Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | FEBRUARY 2016 | 37 In the know Technically speaking: wastewater treatment The spider diagram shows the advantages of liquid lime against caustic soda The use of lime slurry also imparts an enhanced support structure to the generated sludges, facilitating improved separation behaviour from processing equipment when compared to caustic soda. Economic considerations Away from the chemistry and the technical bene ts, economic advantages need to be considered when assessing a possible switch from caustic soda to an advanced lime slurry. Due to the complexity of some industrial processes, some of which are proprietary, a rigid cost comparison cannot be implemented. Consider instead a cost/bene t ratio that addresses the most common elements of the plant setup, which then supports the use of lime. The price/performance ratio favours Neutralac SLS45 as it is o en cheaper to purchase than standard alkaline agents, and it is not subject to ‚ uctuating, extreme price jumps. This is because it is a speci cally produced reagent, and not bound to the manufacture of other products. In addition, as a multi- functional reagent, the utilisation of extra ‚ occulants needs to be taken into account as these tend to decrease in consumption. Plant Parameters/ Capex Investment for silos, pipe work and most pumps are near identical for both reagents, facilitating the easy conversion from one reagent to another. Lime slurries do require agitation, either in the form of a stirrer or a recirculation system, incurring an additional cost. The high solids content of Neutralac SLS45 is however, markedly stabilised, meaning that periodic agitation is su… cient. Consider the physical properties of caustic soda too. Concentrated caustic soda starts to crystallise once temperatures fall below 5°C and so trace heating and lagging needs to be considered. Lower concentrations of NaOH (<28°C) may alleviate this concern, but part of the NaOH still crystallises, leaving behind a residual solution that is a less concentrated reagent for utilisation. Fixed costs Based on working plants elsewhere, it is noted that maintenance and manpower are signi cantly reduced when moving from NaOH to advanced lime slurry. Sloppy sludge generated from NaOH is replaced by drier lter cakes that are easily removed from lter cloths when using lime. Operators are then able to spend more time on activities that generate revenues, whilst transport costs are minimised as well. to meet environmental consent levels before fi nal discharge. The original treatment used caustic soda, and whilst the process was able to treat the waste stream to meet effl uent consent limits, a number of areas where identifi ed for improvement. Fluctuations around the pH set point were to be eradicated giving greater control of pH levels and reagent utilisation. There was also a desire to increase Cr removal effi ciencies from the waste stream as a means to future-proof the plant, and to lower the moisture content of the fi lter cake. A fi eld trial was set up to validate the Neutralac SLS45 proposal, with Lhoist supplying a stand-alone parallel dosing system for the duration of the trial. A ● A Gloucestershire- based surface engineering company specialises in surface fi nishing operations from plating to blasting. Acid from the manufacturing processes, along with wash water, is transferred to the wastewater treatment plant. Varying in accordance with the production schedule, the average pH is below pH 3. A Cr reduction step with sodium bisulphite, pH correction with caustic soda to pH 8-8.5, and the addition of a polymer, prepares the waste stream to fl ow into a series of settling tanks. Solids are le to settle, ultimately destined for a fi lter press, whilst the liquid waste fl ows through a V-notch weir where the pH is tested ' Case Study – Surface treatment ready-to-go system was selected from Lhoist's range of storage vessels and automatic dosing systems, capable of delivering a wide range of fl ow rates, from thousands of litres per hour to less than 10l/hr. The liquid lime reagent showed immediate benefi ts in the treatment of the acidic waste, with Neutralac SLS45 capable of supplying its high neutralising power to the effl uent plant without blocking or settling within the delivery tubing. The new system reacted quickly and eff ectively to changing pH levels while improving Cr precipitation and superior solids- liquids separation. This produced a drier fi lter cake and therefore a more economical solution to its disposal. Price Non-toxic Non-corrosive Safety Environment Handling Neutralisation Potential Reactivity Neutralac® SLS45 NaOH Heavy Metals Removal Fluoride Removal P/ SO4 Removal Flocculation Sludge Dewatering Price Stability 1 2 3 4 5 Price Non-toxic Non-corrosive Safety Environment Handling Neutralisation Potential Reactivity Neutralac® SLS45 NaOH Heavy Metals Removal Fluoride Removal P/ SO4 Removal Flocculation Sludge Dewatering Price Stability 1 2 3 4 5 away from upper discharge limits, yielding consistent compliance, as well as o" ering breathing space on occasions when it is most required.

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