A novel electrochemical process for the recovery and recycling of ferric chloride from precipitation sludge

During wastewater treatment and drinking water production, significant amounts of ferric sludge (comprising ferric oxy-hydroxides and FePO4) are generated that require disposal. This practice has a major impact on the overall treatment cost as a result of both chemical addition and the disposal of the generated chemical sludge. Iron sulfide (FeS) precipitation via sulfide addition to ferric phosphate (FePO4) sludge has been proven as an effective process for phosphate recovery. In turn, iron and sulfide could potentially be recovered from the FeS sludge, and recycled back to the process. In this work, a novel process was investigated at lab scale for the recovery of soluble iron and sulfide from FeS sludge. Soluble iron is regenerated electrochemically at a graphite anode, while sulfide is recovered at the cathode of the same electrochemical cell. Up to 60±18% soluble Fe and 46±11% sulfide were recovered on graphite granules for up-stream reuse. Peak current densities of 9.5±4.2Am-2 and minimum power requirements of 2.4±0.5kWhkgFe-1 were reached with real full strength FeS suspensions. Multiple consecutive runs of the electrochemical process were performed, leading to the successful demonstration of an integrated process, comprising FeS formation/separation and ferric/sulfide electrochemical regeneration

N-nitrosamine rejection by nanofiltration and reverse osmosis membranes: The importance of membrane characteristics

The influence of membrane characteristics on the rejection of eight N-nitrosamines was investigated using one nanofiltration (NF), one seawater reverse osmosis (SWRO) and six low pressure reverse osmosis (LPRO) membranes. The rejection of the two lowest molecular weight N-nitrosamines, namely N-nitrosodimethylamine (NDMA) and N-nitrosomethylethylamine (NMEA), varied in the range from 8-82% to 23-94%, respectively. In general, the rejection of NDMA and NMEA increased with decreasing membrane permeability. The impact of membrane characteristics became less important for higher molecular weight N-nitrosamines. Among the four LPRO membranes (i.e. ESPA2, LFC3, TFC-HR and 70LW) that are commonly used for water reclamation applications, similar rejections were obtained for NDMA (37-52%) and NMEA (69-82%). In addition, rejection values of NDMA and NMEA among two LPRO membranes (i.e. ESPA2 and 70LW) were almost identical when compared under variable permeate flux and feed temperature conditions. However, it is noteworthy that the ESPABmembrane could achieve very high rejection of NDMA (as high as 71%) despite having a similar permeability to the LPRO membranes. Results reported here suggest that membrane characteristics associated with permeability such as the pore size and thickness of the active skin layer can be a key factor determining N-nitrosamine rejection

A novel electrochemical process for the recovery and recycling of ferric chloride from precipitation sludge

During wastewater treatment and drinking water production, significant amounts of ferric sludge (comprising ferric oxy-hydroxides and FePO4) are generated that require disposal. This practice has a major impact on the overall treatment cost as a result of both chemical addition and the disposal of the generated chemical sludge. Iron sulfide (FeS) precipitation via sulfide addition to ferric phosphate (FePO4) sludge has been proven as an effective process for phosphate recovery. In turn, iron and sulfide could potentially be recovered from the FeS sludge, and recycled back to the process. In this work, a novel process was investigated at lab scale for the recovery of soluble iron and sulfide from FeS sludge. Soluble iron is regenerated electrochemically at a graphite anode, while sulfide is recovered at the cathode of the same electrochemical cell. Up to 60±18% soluble Fe and 46±11% sulfide were recovered on graphite granules for up-stream reuse. Peak current densities of 9.5±4.2Am-2 and minimum power requirements of 2.4±0.5kWhkgFe-1 were reached with real full strength FeS suspensions. Multiple consecutive runs of the electrochemical process were performed, leading to the successful demonstration of an integrated process, comprising FeS formation/separation and ferric/sulfide electrochemical regeneration

Effects of feed solution characteristics and membrane fouling on N-nitrosamine rejection by reverse osmosis membranes

Augmentation of potable water sources with reclaimed municipal wastewater is an option of increasing importance for water security in regions and countries where severe water stress occurs. Consequently, the presence of emerging trace contaminants in reclaimed water has been recognised as a critical issue due to their potential adverse health effects. Notable examples of these trace organic chemicals include N-nitrosodimethylamine (NDMA) and several other N-nitrosamines

N-nitrosamine removal by reverse osmosis for indirect potable water reuse – A critical review based on observations from laboratory, pilot and full-scale studies

N-nitrosodimethylamine (NDMA) and several other N-nitrosamines have been identified as probable human carcinogens. Here, we review key aspects related to the occurrence and removal of N-nitrosamines by reverse osmosis (RO) membranes in the context of indirect potable water reuse. A comprehensive analysis of the existing data reveals significant variations in the rejection of NDMA by RO membranes reported in the literature, ranging from negligible up to 86%. This review article provides some insight into the reasons for such variations by examining the available data on the effects of operating conditions on NDMA rejection. Amongst several operating parameters investigated so far in the literature, feed temperature, membrane permeate flux, feed solution pH and ionic strength were found to have considerable impact on NDMA rejection by RO membranes. In particular, it has been recently shown that seasonal changes in feed temperature (e.g. from 20 to 30 C) can result in a significant decrease in NDMA rejection (from 49% to 25%). However, the combined effects of all operating parameters identified in the literature to date can only account for some of the variations in NDMA rejection that have been observed in full-scale RO plants. The impacts of membrane fouling and particularly chemical cleaning on the rejection of N-nitrosamines have not been fully investigated. Finally, this review article presents a roadmap for further research required to optimise the rejection of NDMA and other N-nitrosamines by RO membranes

Removal of magnetic resonance imaging contrast agents through advanced water treatment plants

Stable gadolinium (Gd) complexes have been used as paramagnetic contrast agents for magnetic resonance imaging (MRI) for over 20 years, and have recently been identified as environmental contaminants. As the rare earth elements (REE), which include Gd, are able to be measured accurately at very low concentrations (e.g. Tb is measured at 7 fmol/kg in this study) using inductively coupled plasma mass spectrometry (ICP-MS), it is possible to determine the fate of this class of compounds during the production of purified recycled water from effluent. Coagulation and microfiltration have negligible removal, with the major removal step occurring across the reverse osmosis membrane where anthropogenic Gd (the amount of Gd attributable to MRI contrast agents) is reduced from 0.39 nmol/kg to 0.59 pmol/kg, a reduction of 99.85%. The RO concentrate has anthropogenic Gd concentrations of 2.6 nmol/kg, an increase in concentration in line with the design characteristics of the plant. The increased concentration in the RO concentrate may allow further development of anthropogenic Gd as a tracer of the fate of the RO concentrate in the environment

Modelling the rejection of N-nitrosamines by a spiral-wound reverse osmosis system: mathematical model development and validation

A mathematical model was developed based on the irreversible thermodynamic principle and hydro- dynamic calculation to predict the rejection of N-nitrosamines by spiral-wound reverse osmosis (RO) membrane systems. The developed model is able to accurately describe the rejection of N-nitrosamines under a range of permeate flux and system recovery conditions. The modelled N-nitrosamine rejections were in good agreement with values obtained experimentally using a pilot-scale RO filtration system. Simulation from the model revealed that an increase in permeate flux from10 to 30L/m2h led to an increase in the rejection of low molecular weight N-nitrosamines such as N-nitrosodimethylamine (NDMA) (from31% to 54%), which was validated by experimental results. The modelling results also revealed that an increase in recovery caused a decrease in the rejection of these N-nitrosamines, which is consistent with the experimental results. Further modelling investigations suggested that NDMA rejection by a spiral-wound system can drop from 49% to 35% when the overall recovery increased from 10% to 50%. The model developed from this study can be a useful tool for water utilities and regulators fo rsystem design and evaluating the removal of N-nitrosamine by RO membranes

Rejection of small and uncharged chemicals of emerging concern by reverse osmosis membranes: The role of free volume space within the active skin layer

Free-volume hole-radii of the active skin layer of one seawater and two low pressure reverse osmosis (RO) membranes - namely SWC5, ESPAB, and ESPA2 respectively - were evaluated using positron annihilation lifetime spectroscopy (PALS) with a slow positron beam. The results were related to the rejection of boric acid and eight N-nitrosamines to provide insights to the transport of these small solutes through RO membranes. At pH 8 (which is the experimental pH in this study), these solutes are uncharged. PALS analysis showed that the SWC5 has the smallest mean free-volume hole-radius (0.259 nm) among the three RO membranes investigated here. Correspondingly, the SWC5 membrane exhibited the highest rejection of boric acid and all N-nitrosamines. Results reported here also showed that the rejection of these chemicals increased in the order of increasing molecular volume. In addition, the difference in their rejection amongst the three RO membranes investigated here was most apparent for those (i.e., boric acid and N-nitrosodimethylamine (NDMA)) with a small molecular volume. The EPSA2 and ESPAB were determined to have mean free-volume hole-radius of 0.289 nm. However, the ESPAB membrane had lower water permeability and showed considerably higher rejection of boric acid and NDMA than the ESPA2 membrane. These results suggest that in addition to the mean free-volume hole-radius, other membrane parameters and properties such as the free-volume hole-radius distribution and thickness of the active skin layer can also play a role in governing the rejection of small and uncharged solutes by RO membranes