Improved compact accelerated precipitation softening (CAPS)

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 a nd growth in 2 regions: a) in a pre-prepared slurry made of calcite small p articles and, b) within a CaCO3 layer (cake) formed on the top of the filte r through which water is pumped out. Whilst the largest degree of precipita tion occurs within the slurry, the cake process is a polishing step in whic h calcium concentration is reduced further. Within the dense cake structure , the removal of the smaller calcium concentrations is possible within shor t contact times due to enhanced mass transfer rates made possible by large solution velocities within narrow pores and much larger surface to volume r atio. CAPS was first suggested for water softening [1,2] and later tested f or the possibility of simultaneous silica removal [3]. CAPS was also studie d as a pretreatment for RO [4] with water taken from fish ponds. The capabi lity of reducing SDI, organics and hardness to levels satisfactory for prol onged RO treatment was demonstrated. In [4], water was mixed with CaCO3 par ticles, 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 conjun ction with pressure and electrical driven membrane processes (UF, NF, RO, E D) as an effective pretreatment routine for increasing recovery and decreas ing 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 a ttractive due to compactness and the process more attractive technically an d in terms of cost. The advantages of in-tank filtration were appreciated i n the past and it has been a subject for intensive investigation [5,6]. Lab oratory 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 ini tial CaCO3 Slurry concentration; residence time or pumping rate; pH; backwa sh frequency, duration and mode (dry or wet) and slurry mixing rate was inv estigated and analyzed in terms of Saturation Index (SI) reduction, separat ed effects of the slurry and the cake on the softening action and filter ca ke load.