LEACHATE PRE-TREATMENT: OVERCOMING IRON FOULING DUE TO CHANGING LEACHATE QUALITY

This study evaluates approaches to address iron fouling in leachate pretreatment plant. The leachate pretreatment plant treated leachate that consisted of dissolved organics (aromatics, chlorinated VOCs, oil and grease). Major components of the pretreatment system consisted of oil/water separation and carbon adsorption. In addition to these basic unit operations, the pre-treatment system included pH adjustment and a filtration system (bag filters). However, a few months after start-up the frequency of replacing the bag filters increased due to precipitation/particulate formation. Chemical analysis indicated that Fe(III) oxidation was the result of particulate formation in the filter bags. In order to overcome the frequent replacement of bag filters and reduce the costs, process modifications options were evaluated. Two options that required minimal/no infrastructure modifications that were evaluated included: (a) increase the mesh size of the bag filters; and (b) addition of chelating agents. Increasing the filter bag size (25 to 400 microns) resulted in reduced frequency of filter bag replacement (2 days versus 10 days) with no effect on the carbon adsorption system. However, this approach resulted in frequent disruptions to plant operations and was not considered economical.Amongst the chelating agents considered, citric acid was effective in preventing iron fouling. Bench scale and full-scale implementation successfully demonstrated the effectiveness of citric acid in preventing iron fouling. Frequency of filter bag replacement increased to approximately 30 days. Furthermore, there appeared to be a good mass balance of Fe(II) and Fe(III), before and after the carbon adsorption units suggesting that citrate was not being adsorbed to the activated carbon thereby not effecting its life span. Cost benefit analyses between citric acid and commercial anti scale additives also indicate significant benefits of using citric acid. Based on the results of this evaluation, citric acid substitution was implemented resulting in successful and efficient operation of the leachate treatment plant.