Class "A" Biosolids Municipal Sludge Pasteurization System For Monticello Waste Water Treatment Plant In The City Of Monticello, Minnesota

Class "A" Biosolids Municipal Sludge Pasteurization System
For Monticello Waste Water Treatment Plant In
The City Of Monticello, Minnesota


In view of environmental legislation passed a few years ago by the US Congress, there is pressure on US wastewater treatment facilities to address the problem of discharging harmful pathogens contained in treated municipal liquid waste into the environment. The traditional digestion methods do not produce temperatures high enough to destroy the pathogenic bacteria present in the sludge. Higher temperatures are required for effective pasteurization to produce the Class "A" biosolids. 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 the bacteria is to heat the sludge in a heat exchanger using a traditional steam or hot water boiler. Upon reaching the temperature of 158º F (70º C) sludge is held for a minimum period of 30 minutes to achieve full pasteurization.

The City of Monticello, Minnesota undertook a modernization project of the Monticello Waste Water Treatment Plant, which serves the needs of the local town and two food processing plants. The HDR Engineering Minneapolis office was retained as the engineering contractor for the overall project. Part of the plant modernization included a sludge pasteurization system.

Cold activated sludge enters Inproheat system at 50º F and passes through an Alfa Laval spiral sludge-to-sludge heat exchanger with a cold-side outlet temperature of 115º F. The sludge then enters the SubCom™ heating unit where it is heated to 158º F and passed to the internal flow-through tank retention compartment. Moyno progressive cavity pumps with variable frequency drive speed control are used to maintain level in the retention compartment and pump the sludge through the hot side of the Alfa Laval spiral heat exchanger and to the anaerobic digesters. The system will operate 7 hours per day with continuous sludge flow of 65 USGPM. An Allen Bradley SLC5/04 based system with PanelView 900 color touch screen user interface provides an automated control operating system including system fill, burner startup and preheat cycle (approx. 30 minutes), level and temperature controls, 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 also equipped with a direct contact heat recovery unit, providing an overall thermal efficiency between 94 and 96% based on the higher heating value of the fuel. The system will operate with 500 BTU/ft3 digester gas or alternately with natural gas. The system is connected via the DH+ fibre optic data highway with other plant controllers.

Inproheat's design is based on US made components, and complies with NFPA, CGA, UL, and CSA requirements. Currently, Inproheat owns three US patents and has applied for four new patents related to the above technology system configuration and concept.

The Monticello system is scheduled to arrive on site on 21/22 October with possible startup on natural gas in December / January and switch over to digester gas in March / April 1998.