I tried to reduce the sentences myself though I think you still need to revise it. Thank you very much.
1) We decided to create an effective and inexpensive filter using knowledge of genetic engineering.
2) In particular, we prepared (have prepared)a genetically modified bacteria, the ..., which can survive using arsenic instead of phosphorous in building its DNA.
3) Actually, it contains a recombined plasmid in which we insert (How would you vary tenses?) the necessary sequences to interrupt the pathogenic action of the ... itself and of the main bacteria present in the water.
4) Using plasmid is the most convenient way to transfer very quickly, and without any additional expenditure, the factor we want all the bacteria to have in order to mute their pathogenicity. 5) Moreover, this particular bacteria (?) can survive in presence of arsenic and, therefore, won't die until its action of purification is completed.
1) OK
2) In particular, we prepared a genetically modified bacteria, the ..., which can survive using arsenic instead of phosphorous in building its DNA.
3) Actually, it contains a recombined plasmid in which we inserted the necessary sequences to interrupt the pathogenic action of the ... itself and of the main bacteria present in the water.
4) OK
5) Moreover, this particular type of (?) bacteria can survive in the presence of arsenic and, therefore, won't die until its action of purification is completed.