At large oceans depths (5000m) manganese nodules are formed, these are rock shaped objects that contain various rare earth metals, Royal IHC is currently developing equipment to bring these nodules to the shore. The nodules will be transported to the water surface using a Vertical Transport System (VTS). In the VTS insight of the location of solid concentrations is required: This enables anticipation of the coming flow at the vessel, it is required for controlling the pumps, it will indicate where plugs are likely to be formed and furthermore it will indicate if the aimed production is achieved. For the vertical transport system it has been proposed to measure the volumetric concentration of solids inside the booster stations located every 1000m and predict the propagation of solids in between these measurements, it was found that this configuration has the disadvantage that measurement error in the booster stations results in an error over the whole length of a riser section. An unknown parameter of the slurry flow is the particle diameter, this parameter influences the transport velocity of the solids. These two topics resulted in the following question: "How can the observation of solid concentrations inside a riser be improved?". In order to evaluate this research question an observer is designed for a scaled test setup of the VTS, on this setup designed improvements will be tested. Started is with an observability analysis, it was found that pressure difference measurements over a large distance of the riser will increase the observability of the system. In order to apply the observer to the test setup it has been investigated how the pressure difference measurements translate into a measured concentration, it was found that the pressure drop created by the wall friction of the mixture can be approximated with liquid wall friction. The Ensemble Kalman Filter was used to observer the concentration through the riser. An addition of the observer is an estimator for the particle diameter: A proposal for this observer is to adapt the particle diameter by using a proportional integral of the lag found between the concentration estimate over a large distance and the concentration measurement at that point. The observer has been applied to measurements of a scaled riser section of 140m. It was found that by using a pressure difference measurement over the whole section of a riser, the concentration observation at the top of the riser can by significantly improved. Using the observer it has been enabled to distinguish different mixtures. There are errors in the outcome of the particle diameter estimate due to the fact that at the conditions of the test setup were not optimal, however it has been proven that by applying the particle diameter observer an improvement can be seen in the concentration observation. A sensitivity of the observer was found to be the relation of the pressure difference measurement to the wall roughness of the riser, the effect of this parameter needs to be accounted for by periodically re-determining the wall roughness.