Sludge Treatment Made Easy
Lakeview WWTP reduces odor and maintenance costs by switching from incineration of heat-treated sludge to incineration of untreated sludgeThe Lakeview Wastewater Treatment Plant (WWTP) is located on the shore of Lake Ontario in Mississauga, just west of metropolitan Toronto. Owned by the Region of Peel and operated by the Ontario Clean Water Agency, the plant processes the wastewater from homes and businesses in the towns of Mississauga, Caledon and Brampton. It has a average annual design capacity of 118 million gal per day (mgd).
During the last few decades, the Lakeview WWTP treated sludge generated at the plant as well as imported sludge from the nearby Clarkson plant, which was trucked in liquid form to Lakeview in more than 30 trucks per day. The sludge was heat-treated by Zimmerman Process (Zimpro), decanted, dewatered by up to 38% solid content and burned in three cold-wind-box (CWB) fluidized beds, two of which were installed in 1980 and the third in 1994.
According to William Fernandes, manager of water and wastewater treatment for the Region of Peel, the region decided in 2003 to replace the three existing CWB fluid beds with four Thermylis Hot-Wind-Box (HWB) fluid beds from Degremont Technologies/Infilco.
Frequent community complaints about odors, high maintenance costs generated by the Zimpro and concerns about risk of raw sludge spills, odor and other air pollutant emissions during trucking from the Clarkson facility were the main factors for the replacement.
To meet all needs, the region and the project’s consulting engineers decided to phase out the Zimpro system and replace it with four HWB systems, each disposing 110 tons of dry sludge per day. The first of the four HWB systems became operational in February 2006.
According to Dr. Ky Dangtran, technical manager of thermal process for Degremont, the new system is equipped with an external shell and tube heat exchanger for preheating the combustion air to 1,230°F, thus reducing the auxiliary fuel consumption. It also has a venturi scrubber as an air pollution control device.
Since February 2006, the new HWB system has been running 24 hours per day with very little requirements for auxiliary fuel or makeup sand, according to Fernandes. Following the performance testing, it satisfied the design emission criteria.
Why fluid bed incineration?
Severe sludge standards and lack of available land have made fluid bed incineration widely acceptable for sludge disposal.
In fluid bed incinerators, water is evaporated and organic materials combusted, according to Degremont Technology/Infilco. In that way, odors are eliminated and sludge is reduced to smaller quantities of inert ash (as low as 7% by weight), resulting in reduced land requirements and air pollution.
The Thermylis high temperature fluid bed (HTFB) incinerator consists of three basic zones: a windbox, a sand bed and a freeboard. The term “fluid bed” refers to the strong boiling action of the sand that happens when air is blown through from below. The windbox is refractory lined and equipped with a refractory arch distributor. The refractory arch distributor is equipped with special alloy tuyeres, which ensure even distribution of the air throughout the sand bed. To profit from the turbulent mixing, dewatering sludge and auxiliary fuel (if necessary) are directly introduced into the bed, where they are instantly combusted at above 1,250°F.
In the next stage, combustion gas and evaporated water flow move upward into the voluminous teardrop-shaped freeboard, where the bed material is disengaged. Operating at 1,550°F, the freeboard serves as a gas polisher. It offers a minimum of 6.5-second gas residence time, more than adequate to complete the combustion. The three T’s—turbulence, time and temperature—make the fluid bed incineration the most economical and environmentally sound process to dispose of sludge, according to Degremont Technologies/Infilco.
Because of the concerns regarding the trucking of imported sludge to the Lakeview plant for incineration, engineers and managers decided that the Clarkson WWTP sludge would be dewatered prior to shipping, resulting in a significant decrease in truck traffic in the vicinity—from 30 to 35 trucks per day during current sludge production down to three to four trucks per day, and from 50 to 60 trucks per day during ultimate sludge production down to six to eight trucks per day. This decrease reduced the risk of raw sludge spills, odor and emission of other air pollutants.
The replacement of the Zimpro process with four Thermylis HWB fluid beds resulted in less odor, improved air quality and lower sludge disposal costs.
In addition, the HWB resulted in a reduced demand for auxiliary fuel. According to Dangtran and Fernandes, the new HWB has used little fuel since its startup. When burning the untreated sludge at 27% total suspended solids (TSS), the HWB can save up to 41 MM btu/hr in auxiliary fuel in comparison to the CWB. With all four fluid beds expected to be operational by mid-2009, the Lakeview WWTP will be the largest sludge incineration plant in the world.
“Of extraordinary importance to the success of this project was the spirit of collaboration that was established between the four main players—Infilco, consulting and design engineers Black & Veatch and general contractors Kenaidan and OCWA, who worked from the very first meeting to identify challenges and worked as a team to establish open communication,” said Steve Hailey, project manager at Degremont Technologies.