Wastewater samples are concentrated on a dialysis filter and captured in an elution solution (EPA method 1642). Elution solution has 0.1 g hexametaphosphate, 0.01 ml Tween 80, and 0.001 ml antifoam/liter. It is hard to get the plates to solidify, and when they do, there is much more liquid in the plates than in the spikes I prepare. My phage spikes, run at the same time to monitor precision and recovery, are not filtered, and they always set up perfectly.

This method is well established, and everything I have done to enumerate the phage concentrations and to determine initial precision and recovery, using first the double agar layer method and second the single agar layer method, has worked beautifully. What am I doing wrong?