The Town of Mentone in Kosciusko County, Ind., is known as the Egg Basket of the Midwest. With just over 1,000 residents, Mentone faces challenges common to many other small communities in North America with a dilemma of an aging lagoon-based wastewater treatment facility and new effluent quality limits.

While effluent limits have become stricter in recent years, many communities have struggled to find the technical and financial resources to keep up because technology options for post-lagoon nutrient removal are limited.

Until recently, cost-effective tertiary treatment technologies following cold oxidation ponds or aerated lagoons that can meet low ammonia levels have been rare. Communities had little choice but to abandon their lagoons and construct new mechanical treatment plants, with the accompanying capital and operation costs.

But recent advances in cold-climate nitrification provided the Town of Mentone with an innovative solution for post-lagoon nutrient removal. The community added SAGR (Submerged Attached Growth Reactor) technology to its existing lagoon system to provide nitrification, without taking the existing lagoons offline.

Upgrade for Ammonia

Mentone's treatment facility consisted of a two-cell facultative lagoon system providing secondary treatment. The facility was designed to meet effluent BOD5/TSS limits of 25/70 mg/L, but it could not meet the NPDES limits for total ammonia nitrogen (TAN) of 9.6 mg/L in summer and 10.4 mg/L in winter.

Nelson Environmental collaborated with the town engineering consultant to design an upgraded system that retained the facultative lagoons for secondary treatment, followed by a SAGR process.

The SAGR is an aerated gravel-bed reactor with a horizontal-flow hydraulic profile. The module provides year-round nitrification well beyond most total ammonia permit requirements for influent water temperatures as low as 32.9 degrees F, making it well suited for post-lagoon treatment in cold climates.

An added benefit is effluent polishing to BOD5/TSS levels to less than 10/10 mg/L. In addition, test data from a demonstration facility in Lloydminster, Alberta, showed significant (greater than 90 percent) reduction of fecal coliform to less than 200 CFU, in some cases making additional disinfection unnecessary.

Simple System

The SAGR process is simple to operate. There is no solids return to monitor and adjust and no sludge to waste and handle. The operations and maintenance process is similar to that of a conventional diffused-air aerated lagoon. It is estimated that over the long term, the operator of the Mentone facility will spend an average of 30 minutes per day doing a systems check (visual inspection) and maintenance.

The only moving parts in the system are the blowers supplying oxygen to the SAGR process. A simplified control scheme manages the day-to-day operation of the blowers. The SAGR system at Mentone is comprised of two beds in parallel, each handling 50 percent of the hydraulic loading.

The SAGR process was developed mainly to provide post-lagoon ammonia removal without abandoning existing lagoon treatment. Performance parameters and sizing for the process are based on extensive testing performed on the post-lagoon SAGR in Lloydminster and a demonstration unit that was located in Steinbach, Manitoba.

The process can be used for nitrification following any secondary treatment process, including aerated or non-aerated lagoons. The clean gravel bed has a horizontal-flow distribution chamber at the front end to distribute the influent wastewater across the width of the entire cell.

Saving Energy

The aggregate is submerged, providing the necessary surface area for growth and attachment of a nitrifying biomass within the bed, and it is sized to optimize bacterial growth and hydraulic flow. A horizontal effluent collection chamber at the back end collects the treated effluent and channels it to the discharge structure. Sizing of the bed is based on influent flow and loading rates, expected influent water temperature, and the required rate of nitrification.

In Mentone, no aeration was required in the lagoons to meet the recommended lagoon effluent BOD5 feeding the SAGR process. The blowers for the SAGR are sized to meet the oxygen requirements for nitrification and final BOD polishing only. This saves significant energy that would be required to run blowers for a conventional aerated lagoon system.

It is estimated that this design saves 50 percent on energy versus other systems achieving similar effluent quality. The trade-off is the higher lagoon footprint required for the necessary residence time. Since the capacity was available at the onset of the Mentone system design, the most cost-effective approach was to use the existing lagoon infrastructure. This provided cost savings in both construction and long-term operation and maintenance.

Getting Going

Nelson Environmental provided system commissioning and operational training in March 2011. After a two-week startup window, the Mentone facility is producing effluent averaging 6.5 mg/L BOD, 3 mg/L TSS and 0.3 mg/L TAN year-round. The system design flow is 0.12 mgd.

The facility upgrade using the SAGR process enabled the town to surpass current NPDES requirements. Using the facultative lagoons for secondary treatment and the SAGR process for nitrification and BOD/TSS polishing will lead to an estimated 50 percent operation and maintenance savings compared to a fully mechanical aerated treatment system.

The upgraded system in Mentone provides an example of a cost-effective solution for wastewater operators who face the same regulatory challenges and want to keep their existing lagoon system while maintaining low operation complexity.

About the Authors

Kevin Vieira is involved with client services and Merle Kroeker, P.Eng., is a project development engineer with Nelson Environmental, a provider of water and wastewater solutions based in Winnipeg, Manitoba. The company can be reached at info@nelson environmental.com.