Preparation And Characterization Of Sewage Sludge-Derived Absorbent

The treatment and disposal of excess sludge plays an important role in wastewater treatment. Inadequate disposal of the sludge will significantly reduce the social and environmental benefits of the wastewater treatment. With the expansion of wastewater treatment plants in Wuhan, volume of sewage sludge is expected to accordingly increase. How to treat and dispose the sludge has been a new challenge in pollution prevention in Wuhan. Among the various methods for treating and disposal of the sludge, utilization of sludge for adsorbent preparation is a way to achieve stabilization, harmlessness and reutilization. In this study, we prepared the adsorbents from sewage sludges from two municipal wastewater treatment plants. Based on the iodine number, influences of main factors, including activation agent concentration, activation temperature, and activation time, on the preparation of sludge-derived adsorbents were investigated through both the single-factor experiment and orthogonal array experiments. Optimal preparation parameters were proposed based on the experimental results. In order to improve the performance of sludge-derived adsorbents, fir sawdusts were mixed with sludge to prepare the adsorbent. It was found that the iodine number of adsorbent from the mixed materials largely increased. Box-Behnken center combination experiment and response surface method were employed to determine the optimum preparation condition from sawdusts-sludge material. Adsorbents prepared from single sludge and sludge-sawdust mixed materials were analyzed for specific surface area, pore distribution. In addition, adsorption of methylene blue by sludge-derived adsorbents was carried out. Characteristics and performance of the prepared adsorbents were compared with those of commercial activated carbon. In summary, following conclusions could be drawn:1. With the sewage sludge from LongWangZui wastewater treatment plant as raw material, the influence on the iodine number of activation parameters such as chemical agent, impregnation time, activation temperature, activation time, the ratio of activation agent solution to sewage sludge (V/m) on the final products were investigated. Results showed that ZnCl2as activation agent performed best in terms of iodine number and yield. After the single-factor experiment, the result of orthogonal array experiments revealed that the ranking of influence was:activation temperature> activation agent concentrations>activation time. The optimal preparation condition was:activation temperature of500℃, ZnCl2of20%, activation time of90min. Based on the result of orthogonal array experiments and single-factor experiments on iodine number of the adsorbents, made from sludge in ShaHu wastewater treatment plant, the order of influence was:activation temperature> activation time> liquid-solid ratio> activation agent concentrations. From the analysis of orthogonal experiments, the optimal preparation condition was:activation temperature of500℃, activation time of60min, the liquid-solid ratio of1.5:1(V/m), ZnCl2of15%.2. Comparing with ric straw and tree barks, fir sawdusts, as high-carbon additives, improved significantly the iodine number of sludge-derived adsorbents. Box-Behnken center-united experiment design and response surface methodology were used to optimize the operating conditions of adsorbent from LongWangZui sewage sludge mixed with sawdust in mass ratio of1:2. The optimum preparation condition (ZnCl2of33.7%, activation temperature of499℃, activation time of54.9min) based on iodine number was determined by the fitted quadratic equation. However, when the response was adsorption capability of the methylene blue or yield of adsorbents, the linear model fitted well and the estimated optimum preparation conditions were:50%for ZnCl2concentration, temperature of496℃, activation time of134.7min and34.9%,286℃,87.9min, respectively.3. The specific surface areas of the three resultant adsorbents were175.96,326.54,647.77m2/g, respectively. The adsorbents were used to adsorp methylene blue in water solution. Both Langmuir and Freundlich model were able to fit the experimental data. Adsorption capacity from Langmuir model was65.26,46.66,80.38mg/g, respectively for adsorbents prepared from ShaHu sludge, LongWangZui sludge and sawdust-sludge mixed material. As for the Freundlich model, constant for KF was2.35,12.35,35.27mg/g, respectively. In general, adsorbents derived from single sludge did not exhibit good performance. However, with the aid of sawdusts, performance of the sludge-derived adsorbents was significantly improved. Unfortunately, they were still inferior to the commercial activated carbon. Further studies were needed to improve the characteristics of sludge-derived adsorbents.