Water Journal : Water Journal August 2012
new products & services 92 AUGUST 2012 water water business Nineteen maintenance holes and shafts were also repaired to provide safer access for crews in the future. This involved applying an epoxy protective coating to deteriorated surfaces, replacing several maintenance hole roofs and replacing corroded fittings. "During the repair work, we removed 70 tonnes of silt and debris. Crews decended 25 metres below ground level to access the 1800mm diameter pipe and worked in live flows, mostly at night. Continuous flow monitoring was done to ensure a safer working environment," John said. As part of the project, Water Infrastructure Group worked closely with Sydney Water, Canterbury City Council and the Wolli Creek Preservation Society to repair a stormwater channel located in bushland near the walking path in Wolli Creek Valley in Earlwood. The existing route for stormwater runoff was diverted to prevent erosion and a bridge built to improve access to the walking trail. A SIMPLE TOOL TO OPTIMISE COAGULATION PROCESSES The removal of suspended and colloidal material from aqueous systems forms the basis of a wide variety of production processes. Coagulation and flocculation operations are key elements in these systems. In most cases chemical additives are used to enhance separation and the focus becomes optimising chemical dose rate for quality and cost control. A number of approaches are available for the optimisation and control of coagulants and flocculants. These include streaming current devices, a model predictive algorithm based on optical characteristics and particle charge demand analysers. The suitability of any system to deliver robust measurement and/or control is based on the underlying measurement assumption. A common philosophy is to use streaming current or streaming potential as an indicator of system charge and, therefore, a control point for chemical dosage. In this approach, an arbitrary potential is selected as a desirable endpoint and coagulants are dosed to achieve this. Apart from conductivity dependence, run-ability and reliability issues with many of these simple devices, there is no rational basis for the selection of a control point. As a result, this type of system can be unreliable if there are changes in the upstream process. In addition, simple streaming potential has no value in quantitative lab evaluations of an aqueous system. 1. An increasingly popular approach is to use a model predictive coagulant (MPC) control system based on an inline spectrolyser. The benefit of this system is that the measurement is repeatable and a range of supplementary water properties can be determined. The disadvantage of MPC type systems is that the predicted optimal coagulant control point is only as good as the input data used to generate the model. This puts a predictive system at a disadvantage when natural events significantly change the upstream process but do not form part of the model. 2. Particle charge detectors quantitavely measure the total surface charges in a suspension. A streaming potential is generated in the measuring cell. Based on the polarity of the potential, a polyelectrolyte of opposite charge and known charge density is titrated until charge neutralisation is achieved. Using this technique, it is possible to directly and quantitatively measure the chemical dose rate required to achieve charge neutralisation. Assuming that the charge neutralisation condition is an optimal coagulation condition, this technique can then directly determine the optimum. Since a particle charge detector measures a known physical condition in a suspension and the measurement assumption (charge neutralisation) represents the optimal coagulation point, it is a very powerful tool in process improvement and control. The measured result is independent of upstream process changes and always reflects a known process state. Both laboratory and online versions are available using this technique. The method can be used to: • Detect and quantify particle charges; • Monitor charge demand in colloids and suspensions; • Optimise coagulant and flocculant dosage; • Evaluate the effectiveness of different coagulants and flocculants; • Act as a proxy for gross system pollutant demand (similar to turbidity but covering a much broader particle size range). The starting point in many optimisation projects is in the laboratory, and the PCD- 04 Particle Charge Detector is a widely used particle charge analyser that is described below. The MütekTM PCD-04 Particle Charge Detector measures the surface charge of colloidal, dissolved and finely dispersed substances in aqueous solutions and suspensions. It delivers reliable results even at high conductivity in a broad range of applications. In the paper industry, the PCD is the standard tool for detecting anionic trash levels and for characterising chemical additives. Identification of charge levels is also very important in water and wastewater treatment, the food and beverage industries, ceramics, colours, textiles, mining and pharmaceutical industries. The MütekTM PCD-04 Travel is a true stand-alone unit suitable for production laboratories or frequent travellers. The standard MütekTM PCD-04 instrument can be combined with an external automatic titrator, which is highly recommended for R&D applications. A pH electrode is an option for both the standard MütekTM PCD-04 and the PCD-04 Travel. The surface charges of colloids and suspended solids in water lead to a concentration of oppositely charged ions, the so-called counterions, at the particle surface. If these counterions are sheared from the particle, a streaming potential can be measured in mV between two suitably located electrodes. A streaming Pipe being relined. Figure 1. Measuring cell of the MütekTM PCD-04.
Water Journal September 2012-1
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