Bacteria may be any color, but is usually black, green or brown. Draining the system will reduce microbial activity, but will not eliminate it. The only way to eliminate microbial growth once it has started is to clean and treat the system with a biocide. Wax crystals form as a result of cold temperature precipitation of paraffin. Temperatures below a fuel's cloud point will result in wax precipitation and filter plugging.
Fuel suppliers blend diesel fuel based on local anticipated cold weather conditions.
Particular attention should be given to diesel fuel purchased outside your local area. For example, fuel purchased in the West or South may not be suitable for operating conditions in the Midwest or North. These black, tarry asphaltines are hard and brittle, and are made up of long molecules. Fuel with a high percentage of asphaltines will drastically shorten the life of a fuel filter. Most sediment can be removed by settling or filtration.