Continuous targeting and network design for zero wastewater discharge in water system integration

Zero wastewater discharge (ZWD) networks are targeted and designed in this work by an analytical approach not requiring graphical constructions. The approach is based on three fundamental equations that entail the following requirements for ZWD: (1) the system water loss is entirely made up by freshwater; (2) the net system contaminant load equals the regeneration load; and (3) the net below-pinch contaminant load is picked up by the minimum regenerated water and freshwater. The Unified Targeting Algorithm (UTA) is used to determine the minimum regeneration flowrate and identify the pinch concentration. Necessary conditions are established for pinch jumps and for ZWD feasibility in terms of the maximum post-regeneration outlet concentration. Importantly, continuous targeting is possible over the entire feasible ZWD range through two elegant formulas, wherein (1) the regeneration flowrate is inversely proportional to the concentration reduction during regeneration and (2) the pre-regeneration concentration decreases linearly with post-regeneration concentration provided the pinch is held by the same point. The Enhanced Nearest Neighbors Algorithm (NNA) is utilized to systematically synthesize ZWD networks over the entire ZWD range and ascertain the network topology valid within sub-ranges. Case studies, including one for a paper mill and another for an alumina plant, are presented to illustrate the versatility of the approach in generating superior practical designs for ZWD networks involving fixed contaminant-load and fixed flowrate processes with and without water loss.