Team:Nanjing-China/Design
Design
Pi removal
The widespread use of chemical precipitation for phosphorus removal in wastewater treatment started in Switzerland during the 1950s, in response to the growing problem of eutrophication. Chemical precipitation is in essence a physico–chemical process, comprising the addition of a divalent or trivalent metal salt to wastewater, causing precipitation of an insoluble metal phosphate which is settled out by sedimentation. The most suitable metals are iron and aluminium, added as chlorides or sulphates.
Another commonly used method of phosphorus removal in industry is the biological phosphorus removal, which under certain conditions, activated sludge could take up phosphorus in considerable excess to that required for normal biomass growth. Biological phosphorus removal is achieved in the activated sludge process by introducing an anaerobic and/or anoxic zone ahead of an aerobic stage. In this zone, sufficient readily degradable chemical oxygen demand (COD) must be available, typically by the addition of acetic acid or sodium acetate.
By contrast, the biological phosphorus removal has the advantage of avoiding the abuse of chemicals in chemical precipitation. However, it requires more complex procedures and supporting facilities, not to mention that the sludge from it will finally be dehydrated and compressed, treated as solid waste, when its activity decreases. There is no way to reutilize the phosphorus in it.
To solve so many problems above, we focused on two related genes, ppk1 and ppx, and made some modification for our project. Naturally, polyphosphate kinase (PPK1) can reversibly catalyzes the transfer of terminal Pi from ATP to a growing polyP chain, while gene ppx can express exopolyphosphatase, catalyzing the opposite reaction. It is worth mentioning that ppk1 gene and ppx gene have copies in common bacteria, such as Escherichia coli, Citrobacter freundii and Pseudomonas putida, which is essential to our project.
We overexpressed of ppk1 gene using medium-copy plasmid, thus improving the bacteria’s ability of removing phosphorus in the aerobic conditions, overcoming difficulty of incubating anaerobic sludge that most factories and sewage treatment plants encountered with. As a result, we improved the efficiency and reduced the cost. After the transient expression of plasmid, the ppx gene that the bacteria naturally have would express exopolyphosphatase and release the phosphorus we collected. If we can shrink the releasing system to a small scale, we may achieve the goal of accumulating phosphorus from the sewage. Combined with a chemical method of removing phosphorus, which is existent in the nature--MAP precipitation, we can transform the phosphorus into MAP(Magnesium ammonium phosphate hexahydrate, MgNH4PO4·6H2O), a kind of fertilize, which can function in agriculture. Via these, we make the phosphorus recyclabe and reusable.
N removal
Since wastewater discharges containing nitrogen can be toxic to aquatic life, cause oxygen depletion and eutrophication in receiving water, reducing nitrogen levels from the discharges is necessary. Nitrogen compounds can be removed from wastewater by a variety of physicochemical and biological processes. Because biological nitrogen removal is more effective and relatively inexpensive, it has been widely adopted. In the process of sewage treatment, if there is a certain amount of PO43-, a certain pH and other mixed ion, MAP precipitation can happen spontaneously. In our project, we can add wastewater with high concentration of ammonia, adjust pH and Mg ion content properly after accumulation of phosphorus by bacteria, so, via MAP precipitation, we can remove phosphorus and nitrogen at the same time.