It depends upon the amount. It takes a lot of dead biofilm to give high TOC so chances are you will not see it unless it is really bad. In order to get that amount of biofilm in the water system, it is going to take a pretty high TOC level in water as a nutrient. So you may have high TOC that is likely to come from a pretreatment system that is feeding this TOC into the water system. That is the problem with TOC and distribution of water: it could come from local sources or it could come from incoming water. It is more than likely you have some unit operation that is leeching TOC and the ionizers are notorious for that.