Evaluation of electrodialysis for the treatment of a hazardous leachate

The chloride concentration of a hazardous leachate of approximately 22,000 mg/l is too high for the direct discharge requirement of less than 1000 mg/l into the municipal sewerage system. Electrodialysis (ED) was selected amongst various other alternative technologies for the desalination/concentration of the leachate. Lime pretreatment of the leachate was superior to caustic soda treatment for the removal of scale-forming chemicals (Ca, Ba, Sr, Fe, Mn). Membrane fouling took place during ED treatment. However, membrane fouling should be restored with polarity reversal and mechanical cleaning of the membranes. Batch ED tests have shown that ED performance remained more or less constant during eight desalination/concentration runs. The electrical energy consumption for ion transport and brine volume varied between 9.6 and 11.4 kWh/m3 feed and 17 and 35% of the treated feed, respectively. A very good quality water could be produced with ED treatment of the leachate. The electrical conductivity of the leachate could be reduced from 5,490 to 139 mS/m (97.5% removal). Chloride was reduced from 21,957 to 345 mg/l (98.4% removal). Arsenic was reduced from 8.8 to 1.37 mg/l. The capital cost for a 100 kl/d ED plant is estimated at US$ 0.600 million. Operational costs (ion transport) are estimated at US$ 0.38/m3 leachate.

C Excellent removals of calcium, barium, strontium, iron and manganese were obtained with lime treatment of the leachate. Sludge volume will comprise approximately 10% of the treated leachate. C More membrane fouling was experienced at higher current densities (100–120 mA/cm2) than at lower current densities (20–80 mA/ cm2). It appears that it should be possible to control membrane fouling with polarity reversal and mechanical cleaning of the membranes. C Several batch ED runs on pretreated leachate have shown that ED performance (desalination/concentration characteristics, membrane properties before and after ED treatment), remained more or less constant during eight desalination/concentration runs. This showed that membrane fouling should not be a serious problem. However, long-term pilot tests should be conducted to prove this point.