Wastewater treatment plants, while good for the environment in that they treat and clean wastewater, are nonetheless energy guzzlers. The Electric Power Research Institute and Water Research Foundation released the results of a study conducted in 2013 that concluded that the plants alone consumed an estimated 30 billion kilowatt hours of electricity annually, equivalent to around 0.8 per cent of the total electricity consumed by the United States (US).
And the irony of treating wastewater is that the organic matter found in wastewater holds up to five times as much energy than what the treatment plants use, according to reports from the American Biogas Council. Bringing down the energy and carbon footprint of the treatment facilities through more efficient use of energy and using the discarded energy could lower greenhouse gas emissions.
But in spite of that energy looking as if it’s just sitting there for the taking, cutting down the demand for fossil fuels to be used to run the treatment facilities is not just challenging; it needs a multitude of approaches.
Globally, the water industry is examining and experimenting with new technologies, analysing and assessing them not just for enhancing energy, but also for cost-efficiency and inadvertent repercussions.
Facilities in Denmark and the United Kingdom are already energy-neutral. The East Bay Municipal Utility District in Oakland, California, US, has even surpassed zero energy expectations to selling energy back to the electricity grid.
Innovators are turning their attention to using a broad range of technologies to make their energy use more efficient and bring down the amount of electricity they go through while also generate electricity simultaneously to compensate for what they do use.
Developed countries frequently use four main steps to treat wastewater: There is the primary treatment, where solids are separated from liquid waste; secondary treatment, when bacteria dissipates any dissolved waste containing ammonia and other contaminants while leftover solids are removed from the treated wastewater; an anaerobic stage where the solids separated in the primary and secondary treatments are broken down by microorganisms in a tank that has been sealed without oxygen; and finally the disinfection.
Bacteria may be vital in digesting the sewage and industrial wastewater we generate by absorbing the organic contaminants and other inorganic nutrients like ammonia, but they are tough customers. Certain conditions need to be met if the microorganisms are to survive: Optimum food, oxygen, and temperature at an optimum. Just giving the bacteria the oxygen they require takes up more than half of the energy a wastewater treatment facility goes through. Thus, when plant operators look at improving energy efficiency, they usually focus their attention on lowering energy used on the bacteria.
Conventional facilities usually pump the air into the tank where the microorganisms reside and diffuse the air through miniature holes to create tiny oxygen bubbles easily accessible for the bacteria. Unfortunately, this method squanders a lot of energy as most of the little bubbles containing the oxygen will rise to the top and pop without the bacteria ever using them.
For decades, the water industry has been looking at ways to lower the amount of wasted energy. Presently, the membrane aerated biofilm reactor (MABR) is turning heads as one of the most promising ways to approach the issue.
Rather than forcing the air into the sealed tank, with the MABR, operators implant large cubes packed with porous membrane tubes. A blower then shifts air into the tubes with low pressure. The bacteria gather on the exteriors of the tubes, absorbing the oxygen that comes through while also creating an oxygen concentration cog that encourages oxygen to further spread out.
“Bacteria are actually demanding the oxygen and causing the gradient,” Executive of product management for General Electric’s Water & Process Technologies, Glenn Vicevic, said. The organisation produces a variant of the MABR technology, ZeeLung, and estimates that it can be up to be four times more energy efficient than the traditional forced aeration, as depending on the design of the facility and operation.
Catching energy from the waste present in wastewater is now the norm at the larger water facilities, as they utilise tanks called biogas digesters, or anaerobic digesters, to dissolve organic solids separated during the primary and secondary phases of treatment and transform them into carbon dioxide and methane gas.
Here, bacteria still do the work, but without oxygen. The methane that is produced can then be caught and burned in a biogas machine in order to generate electricity and heat that can either be employed in the operation of the water facility or upgraded to meet the quality of natural gas and sent through a pipeline to an electric plant.
However, only an estimated 35 per cent of US electric facilities use biogas to generate electricity, mostly because majority of the wastewater treatment plants found in the US are comparatively 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,” Manager of the Energy Efficiency Research Office at the California Energy Commission, Virginia Lew, said.
Usually, wastewater facilities that do not produce enough biogas to generate electricity simply burn them off. Although, Lew continued, adding storage areas for the biogas would give them the opportunity to store enough to make generating electricity justifiable. “I think there’s going to be greater emphasis on trying to utilise as much biogas in the future as possible to offset any purchased electricity and to reduce their carbon footprint,” Lew added.
Source: The Guardian