Biosolids Program Produces Energy
While the tunnel and enhanced nitrogen-removal project are being driven by stringent new regulatory requirements, the biosolids upgrade is being driven by a very different factor: sheer economics. The biosolids program will actually make money for DC Water by reducing the cost of the authority's electricity consumption and cutting down the cost of shipping and transporting biosolids out of the plant, says Perry Schafer, vice president of Walnut Creek, Calif.-based Brown and Caldwell, the biosolids program manager.
Currently, DC Water uses lime stabilization to convert wastewater sludge into a class-B biosolid. The new program will replace lime stabilization with a process called thermal hydrolysis and anaerobic digestion. In so doing, Blue Plains will generate a better class of biosolids—that is, class-A biosolids—which will be more widely usable by homeowners and farmers. But those consumers will use less of it: DC Water expects to reduce the amount of biosolids generated by 50%.
DC Water, Brown and Caldwell, and others spent a lot of time studying the thermal hydrolysis system created by Cambi, Norway, to ensure it would work at Blue Plains, Schafer says. Although a proven technology in Europe, it has never been used in the U.S. When complete, the system will be the first thermal hydrolysis installation in North America and the largest in the world, according to DC Water.
The $209-million design-build biosolids project will include four anaerobic digesters, each with a 3.8- million gallon capacity; four Cambi treatment trains and a pre-dewatering centrifuge building.
CDM is the design engineer on the project, and the CDM Constructors Inc./PC Construction Co. joint venture will build the main process train and its supporting infrastructure.
The centerpiece of the system is the Cambi thermal hydrolysis process, which “cooks” the biomass with steam at a high temperature (165° C) and pressure (90 psi). The biomass is propelled through a series of tanks at a high speed, which essentially “explodes the cells and makes them more easily digestible,” and creates more biogas, Schwartz says.
Thus, fewer and smaller digesters can be used, reducing the total cost of the project.
Bids for a second, $70-million contract to design and build a combined heat-and-power facility that will capture the biogas generated by the Cambi process currently are being reviewed. DC Water hopes to make a decision by early November.
The combined heat and power system will take the biogas generated by the Cambi system, convert it to steam, and feed the steam back into the Cambi train, in a sort of continuous feedback loop. The system will generate approximately 13 mw of electricity, the equivalent of approximately $10 million savings annually.