SymBio® Process for Simultaneous Nitrification and Denitrification (SNdN)

A Cost-Effective and Sustainable Tool for Optimizing BNR

Simultaneous nitrification and denitrification (SNdN) is an attractive option for design engineers and scientists as it may offer significant advantages over the conventional processes with separate nitrification and denitrification reactors. For example, SNdN eliminates the need for a separate denitrification tank and mixed liquor recycle. It offers aeration energy savings and improved sludge settling. SNdN was not considered a reliable option for nitrogen removal, until very recently. A lack of practical experience had created such a negative perception. However, introduction of the award winning SymBio® technology in the USA in 1999 by Enviroquip, Inc. of Austin, Texas has established the reliability of SNdN for biological nutrient removal (BNR). Currently 50+ installations worldwide are benefiting from this concept. In the United States, SymBio® process has been selected for retrofitting of nitrifying plants to incorporate denitrification in municipal as well as industrial wastewater treatment plants. The design flow capacity of these plants range from 100,000 gpd to 40 MGD. It has also been selected as the basis of design for several new plants. Total installed capacity of plants using the SymBio
SNdN technology in the United States is approximately 100 MGD.

BioBalance of Denmark originally developed the SymBio® concept in the early 90’s. The main objective of the SymBio® process was to create and maintain conditions for SNdN in a single basin. Initially BioBalance targeted wastewater treatment plants with oxidation ditches but later applied this concept to other configurations like complete-mix, plug-flow as well as sequential batch reactors. Enviroquip has also coupled the SymBio® SNdN concept with its membrane bioreactor (MBR) systems very successfully.

Typical municipal wastewater treatment plants (WWTPs), which are currently nitrifying, can be easily retrofitted to achieve SNdN and reduce the effluent TN levels to 5-10 mg/L using the SymBio® technology. Flow and loading variations, volumetric and aeration capacity as well as aeration control capabilities are considered during evaluation. Steady state as well as dynamic simulations are performed to establish the feasibility of using the SymBio® SNdN concept on any project at no cost to the clients. The SymBio® process offers the following benefits to owners and operators:

• Energy savings, 25-30% compared to nitrifying plants
  • Typical payback period is 2-3 years
• Capital cost reduction for BNR upgrade
  • No additional tanks necessary for incorporating denitrification
  • No need for recycle of mixed liquor
• Reduced sludge production compared to nitrifying plants
• Improved biological phosphorous uptake
• Better pH control due to recovery of alkalinity in denitrification
• Well stabilized sludge, improved settling
• NADH signal provides quick, real-time information for superior process control

The principle behind the SymBio® process control strategy is simple. The idea is to monitor the NADH (nicotinamide adenine dinucleotide), a coenzyme in the living cells, level in the biomass along with the dissolved oxygen (DO) level in the water to precisely predict the changes in the biological oxygen demand on a 24/7 basis. Based on the results, the aeration is automatically controlled to maintain low dissolved oxygen (< 1.0 ppm) for SNdN to occur in the same basin. Due to the low availability of oxygen, mass transfer/diffusion limitation occurs within sludge flocs and the central part of the biofloc turns anoxic while the outer part remains aerobic. This allows nitrification to be the dominant reaction in the outer zones of the flocs while the nitrate produced is simultaneously denitrified at the core (Figure 1).

Figure 1: SymBio SNdN concept using NADH monitoring

The success of this concept relied heavily on an accurate prediction of the changing biological oxygen demand as well as on an effective control of air supply based on the information generated. It was observed that only relying on monitoring of water phase parameters like DO, pH, ORP or alkalinity limited the fine tuning/optimization capabilities of the process control scheme while creating equipment maintenance issues. Direct monitoring of biology through fluorescent measurement of NADH content of the biomass using a sensor resolved these issues. It proved to be extremely accurate and reliable over long-term as well as non-intrusive to the plant operation.

It was known that the level of NADH for any active biomass changes based the metabolic state it is in. For example, the NADH level in a biomass, maintained under a highly aerobic condition, is relatively low. The NADH level tends to increase, as the biomass is exposed to anoxic and anaerobic stages subsequently. This information along with DO measurement along with the dissolved oxygen from the water phase is used for process control in the SymBio® process to maintain SNdN.

Enviroquip’s Big Bear, CA installation has won a couple of prestigious awards from the state of California based on a successful SymBio® operation. Enviroquip has been able to extend the benefits of the SymBio® concept even further in recent years to incorporate enhanced biological phosphorous removal along with SNdN. This reduces the cost of chemical addition for phosphorous precipitation. With the SymBio® Elite process control scheme, plants can achieve higher nitrogen as well phosphorous removal in a single step.