Clogged fuel filters can be caused by bacteria forming in either underground fuel tanks or in the cars fuel tanks.

If & when a fuel filter blocks prematurely i always cut them open and inspect for a black film. If its has a black slimy film in the tank then its suffering with a bacteria which must be treated.

Bacteria in diesel is a well known problem to anyone who works with diesel engines, so what is this bug and why does it contaminate diesel?
Diesel is an organic fuel so it provides an ideal environment for microscopic fungi, yeast and bacteria to feed and grow.

As many as twenty seven (27) varieties of bacteria are responsible for the majority of problems with diesel engines and their performance. There are many differing types of bacteria which can infect systems and form bio-films on steel surfaces. Accelerated corrosion can also occur wherever the bio-film settles, usually in pits or crevices. Unlike general corrosion, it is an attack on a very specific area.

It is very difficult to determine when a system is first contaminated, but once contaminated diesel enters the fuel system, it is very difficult to eradicate.

Diesel bug can originate from the air or moisture, or during tank filling and/or expansion and contraction of storage tanks, the bacteria cover themselves in a protective film (slime) to protect against biocides and can lie dormant in the minute crevices of the metal, rubber and polyurethane coatings of the fuel tanks and fuel systems.

Then, when water is present (a droplet is a lake to a microbe) and the environment hits the right temperature range, they begin reproduction in the area of fuel/water interface.

Microscopic in size, they can develop into a mat easily visible to the naked eye very rapidly. A single cell, weighing only one millionth of a gram can grow to a biomass of 10 kilograms in just 12 hours, resulting in a biomass several centimetres thick across the fuel/water interface.

Each species has its own characteristics:

BACTERIA
Bacteria utilise hydrocarbons and reproduce asexually by binary fission; swelling in size as they feed, they then separate into two cells. In this way, microbes double their numbers every 20 minutes, one spore converting to 262,144 in 6 hours.

SULPHATE REDUCING BACTERIA (SRB)
SRB's are a specific group of bacteria utilising simple carbon, not hydrocarbons, and require the activity of other microbes in a consortium. Aerobic (in the presence of oxygen) or anaerobic (without oxygen) bacteria have a combined effect. The aerobic bacteria (sulphate oxidising) create a film to consume the oxygen first. This allows the anaerobic (sulphate reducing) bacteria to thrive.

SRB's reduce sulphates and produce hydrogen sulphide (a lethal gas). They are directly involved with many microbial corrosion reactions and can cause sulphide souring of stored distillate products. Their action changes the Ph creating an acidic environment, conducive to accelerated corrosion. They attach themselves to the steel as a film and go to work. They derive their nutrition from the surrounding environment and multiply. They are particularly difficult to deal with and produce a sludgy by-product with a strong sulphur odour similar to rotten eggs (hydrogen sulphide).

IRON REDUCING BACTERIA
These also contribute to corrosion, eating steel and reducing ferrite to an oxide through a chemical reaction.

YEASTS
Yeasts prefer acidic environments, such as produced by SRB's. They bud on the parent cell, eventually separating. Reproduction takes several hours.

FUNGUS
Fungi grow in the form of branched hyphae, a few microns in diameter, forming thick, tough, intertwined mycelia mats at fuel/water interfaces.

All of these can and do cause damage to the fuel system.

PREVENTION
Maintain the fuel system by draining water very regularly, keep the tank as full as possible, (especially over-night) and try to ensure your supplier maintains his system well.

CURE
Clean the entire system with a cleaning agent available from or recommend by your diesel supplier.

Thanks for the very thorough rundown of potential problems with diesel contamination Old geezer.

When my vehicle was returned i asked for the old filter with a view to recovering some of the costs of repair from my diesel supplier subject to being able to prove them the cause. I will as you suggest disect the filter & look for signs of contamination

The garage I had replace the filter suggested dosing the tank with an propriety anti-bacterial/water removal product, so that part of your suggestion has already been carried out.

Horneteer

Now that my vehicle is running Ok again I have had time to check out the old fuel filter. I have cut it apart & no sign of any "black slime" suggested by Old Geezer, so can on assume it just blocked with sediment over time.

Can anybody suggest a good code reader ( with reset feature) to buy which will help me diagnose faults without resorting to a trip to the dealers.

Horneteer

Depends on what you intend to do.
Basic engine faults OBD2 readers can be had for less than £50 on ebay.
I swear by the Hawkeye from Brarmach approx £300, expensive, but does all the systems - engine, transmission, brakes, aircon, etc. there are others - Bell do a nice one I am told. Tangiers Orange - gone, missing her
Replaced by Ewok what a mistake - now a happy Disco Sport owner

Now, because of the EURO5 interface's independence, you can have the dealer tools with the same less than £300. Of course, you should have a laptop at your disposal.

By the way, Andy, with Hawkeye, are you able to modify the CCF?

Sorry Alex it's a diagnostic aid, rather than a programming tool.
You can do basic things like reset service indicators & collision reset, it's advantage for me is that you do not need a laptop, just leave it in the spare wheel well, and dig it out whenever you need it.

So if you nip up to the Lake District for a spot of rambler annoying (or shopping!) and the reduced traction message comes up in the pub car park, you can diagnose a faulty wheel sensor, before SWMBO has had time to order drinks. That way you can drive home knowing you are not doing any damage, or in my case start to strip the wheel off while SWMBO does some shopping. Tangiers Orange - gone, missing her
Replaced by Ewok what a mistake - now a happy Disco Sport owner

Wifes TD4 cut out one cold morning about 100m up the road, she got the key out and tried to restart it, it would turn over but not catch. I looked at it, had a reduced engine warning, but still would not start. Later in the day went up with my son to push it home, low and behold it started, seemed to run ok, but the wife was freaked by the sudden failure. I talked to LR dealer, they suggested a crank sensor fault.

I had other suspicions, when bought it was low kms (98K) and had all the service stamps, but after a nasty whining noise I noticed the power steering fluid looked like capuccino, and had obviously never been changed.(did it myself) Figuring a fuel problem I ordered a new filter, and following a good youtube vid by a couple of english guys, changed it. By the way, you dont need a special priming tool, simply empty the existing filter into a jug (about 500ml), and holding your finger tight on the temporary red blanking piece, refill the new filter with a small funnel nicked from the kitchen.

I did this, put it all back together, seems to run fine, I suspect this had never been done before, because the difficult to get at back engine cover nut was still in place (you have to undo a hose to get a 10mm socket on it, and the LR mechanics rarely bother putting it back.)

I got my small angle grinder and cut open the filter cartridge to see what was what, and although I have no idea what it should look like, I doubt it should be the totally black slimy mess I found inside. I suspect a badly clogged filter plus a cold morning caused the problem. About an hours work plus the cost of the filter.