Biodiesel production does not result in a lot of waste. However, depending on the process and feedstocks use, it can produce some wastewater, minerals, resins, solids strained out of used oil and glycerin. Often, waste is not an issue for large producers because they can invest in the equipment needed to recycle and reuse some of these products. For small producers, waste disposal is often a concern.
The biodiesel industry aims for a zero discharge process. Plants that recycle their water claim to have a zero discharge process.
However, like any business, biodiesel producers also have waste streams from other areas, and not just from the production process of biodiesel. These additional waste streams include waste from toilets, water used for washing equipment and floors, packaging waste and general office waste. While these waste streams are not the concern of this article, biodiesel producers who truly want to create zero waste may want to address these additional waste streams.
One way of looking at waste is that it is just a product in the wrong location. Byproducts become waste when no one else wants them.
After biodiesel is made, it is often "washed" with water to remove contaminants, including soap, glycerin, residual methanol and residual catalyst. This can result in as much as one gallon of wastewater per gallon of biodiesel produced.
The amount of wastewater can be reduced by adding an acid to the biodiesel to split the soap. In this case, only .05 to .1 gallon of water is needed per gallon of biodiesel produced.
Most large producers clean and reuse their water. The soaps and catalyst removed from the water are added to the crude glycerin, a byproduct of biodiesel production, which is then sold to a glycerin refiner.
The uncleaned wastewater can also be used for farm irrigation, as long as it does not contain methanol.
Small producers, or producers who have not yet invested in wastewater recycling equipment, often send their wastewater to a municipal sewage treatment plant and pay for this disposal.
Biodiesel is sometimes washed using a waterless method. One method involves using ion exchange resins – styrene plastic beads which remove contaminants from biodiesel. This method results in much less waste -- only 1 pound of beads per 900 gallons of fuel produced.
In theory, the ion exchange resin beads can be recycled, but at the present time, no one is set up to recycle them because the cost of recycling is about the same as that of buying new beads. Therefore, these beads are generally sent to a landfill or burned.
Magnesium silicate is a mineral that can also be used to clean biodiesel in a waterless process. One common brand name for synthetic magnesium silicate is Magnesol. As with ion exchange resins, very little waste is created – equal to about 1 percent of the weight of the fuel.
Once used, this mineral is not reusable in the biodiesel process. It is generally sent to a landfill. Because this mineral is not toxic and the contaminants it has gathered from the biodiesel have nutritional value for animals, the waste magnesium silicate could theoretically be added to animal feed. However, we are not aware of anyone who is currently doing this.
Sometimes biodiesel is produced from used restaurant oil. This oil contains solids such as meat and bone fragments or breading that must be strained out of the oil before processing into biodiesel.
The solids that are strained out of the used oil can be sold as animal feed. Some producers run the sludge from the strained oil through their oil extraction press along with other products from their rendering process. In this way, the solids are combined with the meat and bonemeal, which is then sold as livestock feed.
In its pure form, glycerin is a valuable industrial chemical that is used in many different products. However, many small biodiesel producers lack the resources to refine the glycerin to the 80-percent purity required to sell to glycerin refiners, who will then take it to the 99.5-percent purity level required for most commercial markets.
Glycerin emerging from the biodiesel process is commonly at 50-percent purity. This can be a disposal problem. Since the glycerin contains methanol, it cannot be safely released into the environment. Proper disposal options are essentially limited to anaerobic digestion, or transport to a larger biodiesel plant that can do the necessary refining.