INTRODUCTION
Dairy plants are considered as ‘wet industry’ because
they consume large volumes of water, which is used
for very diverse purposes. As a result, dairy plants
discharge large volumes of wastewater1. Dairy is one
of the expanding industrial sectors in Pakistan with
currently about 17 units engaged in the production of
various dairy products. Interestingly, the majority of
the dairy plants with varying processing capacities are
located near Lahore, which serves as the hub of this
industry in Pakistan. The Pakistan dairy industry is
the fifth largest dairy industry in the world, with dairy
products forming part of traditional Pakistani diet2.
The dairy industry, like most other agro-industries,
generates wastewater characterized by high biological
oxygen demand (BOD) and chemical oxygen demand
(COD) due to their high organic content3. The
discharge of the polluted water is the most significant
contributor to the pollution of environment from the
dairy industry in terms of both quality and quantity;
contamination by the solid waste and waste gases
are less serious1. The dairy-industry wastewater is
primarily generated from the cleaning and washing
operations in milk processing plants. It is estimated
that about 2% of the total milk processed is wasted
into drains4. Dairy wastewater differs widely both
in quantity and quality depending on a given dairyfactory
production characteristics. Many products
in dairy factories are manufactured separately, thus
pollutant contents in the dairy wastewater at a given
time changes together with the application of another
technological cycle of the processing line5. The dairy
industry is one of the most polluting industries, not
only in terms of the volume of effluent generated, but
also in terms of its characteristics as well. It generates
about 0.2–10 l of effluent per litre of processed milk6.
Biological treatment processes offer a costeffective
method to remove organic compounds and
nitrogen from the wastewater. Treatment designs
are continually evolving to provide greater treatment
efficiency, at a lower cost7. Biological wastewater
treatment is the primary method of preparing foodprocessing
wastewater flows for return to the environment.
Increasing industry wastewater loads on
existing plants and more stringent government discharge
requirements have put considerable pressure
on the food-processing industry to refine and understand
better the design and management of biological
wastewater treatment processes8.
Dairy wastewater is generally treated using biological
methods such as activated sludge process,
aerated lagoons, trickling filters, sequencing batch reactor,
upflow anaerobic sludge blanket reactor, anaerobic
filters, etc9. Biological methods, like activated
sludge process, are invariably employed for the