One of the sustainable ways to reduce the cost of waste or wastewater treatment is to generate bio-energy, such as H2 gas, from the organic matter present. Waste biomass contains enough energy to meet a significant fraction of the world’s entire energy demand, if it could be efficiently converted to useful energy forms [19]. According
to one estimation, the energy value of all residual biomass in the United States is 0.2-0.3 TW [20] and conversion of this material to useful forms would meet approx­imately 7% of the USAs’ total annual energy use (~3.3 TW) [19]. The fraction is much higher worldwide, perhaps 25% or more [19]. Major advantages of energy from wastes are the carbon neutrality, renewability, recovery of energy and simulta­neous wastewater treatment. Simple sugars to complex industrial wastewaters have been evaluated to determine their potential as fermentative substrates for the pro­duction of H2. Table 1 shows some of these studies using dark fermentation. Simple sugars such as glucose, sucrose and lactose are readily biodegradable substrates for H2 production but are expensive. Various wastewaters generated from industrial or domestic activities function as good substrates for H2 generation due to the presence of large fractions of degradable organics. Residue like agricultural crops and their waste products, wood and wood waste, food processing waste, aquatic plants, algae, and effluents produced in human habitats can all be used as fermentable substrates

[70]. Many agricultural and food industry wastes contain starch or cellulose, which are rich in terms of carbohydrate content and can also be used for H2 production. The sludge generated in wastewater treatment plants contains large quantities of carbohydrates and proteins which can also be used for energy production. Table 2 shows data on fermentative H2 production using various types of waste.

Unlike wastewater, cellulosic material or solid wastes typically require pretreat­ment to make the organic fraction soluble and bio-available to microorganisms for conversion to H2. Due to its tightly packed, highly crystalline and water-insoluble nature, cellulose is recalcitrant to hydrolysis into its individual glucose subunits [70]. In the pretreatment step, a combination of chemical, mechanical, and enzy­matic processes is typically used. Techniques viz., high temperature, high or low pH, hydrolytic enzymes, microwaves, ultrasound, radiation, and pulsed electric fields are being used for this purpose [19]. Some microorganisms can degrade cellulose effec­tively by using their cellulase enzymes resulting in monosaccharide products that can be converted into H2 with dark fermentation [70].