SubCom® Case Study: Monticello Sludge Pasteurization

Location: Monticello, Minnesota

Year Completed: 1999

Deliverables: 6 MM Btu/h municipal sludge pasteurization system

Wastewater Sludge Heating & Pasteurization Process

Given the environmental legislation passed by the US Congress, there is pressure on the US wastewater treatment facilities to address the problem of wastewater sludge or discharge of potentially harmful pathogens present in the treated municipal liquid waste back into the environment.

The traditional digestion methods do not produce temperatures high enough to destroy the pathogenic bacteria present in the sludge to non-detectable levels through the process of sludge pasteurization. Higher temperatures are required for effective pasteurization to produce Class A biosolids. The US EPA Class A designation to material permits their use as fertilizer on farms, and vegetable gardens and sold as compost. Consequently, all new wastewater treatment facilities or modernization projects have to look for new technological solutions to address this requirement. The traditional method of dealing with bacteria is to heat the wastewater sludge in a heat exchanger using a steam or hot water boiler. Upon reaching the temperature of 158° F (70°C) the wastewater sludge is heated and held for a minimum period of 30 minutes at this temperature to achieve full sludge pasteurization during the sludge heater exchange process.

The City of Monticello, Minnesota undertook a modernization project of the Monticello Wastewater Treatment Plant that serves the local town and two food processing plants.

HDR Engineering’s Minneapolis office was retained as project consultant. Part of the plant modernization included a sludge pasteurization system. Inproheat was involved in the development of the process design concept using proprietary submerged combustion (SubCom®) technology.

Subsequently, after a bidding process, Inproheat was awarded a contract by the project’s prime contractor Adolfson & Peterson of Minneapolis to supply a sludge pasteurization system for the Monticello WWTP.

Cold activated sludge enters the Inproheat system at 50° C and passes through a spiral sludge-to-sludge heat exchanger with an outlet temperature of 115° F.

The sludge then enters the submerged combustion heating-unit where it is heated to 158o F and passed to the internal flow-through tank retention compartment. Progressing cavity pumps with variable frequency control are used to maintain the level in the retention compartment and pump the sludge out of the tank, through the spiral heat exchanger and to the anaerobic digesters at 100° F.

The system is also equipped with a direct contact heat recovery unit (HRU) which maintains the exhaust temperature at 100° F to 110° F.

The products of combustion are collected by the bio-filter odor control system making this installation the first closed loop SubCom®. The system not only preheats the sludge eliminating the need for boiler heating of the digesters but also contributes both evaporated moisture and heat from the products of combustion to the bio-filter, which would otherwise have to be also heated by a boiler.

The system operates with digester biogas as fuel and alternately with natural gas at a continuous sludge flow of 65 USgpm. Due to the closed loop nature of this installation, thermal efficiency of this installation is 100%.

The process is designed to operate 7 hours per day, however the equipment could be used for continuous round-the-clock operation.

An HMI system with a color touch screen user interface will provide an automatic system fill, burner start-up and pre-heat cycle (approximately 30 minutes), level, and temperature control and retention time monitoring.

At shutdown, the system performs an automatic back flush of the heat exchanger, pumps and the submerged combustion chamber. The system is connected via the DH+ fiber optic data highway with other plant controllers.

Inproheat’s design complies with NFPA, CGA, UL, and CSA requirements. Inproheat owns several US and Canadian patents related to the above technology system configuration and concept.

The Monticello system was commissioned in the spring of 1999 and by the end of the year processed 1,800,000 gallons of sludge. Due to its “first-of-a-kind” nature certain operational challenges were expected. These technical uncertainties were mostly related to the thermal performance of the submerged combustion process with 3 to 4% sludge and potential for system plugging. One year of operational experience proved these concerns to be unfounded. The new patented self-cooling and non-plugging combustion system design used for this application provides exceptionally reliable performance both with natural gas and biogas firing.

The problems encountered to-date include: lower than expected plant load, fuel gas and vent system pressure fluctuations, inlet sludge stratification and sludge short-circuiting through plug valves and detention tank. Inproheat, HDR and City of Monticello are working diligently to correct these last deficiencies.

In general, it has been proven that the SubCom® pasteurization system can work reliably in producing Class A biosolids sludge in a wastewater treatment plant scenario.

Submerged combustion is an unparalleled, highly energy efficient method of heating large quantities of liquids and slurries by forcing direct contact of combustion products directly through the liquid.