From biosolids injection to cake production: A small utility’s recipe for the future

From biosolids injection to cake production: A small utility’s recipe for the future
From left, Mark O’Meara and Tim Meloveck of Carbondale Utility; Chad Paulson of SGM look down at a Huber screw press; sales representative Josh Queen of Goble Sampson; John Lewis of Huber. The press near the wall has been in operation for eight years with no significant maintenance yet.

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What does a community do when its liquid biosolids handling equipment breaks down to the point of no repair? If you are on the board of trustees of Carbondale, Colo., you and your Utility Director, Mark O’Meara, do what’s best for customers: plan ahead.

Carbondale upgraded its biosolids process, installing a rotary screw press to dewater the material to 3 percent solids. The equipment has proven easy and cost-effective to operate, and the higher-solids material costs significantly less to haul to area farms than the previous liquid product. 

Agriculture and biosolids 

Tucked into the western Rockies at the heart of the Roaring Forks River Valley, Carbondale is a pioneer town surrounded by open land that makes it a prime location for agriculture, especially potatoes. Carbondale’s crops supplied food to gold and silver miners in the late 19th century and to hotel and retail customers in the early 20th. The deepening of the Great Depression, however, saw businesses and people move out of town, and the demand for the valley’s potato crops disappeared with them.

Coal mining in the Carbondale area came and went, but by the end of the 20th century, snow-capped mountains in Aspen and pristine wilderness habitats in the 40-mile river valley below began to attract tourists, and with them a growing resident population. But instead of potatoes, cattle and sheep dot the valley’s agri-scape, with hay as a major feed crop.

Like many agricultural communities in the early 1980s, Carbondale saw a golden opportunity to use processed biosolids from the local wastewater treatment plant to fertilize farmers’ fields. Enter biosolids injection.

For decades at the 1 mgd Carbondale Wastewater Treatment Plant, waste activated sludge (WAS) went to one of five 100,000-gallon tanks for aerobic digestion and aging. Then the biosolids product was pumped into a tank truck that made repeated runs out to a local farmer’s field. There it filled the tank of the utility’s Ag-Gator applicator (AGCO/Ag-Chem), referred to as the ‘Big A,’ which plowed the 3-percent-solids material directly into the ground. Unfortunately, fuel prices have soared since the early 1980s, pricing the injection out of the market.

Over the last several years, a change in the method of the biosolids reuse has been a key item on Carbondale board meeting agendas. The board considered upgrading or adding cutting-edge technology, such as membrane bioreactors (MBRs) to the process, but that could mean starting over from scratch. Instead, the members chose to make renovations while repurposing some existing operations and infrastructure.

“There’s some neat stuff there and you don’t have to build a new plant,” says O’Meara, a Colorado State University alum with over 30 years of experience in water and wastewater operations along the Roaring Forks River Valley.

The best laid plans … 

Carbondale dedicated a team member to keep the equipment running until the board decided which new biosolids equipment to buy. In the meantime, the city took advantage of grant money to attend to some deferred maintenance, changing out a diffuser, some blower motor pumps, and other items. “That knocked our electric bill down nearly 30 percent,” O’Meara says.

Carbondale Utility staff and the board knew that many utilities were using either a centrifuge or a press to dewater biosolids and turn it into cake. But, like many communities with fewer than 10,000 customers, they thought those options might demand more hands-on equipment operation, flow monitoring, and cake management than was economical.

Little was known about screw presses, but the trustees encouraged Utility Department staff and Project Manager Cooper Best of SGM, an infrastructure design-engineering firm in Colorado, to explore the option.

The old biosolids truck made its last lap in the fall of 2012. The utility hired contractors as an interim solution to haul material to the Ag-Gator. But, despite the care given to keep the Big A running, it suffered terminal engine failure in late February. So, Carbondale representatives made trips to Salt Lake City and Denver Metro plants to contrast and compare biosolids cake processing options.

“The site visits and open forum discussions with the operations staff and manufacturer representatives at these facilities were extremely helpful in our decision process,” says O’Meara. So helpful that, when staff members presented their findings at the February 2013 board meeting, they were confident in their choice of the Huber model ROS3Q-440 screw press.

The screw press feeds biosolids into a horizontal cylindrical screen where a low-speed screw turns, pressing liquids through the screen and pushing cake out the end. A key feature of the unit is that it can be programmed to press a load of biosolids overnight without operator intervention. In the long run, Best says, “The new system will need much less staff time, maintenance and manpower.”

The bigger picture 

The screw press has even more potential. Carbondale does not own the acreage on which the biosolids are being applied. Should the day come that the land is no longer available, the utility could take the cake directly to a landfill — an option not available with liquid biosolids.

The press also has a compact footprint, enabling the utility to repurpose an old building from the original plant to serve as the new press site.

“We’ve actually taken that building and drawn it in 3-D … and then put a virtual Huber press in the room and it fits,” says Best. That revelation apparently was compelling enough for one trustee to recommend putting in a pre-purchase order for the equipment, so that there would be no lag time between the final approved engineering plan and the press installation. 

“This is going to be a workhorse for the plant,” O’Meara says. He is excited that using the press to process biosolids will free up time for his staff so they can focus on other issues, such as mitigating inflow and infiltration in the collections system. “My goal is to be able to run the entire system, to get better efficiency out of the staff we have and fine-tune the maintenance program, getting more done,” he says. 


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