(i) Use of fossil fuel and raw materials to produce biofuels: The whole life cycle of the

production of biofuels involves the use of fossil fuel and raw materials to some extent. Whether the net gain balance out of the fossil input obtained in terms of low emissions is positive or not remains under discussion.

(ii) Availability of land for fuel (food vs. fuel issue): Current biofuel producers do not always have a secure access to raw materials due to limited grain reserves and the fact that the current costs of crude vegetable oil from "food crops" are variable. Bio-based energy industries are also currently in competition with food producers, and we perceive them as being a primary cause of the increase in food prices. In order to make biofuel production profitable and more sustainable, avoiding as much as possible competition with the food market, companies have to focus on second-generation biofuels made from alternative cheap feedstocks (e. g., (waste)-biomass, waste oils and fats, residues, etc.). (Ligno) cellulosic ethanol and biodiesel from waste oils, nonfood crops, or algae emerge as real alternatives to tackle this problem.

(iii) Environmental impact: Despite the fact that some studies carried out to date show first-generation biofuels may offer a low carbon balance, fossil fuel usage and GHG balance, further outputs and environmental indicators must be addressed. Water usage (in the growth of the crops), eutrophication (run off of lawn fertilizers into natural waters), and soil erosion are some of them. Second — and potential third-generation biofuels are more attractive in terms of crop economy.

(iv) Socioeconomic impact: Some sectors of the industry estimated that a robust global biofuel market will be fully established around 2012. The implementation of biofuels will also be highly dependent on the feasibility of the technologies employed for their production and the economics of the processes play a fundamental role in this regard.

At the moment, there is not yet a widely accepted definition of "sustainable biofuels," or a scheme for certification and labeling (Mol, 2007). Nevertheless, some agreement can be observed on four ecological issues that should be included in sustainability schemes such as GHG emissions, energy balance, biodiversity loss, specific environmental effects (i. e., soil condition and water use).The problem is that each feedstock is different and many crops produce their best yields in specific regions of the world or require certain soil or water conditions. These local differences demand specific attention and are not easily generalized. Furthermore, there is wide disagreement on the implementation of international conventions, while inclusion of social criteria is even more difficult (Oosterveer and Mol, 2010).

A holistic approach to valorization of lignocellulosic biomass needs to take into account sustainability of chosen options. If concepts are too heavily orientated toward energy produc­tion or industrial use, this can even be at the expense of environmental protection. If crop residues such as straw are no longer left on field, this will result in depletion of soil organic matter. While anaerobic digestion results in a digestate, which can be brought back to field to supply not only nutrients but also organic matter, thermal valorization and production of second-generation biofuels result in complete consumption of the biomass and consequently a lack of nutrients and organic matter. Soil requirements vary within a wide range and need to be assessed locally. Only lignocellulosic biomass which is in surplus of soil demand for organic matter should be considered for treatment options with complete consumption of the substrate. In regions with concern about declining organic content of soils, anaerobic diges­tion should be given special attention even if the net energy recovery is lower compared to that of alternative technologies with total consumption of the biomass (Sigrid and Morar, 2009).