Stabilization Of Sewage Sludge Eliminated From Heavy Metal And Research Of The Complex Fertilizer Making Process

For issues that urban sewage treatment plants produce large amount of sludge which are difficult to disposal, and the presence of heavy metals in sewage sludge affect its land application, this paper use microwave together with chemical leaching formula to remove heavy metals in the sludge,and discussed the properties of sludge after heavy metals are removed.Anaerobic composting method was adopted to stabilize sewage sludge before and after heavy metals were removed.The main results are as follows:（1）Heavy metals as Zn、Cu、Ni、Pb、Cr、Cd achieved relatively high removal rate after disposed by microwave and chemical leaching formula.The removal rate of Cd, Pb, zinc are more than 50%,Cr 41.12%, and the content of Zn、Cu、Ni、Pb、Cr after disposal was 81.9%、92%、76.4%、76.7% and 87% lower, respectively,than the Class A sludge agricultural standard limit,whereas the content of Cd was 36.8% lower than Class B sludge agricultural standard limit.After treatment,the biological effectiveness of heavy metals are significantly decreased,including that residue state of Cd increased to 55% and potential effective state of Pb decreased 74.4 mg/kg dry sludge. After the heavy metal removing treatment,all the tested indicators except for moisture content,p H and seed germination index can meet the standard of “the urban sewage treatment plant sludge disposal-sludge property for agricultural use"（CJ/T309-2009）;（2）An anaerobic composting of dewatered sludge from sewage plant was conducted, and discovered that composting cycle is slightly longer at the temperature of 35℃than53℃, besides that compost conditions at both temperature show barely evident difference on the condition that both of the sludge were basically rotten in17¹9days.During the composting, ammonia nitrogen concentration rise fast at the beginning, followed by different degree of declination at the 9¹0 days, and 15 days later the concentration gradually reach stability.But at the end of composting ammonia nitrogen concentration decreased slightly. During the medium and high temperature compost, moisture content turning stable at 77% and 78% respectively,after a slow increasing. Seed germination index is above 95% after composting;（3）After the heavy metal removing treatment by chemical leaching formula together with microwave, a sludge anaerobic compost experiment at 53 ℃ are performed using ammonia nitrogen, p H, moisture content, volatile solids, the seed germination index as judge indexes of compost situation.All the indexes tend to stability respectively in 18 22 days, indicating the rotten cycle of composting sludge.The properties of composting sludge are even better than the standard of “the urban sewage treatment plant sludge disposal- sludge property for agricultural use”（CJ/T 309-2009）,in addition that moisture content is a little higher. Moreover,biological effectiveness of heavy metals as Pb and Cu decreased significantly;（4）To make organic-inorganic compound fertilizer,urea,calcium superphosphate and potassium chloride are added into the composting product.The proportion of each component in the final product are as follows,composting products 56.84%, urea18.05% and 19.72% calcium superphosphate, potassium chloride 5.38%.And that N:P₂O₅:K₂O=11:4:5.