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Ngày Đăng : 14/12/2016 - 11:22 AM
Wastewater treatment in developed countries often involves four main steps: primary treatment, which separates solids from liquid waste; secondary treatment, in which bacteria break down dissolved waste that contains ammonia and other pollutants and remaining solids are separated from the treated liquid; an anaerobic step, in which solids from the primary and secondary steps are digested by microorganisms in a sealed tank without oxygen; and, finally, a disinfection phase.
Bacteria play a key role in breaking down our sewage and industrial wastewater by consuming organic pollutants and inorganic nutrients such as ammonia. But keeping them happy isn’t easy. They require particular conditions to thrive: optimal temperature, food and oxygen. Getting bacteria the oxygen they need accounts for one-quarter to more than half of the energy used by a wastewater treatment plant. So plant operators are focused on reducing energy use in this step to boost energy efficiency.
Traditional plants pump air into the tank where the bacteria do their work, diffusing it through small holes to create little oxygen bubbles the bacteria can easily access. This process wastes a lot of energy because most of the bubbles rise to the top and pop without the bacteria using them.
The industry has been trying to reduce this energy drain for decades. One of the most promising approaches is something called a membrane aerated biofilm reactor, or MABR.
Instead of forcing air into the bacteria tank, operators insert massive cubes full of porous membrane tubes. A blower moves low-pressure air into the tubes. Bacteria congregate on the outside of the tubes, sucking up the oxygen that passes through them and creating an oxygen concentration differential that helps more oxygen diffuse.
“Bacteria are actually demanding the oxygen and causing the gradient,” says Glenn Vicevic, executive of product management for General Electric’s Water & Process Technologies, which makes a version of the technology, ZeeLung, that MWRD tested at its O’Brien Water Reclamation Plant in Skokie, Illinois. GE estimates that, depending upon existing plant design and operation, ZeeLung is four times more energy efficient than conventional forced draft aeration.
Boost From Biogas
Capturing energy from waste in wastewater is now standard practice at large plants, which use tanks called anaerobic digesters or biogas digesters to break organic solids from the primary and secondary steps into methane gas and carbon dioxide. Bacteria do the work again, but this time without oxygen. The resulting methane can be captured and burned in a biogas engine to produce electricity and heat that can be used in the plant’s operation. Or it can be upgraded to natural gas quality and put into a pipeline.
Only 35 percent of U.S. plants generate electricity from biogas, in part because most wastewater treatment facilities in the United States are relatively small. “In order to be cost effective, you have to be a fairly large facility, at least 5 million gallons a day of wastewater treatment,” says Virginia Lew, manager of the Energy Efficiency Research Office at the California Energy Commission.
Plants that do not produce enough gas to bother generating electricity often burn it off instead. Adding battery storage could allow them to store enough energy to make using it worthwhile, Lew says. “I think there’s going to be greater emphasis on trying to utilize as much biogas in the future as possible to offset any purchased electricity and to reduce their carbon footprint.”
Another way MRWD is reducing its energy use is with a bacterium called anammox. Ammonia, a pollutant found in wastewater, has been challenging to neutralize in wastewater treatment. It is normally broken down by bacteria during the aeration process in secondary treatment but requires more energy to deal with it than any other pollutant. “Ammonia is your highest air demand in your aeration process,” says MWRD executive director David St. Pierre.