A method of controlling microbial growth in a continuous flowing aqueous system comprising an aqueous transport stream flowing in a closed loop from a processing locus to a production locus using a recycle stream to return the stream to the processing locus, the transport stream transporting an agricultural product from a production locus to a processing locus, the stream having a challenge soil load, the method comprising the steps of:
- (a) maintaining a treatment flow, separate from the transport stream, comprising a fraction of the aqueous transport flow, through a closed treatment zone containing a treatment volume, the transport stream having a challenge load comprising a microbial population, an inorganic and an organic soil;
- (b) contacting the treatment flow in the closed treatment zone with an effective amount of ozone, sufficient to reduce the microbial population in the treatment flow forming a treated volume;
- and
- (c) returning the treated volume to the aqueous stream, the treated volume having an ozone concentration less than 1 part by weight of ozone per each one million parts of the aqueous stream.