Towards achieving SDG-6 in steel industry: A superstructure optimization-based approach

The increasing water scarcity and stringent norms on environmental pollution control impose a demand on the development of sustainable methods for water use in process operations. The steel industry is one such highly water-intensive industry with a water requirement of 2–4 m3/ton of steel and a production of 1950 million tons in 2021 globally. Optimum water network synthesis by recycling, regeneration, and reuse of water, is one of the ways of sustainable management of water in industries. This study explores several opportunities for water network optimization in the steel industry, through appropriate water flow targeting. The opportunities for direct reuse of blowdown water, or water reuse after regeneration of blowdown and process loss water as makeup water, are explored. These opportunities are evaluated algorithmically using a superstructure-based optimization framework. Realistic operational constraints such as the hardness of makeup water and the Langelier Saturation Index (LSI) value of blowdown water are also incorporated in the developed formulation. The interplay between the cycles of concentration (CoC) and blowdown water quality is also studied, first with a representative example problem and then on a representative steel complex network. The advantages of inter-plant water reuse over intra-plant water reuse and the effect of pumping cost on the optimized water network are demonstrated. The result shows that operating cost and freshwater requirement can be reduced to around 50 % by inter-plant reuse and regeneration of blowdown and process loss water through appropriate recycle streams, and by optimizing the operating CoC of the cooling towers in the cooling systems. It is also observed that the cooling towers should be operated locally at the resultant optimal CoC as compared to the conventional method of operating all at the highest possible blowdown concentration to achieve maximum freshwater savings in the water system.