The Site Ion Exchange Plant (SIXEP) (Sellafield, Cumbria, UK) treats spent nuclear fuel storage pond liquors by utilising sand bed filtration to remove Mg containing particulates present in the effluent followed by an ion exchange process to remove Cs and Sr. Clinoptilolite (Na6Al6Si30O72.24H2O) the ion exchange media. After treatment the effluent is discharged to sea. During inspections of the discharge line a significant solid formation was encountered in the mid-2000s. The solid was poorly characterised and limited analysis was performed. At the time biofilm was suggested as the means of adhering a fine fraction of clinoptilolite to the pipeline surface. Three areas of study were investigated in order to identify the source and means of these solids adhering to the discharge pipeline surface. These were: the precipitation of mineral phases from a variety of possible discharge effluent compositions; the dissolution/degradation of clinoptilolite under process like conditions including É£-radiation exposure; and the demonstration of the performance of polyelectrolyte as an adsorption medium when contacted with a clinoptilolite solution onto stainless steel surfaces. Mineral phases from a range of likely SIXEP effluent compositions have been precipitated from static solution conditions. The predominantly aluminosilicate containing solids have similar characteristics (elemental composition and morphology) to the solid from the discharge line, but this is based upon limited and poor characterisation data from the recovered material. Dissolution studies of clinoptilolite displayed a fine fraction release into solution irrespective of the inclusion of a prewashing step. An incongruent release of Si from clinoptilolite was observed which was not considered in the original model put forward for this pipe fouling phenomenon. The presence of colloidal Si was found to increase in irradiated clinoptilolite samples (50 MGy total dose) when compared to un-irradiated material. Polyelectrolyte was found to cause fine fractions of clinoptilolite to adhere to a stainless steel surface (304L) and as a result provides an additional attachment mechanism for solids within the pipeline to what has been previously proposed. Following this work, it is suggested that the primary mechanism to justify this solids formation in the SIXEP discharge line is that the presence of polyelectrolyte and biofilms causes adsorption of clinoptilolite fines and colloids onto the inner pipe surface. Reprecipitation of aqueous species released through incongruent dissolution of the clinoptilolite should be considered as minor bulking agents to the main clinoptilolite fines solid.