WesTech Engineering, Inc.

- Model STM-Aerotor - Biological Nutrient Removal (BNR) System

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The STM-Aerotor™ Biological Nutrient Removal (BNR) System uses Integrated Fixed Film and Activated Sludge (IFAS) technology as part of a process that provides biological nutrient removal for municipal and industrial wastewater treatment.

With every rotation, the STM-Aerotor captures atmospheric air, draws it down into mixed liquor in a steel or concrete basin, and slowly releases it as course bubble aeration. During the rotation, additional cascade aeration elevates the dissolved oxygen in the upper layer of the basin. The combination of the slow rotation of the STM-Aerotor, intense air release, and the addition of a peripheral mixing paddle ensure a thoroughly mixed system.

In addition, The STM-Aerotor includes a large surface area for fixed film growth. The interior and exterior of the special polypropylene discs provide the perfect environment for a variety of attached growth organisms. These organisms will react quickly to an increased food source, or shock load, to eliminate discharge violations during peak or diurnal fluctuations.

The amount of aeration can be controlled using a variable speed drive connected to the rotor, causing it to rotate faster or slower based on the actual oxygen demand.

The STM-Aerotor combines activated sludge and fixed film in a compact biological treatment system that requires a low power input. The unique media of the STM-Aerotor provides both the fixed film surface area and the vehicle for coarse bubble aeration. The STM-Aerotor design allows for efficient aeration without the need for diffusers, air piping, control valves, diffused aeration blowers, or supplementary mixers.

IFAS process systems have been employed for wastewater treatment because they have many distinct advantages. First, the IFAS process can accomplish more treatment than conventional activated sludge in an existing footprint. Second, complete nitrification occurs at much lower sludge ages than conventional activated sludge plants. Finally, the improvement in sludge settling makes operation of an IFAS plant much easier. Most IFAS systems use a free-floating or structurally supported media in a diffused aeration basin, but the STM-Aerotor does not. The STM-Aerotor has all the advantages of the IFAS process without the need for energy intensive diffused aeration or mixing equipment.

The STM-Aerotor has been used in different applications throughout the world. The compact nature of this system works well for small package plants in new communities, resorts, campgrounds, and truck stops. The ability to handle variable loading rates works well for industrial plants such as wineries, food processing, refineries, and dairies. The modular design works well for small treatment plants (less than 5 mgd) that may expand or require nutrient removal in the future.

  • Special polypropylene discs provide perfect environment for fixed film growth
  • No blowers, aeration piping, or diffusers
  • Low HP drives
  • Compact design
  • Stable process
  • Wheels controlled with VFD’s and DO probes
  • Peripheral mixing paddle

  • No odors
  • No noise
  • Low energy requirement
  • Small footprint
  • Improved sludge settling
  • Minimized operator attention
  • Can accommodate seasonal fluctuations in flow rate

  • School Facilities
  • Remote Job Sites
  • Prison Facilities
  • Truck Stops
  • Housing Developments
  • Shopping Malls
  • Campgrounds
  • Restaurants

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Activated Sludge

The STM-Aerotor™ captures atmospheric air with each rotation, draws it down into the mixed liquor, and slowly releases the compressed air as coarse bubble aeration. The amount of aeration can be controlled using a variable speed drive to speed up or slow down the rotor based on the actual oxygen demand.

The aeration of the system is achieved without the use of blowers, aeration piping, or diffusers. During the rotation, additional cascade aeration elevates the dissolved oxygen in the upper layer of the basin. The slow rotation of the STM-Aerotor, intense air release, and the addition of a peripheral mixing paddle combine to ensure a thoroughly mixed system.

Fixed Film

In addition to ensuring effective aeration, the STM-Aerotor also provides a large surface area for fixed film growth. The interior and exterior of the special polypropylene discs provide the perfect environment for a variety of attached growth organisms. The fixed film component increases the “effective” sludge age and improves the sludge settling characteristics. The fixed film will react quickly to an increased food source, or shock load, to eliminate discharge violations during peak or diurnal fluctuations.

Biological Nutrient Removal

The unique STM-Aerotor design allows the process tank to develop both anoxic and/or anaerobic zones within the same basin. The process control system can typically achieve total nitrogen below 10mg/l and total phosphorous below 2 mg/l. For enhanced biological nutrient removal, the STM-Aerotor system can be combined with additional biological selector zones in separate basins to achieve total nitrogen below 3 mg/l and total phosphorus below 1 mg/l.

  • Low HP Drive Unit - The simple drive unit is easily accessible and is controlled by a VFD. Drive lubrication is the only routine maintenance item in the process.
  • Drive Chain - The drive chain is constructed of hardened steel components.
  • Steel Center Shaft - The center shaft provides connecting points for the STM-Aerotor frame assembly.
  • Steel Structural Frame -The steel structural frame is designed to transfer the loads from the media to the supports at the wall.
  • Support Bearings - The specially designed bearing assemblies require no maintenance. The bearings are backed by a ten-year guarantee.
  • Mixing Paddle - To allow the system to be deep enough to develop zones that are oxygen limited, a mixing paddle is added to ensure mixing.
  • STM Media - The special STM media discs maximize the surface area for the fixed film, while optimizing the captured air volume and release depth for efficient aeration.

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Compact Design

Ideal for plants with close neighbors, the STM-Aerotor plant will minimize both noise and odors. Biological treatment is achieved without noisy blowers, while reactive fixed film organisms help eliminate nuisance gases. In addition, new developments usually reserve minimal space for wastewater treatment. The compact design allows the STM-Aerotor plant to fit into tight sites. The low HP drives allow the new plant to operate on single-phase power, eliminating the need for transformers or new power feed. Most importantly, the STM-Aerotor system produces a high quality effluent with complete nitrification, along with single digit biochemical oxygen demand (BOD) and total suspended solids (TSS) values.

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Package Plants

WesTech can design and supply complete package plants to meet the strictest effluent requirements. These package plants can include flow equalization, digesters, disinfection, and tertiary filtration. These compact systems minimize footprint and operator attention. Package plants can range from the most basic control scheme to fully automated systems.

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Flexible Operation

The STM-Aerotor’s ability to accommodate seasonal fluctuations in flow rate makes it ideal for destination resorts, educational institutions correctional facilities and campgrounds. The multiple trains of an STM-Aerotor system can easily be taken off-line or placed on-line based upon demand. The STM-Aerotor wheels are controlled with variable frequency drives (VFD) and dissolved oxygen (DO) probes to optimize the process while minimizing operator attention.

Stable Process Performance

Exceptional process stability and flexibility is ensured, even under high loadings, by combining the activated sludge and fixed film processes. Even the strictest effluent standards can be met consistently.

Solid and Simple Construction

The system provides long-life polypropylene media, a simple low-maintenance drive train, and low-friction support bearings for extended life.

Lower Capital Cost

Tank volume is reduced up to 40% versus conventional activated sludge. No mixers or blowers are required.

Lower Energy Usage

Power consumption is less than 50% of a conventional activated sludge system, and only 40% of a comparable sequencing batch reactor.

Advanced Biological Nutrient Removal

The STM-Aerotor can provide consistent nitrification/ denitrification, as well as biological phosphorus removal.

No Noise or Odor Problems

Simple slow-speed mechanism reduces operating noise. The STM-Aerotor process eliminates aerosols and minimizes odors.

Improved Sludge Quality

The system provides better sludge settling characteristics (lower sludge volume index), better dewatering, and smaller volumes of waste sludge.

Many Applications

The STM-Aerotor is applicable to a wide variety of processes and industries.

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