The sanitary landfill method has been widely accepted for waste treatment and disposal because of its lower cost of operation and maintenance as compared to others. However, landfill leachate is a very strong pollutant. The pollutant composition of landfill leachate is very complicated that often contain organic compounds, heavy metals, ammonia nitrogen, chloride and many other soluble compounds. The type of waste and the age of landfill influence the composition and concentration of contaminants.The concentrations of organic material and ammonia nitrogen are high in fresh leachate, while matured leachate contains relatively lower concentration of organic matter but higher concentration of ammonia nitrogen. The usual content of ammonia in the landfill leachate is 1000 – 4000 mg/L.
Treatment of landfill leachate is very complicated and generally requires various process applications that due to high concentrations of COD and ammonia nitrogen. Anaerobic biological treatment systems are efficient to treat high concentrations of COD, but high concentrations of ammonia nitrogen is considered as the main reason for low efficiency in biological treatment of landfill leachate. Especially, with the release of new standard for pollution control on the landfill site of municipal solid waste, the removal of total nitrogen (TN) and ammonia nitrogen from leachate becomes critical. Many physico-chemical processes which have widely been used can be applied in the pretreatment of ammonium from landfill leachate, such as air stripping, chemical precipitation, adsorption, electric-chemical oxidation, reverse osmosis and other membrane separation processes. These physico-chemical methods was recommended as an effective process for the removal of ammonia nitrogen, however, the physicochemical treatments have many drawbacks such as high cost in the post treatment process and operational process, high energy consumption, low removal of TN.
Supported gas membrane (or called as trans-membrane stripping/chemical absorption) process was widely studied for removal volatile basic or acidic compounds for their dilute aqueous solutions. Supported gas membrane is a separation process thought as a low energy consumption, low cost when compared to conventional separation processes such as air stripping and reverse osmosis, because the driving force of this process is the difference of chemical potential of neutralization reaction which consumes a much less power and no thermal energy (steam). Also, the by-product is an aqueous solution of ammonium salt of high purity and high concentration which can be used as raw materials of fertilizer. Thus, the use of gas membrane process also avoids secondary pollution of ammonia to air as occurring during the air stripping.
However, traditional supported gas membrane is not suitable for the treatment of dirty feed such as landfill leachate or other industrial wastewaters, since the hydrophobic porous membrane will soon lose its initial hydrophobility when contacting with feed containing oil, surfactant, solid suspensions, organic solvent or large organic moleculars, which leads to the leakage between the feed and the absorbing solution and influences the normal operation of the ammonia removal.
A modified supported gas membrane separation process, stabilized gas membrane (SGM) separation process can be used for nearly complete removal of ammonia from landfill leachate. Long-term operation stability of the SGM process can be ensured due to new configuration of membrane modules of the optimal design and effective pretreatment technology including flocculation-coagulation + air floating + membrane ozone oxidation processes. The pilot-scale experiment results showed that the removal rate of ammonia from landfill leachate was over 98% (even more than 99%), drastically reducing the press of following bio-chemical treatment.
A 40-days-long pilot-scale demonstration test in a location of landfill field is still operated normally. During the 40-days test no leakage of the feed or absorbing solution was observed, and the value of the whole mass transfer coefficient of ammonia trough the membrane was almost a constant, which confirms the operation stability and reliability.
The test results showed that when the intial concentration in the leachate was 2000 – 3000 mg/L, an aqueous solution of 28 – 35wt % ammonium sulfate was obtained as a by-product, which can be used directly as fertilizer or is further concentrated and crystallized as solid fertilizer. As a comparison, a by-product with a concentration of less than 23wt% was obtained.
The studies shows SGM process can not only effectively remove ammonia from landfill leachate, but also can significantly reduce the cost of overall capital and operation costs. It was estimated that both the capital cost and daily operation cost can be reduced by 50% respectively.
The above SGM process was also used for the removal and recovery of ammonia from pharmaceutical wastewaters, which usually contain 3000 – 7000 mg/L ammonia and other organic compound. The operation goes well up to now and an aqueous solution of 35 – 43wt % ammonium sulfate was obtained as a by-product.
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