Nevada Bacteria/brown sludge in the Ethylene Glycol. Servicing steps?

Hello,

We have water /water geosystem in Reno, NV that has heavy bacteria contamination in the geo loop. The HVAC contractor used a 7% Ethylene Glycol product without biocide, it's been running for a few years now. There's brown sludge in the Glycol tank. Will adding biocide solve the problem with a simple flush out or will we have to purge and clean the whole system? I'm assuming the various components will be affected, pumps, exchangers, etc... I'm just trying to assess the scope of work and where to focus our efforts. We're gathering the data logs to see if the system performance has dropped over the years. Any servicing documentation that you guys could recommend on this sort of problem would be much appreciated. Thanks.