CHEM-E6125 Environmental Management in Industry Water management in industry Other separation methods in wastewater treatment: Evaporation, refrigeration crystallisation, electrolytic flocculation Professor Olli Dahl Learning objectives: 1. Understand the principles of evaporation, refrigeration, crystallisation and electrolytic flocculation. 2. Be able to explain the advantages and disadvantages of each method and describe situation where each has been used in practice within industry. Slide 1, Wastewaters - other Separation Methods EVAPORATION Evaporation is a unit process in which liquid is transformed by external energy into vapour so that dissolved solids can be separated from the feed or input. Condensate: components having a lower boiling point (bp) than water Feed (water based) Concentrate: Components having a higher bp than water Slide 2, Wastewaters - other Separation Methods 1
EVAPORATION Photo: Tanja Kähkönen, 2003 Slide 3, Wastewaters - other Separation Methods EVAPORATION - BASICS Evaporator types: falling film (laskeva kalvo) and rising film (nouseva kalvo); tube or lamellae types. Slide 4, Wastewaters - other Separation Methods 2
EVAPORATION - BASICS Energy consumption optimisation: evaporation is in series and with liquid preheating Slide 5, Wastewaters - other Separation Methods EVAPORATION METHOD PROS AND CONS Plus side: Reduces wastewater quantities substantially Safe and reliable technology Produced condensate is of uniform quality (contains only easily evaporated components no ions) condensate stripping can often be necessary Minus side: Evaporates harmful emissions (uncondensed gases) Maintenance needed always stoppages Fairly large energy consumption Fouling susceptibility Precipitates formed as dissolved components increase (as dry solids concentration rises) Low solubility salts: calcium carbonate, oxalate, sulphate, barium sulphate Organic components (resins amongst others) Slide 6, Wastewaters - other Separation Methods 3
EVAPORATORS - PRACTICAL EXPERIENCE AssiDomän Frövi mills bleachery Q-stage filter - 2-stage evaporation. Clean condensate used in causticisation and concentrate is burned in the recovery boiler. Slide 7, Wastewaters - other Separation Methods EVAPORATORS - PRACTICAL EXPERIENCE At Meadow Lake BCTMP-Mill (Millar Western) evaporation used in wastewater treatment. Slide 8, Wastewaters - other Separation Methods 4
EVAPORATORS - PRACTICAL EXPERIENCE At Chetwynd BCTMP Mill (Louisiana Pacific) evaporation is used for the treatment of internal wastewaters. The condensate formed is dealt with biologically (BOD causing compounds are removed) and finally filtered prior to re-use in the process. The concentrate formed is burned in a solids incinerator or in the recovery boiler and the ash is taken to landfill. Slide 9, Wastewaters - other Separation Methods Refrigeration Crystallisation Method Wastewater treatment by refrigeration and return of water to circulation. Initially the solution is refrigerated via heat removal during which ice crystals form. After this, the ice crystals are separated from the concentrate and rinsed using clean water. Useable clean water is obtained by melting the ice crystals and the concentrate is sent for evaporation. Method s advantages:: simplicity, low temperature process, where upon easily evaporated substances that easily pass into clean products are reduced, less corrosion and energy consumption is possibly less than in evaporation. Slide 10, Wastewaters - other Separation Methods 5
Refrigeration Crystallisation Method The method has been in use in Louisiana Pacific Ltd s Chetwynd mill, but has been stopped due to the following problems: Ice crystal separation form the waste waters was difficult/troublesome and so it was decided to install a belt press after which crystals managed to be properly separated and cleaned. A second problem caused heat transfer speed growth where crystals became small and tubes were coated with an ice layer. Ice layer reduced heat transfer to zero and this resulted in tubes becoming totally blocked. Further problems caused (amongst others) calcium carbonate precipitation on the surfaces. Because ready solutions had not been readily available it was decided to stop the process and to switch wastewater treatment to one based on evaporation based on evaporation. Slide 11, Wastewaters - other Separation Methods Electrolytic Flocculation The method is a combination of electroflotation and electrolytic precipitation. The method employs direct current and a sacrificial anode (Al or Fe), and proceeds according to the following reactions: The bubbles created/formed are very small 10-16 m (largesurfacearea) Efficiently removes oils, organic compounds and heavy metals, effective method using small quantities of water. In use on oil platforms and at car service stations for oily water treatment. Note! Needs adequate conductivity to work so a salt needs to be added sometimes (NaCl). Slide 12, Wastewaters - other Separation Methods 6
Electrolytic Flocculation In charge tests: foam/froth forms on surface to which dirt is attached. Clean water is left behind in the bottom of the reaction vessel. Photos: Heli Rautjärvi Slide 13, Wastewaters - other Separation Methods Electrolytic Flocculation Continuous operation or steady state tests can be set up as follows: Slide 14, Wastewaters - other Separation Methods Photo: Olli Dahl 7