Development of a semi-batch high density sludge process

The concept of a semi-batch process for the intermittent treatment of effluent containing dissolved heavy metals was proposed. Like the high density sludge (HDS) process, heavy metals are removed by precipitation in the presence of other solids. The effects of cycling, neutralization rate, temperature, and solution composition were investigated. Sludge density, percent solids, sludge viscosity, and settling velocity of the solids generated were used to characterize the system. Experiments were performed on iron (III) and zinc solutions. For the iron system, it was shown that the solids generated were in the form of aggregates rather than discrete particles. Both cycling and lower neutralization rates tended to improve the solids settling velocity. This was attributed to aggregate restructuring under turbulent mixing conditions, and to the local supersaturation conditions created under different neutralization rates. A critical addition rate of 7.5 mL/min was identified, below which little benefit was realized. A modified form of the Richardson-Zaki correlation was developed. With this function, it was shown that the settling velocity of the solids could be described using volume fraction as the single independent variable. This was unexpected, and identifies particle interactions as the dominant factor in determining the settling characteristics of the solids. The zinc system exhibited improvements in the settling velocity with mixing time, while none were realized with the iron solids. The main effect of higher temperatures was to decrease the viscosity of the suspending fluid, giving increases in the solids settling velocity. However, this effect did not fully explain the increase that was found with the zinc solids at higher temperatures