No abstract

A two‐element membrane pressure vessel was installed as a “canary” unit on a full‐scale reverse osmosis (RO) membrane process to monitor scale formation while changes in the plant's pretreatment processes were implemented. Sulfuric acid pretreatment ahead of an RO process was to be progressively reduced and discontinued per the findings of a pilot study. To safeguard the full‐scale membrane process against unforeseen failure caused by scaling when sulfuric acid was eliminated from the RO feedwater, the canary unit was introduced. Concentrate from the second stage of the RO process train served as the feed stream to the canary unit, thereby allowing the canary to provide an earlier indication of scaling in lieu of scaling the full‐scale RO process. The canary unit was intended to act as an early warning to operators who could intervene to protect the RO plant from unforeseen loss of membrane productivity resulting from scaling.

This research identified unintended consequences of integrating ultrafiltration and its required ancillary cleaning systems within conventional surface water treatment facilities. Carboxylic acids, used in ultrafiltration membrane chemically enhanced backwashes, were demonstrated to interfere with ferric chloride and alum coagulation if recycled into coagulation basins at sufficient acid to coagulant (A/C) molar ratios. Tricarboxylic citric acid and monocarboxylic acetic acid were shown to interfere with conventional coagulation process performance. Significant changes in settled water turbidity, true colour and metal concentrations were observed in jar tests designed to simulate fullscale water treatment plant operations. A threshold A/C molar ratio for coagulation interference was identified for three surface waters in the United States based on sedimentation basin performance goals established by the US Environmental Protection Agency. Citric acid interfered with coagulation at A/C molar ratios as low as 0.028; whereas acetic acid negatively influenced coagulation at A/C molar ratios in excess of 18.0.

Dubai Municipality awarded to Porr Besix JV the Project for the Main Tunnel component of the Deep Storm Water System. The tunnel will collect both rainwater and groundwater from approximately 500 sq. km and transfer the captured flow to the sea. The Design Builder JV selected COWI as Designer of the entire Project and IC Consultant as Design Checker for the Tunnels. The Project includes approximately 10.3 km of 10‐meter‐inside diameter tunnel in rock, three construction shafts and one drop shaft. The main tunnel will convey stormwater and groundwater flows from the EXPO 2020 area near the intersection of Sheikh Mohammed Bin Zayed Road and Jebal Ali Lehbab Road to the sea close to the EGA facility. The tunnel will follow beneath the road easement along Jebal Ali Lehbab Road and along Sheikh Zayed Road and continue to the pumping station. The tunnel traversed through the Barzaman and Fars formation with an overburden of 33 m with maximum water pressure of 4.4 bar and was excavated by EPB TBMs. This project is characterized by its dimensions with an internal diameter of 10 m and 350 mm of segment thickness, and by the use of steel fibre reinforced concrete in the precast segmental lining. The use of fibres aims to reduce the CO2 footprint obtaining an optimized design from the environmental point of view. These facts are associated to a complex design of precast segments, in order to ensure their structural competence and their integrity according to the durability requirements, under large thrust forces (temporary loads) and permanent load. Hence, considering such complexities, the structural design has been carried out producing a 3D structural model by means of a sophisticated FEM structural software. Results of the model allow to identify areas of the segment where spalling and bursting stresses are generated along circumferential joints and maximum value of those stresses in the temporary load cases. Moreover, a structural design verification of the segment has been undertaken considering the contribution of steel fibres class 4c, as it is set up in the FIB model code, aiming to ensure that the precast segments are structurally competent and fulfil the durability requirements of the Project. The article details the design approach and the independent checker design verification approach. The experience gained during construction is also reported, describing challenging aspects of the Tunnel execution and an analysis of the lining damages.

A two-element membrane pressure vessel was installed as a "canary" unit on a full-scale reverse osmosis (RO) membrane process to monitor scale formation while changes in the plant's pretreatment processes were implemented. Sulfuric acid pretreatment ahead of an RO process was to be progressively reduced and discontinued per the findings of a pilot study. To safeguard the fullscale membrane process against unforeseen failure resulting from scaling when sulfuric acid was eliminated from the RO feedwater, the canary unit was introduced. Concentrate from the second stage of the RO process train served as the feed stream to the canary unit, thereby allowing the unit to provide an earlier indication of scaling in lieu of scaling the full-scale RO process. The canary unit was intended to act as an early warning to operators who could intervene to protect the RO plant from unforeseen loss of membrane productivity resulting from scaling. Expanded SummaryThe increasing availability of proprietary scale inhibitors that can inhibit and control carbonate, sulfate, and fluoride scales of calcium, strontium, and barium, in addition to inorganic foulants composed of iron, aluminum, and reactive silica, has resulted in a re-evaluation of the need to suppress the pH of feedwater to RO membranes (Abdulrazaq, 2011;Ning & Netwig, 2002;Butt et al, 1995). With this in mind, a Florida utility operating a 4.5-mgd, two-stage brackish water RO process, operating at 75% recovery rate, conducted pilot-testing to evaluate the elimination of sulfuric acid in its pretreatment process (Tharamapalan et al, 2013). The pilot-scale evaluation was carried out over a fourmonth period. Upon successful operation of the RO pilot without acid pretreatment, the researchers, in cooperation with the utility, instituted a robust monitoring system as the acid use was progressively eliminated from its process. A two-membrane element pressure vessel was proposed and developed as a sentinel against unforeseen failure during the staged elimination of sulfuric acid from the full-scale plant.The two-membrane element pressure vessel was called a canary unit. It was designed to serve as a sacrificial membrane so that if scaling occurred, the membranes in the unit would fail before the full-scale process. To accomplish the goal of the canary unit serving as a scaling-potential indicator, the unit was connected downstream of the second stage of one of three 1.5mgd RO process trains that made up the RO plant. With only high-pressure feed pumps before the first stage and no intermediate boosting in the process system, the feed pressure to the canary unit was the residual concentrate pressure from the second stage. METHODSPilot-testing of the acid-elimination plan was carried out first under the same conditions as the full-scale plant with a 2-mg/L dose of a scale inhibitor. The staged elimination program during the pilot study involved three incremental pH steps to bring the RO feedwater pH up to ambient water pH levels-5.8 (prevailing pH suppressed condition), 6.3, 6.6, ...