Cloudy or hazy biodiesel contains water, so a finished batch should be clear, golden and bright when held to the light; if it is hazy, let it settle and dry it by heating to 100°C. A clear batch can still hold dissolved water, and water in fuel feeds microbial sludge at the water-fuel boundary, so store the fuel sealed, full and cool and drain any water from the tank bottom.
Part of Biodiesel Production
Clarity is the visual check for water in biodiesel; water content decides whether stored fuel stays usable.
Haze Is Water, But Clear Does Not Mean Dry
The parent’s test: good biodiesel is clear, with no cloudiness or haze, and haze means water. Fine droplets of water scatter light and make the fuel look cloudy. Water also arrives from washing, which is why the last step is heat. But biodiesel is known to take up more dissolved water than petroleum diesel (a recalled property, not a figure from the corpus), and dissolved water is invisible, so a clear fuel can still be wet. It comes out of solution as the fuel cools, or in storage.
What You Need
- A clear glass jar and good light
- A thermometer and a heat source for drying (water bath or indirect heat)
- A sealed storage drum that is filled as full as possible, with a bottom drain or siphon
- A dark, cool place
Method
Step 1: Look. Fill a clear jar and hold it up to the light. Cloudy: water. Clear: pass the first check.
Step 2: Settle. Leave the fuel still for a few days. Water drops to the bottom; draw the fuel off from above.
Step 3: Dry. Heat to 100°C for 15 minutes (parent), stirring gently, outdoors and only after washing is complete, since methanol vapour from poorly washed fuel is flammable. Cool before sealing, because warm fuel in a closed container draws water in as it cools.
Step 4: Check for water at the bottom. Draw a sample from the lowest point of the tank and look for a separate layer or droplets.
Step 5: Store full. A full tank leaves little air from which water can condense.
Why It Matters
Microbes grow where water meets fuel and form slimes that block filters and injectors (Diesel & Heating Oil, whose guidance to keep tanks full, filter and drain water applies to biodiesel too). Water also corrodes tanks and, in the cold, forms ice crystals that block lines, which compounds the wax problem described in Cold Weather Performance.
Common Mistakes
- Taking clear for dry. Dissolved water is invisible.
- Drying a still-wet, unwashed batch and fuming off methanol.
- Sealing hot fuel in a cold drum.
- Storing in a half-empty drum where condensation collects.
Fuel heated for drying is flammable
Heated fuel with any methanol left in it gives off flammable, toxic vapour. Work outdoors, away from any flame, with the vessel covered, and use a water bath or indirect heat; do not leave it unattended (Petroleum Safety Protocols covers heating flammable liquids).
What’s Next
- Learn next: Cold Weather Performance
- Combine with: Washing & Drying, where the water comes from