Richard, if this is only a solution of food grade phosphate and potable water (possibly chlorinated?), and there is no other contamination of the solution from other sources, there should be no or negligible bacterial growth in the solution. Phosphate compounds by themselves do not support bacterial growth--in fact, some types (SAPP, etc.) are antagonistic towards growth, although not what you would call bacteriostats.
Phosphate solutions are commonly made up ahead of time in the meat and poultry industry and used this way. We prefer that they be used quickly and kept refrigerated so as to retard hydrolysis into ineffective molecular forms--not to retard bacterial growth, but keeping a solution around for one or two shifts is not unheard of. If you are seeing bacterial growth, then there is some point of contamination that must be present with a food source and organisms.
If you have further questions, you may contact me directly.
Regards,
Paul
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