Nutrient removal can be expensive. To meet new nitrogen limits for the 7.1 mgd wastewater treatment plant in Amherst, Mass., a state study determined that a $61 million upgrade was needed. Instead, the city is getting the job done for $75,000 by making informed process changes.

Instead of pursuing the traditional facility planning, design and construction pathway to permit compliance, the Amherst staff, led by Jim Laford, superintendent, experimented with new ways of operating their 1979 vintage facility.

After a few failed attempts at dialing in ammonia and nitrate removal, the staff found that cycling the plant’s mechanical aerators to alternate highly aerobic and marginally anoxic conditions provided the best treatment. After four months of experimenting with process changes, effluent total nitrogen is 8 mg/L (roughly equal concentrations of TKN, nitrate, and nitrite). Effluent ammonia averages 0.5 mg/L. The new permit limit is 546.5 pounds per day total nitrogen, amounting to 9.2 mg/L at design flow.

Operating parameters are not yet established for all conditions, but early results are encouraging. The team expects to satisfy permit conditions at a $60 million savings, with no increase in operations and maintenance costs.

Varied influent

With nearly one-third of the flow and loading coming from area colleges (the University of Massachusetts, Amherst College and Hampshire College), Amherst’s influent is highly variable. The treatment plant operates in a near-complete mix mode with a hydraulic retention time of about six hours. Two of three aeration trains are operated routinely, and two of three final clarifiers are typically used.

Amherst’s process control strategy starts with dissolved oxygen (DO) probes near the midpoint of the two in-service aeration tanks — they help control the cycling of the mechanical aerators. Tanks independently cycle between oxygen-rich aerobic and oxygen-poor anoxic conditions.

The plant’s SCADA system maintains aerobic conditions for a set period of time once the target DO setting is reached. During aerobic treatment, the speed of each mechanical aerator is independently controlled to maintain a target DO level and thus optimize ammonia removal. Oxygen reduction potential (ORP) probes near each of the six operating mechanical mixers provide ongoing information on tank conditions.

After aerating for the desired time, the mechanical aerators slow down to a mixing speed to provide anoxic conditions and support biological nitrate removal. Using SCADA trend graphs for guidance, Duane Klimczyk, assistant chief operator, makes frequent adjustments to the SCADA settings to optimize ammonia and nitrate removal.

Because of the relatively short hydraulic retention time, the staff found it necessary to raise the aeration tank DO to 2.0 mg/L to provide effective ammonia removal. During aeration, the ORP reading climbs above +100 mV. The facility readily removes nitrate (denitrification) with a short anoxic cycle and a relatively high anoxic ORP reading of +50 mV.

Repeatable result

As remarkable as Amherst’s achievements may seem, they are not unique. Experience demonstrates that most municipal treatment plants can get similar results by using existing equipment differently. Dozens of treatment plant managers are operating biological treatment plants not designed for nutrient removal to produce effluents with total nitrogen at 5 to 8 mg/L, or total phosphorus at 0.5 to 1.0 mg/L, or both.

For example, 10 Connecticut municipalities are achieving the same 6 mg/L total nitrogen without upgrades as 48 facilities upgraded at an average cost of $6.15 million. The equipment cost for the non-upgraded facilities averaged less than $50,000. Because most of the process changes involve the creation of anoxic zones, most are aerating less and are therefore using less electricity.

The 4.0 mgd facility in Keene, N.H., is seasonally producing effluent with total phosphorus averaging 0.2 mg/L without filtration. The team in Montague, Mass., is using a creative sequenced aeration strategy to reduce total nitrogen to 7.0 mg/L and effluent total phosphorus to 1.0 mg/L. In Conrad, Mont., cycling of aeration in a manner similar to Amherst’s produces effluent with total nitrogen averaging 4.0 mg/L.

Making it happen

In Amherst’s case, Hach ORP probes were installed alongside each of the plant’s six in-use mechanical aerators; six Hach DO probes were already in place. The new in-line instrumentation is connected to the plant’s Control Logic SCADA system.

Mike Moore, chief electrician/programmer, programmed the RSView SE software (Rockwell Automation) to provide real-time information and automatic control of the variable-frequency drives that regulate the speed of the mechanical aerators.

Daily effluent testing for ammonia, TKN, nitrite, and nitrate is performed by Ashley Warren, lab analyst, using a Hach DR3900 spectrophotometer. Effluent pH and alkalinity are monitored using an Orion 420A laboratory instrument. To support the staff in optimizing treatment, Amherst’s consultant provides almost daily data review and visits the site twice monthly to collaborate with the staff.

A 2008 computer modeling of the plant concluded, “there are no operational or minor modifications/retrofits that could be implemented at this facility to consistently achieve nitrogen removal. The existing facility has half of the necessary volume at the current flows.”

However, notwithstanding the variable flow, plant bacteria fail to recognize that they are incapable of providing nitrogen removal to an average flow of 4.2 mgd using two of the facility’s three aeration tanks and two of the three final clarifiers. Computer modeling determined that five aeration tanks and four final clarifiers would be required.

Essential instruments

Amherst’s is not the only treatment facility that performs better than computer modeling found possible. Experience with dozens of treatment facilities shows that better-than-theoretically-possible nitrogen and phosphorus removal can be realized by:

• Gathering appropriate real-time data
• Reviewing the data frequently
• Using the data to take timely and appropriate actions

Nitrogen and phosphorus removal works best when the biological habitats are closely monitored and controlled. In-line equipment recommended for tracking environmental conditions includes DO, ORP, TSS, pH and alkalinity monitors. Instruments to provide vital feedback on treatment effectiveness include:

• Nitrite, nitrate and ammonia analyzers for facilities with nitrogen removal objectives
• In-line ortho-phosphate effluent TSS monitoring for facilities with phosphorus removal requirements

Amherst is using real-time data for decision-making, backed by consultant guidance. The facility is meeting its nutrient obligations with minimal capital investment after thoughtfully experimenting with new process control strategies.

About the author

Grant Weaver, P.E., is a Class IV wastewater operator and owner of The Water Planet Company in New London, Conn. He can be reached at grantweaver@thewaterplanetcompany.com.