The customer operated two 10,000 gallon central coolant systems - one with soluble oil coolant and one with semi-synthetic coolant. In the central tank, tramp oil was removed and the coolant was side treamed filtered through an indexing cloth filter. Filter bags at each machine provided additional filtration of the coolant. Flow rate was 4,200 gpm. The customer was using large quantities of biocides (bactericides + fungicides) and yet the microbes were frequently out of control. Fungus was evident in many places particularly on bag filters, hydraulic hoses and in the central trenches and individual machine sumps. The situation was most severe with the soluble oil.