Carrousel - Systems For Biological Nutrient Removal

• They were first introduced into the US market by Ovivo in 1979. Ovivo has continually added and improved the design to meet today’s expectations for both performance and energy consumption.
• The designs today are radically different from the original designs. Advances in research for biological wastewater treatment have allowed Ovivo to further perfect the design and make it more efficient.
• Designs today are geared to a process unit approach. This allows for a new view of the systems which incorporates controls and instrumentation to give the operator and management a complete overview of the system’s performance.
• The systems are very operator friendly and whether they are controlled manually or through a control package, their performance is maintained. They have consistently won awards from the EPA for operator’s performance over the last three decades.

The Carrousel systems are laid out in two different configurations; the hot dog layout and the foldover layout. Either one of these layouts performs in the same manner. They are designed in these configurations to allow for the best available use of the space provided for the project.

Over the last thirty plus years the Carrousel systems have consistently produced results that meet or exceed our customers’ expectations. In effect, the more advanced systems have produced results that have achieved effluents that approach the LOTs (Limits of Technology).

• The Basin
• The Aerator
• The Internal Recycle Gate
• The Mixers
• The Controls

The Basin
The aeration basin is the first step in the design of the biological treatment process for the Carrousel systems. The basins are designed based on the model for design of oxidation ditches with surface aerators developed by the inventor of this technology. It takes into consideration velocity and OTE (Oxygen Transfer Efficiency) in its model. DHV, the inventor has also tested and certified various impellers for use in oxidation ditch technologies thus providing an independent source for approval of impellers.  The impellers are selected based on HP, depth, width and length to maintain optimum propulsion in the basin. This is critical when designing systems for BNR.

The Aerator
The aerator was originally designed as a single impeller for propulsion and oxygenation. This sufficed for the application for BOD reduction and nitrification. With the advent of more stringent effluent limits, the design of the aerator was improved. Ovivo was the first and is the only manufacturer that supplies a dual impeller in this application. The purpose of the dual impeller is to provide the operator with better mixing at a wide range of HP draws. In doing so, the impeller provides more efficient mixing at lower HP draws. It also allows for more effective use of the basin volumes where settling can occur. The aerators also have a better OTE typically 3.65 lbs O2/HP Motor-HR from the originally designed aerators.

The Internal Recycle Gate
The internal recycle of nitrates for denitrification is a fundamental aspect of the MLE process for activated sludge. The process requires the recycling of nitrates to an anoxic tank for the denitrification process. Ovivo invented the internal recycle gate by incorporating the anoxic tank into the Carrousel basin. This process called the DenitIR system allows for a slipstream from the aeration basin to enter into the anoxic basin located in front of the aeration basin. By doing this Ovivo has achieved two important advantages. The first is the elimination of an internal recycle pump. This is a cost savings for the operator. The second is a limitless amount of internal recycle into the anoxic basin. The design of pumped recycle was limited to 4Q which was the economical limit for pumping. Using the propulsion from the aerators in the Carrousel basin, the pump is eliminate and the amount of nitrates recycled into the anoxic basin can be controlled by the Internal Recycle( IR) gate. Modelling through use of the Biowin software has shown that there is an optimal recycle rate that provides the best denitrification in an anoxic tank. Knowing this, Ovivo has further enhanced its Internal Recycle (IR) gate and now supplies a motorized gate for this process.

The Mixers
Mixers are used in the enhanced nutrient removal configurations of the Carrousel systems. Ovivo uses submersible mixers for this application. They provide the best mixing/HP requirements of any mixer. This is important when designing a system to meet effluent requirements and reducing operating costs. They also reduce the requirement maintenance for the equipment.

The Controls
Ovivo was the first to introduce aerator controls through dissolved oxygen (D.O.) for oxidation ditches with surface aerators. This was in response to customers’ needs to optimize energy consumption and also to improve performance on BNR systems.  The controls were basic; simple control of the D.O. by adjusting aerator speed through VFDs. This has proven to be effective for the requirements. However, the need of operators to have a good sense of the costs of operating these systems has increased. Ovivo has developed the Oculus™ control system to address the needs of operators for today and the future. The Oculus system is an “all seeing” control system. It allows the operator to monitor the process through instrumentation and to adapt the D.O.control set up to minimize power consumption. The operator and plant manager can now determine the best operating mode for their process, achieving the effluent permit and minimizing power consumption.

• Over 30 years of experience designing BNR systems with over 700 installations, over 300 of which are BNR plants.
• Process knowledge and resources that is available to our customers for the lifetime of their plant.
• Being a resource for customers to look at their oxidation ditch and developing a solution to upgrade their plant to new effluent permits.
• Ovivo has a worldwide network of Centers of Excellence where we provide our customers with the latest and most accurate resource to solve their issues.
• The ability to take existing oxidation ditches not designed by Ovivo and converting them to our Carrousel system designs to give our customers Optimum performance.