Significant operational challenges. That’s something of an understatement when making wastewater treatment enhancements at an international research university that after 170 years continues to expand.

Known as Ole Miss, the University of Mississippi approached Engineering Solutions Inc. (ESi) in 2010, seeking guidance on an upgrade to its wastewater facility, built in 1972 as a dual-basin extended aeration plant.

ESi designed a project to convert one of the basins to a 0.75 mgd oxidation ditch (Lakeside Equipment), replace the clarifier equipment and recirculation pumping, upgrade the headworks and convert the other aeration basin to flow equalization.

The new oxidation ditch uses a closed-loop-reactor process consisting of reactors with a single feed point for raw wastewater and return activated sludge. The basic design uses a racetrack configuration that provides a straight-line flow between the headworks and the final clarifiers.

At the core of the closed-loop-reactor process is the horizontal Magna Rotor (Lakeside), which sustains a high population of microorganisms in the reactor to provide simple process control. It delivers precise oxygen input to the biological process through adjustment of rotor immersion by raising or lowering the level-control weir and by adjustment of the rotational speed.

Variable flows

The university’s wastewater facility is different in several ways, according to Mike Falkner, ESi principal and civil engineer. “First, there are very wide swings in on-campus population depending on the school calendar. This can range from almost no campus population during the Christmas break, to normal student loading and then to home football games, for which the Ole Miss Rebels attract sellout crowds in excess of 60,000.”

Influent flows can range from 0.3 to 0.4 mgd to an average of 0.6 to 0.7 mgd, to a high of 1.2 mgd. This creates challenges in maintaining a stable biomass within the treatment reactor.

“Also, due to a campuswide push to convert to low-flow plumbing fixtures, wastewater flows were not increasing as a linear function of population,” Falkner says. “However, the concentrations of contaminants such as ammonia in the waste stream continued to rise, often to levels difficult to effectively treat using normal municipal processes. The rising ammonia levels were not identified in pre-design sampling for the 2010 plant rehab project. The source has been confirmed by on-campus sampling conducted as an independent study.”

Exploring alternatives

As the campus population continued its rapid growth, the Physical Plant Department in 2013 asked ESi to investigate wastewater treatment options. At that time, the university operated a separate single-oxidation-ditch treatment facility, permitted for 0.95 mgd and scheduled for upgrade in the 2015 project to the dual-basin configuration with nutrient removal capability.

Faced with a growth rate that would overwhelm the wastewater facilities as early as 2017, and experiencing periods of overload before that, the university sought the most feasible approach from economic, environmental and social perspectives to last the next 20 years.

ESi saw that while campus water conservation had been beneficial in many respects, it had placed higher demands on the wastewater facility, especially when required to meet an ammonia nitrogen discharge limit of 2 mg/L.

One alternative was to send wastewater to the city of Oxford treatment plant. The city had capacity to treat the university’s flow, but that would mean constructing some 12,500 feet of 24-inch gravity sewer to make the connection, including a bore under a state highway. The potential benefits included lower capital cost, elimination of operating and maintenance expenses for the university’s plant, and less environmental liability. It would also allow the university to clear the treatment plant site, on a prime property next to the football stadium. 

However, the analysis showed that the drawbacks outweighed the benefits. The city’s commercial treatment rates were high, and the university would be vulnerable to rate increases. There would also be charges for maintenance and for expansion and replacement of existing collections system components.

Opting for expansion

After lengthy consideration, ESi recommended increasing the university plant’s capacity while maximizing existing assets. ESi proposed a second Lakeside oxidation ditch along with the addition of anoxic tanks upstream of both ditches for enhanced biological nutrient removal. This would help maintain compliance with more stringent NPDES permit limits. 

The proposed new process train would need slightly more capacity than the existing ditch, and new headworks would be built to optimize the flow balance between the two. Other components would also have to be modified to handle pumping, sludge treatment and chemical treatment.

The plan meant the university retained full control of wastewater treatment. Annual debt service plus operational costs would be substantially lower than for off-campus treatment. The capital cost was higher and the treatment plant would remain next to the stadium, but visual and odor impacts would be attenuated by using covers and air scrubbers.

Falkner observes, “The most recent project added a third clarifier, new screening and grit removal systems, a headworks odor control scrubbing system, an aerobic digester and a UV disinfection system.”

The equipment includes the SpiraGrit system (Lakeside), which has no submerged bearings and so is easy to maintain. The compact system efficiently removes grit over a wide range of flow rates in a mechanically induced vortex. Rotating paddles maintain the flow velocity in the vortex chamber, keeping organics in suspension while allowing heavier grit to settle.

Lakeside also supplied a Raptor Micro Strainer, which captures small debris that passes through other screens. The screenings are washed, compacted and dewatered to 40 percent solids, reducing volume by half and weight by two-thirds, lowering reducing disposal costs.

Still growing

The university’s growth was underlined in fall 2016 when enrollment reached an all-time high of 24,250 students. Enrollment has grown by 40.5 percent over the past decade and by 13.1 percent in the past five years alone.

“The current configuration has a flow capacity of 1.5 mgd with a peak of 2 mgd,” Falkner says. “It is designed to be highly energy efficient because the aerators are controlled based on dissolved oxygen levels in the mixed liquor using variable-frequency drives. This means they only inject as much oxygen and mixing as needed to maintain high-quality effluent.

“Investing in Lakeside’s robust engineering was very much a part of our commitment to the long-term well-being of the university’s wastewater treatment facility. It is tough, reliable equipment that requires only basic maintenance.”