Page 79 - Industrial Wastewater Treatment, Recycling and Reuse

56 Industrial Wastewater Treatment, Recycling, and Reuse

3
The dye industry generates wastewaters to the tune of 8–75 m waste-
water per ton of product, and dyes such as reactive dyes are prominent along
with dyes and pigment in producing large quantities of liquid effluent. Typ-
ically, the raw materials used in the dye industry include organic compounds
such as benzene, xylene, toluene, aniline, anthtraquinone, and naphthalene,
along with other chemicals such as acids (sulfuric, nitric, HCl, acetic acid),
ammonia, sodium hydroxide, and sodium/potassium salts (carbonates and
sulfates). Some solvents, such as alcohols, are also used in the process and
have their presence in the liquid effluent even after their recovery. There
is significant use of components, mainly as catalysts or in complexing, con-
taining heavy metals such as copper, nickel, zinc, and iron, traces of which
eventually find their way into liquid effluents. Many times metal content as
high as 3000–4000 mg/L is found in the wastewaters of such industries. The
wastewaters are also characterized by high COD, ranging from a low of 3000
to as high as 32,000 mg/L or even more. The wastewaters are many times
characterized by high ammoniacal nitrogen content with values as high as
6000–11,000 mg/L. Ammoniacal nitrogen can create complexities in the
selection of suitable wastewater treatment methodologies.
The treatment methodology for dye industry wastewaters is complex and
diverse depending on the specific characteristics of the effluent; by and large,
no general solutions are available. A number of physico-chemical
methods—filtration, coagulation, adsorption, and biological methods—
are generally employed in combination. It is recommended that compo-
nents that can be recovered, such as solvents, should be recovered prior
to treatment. By-products such as gypsum salt, iron sludge, spent acid/dilute
acid solutions of HCl or other acids, and ammonia solutions can be recov-
ered and can be value-addition to the wastewater treatment process. The
lowering of salts/recovery of salts also reduces the load on wastewater treat-
ment. An effluent having high ammoniacal nitrogen content is usually dif-
ficult to treat using conventional methods and needs to be separated from
common effluent for separate processing. Process changes in the operations,
such as replacing precipitation of reactive dyes by salting out with spray dry-
ing, can drastically reduce the wastewater generation, which otherwise is
huge and has high COD and TDS. Many times, the reactive dyes are con-
sidered refractory pollutants that are difficult to degrade using conventional
physico-chemical and biological methods. Newer methods of treatment
such as AOPs and cavitation can be attractive alternatives in such cases.
The overall Indian production of dyes is expected to grow significantly in
the future. However, strict effluent treatment and environmental norms
form an impending bottleneck for the growth of this industry sector.