Produced water (PW) is wastewater generated from oil exploration, and requires treating for oil and suspended solids removal. The viability of an effluent treatment unit process for this duty is dependent both on its efficacy, in terms of oil removal and - for offshore applications especially - its size, in terms of its area (F-A, m/h) and volume (F-V, h(-1)) footprint per unit volume flow. The incurred footprint applies to both the individual unit (vessel, column or tank) and the collection (or array) of units/vessels in a skid. An assessment of unit process footprint based on available information has been conducted, in particular to the case where high-quality treated water is required for reinjection. The analysis encompasses technical data from specific proprietary technologies as well as generic information for process technology types. Technologies considered comprised hydrocyclones (HCs), induced gas flotation (IGF), media (nutshell) filtration (NSF), and crossflow membrane filtration (CMF). The analysis revealed the HC to incur the smallest area footprint, less than half that of an IGF, notwithstanding only similar to 0.15% of the total skid volume being used for the actual separation process. The CMF had a slightly smaller area footprint and less than half the volumetric footprint of the NSF, if the requirement for backflushing is considered. The fitting of the modular HC and CMF technologies to a skid incurs a considerable increase in the footprint, particularly for the HC where the volume occupancy is increased by an order of magnitude. It was concluded that spatial efficiency gains could be attained for modular processes if spacing of the HC vessels or membrane modules can be reduced, contributing significantly to the viability of CMF in particular. (C) 2014 Elsevier B.V. All rights reserved.