Page 507 - Industrial Wastewater Treatment, Recycling and Reuse

Simulation, Control, and Optimization of Water Systems in Industrial Plants 477

the wastewater generation. Mathematical modeling helps to reduce demi-
neralized and freshwater consumptions, which will ultimately affect the cost
of water used during processing.
Dakwala et al. (2011) studies the starch industry using water pinch tech-
nology to reduce the demineralized (DM) water flow rate and, subsequently,
the wastewater flow rate. The wastewater problem is viewed as a single con-
taminant problem, and all the three modes of water integration, that is,
reuse, regeneration-reuse, regeneration-recycle, are demonstrated. The
DM water consumption is 50 tph before modification. After modification
using water pinch, DM consumption reduces to 31.9 tph (reuse only),
21.6 tph (regeneration-reuse), and 12 tph (regeneration-recycling).
Their procedure for wastewater minimization was based on the concen-
tration interval diagram and the concentration composite curve as intro-
duced by Wang and Smith (1994). The results obtained using
that procedure compared well with the results obtained from the well-
established software ASPEN WATER, which uses a mathematical
programming approach based on MINLP (Mixed-Integer Non-Linear
Programming). The cost-benefit analysis illustrates that the profit obtained
in the case of reuse is substantial, and the payback periods for the
regeneration-reuse and regeneration-recycling are 1.8 and 1.1 months
respectively.

12.2.6 Zinc Refinery
The metal and mining industries consume large amounts of energy and
water and produce large volumes of waste. Water is probably the most
widely used raw material in the process industry.
Bhikha et al. (2011) conducted a study involving reduction of water con-
sumption at a zinc refinery. They used both simulation and water pinch
technology for this purpose. The aim of the simulation was to obtain a work-
ing mass balance of the operation, from which different water minimization
scenarios could be investigated. The simulation process was critical because
water quality affects key threshold concentrations that, if exceeded, affect
final zinc product purity. Due to the complexity of the process, with numer-
ous recycles and variables, a number of simulations were performed. Each
simulation increased in complexity until a final working simulation was
obtained that modeled the operating conditions of the plant as closely as pos-
sible. The study showed that supplementation of demineralized fresh water
with demineralized effluent water results in a 14% reduction in water usage,