Methods and systems for on-site, continuous generation of peracid chemistry, namely peroxycarboxylic acids and peroxycarboxylic acid forming compositions, are disclosed. In particular, an adjustable biocide formulator or generator system is designed for on-site generation of peroxycarboxylic acids and peroxycarboxylic acid forming compositions from sugar esters. Methods of using the in situ generated peroxycarboxylic acids and peroxycarboxylic acid forming compositions are also disclosed.