Study On Pretreatment Process Based On The Characteristics Of Organic Pollutants In Landfill Leachate

The sanitary landfill is the main treatment technology for municipal solid waste disposal currently. Landfill leachate generated at a landfill contains high concentration of organic pollutants, not only the recalcitrant ones with macromoleculars, but also toxic ones with small moleculars. All above will reduce or even inhibit the activity of microorganisms in biological treatment. Therefore, the leachate of Chang'an landfill in Chengdu was collected to research in this experiment. Based on a comprehensive analysis of organic pollutants by GC-MS, new anaerobic biodegradability index for high concentration organic wastewater was proposed in theory, and whose applicability was compared with the traditional biodegradability index. Landfill leachate was pretreated by the combination of lime sedimentation, coagulation and oxidation method for removing organic pollutants. The importance of removing low content of refractory organic pollutants for improving leachate biodegradability was illustrated. The influencing factor and reaction pathway were also studied when phenol, a typical organic matter in leachate, was oxided by sodium hypochlorite (NaClO).Organic pollutants in leachate were extracted by ultrasonic with CH2Cl2 and determined by GC-MS. Compared with the traditional liquid-liquid extraction, ultrasonic extraction could improve extraction rate of organic pollutants effectively. Capillary column stationary phase was researched on GC-MS measuring in order to offer an accurate method for the measure and analysis of organic matter in leachate. The results of study were as follows:polar capillary column HP-1NNOWAX was applicable for the analyse of polar organic pollutants such as carboxylic acids and alcohols, which were the major components of early landfill leachate. Non-polar capillary column HP-1MS could improve detectable rate and credibility of organic pollutants such as alkanes and esters whose content was low in early leachate and high in old leachate. Because the Leachate of Chang'an landfill in Chengdu was a mixed leachate, polar capillary column and non-polar capillary column should be used together.On the basis of theoretical analysis, that CODCH4/COD was regarded as wastewater's anaerobic biodegradability index was presented, and the bound of numberical value was laid down. At the same time, the anaerobic biodegradability of landfill leachate was appraised by CODCH4/COD through biochemical methane potential (BMP) test. When the anaerobic biodegradability index respectively were CODCH4/COD and BOD5/COD, a similar conclusion was obtained. But CODCH4/COD was greater than BOD5/COD for the same sample. The possible explanation was that total biochemical oxygen demand (BODu) was smaller than total chemical oxygen demand (COD) in aerobic condition, and with organic pollutants converted completely into methane CODCH4 was equal to COD in anaerobic condition. In addition, the test time was only 5 days for BOD5 and above 30 days for BMP where microbial flora had enough time to adapt to wastewater. Thus CODCH4/COD was feasible for the biodegradability evaluation of wastewater such as landfill leachate. Further GC-MS analysis showed that the type and nature of organic pollutants had an direct effect on leachate biodegradability. In high biodegradability (BOD5/COD=0.3～0.4) leachte A, the detected organic pollutants in leachate were mainly composed of carboxylic acids and their content was about 90%. While in low biodegradability (BOD5/COD<0.15) leachte B, the detected organic pollutants in leachate were mainly composed of alkanes and their content was about 50%. In spite of the high BOD5/COD value, mixed leachate A should be pretreated by physic-chemical methods before biological treatment like leachate B for increasing biodegradability.This study selected landfill leachate A as research subject of multiple pretreatment. Lime sedimentation and cogulation was used at the first stage. Because of the weak alkaline of saturated limewater and buffer action of landfill leachate, adding some NaOH was helpful for the lime sedimentation. Compared to ferric chloride(FC) and ferric sulfate(FS), poly aluminium chloride(PAC) was the better flocculant. The combined use of aid flocculating polyacrylamide(PAM) and PAC not only increased the removal rate of COD, but also was helpful to the sedimentation of flcos. The organic matter in leachate A which was pretreated before and after was analyzed by GC-MS. The results showed that some refratory organic compounds, in particular low content of them, could be removed by lime sedimentation and coagulation. Meanwhile, removal of UV260 absorbance which characterize refractory orgaic matter in leachat was 48.67% after pretreatment.For further decreasing the concentration of small molecular recalcitrant or toxic organic pollutants, chemical oxidation method was used for the effluent of limewater sedimentation and coagulation pretreatment. The effect of pH, oxidant dose, oxidation time and reaction temperature on the preoxidation of NaCIO, Calcium hypochlorite [Ca(ClO)2] and potassium permanganate (KMnO4 was respectively investigated. Contrast to Ca(C10)2 and KMnO4, NaClO was the best oxidant. Orthogonal experiments were designed to optimize the oxidation conditions of NaCIO based on the single affected factor. The results discovered that the influences of operating factors followed the order of oxidant dose>oxidation time> reaction time. And NaCIO dose had a drastic effect. GC-MS analysis showed that with peak height of low content orgaic pollutants decreased obviously, peak height of some small aliphatic acid increased in TIC of orgaic pollutant in leachate. The cause was that NaClO transformed some recalcitrant or toxic compounts into biodegradable products, or even to CO2.Phenol was a typical organic pollutant in landfill leachate. The degradation of phenol by oxidation using sodium hypochlorite were respectively investigated with ammonia nitrogen(NH3-N) and without NH3-N. And the reaction pathway during the oxidation processes were deduced. The results showed that there were only five chlorophenols such as 2-monchlorophenol(2-MCP),4-monchloropthenol(4-MCP),2,6-dichlorophenol(2,6-DCP), 2,4-monchlorophenol(2,4-DCP) and 2,4,6-trichlorophenol (2,4,6-TCP) without NH3-N. In the range of 6-9, the variation of same chlorophenol concentration with reaction time showed similar trend, but the reaction rate was different at different pH. And the successive chlorination process could be facilitated in slightly alkaline and alkaline media. The increase of chlorine dose was not only beneficial to the chlorination of phenol, but also the conversion and degradation of chlorination products. In NH3-N system, there was a competion between ammonia and phenol after sodium hypochlorite was added to wastewater. Breakpoint chlorination curve was as follows:When the chlorine/ammonia weight ratio (Cl/N) rose from 5.35 to 27.67, the trend of NH3-N removal lagged behind and residual chlorine continuously decreased with no discont point. With increasing NH3-N concentration, the degradation of phenol was inhibited:on the one hand, the removal rate of phenol declined; on the other hand, a series of chlorophenols whose amount and sort showed an increase before decreasing were detected. Analysis of products by HPLC revealed that chlorophenols were only two chlorophenols(2-MCP and 4-MCP) at least, and five chlorophenols(2-MCP, 4-MCPl,2,6-DCP,2,4-DCP and 2,4,6-TCP) at most.2-monchlorophenol and 4-monchlorophenol were the highest activity of forming trihalomethanes (THMs), While 2,6-dichlorophenol,2,4-monchlorophenol and 2,4,6-trichlorophenol were the highest activity of forming haloacetic acids (HAAs).The experimental results provided certain reference value for dealing with organic wastewater with low biodegradability or toxicity by sodium hypochlorite in NH3-N.