Page 75 - Energy from Toxic Organic Waste for Heat and Power Generation

Toxic Waste From Leather Industries 61

Azo dyes, the synthetic dyestuffs based on nitrogen, are used in leather
industry for dyeing the leather articles. The toxicity of several azo dyes has
been mentioned earlier [43,44]. Many azo dyes on cleavage produce car-
cinogenic and allergenic aromatic amines. The EU Azo Colorants Directive
(2002) [45] has given a list of azo dyes that should not be used in leather
articles as they may release one or more prohibited aromatic amines in
detectable concentration above 30 ppm in the finished articles or in the
dyed components. Organotin compounds like dibutyl tin used in leather
finishing as a catalyst may contain tributyl tin as an impurity which is highly
toxic and has hormone like activity [46]. No restriction has been set yet for
synthetic tannins, although there have been extensive impact assessment and
treatment applications for individual chemicals such as cresols and phenols
[47,48].
Thus, from the preceding sections, it is evident that leather process-
ing involved the usage of highly toxic and carcinogenic chemicals. The
waste from the leather processing includes the above-mentioned organic
and metallic substances. The waste from leather industry is categorized into
solid wastes and wastewater effluent with the presence of VOC and toxic
chemicals.

5.5 HEAT AND ENERGY GENERATION FROM LEATHER
PROCESSING WASTE
The heat and energy generation can be achieved by treating the waste-
water effluent using upflow anaerobic sludge blanket (UASB) technology.
The solid waste generated from the leather industry is primarily due to the
animal skin used as the raw material. The tannery wastes are subjected to
biomethanation for producing biogas for further use.

5.5.1 UASB Technology With Sulfur Recovery Plant
The UASB technology has been well-known for treating wastewater pri-
marily because of its low sludge production and less requirement of energy
as well as area. The major advantage of this technology is that it provides
a potential possibility to generate energy rather than consuming the same
while treating the wastewater [49].
Despite the aforesaid advantages, there is a major constraint of using
this technology for treating tannery waste water because of the high sul-
fate content, which results in generation of methane gas, contaminated
with hydrogen sulfide, thereby restricting its use as a source of energy.