MBBR SND: How Simultaneous Nitrification Denitrification Works in Wastewater | MBBR Technology

How Does Mbbr Technology Achieve SND?

SND for Biological Nitrogen Removal

Simultaneous Nitrification and Denitrification (SND) is a technology where nitrification, denitrification, and carbon removal occur concurrently within a single reactor. It challenges the traditional belief that nitrification and denitrification cannot happen simultaneously, particularly demonstrating that denitrification can occur under aerobic conditions, making SND feasible.

Nitrification consumes alkalinity, while denitrification produces it. Thus, SND effectively maintains stable pH levels in the reactor without the need for acid-base neutralization or external carbon sources. It reduces reactor volume, shortens reaction time, and minimizes sludge flotation in secondary sedimentation tanks by lowering nitrate concentrations. As a result, SND has become a research hotspot in biological nitrogen removal. The feasibility of SND is currently explained by three main perspectives:

1. Macroenvironment Perspective:
   This view argues that perfectly uniform mixing is unattainable. Uneven distribution of dissolved oxygen (DO) in the reactor creates aerobic, anoxic, and anaerobic zones. Denitrification occurs in anoxic/anaerobic environments, while organic matter removal and nitrification take place in aerobic zones, enabling SND within a single reactor.
2. Microenvironment Perspective:
   This theory attributes SND to the anoxic microenvironments within microbial flocs. Due to oxygen diffusion limitations, a dissolved oxygen gradient forms inside the flocs, creating microenvironments conducive to simultaneous nitrification and denitrification.
3. Biological Perspective:
   This approach emphasizes the role of specialized microbial communities. Some nitrifying bacteria can perform both nitrification and denitrification. For example, Dutch researchers have identified Thiobacillus denitrificans, capable of aerobic nitrification and denitrification. Other bacteria collaborate in sequential reactions to convert ammonia to nitrogen gas, enabling complete nitrogen removal under uniform conditions.

Current microbiological research on biological nitrogen removal is extensive but incomplete, and understanding of SND remains evolving. The microenvironment theory is widely accepted: due to dissolved oxygen gradients, the outer surface of microbial flocs or biofilms harbors aerobic nitrifying and ammonifying bacteria, while the inner regions become anoxic as oxygen diffusion is limited and consumed, allowing denitrifying bacteria to dominate. This explains the coexistence of diverse bacteria in a single reactor but faces a challenge: organic carbon availability. Organic carbon serves as an electron donor for heterotrophic denitrification but inhibits nitrification. As organic carbon is first consumed in aerobic layers, denitrifying bacteria in anoxic zones may lack sufficient electron donors, potentially reducing denitrification rates and SND efficiency. Thus, the mechanism of SND requires further refinement.

SND in MBBR Biofilm Systems

The Moving Bed Biofilm Reactor (MBBR) is an advanced reactor system combining suspended-growth activated sludge and attached-growth biofilm processes. Its core design principle involves adding suspended carriers with a density close to water directly into the reactor to serve as active microbial substrates. These carriers frequently interact with wastewater, facilitating the growth of biofilm on their surfaces (biofilm attachment), which enhances mass transfer of pollutants, dissolved oxygen, and microbial activity. Thus, MBBR is often referred to as a "moving biofilm."

Based on current research on SND mechanisms, integrating microenvironment and biological theories, the potential reaction model for SND within MBBR biofilms involves collaboration between:

• Aerobic layer microorganisms: Aerobic ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), and aerobic denitrifying bacteria.
• Anoxic layer microorganisms: Anaerobic ammonia-oxidizing bacteria (Anammox), autotrophic nitrifying bacteria, and denitrifying bacteria.

This synergy achieves efficient nitrogen removal.

MBBR relies on aeration and hydraulic flow to fluidize the carriers, creating a hybrid system with both suspended activated sludge and attached biofilm. This maximizes the advantages of both planktonic and sessile biomasses, providing macro- and micro-scale aerobic and anaerobic environments. It resolves conflicts between autotrophic nitrifiers, heterotrophic denitrifiers, and heterotrophic bacteria for dissolved oxygen (DO) and carbon sources. Consequently, MBBR achieves kinetic balance between nitrification and denitrification, creating ideal conditions for SND and enabling efficient nitrogen removal through simultaneous nitrification and denitrification.

Factors Influencing SND in MBBR Systems

The key to achieving SND in MBBR lies in controlling the kinetic balance between nitrification and denitrification, resolving competition for dissolved oxygen (DO) between autotrophic nitrifiers and heterotrophic bacteria, as well as competition for carbon sources between denitrifiers and heterotrophic bacteria. Primary controlling factors include:

• Carbon-to-Nitrogen Ratio (C/N)
• Dissolved Oxygen (DO) Concentration
• Temperature
• pH (Alkalinity/Acidity)

Specific Influencing Factors:

• Impact of Carriers on MBBR Performance
• Impact of Dissolved Oxygen (DO) on MBBR Efficiency
• Impact of Hydraulic Retention Time (HRT) on MBBR Processes
• Impact of Water Temperature on MBBR Systems
• Impact of pH on MBBR Performance
• Other Factors Affecting MBBR Efficiency

If you need more details about it, pls contact us!