Superfine Powdered Activated Carbon (S-PAC) Coupled with Microfiltration for the Removal of Trace Organics in Drinking Water Treatment

Anthropogenic contaminants - such as pharmaceuticals and personal care products - are an area of emerging concern in the treatment of drinking water. An integrated activated carbon membrane coating consisting of superfine powdered activated carbon (S-PAC) with particle size near or below one micrometer was explored to enhance removal of trace synthetic organic contaminants (SOCs) from water. S-PAC was chosen for its fast adsorption rates relative to conventionally sized PAC and atrazine was chosen as a model SOC. S-PAC and microfiltration membranes have a symbiotic relationship; membrane filtration separates S-PAC from water, while S-PAC adds capacity for a membrane process to remove soluble components. Three aspects of S-PAC in conjunction with membranes were examined, fouling by S-PAC on the membrane, effects of S-PAC production on material parameters, and modeling of S-PAC adsorption with and without a membrane. Fouling caused by carbon particles can result in marked reduction of filtration rate and an increased cost of operation. Since larger carbon particles foul less than smaller particles, while smaller carbons have faster adsorption performance, states of carbon aggregation were tested for filtration. Particles aggregated using the coagulant ferric chloride resulted in improved flux, while aluminum sulfate and polyaluminum chloride resulted in the same or worse filtration rates. A calcium chloride control showed that increased effective particle size via divalent bridging was very successful in reducing fouling. While particle size increased with conventional coagulants, the unflocculated metal precipitates likely contributed to membrane fouling. The methods of producing S-PAC determine material properties that affect both adsorption and filtration performance. In-house S-PACs - including multiple sizes of several carbon types - were prepared by wet bead milling and measured for both physical and chemical material parameters. Physical parameters, aside from particle size, did not change deterministically with milling duration, although stochastic changes were observed. Chemical measurements revealed a heavily oxidized external particle surface resulting from a high energy milling environment. Surfaces of interior pores appeared to be unaffected. Adsorption via batch kinetics and adsorption via S-PAC coating were modeled with analytical and computational models, respectively, using experimental data produced from the in-house S-PACs. The experimental data showed that removal of atrazine by S-PAC membrane coating correlated most strongly to a combination of oxygen content and the specific external surface area, while membrane fouling correlated to particle size and the specific external surface area. Batch kinetics data were modeled with the homogeneous surface diffusion model (HSDM) while membrane coating data were modeled with computational fluid dynamics (CFD). The fitted models required isotherm parameters indicative of an adsorbent with more capacity than was measured for S-PAC experimentally. Lastly, surface diffusion coefficients were neither constant nor varied with any measured material parameter. However, both model parameters correlated with overall atrazine removal, which indicates that model fits are related to performance, but it is not yet clear how they are connected

Dissolved Carbon Dioxide for Scale Removal in Reverse Osmosis

Membrane fouling is a major operational issue in reverse osmosis (RO) desalination plants. In particular, plants treating brackish groundwater can encounter troublesome inorganic scales, including carbonates, sulfates, and silicates. A novel cleaning method is proposed to remove inorganic scales from fouled RO membranes usinag dissolved CO 2 . As CO2 molecules encounter membrane foulants, the surfaces serve as nucleation sites for small bubbles to form and shear off foulants. Dissolved CO2 solutions were prepared by bubbling CO2 gas into water held in a pressure vessel. Gas dissolution was confirmed by enhanced exit velocities for water containing CO2 , due to the increase in volume from exsolution, when compared to water containing less soluble N2 . A dissolved CO2 solution was effective in removing scale from RO membranes through bubble nucleation. Membranes scaled with CaCO3 were cleaned for 10 minutes with a once-through dissolved CO2 solution of approximately pH 4.5, achieving an average 80% flux recovery. Controls were performed with other cleaning regimes to isolate effects from pH and air scouring present in CO2 cleaning. An HCl solution at pH 3 provided an average flux recovery of 79% after circulating through the system for 30 minutes, while an HCl solution at pH 4 only gave an average 20% flux recovery. Trials using N2 gas in place of CO2 only produced a 6% flux recovery on average. Lowering the pH of the N2 solution to pH 4 with HCl boosted cleaning slightly to an average 8% flux recovery. Thus, the low pH of the CO2 solution at pH 4.5 and bulk phase air scouring are minor mechanisms in scale removal. In addition, membranes scaled with calcium silicates were not cleaned using dissolved CO2 - only NaOH at pH 12 plus sodium dodecyl sulfate provided significant cleaning. Future work should be done with additional scale types to narrow in on the mechanism for cleaning by dissolved CO2