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 ± 4.2 A m(-2) and minimum power requirements of 2.4 ± 0.5 kWh kg Fe(-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.

Understanding colloidal FeSx formation from iron phosphate precipitation sludge for optimal phosphorus recovery.

The use of sulfide to form iron sulfide precipitates is an attractive option for separation and recovery of phosphorus and ferric iron from ferric phosphate sludge generated in wastewater treatment. The key factors affecting the simultaneous generation and separation of iron sulfide precipitates and phosphate solution from ferric phosphate sludge have so far not been thoroughly investigated. This study therefore focuses on the recovery of phosphorus from synthetic sludge by controlled sulfide addition under different operating conditions. The factors that affect the phosphorus recovery, as well as the optimal process conditions to achieve an effective solid-liquid separation, were investigated. The separation of the FeSx particles is a significant challenge due to the colloidal nature of the particles formed. Faster separation and higher phosphorus recovery was achieved when operating at pH 4 with dosing times of at least 1h. At this pH, phosphorus recovery of 70±6% was reached at the stoichiometric S/Fe molar ratio of 1.5, increasing to over 90% recovery at a S/Fe molar ratio of 2.5. Zeta potential results confirmed the colloidal nature of the iron sulfide precipitate, with the isoelectric point around pH 4, explaining the fast separation of the FeSx particles at this pH.

Impact of effluent organic matter on low-pressure membrane fouling in tertiary treatment.

This study aims at comparing low-pressure membrane fouling obtained with two different secondary effluents at bench and pilot-scale based on the determination of two fouling indices: the total fouling index (TFI) and the hydraulically irreversible fouling index (HIFI). The main objective was to investigate if simpler and less costly bench-scale experimentation can substitute for pilot-scale trials when assessing the fouling potential of secondary effluent in large scale membrane filtration plants producing recycled water. Absolute values for specific flux and total fouling index for the bench-scale system were higher than those determined from pilot-scale, nevertheless a statistically significant correlation (r(2) = 0.63, α = 0.1) was obtained for the total fouling index at both scales. On the contrary no such correlation was found for the hydraulically irreversible fouling index. Advanced water characterization tools such as excitation-emission matrix fluorescence spectroscopy (EEM) and liquid chromatography with organic carbon detection (LC-OCD) were used for the characterization of foulants. On the basis of statistical analysis, biopolymers and humic substances were found to be the major contribution to total fouling (r(2) = 0.95 and r(2) = 0.88, respectively). Adsorption of the low molecular weight neutral compounds to the membrane was attributed to hydraulically irreversible fouling (r(2) = 0.67).

Occurrence of N-nitrosodimethylamine precursors in wastewater treatment plant effluent and their fate during ultrafiltration-reverse osmosis membrane treatment.

The formation of N-nitrosodimethylamine (NDMA) is of major concern among wastewater recycling utilities practicing disinfection with chloramines. The NDMA formation potential (FP) test is a simple and straightforward method to evaluate NDMA precursor concentrations in waters. In this paper we show the NDMA FP results of a range of tertiary wastewater treatment plants that are also the source for production of recycled water using an Ultrafiltration - Reverse Osmosis (UF-RO) membrane process. The results indicate that the NDMA FP of different source waters range from 350 to 1020±20 ng/L. The fate of these NDMA precursors was also studied across the different stages of two Advanced Water Treatment Plants (AWTP) producing recycled water. These results show that more than 98.5±0.5% of NDMA precursors are effectively removed by the Reverse Osmosis (RO) membranes used at the AWTPs. This drastically reduces any potential for re-formation of NDMA after the RO stage even if chloramines may be present (or added) there.

Operating boundaries of full-scale advanced water reuse treatment plants: many lessons learned from pilot plant experience.

Three Advanced Water Treatment Plants (AWTP) have recently been built in South East Queensland as part of the Western Corridor Recycled Water Project (WCRWP) producing Purified Recycled Water from secondary treated waste water for the purpose of indirect potable reuse. At Luggage Point, a demonstration plant was primarily operated by the design team for design verification. The investigation program was then extended so that the operating team could investigate possible process optimisation, and operation flexibility. Extending the demonstration plant investigation program enabled monitoring of the long term performance of the microfiltration and reverse osmosis membranes, which did not appear to foul even after more than a year of operation. The investigation primarily identified several ways to optimise the process. It highlighted areas of risk for treated water quality, such as total nitrogen. Ample and rapid swings of salinity from 850 to 3,000 mg/l-TDS were predicted to affect the RO process day-to-day operation and monitoring. Most of the setpoints used for monitoring under HACCP were determined during the pilot plant trials.

Advanced oxidation for indirect potable reuse: a practical application in Australia.

December 2008 marked the completion of Stage 2B of the Western Corridor Recycled Water (WCRW) Project in South East Queensland, Australia. With a maximum combined production capacity of 232 million litres of purified recycled water a day, it is the third largest recycled water scheme in the world and the largest in southern hemisphere. A seven-barrier approach has been used to ensure very highest quality, safe water is produced at all times for the purpose of indirect potable reuse. Three of these barriers occur in the advanced water treatment section of the WCRW Project: micro- or ultra-filtration (MF), reverse osmosis (RO), and H(2)O(2)/UV advanced oxidation. In addition to providing very efficient disinfection, the advanced oxidation process specifically aims at destroying compounds not fully rejected by RO that are potential health hazards. This includes N-nitrosodimethylamine (NDMA), which is a potential carcinogenic product likely to be formed by chlorination or chloramination of wastewaters. As in many other countries, Australia has adopted a stringent guideline limit for this compound of 10 ng/L in purified recycled water. After 16 months of operations of the WCRW Project's first plant, the advanced oxidation system has been proven effective in removing NDMA and ensuring 100% compliance with the regulation at a controlled cost.