The Adsorption Of Bio-charcoal From Sludge On Heavy Metals And Its Effects On Sewage Sludge Composting

With the increasing number of wastewater treatment plants, more and more municipal sewage sludge is produced. Due to the lack of reasonable and effective disposal technologies, large amounts of sewage sludge accumulated caused secondary pollution. Since sludge is rich in nitrogen, phosphorus, potassium and other elements, the agricultural use is the most effective way of its resource utilization, but the toxic and hazardous substances such as heavy metals in the sludge limit its application in the land. So there is an urgent need for a safe disposal option. In this thesis, for this reason, the adsorption of bio-charcoal from sewage sludge on Cr (Ⅵ) and Cu (Ⅱ) was explored and the influence of bio-charcoal on nitrogen loss, maturity time and heavy metal bioavailability during the sludge composting was also discussed. The main conclusions were showed as follows:(1) Based on the adsorption capacity of Cr (Ⅵ), the conditions of bio-charcoal preparation were optimized. The results were as follows:the activation temperature of500℃、40%of the concentration of H2SO4、impregnation ratio of1:1、impregnation time of8h、carbonization time of30min. According to the analysis of IR spectra, there may be hydroxy1、carboxy1、lactone and structure of Si-O-C on the surface of bio-charcoal.(2) The adsorption conditions of bio-charcoal on Cr (Ⅵ) and Cu (Ⅱ) was optimized through single factor experiments. The maximum adsorption capacity of bio-charcoal on Cr (Ⅵ) was61.3mg g-1under the condition of25℃、pH1.0and dosage of1.4g L’1. The adsorption capacity increased with the decrease of pH. The thermodynamic analysis of△H0>0and△G0>0illustrated that the adsorption of bio-charcoal on Cr (Ⅵ) was an endothermic process and didn’t occurre spontaneously. The maximum adsorption capacity of bio-charcoal on Cu (Ⅱ) was136.5mg g-1under the condition of35℃、pH5.6and dosage of0.4g L"1. The adsorption of bio-charcoal on Cr (Ⅵ) and Cu (Ⅱ) were fitted well with second-order kinetics, which indicated that the adsorption was mainly chemical. The fitting of Langmuir isotherm showed that the surface of bio-charcoal was uniform and the adsorption was monolayer.(3) The sludge composting results indicated that the additive bio-charcoal in the compost could reduce the loss of total nitrogen and the effect was proportional to the amount of bio-charcoal added. The inhibition of nitrogen losses was most significant with8%bio-charcoal added. After40days of composting the nitrogen loss rate was25%in compost with8%bio-charcoal and was44.3%in control compost. The amendment of bio-charcoal and thermophilic bacteria could speed up the compost heating rate, improve the temperature and accelerate the composting. The incorporation of bio-charcoal into the sludge composting material could inhibit the bioavailability of heavy metals in the compost. The proportion of chromium and copper in the unstable state decreased10%and1.3%, separately, in compost with8%bio-charcoal. However, in the no bio-charcoal amendment control, the proportion of copper in the unstable state increased2%and the proportion of chromium in the unstable state only decreased4%. This may be assigned to hydroxyl, carboxyl and other groups on bio-charcoal surface. The network structure formed through hydrogen bonds between these groups and the reaction between surface groups and metal ions or heavy metal oxides could reduce bioavailability of heavy metals.