Page 166 - Membranes for Industrial Wastewater Recovery and Re-Use

140 Membranes for Industrial Wastewater Recovery and Re-use

peroxide can cause toxicity problems in biological treatment processes (Cooper,
19 78). Only light bleaching, if any, is required when processing 100% synthetic
or woollen goods, and the generated wastewater is not a significant source of
pollution in such cases (Nolan, 1972).

Mercerising
Mercerisation is performed almost exclusively on pure cotton fabrics, which are
treated by a concentrated caustic bath and a final acid wash to neutralise them.
Its purpose is to give lustre and also to increase dye affinity and tensile strength.
Mercerisation wastewaters have low BOD and total solids levels but are highly
alkaline prior to neutralisation (Cooper, 1978; Nolan, 1972). The low BOD
content arises from surfactants and penetrating agents used as auxiliary
chemicals (Table 3.21).

Carbonising
Carbonisation is performed on woollen items to remove traces of vegetable
matter. The process can be carried out either in conjunction with raw scouring
or at the fabric processing stage, depending on the level of impurities and the end
use of the wool (Cooper, 19 78; OECD, 198 1). Carbonising consists of soaking the
material in dilute sulphuric acid followed by neutralisation with sodium
carbonate. The material is then dried and the brittle cellulosic matter
mechanically removed. The generally low levels of organic materials in
carbonisation effluents are due to vegetable matter, whilst the acid treatment
yields high levels of dissolved solids. Performing carbonisation in conjunction
with raw wool scouring leads to a reduction in the total pollution load of the
scouring waste stream (OECD, 1981).

Fulling
Pulling stabilises woollen fabrics and gives them a thicker and more compact
appearance. It is carried out with soda ash or sulphuric acid in the presence of
detergents, sequestering agents, metallic catalysts and hydrogen peroxide in
conjunction with mechanical agitation (Nolan, 19 72: OECD, 1981). The fulling
solution is then drained and the treated product extensively washed to remove
the remaining chemicals. Fulling wastes in combination with effluents generated
by subsequent washing operations present, after raw wool scouring, the largest
source of BOD in wool processing wastewaters (Cooper, 1978: Nolan, 1972).
Most of the BOD arises from soap, detergents and lubricants and oils added to the
wool during the production process.

Dyeing
Dyeing is carried out to add colour to fabrics or yarn. Identification of generic
types of dyeing wastewaters is complicated by the diversity of both the dye
chemistry and the operational modes of the dyeing process itself. Although
rarely toxic, these wastewaters demand special consideration since they are
arguably the most problematic of all textile wastewaters, for a number of
reasons: