Biological Nutrient Removal: NitriMORE Benefits

Biological Nutrient Removal Water Treatment Plant

NitriMORE Offers Operational Flexibility & Cost Savings

The conventional design of Biological Nutrient Removal (BNR) systems is “handcuffed” by the need to operate at a Solids Retention Time (SRT) for the Aerobic Basin, SRTaerobic ,that is long enough to prevent washout of the nitrifying bacteria.  In other words, the SRTaerobic equals the SRTnit, as shown below.  The design space for conventional BNR systems also requires a mixed liquor suspended solids (MLSS) level (2,000-4,000 mg/L) that does not impede aeration or sludge settling in the secondary clarifier.

Nitrimore design space of Biological Nutrient Removal
Coupled SRT for Aerobic & Nitrification Limit Conventional Design Space or BNR.

Nitrifying Bacteria Form Microcolonies

A molecular method called fluorescence in situ hybridization (FISH) evaluated the four major types of nitrifying bacteria in the freshly settled sludge in secondary clarifiers of different biological nutrient removal (BNR) systems.  Detached microcolonies of nitrifying bacteria settle at a slower rate than the rest of the sludge flocs.  These observations led to a new hypothesis.

New Hypothesis on Nitrifying Bacteria Microcolony Formation

Nitrifying bacteria form microcolonies on the surface of flocs that detach due to exposure to shear from the Return Activated Sludge (RAS) pump.  This microcolony formation process, as shown below, results in a distribution of three distinct types of sludge flocs or particles are present in the settled sludge of the secondary clarifier.

For BNR systems, the internal recycle results in microcolony growth on the surface of the sludge floc, while the RAS pump detaches the microcolony.  These observations produced two patents that described a novel way to uncouple the solids retention time (SRT) of the aerobic basin (SRTaerobic) from the nitrifying bacteria (SRTnit) by selective wasting of sludge with low levels of nitrifying bacteria.

Nitrifying bacteria microcolony formations
Microcolony formation process generates three types of sludge. Nitrifying bacteria grow and form microcolonies.  These microcolonies are present in sludge as attached to sludge flocs (I and II) or detached (III). The internal recycle promotes growth of small attached microcolonies (I) to large attached microcolonies (II). Shearing conditions of the RAS pump promotes the detachment of large microcolonies (III).

NitriMORE Recovers Microcolonies of Nitrifying Bacteria

The figure below shows a typical inclined plate settler modified for NitriMORE. For a typical inclined plate settler, sludge enters below a series of inclined plates. The solids settle on the plates and slides down to the bottom of the unit. The effluent has low solids concentration and flows out the top of the inclined plates. With NitriMORE, the Inclined Plate Settler produces two solids streams from the traditional WAS.  The effluent is the light fraction that contains the recovered nitrifying bacteria, which returns to the treatment train. The heavy fraction is the new WAS, which exits the treatment train for downstream processing.

Control of the system SRT is possible with knowledge of the suspended solids concentration of the new WAS. NitriMORE has on-board probes to measure the suspended solids of the effluent and new WAS. In addition, variable speed pump increase or decrease the new WAS flow rate to achieve a target solids residence time.

NitriMORE schematic diagram of Biological Nutrient Removal
Basic Schematic Of NitriMORE

NitriMORE Breaks Biological Nutrient Removal Design Rules

NitriMORE is new unit process that offers operational flexibility for better BNR performance and cost savings.  Uncouple the SRTnit and SRTaerobic for new design and operation possibilities, as shown in Figure 4.  Maintain the current SRTnit and reduce SRTaerobic, which reduces the size of the aerobic basin or aeration costs.  Operate at a much higher SRTnit, while maintaining current SRTaerobic for improved nitrification performance.  Use this approach during winter time operation or to improve nitrification during peak ammonium loading.  Operate at a higher SRTnit and lower SRTaerobic to achieve multiple objectives.

NitriMORE Design space, aerobic basin, expansion
NitriMORE Expands The Design Space Of BNR Systems.

Simple Integration of NitriMORE Into Your BNR System

NitriMORE integrates into a Modified Ludzack-Ettinger treatment train, as shown below, by installing NitriMORE in the existing WAS line.  Additionally, it diluts the WAS with secondary effluent before feeding to the NitriMORE unit.  In some locations, the NitriMORE unit could use MLSS from the Aerobic Basin and use the existing WAS pump to transfer the new WAS (heavy fraction) to the downstream solids processing facility.

NitriMORE treatment integration
MLE Process (top) With NitriMORE (bottom).

Features Of NitriMORE

Wet Tech Environmental works with equipment vendors to provide NitriMORE to your specifications.  Annual maintenance of NitriMORE is available through a nationwide network of authorized service technicians.

Rapid installation requires a concrete pad and electrical service for the on-board pumps and control system.  Custom length hoses ensure efficient sludge transfer.  The small unit size ensures flexibility for your facility.

Benefits of NitriMORE

NitriMORE is simple to operate and integrates into an existing WAS process.  Start with a small NitriMORE unit for aeration cost savings and add more NitriMORE units for additional capital cost savings and/or better nitrification performance.  Meanwhile, operation remains at a lower SRT while maintaining complete nitrification produces more WAS for anaerobic digestion.  For facilities with anaerobic digester capacity, the additional WAS produces more biogas for generating electricity for on-site usage.  Beyond a lower cost, NitriMORE also preserves land for future expansion.

For more information, watch this video on Biological Nutrient Removal from The Water Research Foundation.

For any other questions related to NitriMORE about how it could help your Biological Nutrient Removal (BNR) system meet treatment goals, lower operational costs, or reduce capital expense for expansion, please write to We can also provide a White Paper that provides details on how to use NitriMORE in multiple ways.

Leave a Reply