Countercurrent stacked floc blanket reactor
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Fall 2016
The presence of contaminants such as arsenic and fluoride in drinking water is a major health concern for rural communities around the world. Some of the most severe cases are in South-Asian countries such as India and Bangladesh, several regions in South America, and Africa. Arsenic and fluoride are natural occurring elements widely distributed throughout the environment and are highly toxic in their inorganic form. People in these parts of the world are exposed to elevated levels of inorganic arsenic through contaminated groundwater, which is often used for drinking, food preparation, and irrigation. Long-term exposure to these inorganics can lead to chronic poisoning, severe cases of which may lead to organ failure and internal hemorrhaging.
A previous research team was able to successfully remove arsenic from contaminated synthetic groundwater by running the water through a sand filter in which the sand was coated in a coagulant, specifically polyaluminum chloride (PACl). This process, however, required frequent filter backwashing due to clogging seen through large pressure differentials. Despite being able to successfully remove arsenic from contaminated groundwater, the process was deemed inefficient due to its excessive consumption of treated water.
A potential solution to this problem of water wastage is to utilize a facet of AguaClara’s sedimentation tanks--the floc blanket. The floc blanket is a consistently suspended and highly concentrated collection of flocs in sedimentation tanks. The Countercurrent Stacked Floc Blanket Reactor (CSFBR) team is researching to see if heavy metals such as arsenic and fluoride can be removed from contaminated waters by running contaminated water through multiple floc blankets, which have been previously loaded with the coagulant PACl.
The goal of the countercurrent stacked floc blanket reactor invention is to develop a novel reactor that efficiently removes fluoride and arsenic while reducing the volume of wastewater produced by producing a concentrated solid waste stream.
Filters loaded with PACl precipitate are efficient at removing arsenic (and presumably fluoride) in part because they are plug flow reactors and thus upstream PACl precipitate is in equilibrium with influent contaminant concentration and downstream PACl precipitate is in equilibrium with effluent contaminant concentration. This allows efficient use of the PACl because most of the PACl ends up being in equilibrium with influent contaminant levels and thus the maximum mass of contaminant is absorbed per mass of PACl precipitate
Floc blankets have the potential to hold a large mass of PACl precipitate without generating high head loss and floc blankets can concentrate flocs in a floc hopper and thus eliminate the need for the wasteful process of backwashing. The disadvantage of floc blankets is that the fluidized bed is completely mixed over the residence time of the flocs. Thus flocs at the top of the floc blanket would include flocs that are in equilibrium with influent contaminant levels and thus the removal efficiency will be poor. To improve performance we propose to have countercurrent flow through 3 floc blankets with PACl injection and flocculation occurring at the influent to the 3rd floc blanket and the wasted flocs from the 3rd floc blanket being injected through a venturi (or other means) into the 2nd floc blanket. Similarly the wasted flocs from the 2nd floc blanket will be injected through a venturi into the first floc blanket. The wasted flocs from the first floc blanket will be in equilibrium with the influent contaminant and will be the waste stream from the treatment process.
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