(269d) Biodiesel Production and Its Implication On Water Resources

Biodiesel, defined as mixture of monoalky esters, has been well established as the alternative fuel in the US. The burning of biodiesel offers the opportunity to significantly reduce most of the air pollutant emissions. In addition, from a larger perspective, biodiesel industry not only creates thousands of jobs in the related area but also helps to fulfill the goal of achieving energy independence proposed by Energy Independence and Security Act (EISA). Under such a context, the biodiesel industry has experienced a boom since 2004 with the annual production capacity increased from 25 million gallons to 700 million gallons. In addition, the new Renewable Fuel Standard (RFS2) has set a mandate of 1 billion gallons of biomass-based diesel for the obligatory parties in 2012, which tends to further promote the development of the biodiesel industry.

However, the boom of biodiesel industry has raised the increasing concern about its sustainability, among the issues of which water-intensity is of particular interest. Although literally biodiesel production involves few water consuming scenarios, from a boarder perspective, including the feedstock development water usage, the water intensity of biodiesel manufacturing should not be overlooked. In this study, a holistic review of the water consumption scenarios was performed and relevant data was collected through a variety of approaches, including literature review, industrial survey and personal communication.

First of all, the survey indicated that throughout the US, about 16.98% of the biodiesel plants, which account for 11.37% of the total annual production capacity, are located in the water-stressed area where the water resource is vulnerable.

In addition, the result showed that the water consumption data for biodiesel feedstock development stage varies significant from case to case, the reason of which may be the spatial and temporal difference in water resource distribution. Also, the inconsistence among the allocation methods, assumptions and/or modes involved in the individual studies may be the cause as well. So a sample calculation was conducted to give a nationwide estimation for the water consumption during feedstock growth soybean-derived biodiesel.

The water consumption during biodiesel production process, on the other hand, is much less and the literature review and data collection indicated that washing water and cooling tower makeup are the major water consuming scenarios during biodiesel manufacturing. The washing water consumption ranged from 2-10% on a volumetric basis and the number for cooling tower makeup was 3-33%, which was highly dependent on the individual configuration of the plants.

As a water saving practice, drying purification process has been considered and applied in some of the commercial biodiesel plants to replace the traditional water washing step. Ion-exchange resin and magnesium silicate are the two materials that are most widely used and both of them have their own pros & cons so that neither one has the predominant advantage over the other.

Finally, the literature review on the wastewater and storm water discharge of biodiesel manufacturing revealed that the BOD and COD of the water are extremely high and thus the water should be properly treated in case of the pollution otherwise caused by the direct discharge into municipal sewage system.