Water & Wastewater Treatment

36 | FEBRUARY 2016 | WWT | www.wwtonline.co.uk In the know Technically speaking: wastewater treatment R estrictions for discharging wastewater into the environment have been tightened all over Europe, and in addition to limiting the release of organic and inorganic substances, pH threshold values are de ned within the regulations governing direct and indirect discharge. The selection of chemicals used for the neutralisation of acidic waste streams is almost as important as the design of the neutralisation system Neutralisation solutions While caustic soda is widely used in neutralising acidic wastewater streams, advanced liquid lime slurries are now able to off er a compelling alternative XAVIER MEAR PRODUCT MANAGER LHOIST UK itself. Typically, the considerations span areas of health and safety, storage, speci c consumption and ultimately, cost. But how about performance? Does it do what I need it to do? Can it do it better? Sodium hydroxide, or caustic soda, is the most widely used alkaline neutralisation reagent, e ective for both strong and weak acids. However, it is calcium hydroxide, in the form of liquid lime that o ers signi cant advantages when an e€ uent stream needs to undergo several process steps before being compliant for site discharge. This is particularly true where the precipitation of metals or ‚ uorides is the aim. Classic cases of lime separating out from solution, blocking both storage tanks and delivery lines, have meant that some sites have opted for the easier handling of caustic soda, and favouring additional chemicals to complete the treatment. However, advanced lime products are now available – such as Lhoist's Neutralac SLS45 – that combine the advantages of a ‚ uid reagent such as caustic soda, with the bene ts that calcium hydroxide brings. This liquid lime, multi-functional reagent di ers extensively from the common types of lime slurries available and can compete with 50 % caustic soda. Developed especially for the treatment of wastewater from municipal and industrial processes, Neutralac SLS45 is characterised by a high solid content, but a low viscosity, and can manifest itself as an e ective reagent even at those sites that are dosing <15l/hr caustic soda or where previously, consideration of lime would be linked with historic issues of dust and blocked lines. Neutralisation benefi ts The e€ uent plant represents an important part of the manufacturing process. As a bottle-neck, it can impede manufacturing throughput; operations below par halt it altogether if the treatment is unable to meet the demands of environmental compliance. Indeed, foresight that considers impending tightening of compliance limits, or looking forward to growing manufacturing output but still relying on a treatment plant from yesteryear, can be causes for reconsidering reagents. Frequently, the wastewater stream is loaded with additional contaminants from the production process that need to be removed prior to discharge. As a multi-functional reagent, advanced lime slurry precipitates ‚ uorides, sulphates and phosphates from the wastewater. Heavy metal ions in the wastewater are precipitated in the form of very stable hydroxides, whilst the reagent also contributes to the decomplexation of metal compounds. Increases in contaminant removal e• ciency translate as moving safely A schematic of liquid lime slurry treatment

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