Food Processing Plant, Wisconsin

Project Background & Challenges

The southern Wisconsin shore of Lake Michigan is home to many large industrial companies valuing the proximity to both Milwaukee and Chicago, which can put a strain on the area’s wastewater treatment infrastructure. At the same time, protecting local watersheds and ecosystems is a priority; even the city of Kenosha, located in the heart of this region, is named for the indigenous word for a local species of fish. As a result, regulators and regional treatment plants are pushing the responsibility back on industry.

At one of the region’s food processors, for example, there are a series of covered and uncovered ponds, followed by a Dissolved Air Flotation (DAF) system with chemical addition for phosphorus control. The wastewater then passes through a recirculating sand filter.

However, in 2015, scheduled testing showed this food processor’s lagoon-based wastewater treatment facility was no longer meeting state regulated ammonia limits that go as low as 1.3 mg/L. This required the company to truck their wastewater away for disposal on a weekly basis, the high cost of which prompted the food processor to seek out a better solution.

The Nexom Answer

The company in question explored all their options. At first, they considered upgrading to a mechanical plant in order to achieve their ammonia limits (right), until a conversation with Nexom engineers offered a more cost-effective solution.

Nexom proposed upgrading the existing lagoons cost-effectively to achieve the ammonia limits set for the food processor. This would involve placing a SAGR® submerged attached-growth reactor after the lagoons for ammonia nitrification, as well as additional BOD and TSS effluent polishing. This would enable the food processor to work with and enhance their existing infrastructure, rather than abandoning it.

The SAGR is a simple yet powerful tool, designed to help wastewater treatment plants to achieve their ammonia limits safely and consistently. While it can be used successfully in any climate, where the SAGR really shines is in its ability to handle cold-weather locations. Consisting of a clean rock bed, with evenly distributed wastewater flowing through the length of the cell and aeration across the cell floor providing aerobic conditions for nitrification, the system solves the problems posed by lagoons in cooler temperatures. This includes providing the necessary oxygen to support nitrifying bacteria even in cold temperatures, ample surface area for bacteria to grow and thrive on, and the ability to prebuild and store bacteria while the water is warm.

Each SAGR bed is sized based on several parameters, each of which is specific to the site it is being installed on. This includes loads and flows, available space, site dynamics, and more. For this reason, there are no “standard sizes” for the SAGR. In many cases, it can even reduce the overall footprint of the lagoon system because the attached-growth environment allows for a higher concentration of helpful bacteria. The SAGR bed at this specific Wisconsin food processor was sized to handle levels of 15 mg/L of BOD and 25 mg/L TSS in the influent, and to bring those numbers down below < 10 mg/L, as well as achieve a Total Ammonia Nitrogen (TAN) limit of < 1 mg/L.

Solving the Phosphorus Deficiency Problem:

Following initial commissioning in 2017, the system operated as expected, capably meeting the ammonia requirement of < 1 mg/L by achieving a 0.07 mg/L in the effluent. However, although the SAGR appeared to be operating normally, the ammonia levels began to increase. While initially not a concern, the ammonia levels eventually rose to the point where the plant was briefly out of compliance and it was necessary to resume trucking a portion of their wastewater out for treatment.

Nexom engineers worked closely with the team at the food processor, testing and adjusting the system to isolate the cause for the increase. It was determined that at certain times of the year, the phosphorus levels in the influent entering the SAGR were lower than the minimum level required to sustain biomass, causing the biomass to grow slower than expected and resulting in elevated ammonia levels.

Due to the plant’s ultra-low phosphorus limit of 0.075 mg/L, the existing treatment system was operated as such that ortho phosphate (the main bioavailable form of phosphorus) was removed to non-detectable levels prior to the SAGR. In order to overcome this issue, adjustments were first made to the phosphorus removal system, therefore allowing more phosphorus to stay in the wastewater until it reached the SAGR. When measured levels were still too low, Nexom engineers recommended mixing phosphoric acid directly into the SAGR influent to allow for the appropriate balance of nutrients to aid in biomass growth.

The system operators implemented a mixing tank, with a chemical dosing system that they already had on site. Based on initial recommendations from Nexom engineers, a small amount of phosphoric acid (the equivalent of 20 mL, or 1.35 tbsp) was mixed and dosed daily into the SAGR influent. After some initial testing, it was discovered that dosing of phosphorus was only required during the biomass growth phase in fall. This enabled the system to grow and maintain the amount of biomass needed to achieve and maintain ammonia compliance the entire year.

Upgraded System Performance

The SAGR system for the food processor’s wastewater treatment plant was first commissioned on July 6, 2017.

While the site struggled initially with keeping its ammonia levels consistent, by working together with engineers from the Wisconsin food processor, Nexom was able to effectively assess and adjust the system to meet their needs.

Since adding the phosphorus dosing in early winter 2018, the SAGR-enhanced wastewater treatment plant has had a clean compliance record. The food processor has been able to fully eliminate the cost of hauling their wastewater away for treatment, and effluent ammonia levels have stayed well within acceptable levels, all without the capital costs, complexity or O&M requirements of a mechanical plant.