**3. The impurities present in crude glycerol**

strategy for biofuel production. This is possible due to exceptional conditions for the cultiva‐ tion of oilseeds for oil extraction. The selection of feedstock depends strongly on potentialities

When biodiesel is produced from vegetable oils and animal fats through transesterification process, high amounts of waste are created. This waste chiefly consists of crude glycerol, which has limited commercial value, unless expensive purification processes are performed. Even when purified, there is such a large global overproduction that traditional markets find difficult to absorb it. Each ton of biodiesel produced generates approximately 100 kg of crude glycerol. However, the amount of crude glycerol generated in biodiesel production can vary from 1% to 85% (v/v), depending on operating conditions of the transesterification plants. Thus, to overcome these issues it is necessary to discover new uses for this significant residue.

Ethanol has been primarily produced from sugarcane in Brazil since 1975, encouraged by the implementation of the National Program for Alcohol (1975–1985). Brazil produces (in 2011/2012) nearly 571 million tons of sugarcane, which is processed by sugar mills to produce 36.9 million tons of sugar and 22.9 billion liters of ethanol [6]. The ethanol could potentially be used in the esterification process of biodiesel [7] to develop cheaper and more environmentally friendly processes. Several studies have been developed to obtain hydrogen gas, as a renew‐ able energy source that can be generated from waste glycerin, a byproduct of biodiesel

In this sense, this chapter presents a comparative study on biological hydrogen production from crude glycerol, the microorganisms involved in the biological processes for hydrogen production and some of the strategies applied in the literature for the improvement of these systems. It will contribute to innovation in research into the reuse of crude glycerol, in a sustainable manner, thus leading to potential cost reduction and clean energy generation.

Glycerin is the principal byproduct of biodiesel production, with waste streams containing at least 95% glycerol; however its purity can vary depending on the efficiency of the production

Glycerol is generated by the process for obtaining biodiesel from vegetable oils or animal fats. This process often uses the addition of catalysts, such as sodium hydroxide, and alcohols, such

However, during the transesterification process a high volume of glycerol is produced as a

Pure glycerol can be used in many different applications, mainly in textile, chemical, pharma‐ ceutical and food industries. However, to use it in these applications is necessary a degree of purity higher than 95% [9]. To achieve this, the crude glycerin must be submitted to a purifi‐

of glycerin is generated [8].

as methanol or ethanol, with reactors maintained under heat and agitation (Figure 1).

of produced biodiesel, approximately 10 m3

**2. Glycerol: A byproduct of biodiesel production**

cation process, often resulting in high financial costs.

of each region [5].

478 Biofuels - Status and Perspective

production.

process [8].

byproduct: for 90 m3

The main impurities present in crude glycerol are soap, free fatty acids, methanol, unreacted triglicerides, diglycerides and monoglycerides [10]. The exact composition of the waste glycerol often differs depending on the manufacturing process employed (Table 1). Silva et al. [12] observed methanol, water and sodium chloride, in samples of crude glycerol during the biodiesel production in a Brazilian plant with soybean oil as raw material.

Crude glycerol should be pre-treated before being used as feedstock. Methanol or ethanol can be removed using heating or distillation processes. Soap is another impurity present in the crude glycerol and can be removed by precipitation from the liquid medium through pH adjustment. Sodium ions can be removed from crude glycerol by neutralization with addition of phosphoric acid and lime in excess, in order to crystallize/precipitate hidroxyapatite [17]. However, these treatments are costly and not economically justifiable. Alternatively, there are many studies covering the use of crude glycerol for bio-hydrogen production without pretreatments [11]. These bioconversions of the crude glycerol may be suitable and economically attractive alternatives to the industrial processes.


**Table 1.** Impurities present in the crude glycerol during the biodiesel production
