Project Background & Challenges

In 2016, the CDP of Absarokee in Stillwater County, MT began looking for solutions to upgrade their existing three cell aerated lagoon. As their serviced population and flows began to increase beyond the original system design loading, effluent water quality became a concern at the high design flow during irrigation season.

In addition to BOD and TSS removal, nitrification would now be required, adding additional process steps to the existing system. The design goal for the system was to meet the new effluent requirements, while maintaining the operational simplicity and process stability of a lagoon system.

The unique challenge at Absarokee was the limited property boundary. Setback requirements from existing wells in the area dictated the available footprint.  Early design solutions considered raising the lagoon berms and increasing lagoon slopes to squeeze as much treatment volume as possible into the small available footprint; however, it was not possible to gain enough volume in the lagoons while staying within the property boundaries and site setback constraints.

The Nexom Answer

Great West Engineering reached out to Nexom to provide recommendations on a system upgrade for Absarokee that would improve BOD reduction and provide cold water nitrification. Nexom’s complete suite of available biological processes allowed a solution to be developed that leveraged multiple technologies in sequence for effective staged treatment while retaining relative operational simplicity. After a few design iterations, Nexom concluded that the desired BOD removal would not be possible within the existing lagoon cells. A combination of adding a BioPorts™ Moving Bed Biofilm Reactor (MBBR) to provide BOD reduction between lagoon cells, and a post-lagoon SAGR^® submerged attached-growth reactor to provide cold-water nitrification was chosen as the best solution.

Lowering BOD and TAN in a Limited Footprint

The unique requirements at Absarokee required the implementation of three distinct biological processes working together to achieve the treatment goals:

optAER^® Fine Bubble LAGOON Aeration

optAER fine bubble partial mix aeration with floating laterals was implemented in cells 1a and 1b. The primary purpose of the aerated ponds at the front of the process was to settle inorganic solids prior to the MBBR, while providing oxygen and contact time to natural bacteria. This ultimately converts the wastewater contaminants to carbon dioxide, water, and inert ash and nitrates. The primary aerated ponds effectively control odors and provide internal sludge digestion. The fine bubble diffusers are suspended near the bottom of the cells. Through the rise of the bubbles and subsequent mixing, convection cells are created between the diffusers. This combined with the slow rate of bubble rise contributes to the overall efficiency of the system. Baffles were added to lagoon cells 1 and 2 to lengthen the flow path increase retention time.

Bioports™ Moving Bed Biofilm Reactor (MBBR)

The limited land footprint available at Absarokee meant that there was not enough lagoon volume available for full BOD removal. The Nexom solution was adding an MBBR process to provide secondary BOD removal. The BioPorts MBBR is an advanced biological process for efficient and reliable active treatment that fits within a small footprint. Biofilms are self-maintaining and continuously replace old biomass with new biomass. HDPE media cultivates biofilm comprising specialized microorganisms that provide intensified treatment. The media is retained in designated treatment zones by wedge wire screens while peak flows pass through unimpeded. The aeration system comprises of an engineered grid of diffusers that has been optimized for energy efficiency and effective mixing while utilizing maintenance-free coarse bubble diffusers.

However, moving biofilm processes can expel solids that would affect the design and performance of the downstream SAGR cell. To combat this, the MBBR process was placed downstream of lagoon cell 1, and upstream of lagoon cell 2, to allow for settling of solids between the MBBR and the SAGR processes.

SAGR® Cold Water Post-LAGOON Nitrification

Within the footprint of the existing cell 3, three SAGR® cells were constructed for post-lagoon cold-water nitrification and BOD/TSS polishing. The SAGR submerged attached-growth reactor is a patented process designed to provide nitrification (ammonia removal) in cold to moderate climates. The SAGR is a clean gravel bed with evenly distributed wastewater flow across the width of the cell, and a horizontal collection chamber at the end of the treatment zone. Linear aeration throughout the floor of the SAGR provides aerobic conditions that are required for nitrification. The gravel bed is covered with a layer of wood chips or shredded rubber tires for insulation.

Site Construction

Montana company KLE Construction began construction on the improved wastewater treatment system in late 2020, with all Nexom equipment on site by early 2021. The system is expected to begin operation in 2022.

Construction included reshaping of lagoon cells 1 and 2, construction of the MBBR and SAGR basins, and installation of Nexom supplied equipment. A new building was constructed to house equipment and air supply blowers. The upgrade also required rehabilitation of the existing UV system to meet permit requirements.

Upgraded System Performance

In addition to the desired treatment outcomes, Nexom’s processes will result in relatively low operational and maintenance costs and procedures. All maintenance on the optAER lagoon equipment can be performed from a boat with a 2-person crew, without taking the system out of service. In-basin aeration equipment for the MBBR and SAGR systems are maintenance free. The process provides Absarokee with a simple system capable of meeting their new effluent limits within the compact footprint of the existing lagoon site