The VertiCel™ process from Siemens Water Technologies is a series of Vertical Loop Reactor (VLR™) tanks followed by fine-bubble diffuser reactor tanks.  The VertiCel process is an activated sludge process employing aerated-anoxic tanks, mixed and aerated with disc aerators, followed by reactors aerated with fine-bubble diffusers in the second part of the process.  The VertiCel process uses two types of aeration devices (disc aerators and fine-bubble diffusers) to optimize the overall process aeration efficiency.  Power costs of the VertiCel process are 20 to 30% lower than those of other BNR processes.  The VertiCel process uses deep rectangular reactors for reduced space requirements and can easily be retrofitted into existing rectangular aeration basins.  This configuration presents a good retrofit opportunity for countries like China that are facing new more stringent BNR requirements and a growing need to conserve power. 

The VertiCel process saves power in two fundamental areas.  The aerated-anoxic reactors in the first part of the process provide the environment for simultaneous nitrification/denitrification.  This allows savings in power via a denitrification oxygen credit as well as increased oxygen transfer efficiency by providing oxygen at a zero dissolved oxygen concentration.  The second and most important component of the process is the use of mechanical disc aerators in the first part of the process, followed by fine-bubble aeration at the end of the process to optimize the overall process aeration efficiency.  This key design feature allows the alpha associated with each aeration device to be optimized across the process, thus minimizing the overall energy required to treat the wastewater.

The VertiCel process is economical to build and requires no more operator attention than any other BNR process.  This paper includes a discussion of theory for nutrient removal and minimization of power costs, along with an example of the savings possible with the VertiCel process when compared to a conventional anoxic/aerobic fine-bubble BNR design.

Comparison of Aeration Devices
 
To understand the benefits of the VertiCel process, it is necessary to look at the pros and cons of the two types of aerator devices it contains – fine-bubble diffusers and disc aerators – specifically in regards to energy consumption, maintenance, and mixing efficiency.

Many engineers favor fine-bubble diffusers for their energy efficiency, as these are the most efficient oxygen transfer devices in clean water.  However, wastewater contains surfactants and other impurities that reduce the value of alpha for fine-bubble diffusers.  Surfactants reduce the surface tension in the liquid which results in larger bubbles that transfer oxygen less efficiently.  Alpha is a coefficient used to estimate the ratio of oxygen transfer in wastewater under actual conditions (AOR) to oxygen transfer in clean water at standard conditions (SOR).  Disc aerators operate in a much more turbulent mixing regime.  The decreased surface tension resulting from surfactants in the wastewater create smaller water droplets which may actually increase the disc aerators’ ability to transfer oxygen to the wastewater.  When the impact of surfactants on alpha is taken into account, the overall energy requirements for fine-bubble diffusers are slightly lower than those for disc aerators. 

WWTPs are also concerned with maintenance.  Fine-bubble diffuser systems require periodic basin dewatering to clean and/or replace the diffusers.  The USEPA Fine Pore Manual recommends cleaning diffusers every 2 to 12 months.  Disc aerators can be serviced without dewatering the aeration basin, and they often operate for 10 years or more without the need for any significant maintenance (such as a bearing replacement).   In addition, over time fine-bubble diffusers can typically lose 10 to 30% transfer efficiency from fouling.  This is most pronounced in the front of the process where surfactants are highest.  The VertiCel process reduces the impact of fouling by locating the fine-bubble diffusers in the latter part of the process. 

Most WWTPs are designed for a projected future capacity, and as such, many plants begin operation by treating less than half of their design capacity.  Many aeration devices like fine-bubble diffusers are difficult to turn down during underloaded conditons.  This will make it difficult to maintain the aerated-anoxic conditions necessary for simultaneous nitrification and denitrification while still maintaining the ability to keep the aeration basins mixed.  Treatment plants using multiple reactors with fine-bubble diffusers, such as the Mandarin WWTP in Jacksonville, Florida, USA, have been designed for simultaneous nitrification and denitrification, but the amount of denitrification has been limited by the ability of the fine-bubble diffusers to maintain mixed liquor solids in suspension at reduced airflow rates.  In contrast, disc aerators in oxidation ditches can maintain aerated-anoxic conditions at very low organic loading rates (as low as 0.1 kg of BOD per m3 per day). 

Table 1 summarizes the above comparison of aeration devices.  Upon review of the pros and cons listed in the table, it becomes clear that while disc aerators are ideal for aerated-anoxic tanks containing surfactants, fine-bubble diffusers are better for the final stages of the process where surfactants have been biodegraded and where bio-fouling will occur at a lower rate. 

Energy Benefits of Aerated-Anoxic Reactors
 
Mixing is an important criterion depending on the BNR process.  Variations of the Ludzack-Ettinger process are often used for total nitrogen removal, as shown in Figure 1.  While the Ludzack-Ettinger process is effective at removing nitrogen, it requires a fair amount of energy input for mixing in the anoxic zone and for recirculating nitrates from the aerobic zone back to the anoxic zone. 

Multi-stage reactors such as those of the VertiCel process, with aerated-anoxic zones at the front end and aerobic zones at the back end, are a more efficient means of total nitrogen removal (Figure 2).  By keeping the amount of oxygen supplied in the initial reactor less than the demand of the micro-organisms, anoxic conditions can be maintained so that nitrification and denitrification occur simultaneously in the same reactor.  Although not discussed in this paper, enhanced biological phosphorus removal has also been demonstrated in multi-stage reactors utilizing aerated-anoxic zones. 

Even if denitrification is not required to meet an effluent permit, operation of aerated-anoxic zones may be desirable, because denitrification in these zones will reduce energy consumption and recover alkalinity.  While the recovery of alkalinity by the denitrification process is well understood and documented, the energy savings associated with aerated-anoxic zones are not reported widely in technical literature on the subject of BNR. 

There are four components to the energy savings of an aerated-anoxic reactor.  First, oxygen demand is reduced when nitrates are used to oxidize some of the BOD instead of oxygen added by aeration devices.  Second, if the dissolved oxygen (DO) concentration in the aerated-anoxic reactor remains close to zero, the oxygen dissolves more readily, resulting in a higher field correction factor for converting SOR to AOR.  Third, if aerators in the anoxic zone provide enough mixing energy to keep the mixed liquor solids in suspension, separate mechanical mixers are not required.  And fourth, if ammonia is converted to nitrate in the same reactor in which nitrates are converted to nitrogen gas, then energy need not be expended to transfer the nitrates to another reactor.

Case in Point: a Design Example
 
An example might involve converting a fine-bubble plant into a VertiCel process.  The system would have the following design parameters:  average flow – 68,000 m3/d; BOD load – 15,200 kg/d; TKN load –2,700 kg/d; and two trains of five aeration tanks, each measuring 9 m W x 61 m L x 6.4 m SWD (sidewater depth). 

As illustrated in Figure 3, two of the existing fine-bubble tanks would be converted into VLR tanks.  This would allow the plant to meet an effluent total nitrogen limit of 10 mg/l using 37.5% less energy than before the upgrade and 30% less energy than a conventional design with an anoxic tank followed by aerobic treatment.    



Table 2 details the design basis and overall power for each option.  The table considers the efficiency for new fine-bubble diffusers and does not consider fouling that will occur over time which will reduce the fine-bubble transfer efficiency by an additional 10 to 30%. 

To convert a fine-bubble tank to a VLR tank, a horizontal baffle (usually concrete) is added at mid-depth.  Disc aerators are added at the surface to impart a unidirectional flow pattern in a continuous circuit above and below this baffle (Figure 5).  Coarse-bubble diffusers can also be located beneath the horizontal baffle to provide supplemental air if required. 

VertiCel processes are currently operating or are in construction at more than 15 WWTPs worldwide, with many more under design.  The largest VertiCel process in the world, which is designed to treat 450,000 m3/d, is located in China. 

Conclusion
 
The estimated power savings of the VertiCel process are substantial, due to a combination of factors.  By allotting a large portion of the volume to be operated under aerated-anoxic conditions, a significant amount of oxygen can be recovered through denitrification without the need for non-aerating mixers or internal recycling.  Delivery of a large amount of oxygen under aerated-anoxic conditions improves the field correction factor of the aeration system for the front end of the plant.  Using disc aerators in the first part of the aeration process and fine-bubble aeration in the latter stages allows the alpha factor for both devices to be optimized.  Using channel or looped aeration for the upfront surface mechanical aeration allows the aeration to be optimized for “aerated-anoxic” conditions. 

Because there is such a large reduction in power requirements, the installed power of a VertiCel process will be substantially less than that of a conventional fine-bubble system – which will also make the former’s hybrid aeration design attractive on capital costs.  The multiple reactor features of the VertiCel process also offers operating flexibility.  The VertiCel hybrid aeration design presents a good retrofit opportunity for countries like China that are facing new more stringent BNR requirements and a growing need to conserve power.

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