Temperature restrictions on aqueous effluents dictate that streams with a temperature higher than the permitted level needed to pass through cooling systems to reduce the effluent temperature before discharge. This paper introduces methods for the design of effluent cooling systems. Inappropriate mixing of effluents with different temperatures reduces opportunities to recover heat from effluents and degrades driving forces for cooling systems. A new systematic method is introduced for the segregation strategy for effluents to deal with effluent temperature problems most effectively by a combination of heat recovery and effluent cooling. This can lead to distributed effluent cooling systems. The design procedure sets targets before design. A design procedure then allows the targets to be achieved by following design rules for distributed cooling. An optimisation model has been developed to search for the most economic design of cooling systems. A case study involving retrofit is presented to illustrate the design methodology and the optimisation model of cooling systems.