Hi All,
I am a registered Biodiesel manufacturer in Minnesota. While all the comments so far in this thread are well informed, I would like to add a few things due to an amazing amount of bad information in the world. As a manufacturer and distributor I have seen 1st hand every kind of real and imagined disaster. The imagined kind seems to have the greatest longevity - go figure. I know I should not make it my mission to fix it.
1) Every fuel has bio enemies including gasoline and jet fuel.
2) Ultra low sulfur diesel introduced an entirely unforeseen bio vulnerability among other issues like cold flow and viscosity problems that did not exist before.
3) 20% Biodiesel is no less vulnerable then 99% or 100% ULSD. I am baiting the backyard fuel chemists. If you disagree....prove it!
4) Infections tend to start at the terminal and not in your tank.
5) Major fuel retailers are far less likely to be infected then smaller retailers that get their fuel from local distributors. (time in tank + air + water)
6) There are several kinds of possible infection including several types of algae, bacteria and other micro organisms.
7) The root cause of infection is water, air and time. Your fuel is the food they want but they must also have water and air to survive.
The petroleum industry is very proactive about pre-treating - but they will rarely reveal what is in their proprietary blend. Retailers generally can not answer a question.
9) Farm tanks are most vulnerable
10) I am seeing fewer "real" infections now then we did in 2008. I have responded to many claims, looked in many tanks and found many problems. Very few of them were actually biological as claimed.
Actively look for it! If you clog a filter, cut it open and scrape the junk out from the pleats.
If it is redish or green - most likely algae - red dyed fuel hides the symptoms for awhile.
If it is whitish and pasty- it is most likely water/biodiesel at all temperatures. Possibly paraffin fall out from ULSD if it has been -0 F.
If it is black or grey - it is most likely residue from your fuel lines dissolving in Biodiesel. Most common in vehicles mfg prior to 1995.
Other biologicals and/or contamination can be very complex and difficult to diagnose.
Water issues and natural rubber problems are far more likely and often mis-diagnosed as biological.
Water is the biggest problem. While it can lead to biological attack, it will cause problems before a new infection takes place. ULSD can only hold 200 ppm in solution. heavy water then will always appear as free water in the bottom of a diesel tank. Biodiesel can hold nearly 10x that of ULSD in solution or emulsion. ULSD and BD are highly miscible. Meaning they blend well and stay blended at all temperatures. However, when you add water, the BD will start absorbing it, become heavy and fall out to the bottom. It is easy to spot as even small amounts will cause the blend to go cloudy. Large amounts will result in pasty white goo on the bottom of the tank. this goo is easily caught in your average fuel filter - clogging them quickly. Strangely, diesel engines don't hate a little water. But filters do!
Biodiesel is a solvent to natural rubber and will dissolve it. Vehicles and equipment made before 1995 had allot of natural rubber in the gaskets, o-rings, filters, filler boot and injection pumps. Some of these parts will last forever and others a matter of months. But beware, the replacement for natural rubber (EDPM and High Nitrile rubber) are not 100% compatible though they are marketed as such. They will slow it down. Only Viton is thought to be 100% and it is expensive. There are now Viton kits available to rebuild many common injection pumps.
So, if you are going to store Biodiesel or a blend, let the tank settle awhile, drain the bottoms off, close any breathers and close the top tightly. You may also want to keep the tank full in order to eliminate air spaces. No water, no air exchange - even if the tank has a biological issue, it will not grow quickly. Just good tank hygiene. Before you store a tank, there is a paste you can buy. Smear a little dab on the bottom of a stick and drop it to the bottom of your tank. Withdraw the stick. If it changed color, you have water. Inexpensive prevention and maintenance. When you open the tank after storage, test it again. All tanks are vulnerable to condensation. If needed, drain the bottoms again until it runs clear. Storing for very prolonged periods (years) is never a good idea. Both BD and ULSD will start to break down eventually. Look up "Oxidative Stability". There are additives for this as well.
Last, I get way more calls these days from folks using WVO or WMO and all the variations. These things are way more complicated then we are led to believe by the people who are trying to sell us conversion kits and filtration systems.
If you ever see something you can not identify and/or fix, please contact me. It could be educational for me as well. Most things I can spot in a good photograph but if needed, I have a lab. I am particularly interested in the folks out there that are distilling plastic into fuel. I have not seen any of that yet.
Best regards - please don't send me hate mail!