Deacidification through calcium carbonate dosing in combination with ultrafiltration

Soft acidic waters are often treated for drinking water purposes by using limestone filters to attain chemical equilibrium. The present study investigated the process parameters of a relatively new process combination in which powdered calcium carbonate (CaCO3) was added prior to an ultrafiltration (UF). In order to reach the targeted pH value (≥7.8), dosing concentration, type of material and retention time were evaluated in pilot-scale experiments. The deacidification followed the same kinetics as for limestone filtration and yielded similar filtrate characteristics with dosing concentrations of 20 and 40 g/L CaCO3. No significant increase in transmembrane pressure was observed during the operation of a pilot-scale UF module at low flux (34 L m−2 h−1). Critical flux was determined in a lab scale to evaluate the potential impact of CaCO3 particles on the UF operation. Stepping-flux experiments revealed the presence of fouling only at high-dosing concentrations, resulting in a critical flux of 55 L m−2 h−1. At a higher flux, a CaCO3-fouling layer was formed, which decreased the membrane's permeability by 20% over 5 h. Considering that effective air-enhanced backwash and acidic chemical cleanings will be implemented in large-scale applications, the investigated process combination promises to be an appropriate treatment technology for turbid and soft acidic waters.

Simulating fouling impact on the permeate flux in high-pressure membranes

Porous high-pressure membranes have been widely used for saline water desalination. However, fouling (concentration polarization) extensively reduces permeate flux in reverse osmosis (RO) and/or nanofiltration (NF) modules. Fouling arises from pore blocking, organic adsorption, cake formation, inorganic or biological precipitation reducing water flux. Herein, we investigated the effect of feed water with various NaCl concentrations on fouling of RO and/or NF and the permeate water flux. A parabolic (or diffusion) partial differential equation (PDE) was used to model salt concentration profile or gradient inside the membrane. Subsequently, the numerical PDE equation, solved by the forward finite difference (FFD) explicit method, estimated flux decline rates resulted from NaCl fouling. It was found that salt accumulation occurs at the feed-side with a noticeable decrease in flux as fouling increases. Previous works reported similar findings as those identified from our analysis: (1) fouling increases with feed concentration and surface roughness, (2) fouling becomes intensified with higher pressure and flux, (3) fouling from long operation times can reduce flux by 65% within 24 h, (4) NaCl fouling can decrease flux rates by 70% (67-22 LMH) for brackish water with an initial concentration of 10000 ppm, and (5) reversible organic fouling may be avoided from lowering flux rates below the membrane critical flux. Results showed fouled RO modules would decrease flux rates from the increased surface polarization, where reverse flow (negative flux) was estimated for feed-side accumulations >10000 ppm for waters with an initial NaCl concentration of 10000 ppm and average diffusivity of 1.3×10-6 cm2/s.

Analytical Modeling of Particle Interference in Micro-abrasive Jets

Two existing analytical models of particle interference in non-divergent particle jets were modified to include radial symmetry of the particle jet across the nozzle axis, inter-particle spacing, and more complex rebound geometry. Two novel experimental techniques for obtaining the particle spatial and velocity distributions across a micro-abrasive jet were then devised and rigorously tested. One of the above mentioned analytical models was then chosen to be further modified to include the above-mentioned modifications along with the effect of a divergent jet, the ability to simulate high flux cases, and experimentally obtained particle spatial and velocity distributions. All of the models were tested at various jet conditions. The results of the models were quantitatively compared to a previously developed computer simulation and were found to qualitatively agree with previous experimental observations. The modified models allow the critical flux below which inter-particle interference is likely to occur to be determined.

Analytical Modeling of Particle Interference in Micro-abrasive Jets

Two existing analytical models of particle interference in non-divergent particle jets were modified to include radial symmetry of the particle jet across the nozzle axis, inter-particle spacing, and more complex rebound geometry. Two novel experimental techniques for obtaining the particle spatial and velocity distributions across a micro-abrasive jet were then devised and rigorously tested. One of the above mentioned analytical models was then chosen to be further modified to include the above-mentioned modifications along with the effect of a divergent jet, the ability to simulate high flux cases, and experimentally obtained particle spatial and velocity distributions. All of the models were tested at various jet conditions. The results of the models were quantitatively compared to a previously developed computer simulation and were found to qualitatively agree with previous experimental observations. The modified models allow the critical flux below which inter-particle interference is likely to occur to be determined.