Biosolids created at municipal wastewater treatment plants can generate odors, which can cause complaints among operators and the community. Belt filter presses, a technology long used to dewater or reduce the volume of these biosolids, have historically been challenged to contain these odors. That’s why facilities and press makers have taken steps to address odor issues. 

A belt filter press works like an old-fashioned wringer washer. Sludge comes out of the holding tank into the top of the press where operators add a flocculating polymer to help separate the solids from the water. They pump the slurry onto gravity belts, called that because water flows down through the belt by gravity, leaving the solids on top. Then the solids drop onto two pressure belts that carry them through a series of rollers, which squeeze out more and more water. When the sludge reaches the end, a bladed scraper peels off the dewatered sludge onto conveyor belts that take it outside for disposal. 

According to the U.S. EPA, odor complaints at wastewater treatment plants and biosolids end-use sites can interfere with implementation of the most cost-effective biosolids management options. As a result, the EPA advises that odor-control measures be included when designing dewatering facilities — advice many plants have taken to heart. 

“There are always odor issues within the room that houses the belt filter press because of the hydrogen sulfide that the solids emit,” explains Brian Lavallee, a veteran wastewater treatment operator for the West Warwick (R.I.) Sewer Authority. “We’ve used all different kinds of things to get rid of the odors. Right now, we’re using a 31 percent solution of sodium chlorite; we also use potassium permanganate off and on to reduce the odors.” 

The West Warwick wastewater plant runs its two open belt filter presses every day, processing 60,000 to 70,000 gallons of biosolids per of shift, two shifts per day. After processing the biosolids, the plant makes it available throughout Rhode Island for land application. In fact, the facility uses two 20-yard dump trucks to haul it to farmers or nurseries or gardeners who apply it as a soil additive to grow hay, or grass or even giant pumpkins. 

Lavallee says that because of a major flood in 2010 that destroyed its scrubbers, the 10 mgd facility has no scrubbers in the room to reduce the odors. As biosolids are deposited onto the press to be dewatered, chemicals are pumped into it, helping to reduce, but not completely eliminate the odors — an issue common to open belt filter presses. 

Keith Williams, national municipal sales manager for Alfa Laval, a manufacturer of open and enclosed belt filter presses, says odor-control solutions will vary with treatment process equipment. “Depending on the type of technology you’re using, you may be able to contain the odor within the dewatering equipment, potentially reducing the odor-handling system size and cost,” he says. “If odor is an issue, you may need to put in an odor-controlling system to treat the air in the room. If you have an enclosed-type device, you can treat the area of air volume within the piece of equipment by putting a vacuum on it to extract the fumes.” 

Williams says the demand for enclosed belt filter presses typically isn’t as large because operators prefer open presses. Why? Because they can see what’s going on, spot problems and easily make adjustments. In an enclosed device, such as an enclosed belt press, centrifuge or screw press, there is a perception that it’s much more difficult for the operator to see how the equipment is functioning from a process efficiency standpoint and adjust accordingly. 

“With an open belt filter press, you can add chemicals to the sludge to reduce the odor,” Williams says. “Or you can put in an odor-handling system, such as mounting a large hood directly above the top of the press in what’s called the gravity zone and draw a vacuum on it, like the hood above your kitchen stove. It’s up to each water or wastewater plant to determine what works best — and is the most cost-effective solution — for them.”