Clarifier Upgrade Challenged by Municipal Water Treatment Plant’s Setting
Hester Park overlooks the Mississippi River in St. Cloud, Minnesota. It is a picturesque space dotted with Works Projects Administration-era stone bridges and walking trails. Hidden underneath the grass fields and play areas of the park is the municipal water treatment plant, which was built into the hillside in the 1950s. Its size is deceptive -- only a third of the facility is visible to the park visitors. People who come to the park enjoy their activities without realizing that the municipal water treatment equipment, such as clarifiers, filtration, and chemical treatment, is situated directly beneath them.
When it was time to upgrade one of the plant’s clarifiers, the equipment choice would factor in the plant’s significant size yet challenging location, along with the plant’s influent, aging equipment, and need for energy conservation.
The plant’s unique location, said Brett Boissevain, WesTech engineer,
created unusual challenges when equipment installations were necessary. The building is deceivingly small from the outside. It is pretty extensive once you make it past the front lobby.
Factors to Consider With the Upgrade
River as a Drinking Water Source
Because the St. Cloud plant’s location is adjacent to the Mississippi River, the City conveniently pumps its water from the neighboring waterway. But compared to a groundwater influent, the river is a particular challenge as a water source.
The Mississippi is a source of drinking water for millions of people, but the waterway is known for its sediment and excess nutrients (primarily nitrogen and phosphorus from fertilizer runoff), especially in the upper section where St. Cloud is located. The size and dynamic nature of the Mississippi River Basin have made the comprehensive elimination of the sedimentation problems impossible.
Treatment plants along the waterway, like St. Cloud, that use the Mississippi as a water source have particularly challenging river water conditions to factor into their treatment plans.
The City of St. Cloud located its first water treatment plant in Hester Park in 1907 with 19 wells, a pump house, and a filtration plant. The current facility replaced that plant in 1954 and provided capacity for 9 MGD. An expansion completed in 1994 increased the capacity to 16 MGD, but these improvements took place over two decades ago. As with many water treatment facilities across the U.S., the St. Cloud plant staff provided creative solutions to make sure their aging infrastructure remained effective and dependable through the years. In this case, the plant installed three separately-designed trains of water treatment that accomplish the same goal.
The water is first pulled from the river via pumps at the intake station. Immense, 100,000-gallon clarifiers provide for solids separation of the water. After filtration, the water is then chemically treated to remove contaminants. Under regular conditions, all three trains are not in use at the same time but exist as redundant back-ups.
Taxpayers Demand Lower Energy Use
Reducing energy use at the plant is both a budgeting and public relations initiative. The St. Cloud facility was under pressure to reduce operational costs and adapt their treatment processes to become more energy efficient so as not require a tax hike for the growing cost of operations.
A facility like St. Cloud requires pumps, motors, and other equipment that run 24 hours a day, and the plant is one of the largest consumers of energy in the community. According to the EPA, wastewater and water facilities account for 3-4 percent of total energy use in the country. Energy reduction at the plant would have significant impact in city energy use.
The staff identified that there was a potential energy reduction with the clarifier in one train if the two-step process for flocculation and sedimentation was reduced to one.
Combining Steps With the Solids CONTACT CLARIFIER™
To meet the goal of reducing the two-step process for handling solids, the facility chose to improve efficiency by installing the Solids CONTACT CLARIFIER.
This new type of clarifier improved their clarification process because it repeatedly recirculates solids that have already settled, so less chemical input is needed. A conventional system (like the existing trains on site) is just once through, where settled solids are removed and sent to dewatering. Because the Solids CONTACT CLARIFIER operates with solids recirculation, this presents some unique operating requirements compared to a conventional system.
In addition, the Solids CONTACT CLARIFIER uses less space than conventional flocculation and sedimentation trains, and the staff was able to adapt the present tank space to install the new clarifier. While a solids CONTACT CLARIFIER provides both enhanced flocculation and a high rate chemical precipitator, this single unit can do even more. It implements mixing, internal solids recirculation, gentle flocculation, and gravity sedimentation, and the results are better than conventional treatment methods.
By installing the clarifier, St. Cloud was able to save significant resources – money and energy – in terms of operational costs.
While the new clarifier would help the St. Cloud plant reach its goals, the unique site made the installation challenging. Since the plant is essentially underground, fitting the equipment into the limited space was difficult. The plant is well established beneath the popular park so disturbing the area as little as possible was important.
There were two 8 x 8-foot doors for the new clarifier to pass through, so the equipment had to be taken apart and fed through each door. The round clarifier then needed to be placed in a square tank. This meant that specially-designed pieces were installed in the corners of the tank to keep the sediment from collecting there.
Maneuvering in the tight space was tough, said Boissevain.
We had to carefully plan our equipment design before we even began fabricating.
Operating the new clarifier required a shift in thinking for the operators at the plant. They had to stop thinking like they were running a standard clarifier. There were frequent calls to WesTech to make sure the equipment was running the way it should. WesTech was aware of the learning curve and was onsite frequently.
The responsiveness paid off, Boissevain said.
The team there learned to trust us when we explained that things can be different with the Solids CONTACT CLARIFIER. They worried that their drains would clog or that modifications we were suggesting were not having any affect.
Adam Bourassa, St. Cloud Water Services Manager, said:
The staff for site visits and technical support has been spectacular. The best equipment reps we dealt with for our project.
The goal of the new clarifier installation was to reduce water hardness from 188 mg/L avg to <110 mg/L (as CaCO3), with an effluent turbidity less than or equal to 5 NTU for flows up to 8 MGD.
The clarifier does surpass our treatment goals, and we know it will improve as we get more experience operating it, Bourassa said.
It’s pretty exciting, said Patrick Shea, City of St. Cloud Public Services Director.
A more efficient clarifier, creative implementation, targeted staff training, and exceptional customer support resulted in a successful upgrade of a plant clarifier to reduce energy costs, meet the treatment demands of the influent, and provide clean water to a riverside community, all while maintaining the pristine setting of the site.