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Water for Energy and Fuel Production
inhibited the nitrogenase activity. They generated the mutant named AR-3 that can
produce hydrogen in the medium containing even 4 mmol/l NH 4
+ due to the release of
the inhibition of NH 4
+ to the nitrogenase activity. Under suitable conditions, they also
showed that the hydrogen generation rate of AR-3 from tofu wastewater could reach
14.2 ml/l/h. It was increased by >100% compared to that of wild-type R sphaeroides.
8.4.12 FruiT WASTe
Methane can be produced from waste orange peel. The thermophilic anaerobic
digestion of industrial orange waste pulp and peel with a subsequent aerobic posttreatment of the digestate has been successfully demonstrated by Kaparaju and
Rintala [90]. In this study, in anaerobic batch cultures, the methane production rate
of 0.49 m 3 /kg volatile solids, and in a semicontinuous process, the methane production rate as high as 0.6 m 3 /kg VS was generated. This did require the pH adjustment
from 3.2 to 8.0 by CaCO 3 addition. An aerobic follow-up treatment with activated
sludge produced CO 2 and water and converted ammonia into nitrate. The removal of
nitrogen required an additional denitrification step. The process can be adapted to
other fruit and vegetable wastes such as mango, pineapple, tomato, jackfruit, banana,
and whole orange. The methane production rate from these fruits can be improved
by using selected strains of Sporotrichum, Aspergillus, Fusarium, and Penicillium
[5,90,91]. These fungal pretreatments enhanced the availability of nutrient in the
medium, decreased the concentrations of antimicrobial components, and enabled the
higher loading rate utilization.
Besides other fruits and vegetable wastes, the wastewater from pressing of these
byproducts is also good substrate for methane production [91]. This wastewater
is generated by pressing the rind of orange peel and it contains a large amount of
organic matter and alkalinity because in the pressing process, Ca(OH) 2 is used as
binder. Before the anaerobic treatment, the waste is pretreated by aluminum phosphate flocculent to remove solids that can hinder the anaerobic treatment and to
reduce the pH from 11.21 to 5.5. In the batch process, this treatment removed 84%
of soluble COD and generated 295 ml of methane per gram of COD removal. The
presence of antimicrobial components reduced the methane production when COD
loadings were high [5,91].
8.5 CO-diGestiOn
Co-digestion is the simultaneous digestion of a homogeneous mixture of multiple
substrates. The most common situation is when a major amount of basic substances
is mixed and digested together with minor amounts of a single or a variety of additional substrates. As pointed out by Braun et al. [52], Braun [53], and Braun and
Wellinger [54], co-digestion can improve the overall nutrient balance and digestion,
and create an additional biogas and fertilizer. It can also equalize the particulates,
floating and settling materials, and acidity in settlers by a suitable dilution by manure
and sewage sludge to the agricultural waste. The co-digestion can, however, create
an increased COD in the digester effluent, may require more pretreatment of the
waste, and increase the mixing requirement in the digester. It may also cause an