Optimization of Calcium Fluoride Crystallization Process for Treatment of High-Concentration Fluoride-Containing Semiconductor Industry Wastewater.

This study utilized a fluidized bed reactor (FBR) for fluoride removal from high-concentration fluoride-ion-containing simulated semiconductor industry wastewater and recovered high-purity CaF2 crystals. The effects of hydraulic retention time (HRT), pH, Ca2+ to F- ratio, upflow velocity, seed size and seed bed height were investigated by performing lab-scale batch experiments. Considering fluoride removal and CaF2 crystallization efficiency, 5 h HRT, pH 6, seed height of 50 cm and [Ca2+]/[F-] ratio of 0.55 (mol/mol) were found to be optimum. The effect of the interaction between the important process parameters on fluoride removal was further analyzed using response surface methodology (RSM) experimental design. The results showed that all the individual parameters have a significant impact (p = 0.0001) on fluoride removal. SEM-EDX and FTIR analysis showed the composition of the crystals formed inside FBR. HR-XRD analysis confirmed that the crystalline structure of samples was mainly CaF2. The results clearly demonstrated the feasibility of silica seed material containing FBR for efficient removal and recovery of fluoride as high-purity calcium fluoride crystals.

Process Intensification for Enhanced Fluoride Removal and Recovery as Calcium Fluoride Using a Fluidized Bed Reactor.

This study explored the feasibility of fluoride removal from simulated semiconductor industry wastewater and its recovery as calcium fluoride using fluidized bed crystallization. The continuous reactor showed the best performance (>90% fluoride removal and >95% crystallization efficiency) at a calcium-to-fluoride ratio of 0.6 within the first 40 days of continuous operation. The resulting particle size increased by more than double during this time, along with a 36% increase in the seed bed height, indicating the deposition of CaF2 onto the silica seed. The SEM-EDX analysis showed the size and shape of the crystals formed, along with the presence of a high amount of Ca-F ions. The purity of the CaF2 crystals was determined to be 91.1% though ICP-OES analysis. Following the continuous experiment, different process improvement strategies were explored. The addition of an excess amount of calcium resulted in the removal of an additional 6% of the fluoride; however, compared to this single-stage process, a two-stage approach was found to be a better strategy to achieve a low effluent concentration of fluoride. The fluoride removal reached 94% with this two-stage approach under the optimum conditions of 4 + 1 h HRT combinations and a [Ca2+]/[F-] ratio of 0.55 and 0.7 for the two reactors, respectively. CFD simulation showed the impact of the inlet diameter, bottom-angle shape, and width-to-height ratio of the reactor on the mixing inside the reactor and the possibility of further improvement in the reactor performance by optimizing the FBR configuration.

Sequential combination of nanofiltration and ettringite precipitation for managing sulfate-rich brines.

The sequential combination of nanofiltration (NF) and ettringite precipitation to manage sulfate-rich brine is proposed. In this study, NF experiments clearly demonstrated that sulfate-containing wastewater was effectively concentrated by the NF process (concentrate factor, CF > 5) with insignificant membrane fouling. Ettringite precipitation was implemented as an alternative to lime precipitation to process sulfate-rich brine resulting from the NF operation. More than 93% of the sulfate ions were removed by ettringite precipitation, whereas lime precipitation removed less than 28% under the same conditions due to the difference in their solubility. However, with highly concentrated NF brine (CF > 5), the pH and sulfate concentration of the supernatant were higher than the discharge limit. Therefore, optional blending of the supernatant after ettringite precipitation with the NF permeate was proposed to satisfy the discharge limit for sulfate. The sequential operation consisting of NF and ettringite precipitation enables sulfate-rich wastewater to be treated effectively, minimizing its negative impact by reducing the brine volume and enabling the water to be reused.

Electroflotation clarifier to enhance nitrogen removal in a two-stage alternating aeration bioreactor.

Stringent water treatment criteria and rapidly growing pollutant loads provoke the demand for retrofitting wastewater treatment plants towards a higher capacity. In this study, we assess a two stage alternating aeration (AA) bioreactor equipped with electroflotation (EF) clarifier, for nitrogen removal within a short hydraulic retention time (HRT). The EF under steady solids loading required a minimum unit height and gas: solids ratio of 0.006 for efficient clarification. The separated sludge blanket was further thickened with retaining stability when the cyclic solids loading was smaller than 1.0 kg m(-2). In the continuous operation of the bioreactor, the returned activated sludge concentration increased to more than 18,000 mg L(-1), while the effluent suspended solids concentration was lowered below 5 mg L(-1). Under influent chemical oxygen demand (COD)/total inorganic nitrogen (TIN) concentration of 300/30 mg L(-1), the TIN removal efficiency was near 70% with cycle time ratios of 0.17 and 0.27. Under higher influent COD concentration of 500mg L(-1), TIN removal efficiency was found to be 73.4% at a carbon:nitrogen (C:N) ratio of 10 and even higher (80.4%) at a C:N ratio of 16.6. The increased mixed liquor suspended solids concentrations (> 6000 mg L(-1)) under the high COD loading were efficiently maintained by using the EF clarifier. The results of this study demonstrate that an EF clarifier with a HRT of less than 1 h can support reliable nitrogen removal in the AA process that has a HRT of 6 h, even under increasing influent loadings.

Alleviation of membrane fouling in a submerged membrane bioreactor with electrochemical oxidation mediated by in-situ free chlorine generation.

The control of membrane fouling is still the biggest challenge that membrane bioreactor (MBR) for wastewater treatment faces with. In this report, we evince that an in-situ electrochemical free chlorine generation is effective for membrane fouling mitigation. An electrochemical oxidation (EO) apparatus with perforated Ti/IrO2 anodes and Ti/Pt cathodes was integrated into a conventional MBR with microfiltration module (EO-MBR). The membrane fouling characteristics of EO-MBR fed with synthetic wastewater were monitored for about 2 months in comparison to control MBRs. In the EO-MBR at a direct current density of 0.4 mA/cm(2), the frequency of membrane fouling when the trans-membrane pressure (TMP) reached 30 kPa was effectively reduced by 40% under a physical membrane cleaning regime. The evolution patterns of TMP together with hydraulic resistance analysis based on resistance-in-series model indicated that the electrochemically generated active chlorine alleviated the physically irremovable membrane fouling. Further analysis on extracellular polymeric substances (EPS) of sludge cake layer (SCL) revealed significant reductions of protein contents in soluble EPS and fluorescence emission intensities from humic acids and other fluorophores in bound EPS, which in-turn would decrease the hydrophobic accumulation of organic foulants on membrane pores. The chlorine dosage from the EO apparatus was estimated to be 4.7 mg Cl2/g MLVSS/day and the overall physicochemical properties (bio-solids concentration, floc diameter, zeta-potential) as well as the microbial activity in terms of specific oxygen utilization rate and removal efficiency of dissolved organic carbon (>97%) were not affected significantly. A T-RFLP (terminal restriction fragment length polymorphism) analysis suggested noticeable shifts in microbial community both in mixed liquor and sludge cake layer. Consequently, our electrochemical chlorination would be an efficient fouling control strategy in membrane-based water treatment processes where additional electricity consumption and cathodic scale deposition are not of serious concerns.

Continuous clarification and thickening of activated sludge by electrolytic bubbles under control of scale deposition.

Electroflotation (EF) was investigated as a final clarification of an activated sludge process, to intensify its novel clarification and thickening efficiency. During operation of a biological reactor combined with an EF clarifier, deterioration of clarification efficiency was observed. Scale deposition on electrodes caused a coarse electrode surface, significantly increasing the size of the electrolytic bubbles. The average bubble size was initially 34 microm and increased to 80 microm after bulk cell electrolysis for 150 h. X-ray diffractometry and scanning electron microscopy further characterized the scale deposition as a cluster of calcite (CaCO(3)) and brucite (Mg(OH)(2)). Switching the polarity of electrical current clearly alleviated the increase of bubble size, when applied before scale growth. Under the control of scale deposition, excellent clarification was observed, with the effluent turbidity consistently lower than 2 NTU. An efficient thickening, with the concentration of return activated sludge higher than 15 g L(-1), was additional advantage of the EF clarifier.