Page 77 - Energy from Toxic Organic Waste for Heat and Power Generation

Toxic Waste From Leather Industries 63

electricity for export and to cover its own running power. The technology
needs constant monitoring when put into use to ensure that the sludge
blanket is maintained, and not washed out (thereby losing the effect). The
heat produced as a by-product of electricity generation can be reused to
heat the digestion tanks.
The blanketing of the sludge enables a dual solid and hydraulic (liquid)
retention time in the digesters. Solids requiring a high degree of digestion
can remain in the reactors for periods up to 90 days [52]. Sugars dissolved
in the liquid waste stream can be converted into gas quickly in the liquid
phase which can exit the system in less than a day.
With UASB, the process of settlement and digestion occurs in one or
more large tank(s). The effluent from the UASB, which has a much reduced
BOD concentration, usually needs to be treated further, for example with
the activated sludge process, depending on the effluent quality require-
ments. UASB reactors are typically suited to dilute waste water streams (3%
TSS with particle size >0.75 mm).

5.5.2 Biomethanation for Solid Waste Disposal

Solid wastes generated by the leather processing industry are posing a major
challenge. Appropriate technology has been developed for the profitable
disposal of these solid wastes. Tannery fleshings, which are the major solid
wastes emanating from the beam house of a tannery, are subjected to bio-
methanation. It is a process whereby the fleshing is liquefied completely
biologically and the resulting liquefied fleshing is treated in anaerobic re-
actors to produce biogas. Any anaerobic reactor like UASB reactor can be
used for this purpose. Depending on the microbes present in the system, the
generated gas may contain methane as well [49].
Biomethanation is a process by which organic material is microbiolog-
ically converted under anaerobic conditions to biogas. Three main physio-
logical groups of microorganisms are involved: fermenting bacteria, organic
acid oxidizing bacteria, and methanogenic archaea. Microorganisms de-
grade organic matter via cascades of biochemical conversions to methane
and carbon dioxide. Syntrophic relationships between hydrogen producers
(acetogens) and hydrogen scavengers (homoacetogens, hydrogenotrophic
methanogens, etc.) are critical to the process. Determination of practical
and theoretical methane potential is very important for design for optimal
process design, configuration, and effective evaluation of economic feasi-
bility. A wide variety of process applications for biomethanation of waste-
waters, slurries, and solid waste have been developed. They utilize different