser information, retention-screen details, and a description of what changed when capacity increased.

HydroNovus can provide a preliminary MBBR ammonia-removal and retrofit review to help identify whether the strongest evidence points toward biological loading, oxygen transfer, alkalinity, temperature, mixing, hydraulics, biofilm commissioning or another process constraint.

Final process design, chemical dosing, equipment sizing and performance guarantees should be confirmed using project-specific engineering data.