CAPS is a softening process in which adjusting water pH to the range 8-10.5 reduces calcium and carbonate alkalinity by accelerated CaCO3 nucleation and growth in 2 regions: a) in a pre-prepared slurry made of calcite small particles and, b) within a CaCO3 layer (cake) formed on the top of the filter through which water is pumped out. Whilst the largest degree of precipitation occurs within the slurry, the cake process is a polishing step in which calcium concentration is reduced further. Within the dense cake structure, the removal of the smaller calcium concentrations is possible within short contact times due to enhanced mass transfer rates made possible by large solution velocities within narrow pores and much larger surface to volume ratio. CAPS was first suggested for water softening [1,2] and later tested for the possibility of simultaneous silica removal [3]. CAPS was also studied as a pretreatment for RO [4] with water taken from fish ponds. The capability of reducing SDI, organics and hardness to levels satisfactory for prolonged RO treatment was demonstrated. In [4], water was mixed with CaCO3 particles, the slurry was circulated through a microfiltration module and the clear and softened permeate was then RO treated with a recovery rate above 80%. CAPS may be used as a stand-alone water treatment process or in conjunction with pressure and electrical driven membrane processes (UF, NF, RO, ED) as an effective pretreatment routine for increasing recovery and decreasing fouling rates. In this work, a new concept for CAPS, which comprises in-tank mixing and filtration is presented. This makes the CAPS device more attractive due to compactness and the process more attractive technically and in terms of cost. The advantages of in-tank filtration were appreciated in the past and it has been a subject for intensive investigation [5,6]. Laboratory CAPS units were run continuously (up to 250 h) and for shorter time periods in order to investigate tap water softening. The effect of the initial CaCO3 Slurry concentration; residence time or pumping rate; pH; backwash frequency, duration and mode (dry or wet) and slurry mixing rate was investigated and analyzed in terms of Saturation Index (SI) reduction, separated effects of the slurry and the cake on the softening action and filter cake load.