Pulp and paper mills are facing the possibility of stricter effluent discharge limits. End-of-pipe
treatment for discharge no longer guarantees compliance, nor is it the most cost-effective way of
solving mills' effluent problems. In this dissertation, water pinch analysis is used as a tool to
determine the optimum effluent treatment conditions to ensure compliance at the least cost to the mill.
It is also shown that the environment and the mill can benefit simultaneously if the correct effluent
discharge philosophy is implemented.
Mill simulation results were used to set up a water pinch analysis model. Maximum permissible inlet
concentrations were specified for all process units. Mass transfer equations were used to describe the
relationships between inlet and outlet concentrations of the process units. A number of generic
effluent treatment units with preset performance specifications were added to the pinch model. These
treatment units can be sized and used in an optimal way by the pinch model to obtain an optimum
effluent treatment and recycling scheme. Capital and operating costs for different treatment units were
included in the analysis. The capital cost for treatment units decrease as the volume treated by the unit
decreases. The operating cost is generally expressed in terms of volume; however, certain treatment
units have treatment cost expressed on load treated rather than volume.
The validity of the results obtained from WaterPinch™, the pinch analysis software used for building
the pinch model, was checked by using a process simulation package, WinGEMS™, to simulate the
proposed effluent treatment scenarios. This step ensured that the mass transfer relationships used in
the water pinch model were valid. This was an important part of the work, as the results generated by
the optimisation model have to be reliable in order to make the results obtained applicable to the mill.
The verified water pinch model was used to find optimum treatment plant layouts for different
effluent discharge volume and concentration specifications. This resulted in an optimum-cost profile
for a range of effluent discharge volumes and concentrations.
Optimum-cost profiles could be a decision making tool in the negotiation between the mill and the
regulatory authority to set effluent discharge regulations in such a way that the environment benefits
without unnecessarily restricting economical and social development of the region.
Using optimum-cost profiles, the differences between a load-based and a concentration-based
discharge permit was illustrated. Comparing the pinch analysis results for these scenarios showed that
the mill has no financial incentive to reduce effluent volume if a concentration-based permit is in
place. However, a load-based permit could make it financially viable for a mill to reduce effluent
volume and load rather than to simply treat and discharge. It is also shown that both the mill and the
environment (river) benefit from a load based permit.
The impact of possible future waste discharge charges on the economical feasibility of various
effluent treatment options is also investigated. The results indicate that the implementation of waste
discharge charges will only benefit the environment if it is linked with a load-based effluent discharge
permit. This illustrates the usefulness of pinch analysis as a basic risk analysis and risk management
tool. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/9040 |
| Date | January 2005 |
| Creators | Mansfield, Maryna. |
| Contributors | Brouckaert, Christopher J. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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