The District of Columbia Water and Sewer Authority's wastewater treatment plant is going green in a big way. The authority, known as DC Water, is in the early stages of projects that will help meet stringent regulatory requirements for nitrogen as well as make improvements primarily aimed at improving water quality in the Chesapeake Bay. One of the projects is to reduce total nitrogen in the plant system to under 4 milligrams per liter (mg/l) from current levels of about 5 mg/l. The second approach will build a system of tunnels to accommodate and store peak water flows so the plant can fully treat more of the water that currently overflows the plant's capacity during heavy storms.
However, it is the third leg of these project that is leading innovation in water treatment. The Cambi thermal hydrolysis process will be used to "cook" sludge material under high pressure and steam to generate a better class of biosolids as well as about 13 MW of power at the plant. Blue Plains will be the first plant in North America to use the process, according to DC Water. Although a proven technology in Europe, it has never been used in the U.S.
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. When complete, the system will be the first thermal hydrolysis installation in North America and the largest in the world, according to DC Water.
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.
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