**4.2 Biological hydrogen production**

Hydrogen is a clean, recyclable, and efficient energy carrier. The possibility of converting hydrogen into electricity via fuel cells makes the application of hydrogen energy very promising (Chang et al., 2002).

Hydrogen production via dark fermentation is a special type of anaerobic digestion consisting of only hydrolysis and acidogenesis. It leads to the production of hydrogen, carbon dioxide and some simple organic compounds [VFA and alcohols]. These readily degradable organic compounds can be used for further methane production. (Bartacek et al., 2007)

Much interest has recently been expressed in the biological production of hydrogen from waste streams by dark fermentation. Biological hydrogen production shares many common features with methanogenic anaerobic digestion, especially the relative ease with which the two gaseous products can be separated from the treated waste.

From hydrogen-producing mixed cultures, a wide range of species have been isolated, more specically from the genera Clostridium (*Clostridium pasteurianum, Clostridium saccharobutylicum, C. butyricum*), Enterobacter (*E. aerogenes*) and Bacillus under mesophilic conditions; and from the genera Thermoanaerobacterium (*Thermoanaerobacterium thermosaccharolyticum*), and Caldicellulosiruptor (*Caldicellulosiruptor saccharolyticus*, *C. thermocellum, Bacillus thermozeamaize* ) under thermophilic or extremophilic conditions.

However, the low efficiency of the hydrogen production process remains the main limiting factor. Much research will be needed to be carried out to reach hydrogen yields comparable with the theoretical efficiency maximum. Although a relatively high efficiency has been reached using pure substrates, the low hydrogen yield with complex (real) substrates remains a great challenge.

Olive Oil Mill Waste Treatment: Improving the Sustainability

**5.1.2 Three-phase olive mill wastewaters (3POMWW)** 

and volatile solids removal (Dareioti et al., 2010).

the phenolic extracted 3POMWW (Khoufi et al., 2006).

treatment of 3POMSW.

in the olive oil mill itself.

good results.

solution.

of the Olive Oil Industry with Anaerobic Digestion Technology 283

complex structures. Therefore, anaerobic digestion is not a suitable technology for the

Anaerobic digestion is a promising alternative for the treatment of 3POMWW. It allows for the disposal of these wastewaters achieving considerable organic material removals and producing renewable energy in the form of biogas, which could be used as an energy source

Certain components of 3POMWW such as poly-phenols, pH, oil, etc. may inhibit the AD process. Martín et al., (1991) obtained methane yields of 260 mL CH4 g-1 COD for 3POMWW. Borja et al. (1995b) improved the methane production using a pre-treatment stage with *Geotrichum candidum*, *Azotobacter Chroococcum* and *Aspergillus terreus*. The latest study reported methane yield coefficients of 300 (*Geotrichum*-pretreated 3POMWW), 315 (*Azotobacter*-pretreated 3POMWW) and 350 (*Aspergillus*-pretreated 3POMWW) mL CH4 g-1

3POMWW have a low nitrogen content which limits the AD process due to the fact that the microorganisms need this element for their metabolism. In this way, co-digestion with rich nitrogen substrates may improve the biodegradability of the mixture. Azbar et al., (2008) studied the co-digestion of 3POMWW with laying hen litter obtaining a significant improvement in the biodegradability of 3POMWW. Co-digestion with liquid cow manure [20% 3POMWW, 80% liquid cow manure (v:v)] also showed good results in terms of COD

Another option is the combination of catalytically oxidized olive mill wastewaters (by Fenton's reagent) plus anaerobic digestion. El-Gohary et al. (2009) found that the digestion of catalytically oxidized 3POMWW followed by a classical upflow anaerobic sludge blanket reactor (UASB) and a hybrid UASB as a post-treatment step is a promising alternative.

Other treatments envisage the combination of an initial liquid-liquid extraction with ethyl acetate for exploitation of the phenol content, followed by aerobic or anaerobic digestion of

The use of sand filtration and subsequent treatment with powered activated carbon in batch systems has also been studied as a pre-treatment. This pre-treatment allowed COD removal efficiencies of 80%-85% for an HRT of 5 days and at an OLR of 8 g COD L-1 d-1. A methane

The separation of the digestion phases, hydrolytic-acidogenic reactor and methanogenic reactor, in two completely independent reactors can also be considered as a way to improve the AD digestion of these wastes. Bertín et al. (2010) studied different acidogenic configurations of biofilm reactors using ceramic filters or granular activated carbon with

The latest research studies report AD as a promising technology for the treatment of 3POMWW, leading to sustainable waste treatment and an environmentally friendly

yield of 300 mL biogas g-1 COD removed was achieved (Sabbah et al., 2004).

COD against the 260 mL CH4 g-1 COD obtained for the untreated 3POMWW.

#### **4.3 Microbial fuel cell technology**

The microbial fuel cell (MFC) is a new energy technology in which microorganisms produce electricity directly from renewable biodegradable materials (Logan et al., 2006). During microbial oxidation of biodegradable matter, not only are protons and oxidized products formed but electrons are remarkably transferred from the bacteria towards a solid electrode. The electrons flow through an electrical circuit towards the cathode where a final electron acceptor is reduced resulting in generation of electrical power (Figure 3).

Although interest in microbial fuel cells was relatively high in the 1960s, research has been limited as the cost of other energy sources remained low and the available microbial fuel cells lacked efficiency and long term stability. However, in the past seven to eight years there has been a resurgence in microbial fuel cell research. In fact, the efficiency of this energy conversion is potentially higher than the actual waste treatment technology for energy recovery, such as anaerobic digestion or incineration (Logan et al., 2006).

Microbial fuel cells have been validated at lab-scale with simple organic substrates, pure culture and highly controlled experimental conditions. Organic substrates as volatile fatty acids and more recently wastewater have generated high energy production (Catal et al., 2008; Clauwaert et al., 2007; Clauwaert et al., 2008; Rabaey et al., 2003; Rabaey & Verstraete, 2005). Over the last 10 years, the improvement in the design of microbial fuel cells has increased electrical generation 10,000 times (Debabov, 2008). However, full scale application has not yet been developed.

Fig. 3. Microbial Fuel Cell (MFC) set up
