Removing Cadmium From Pharmaceutical Sludge By Bioleaching

Sewage sludge is the sediment of sewage disposal process,which occupied 15%-30% of sewage disposal quantity. As the world population increasing quickly and economy growing fast,the problem of environmental pollution has become increasingly worse. In recent years, many municipal sewage treatment plants have been built, which effectively alleviate the pollution of water environment caused by the discharge of municipal sewage and industrial wastewater. At the same time, heavy metal is the limit factor of resource use.The methods to remove heavy metal contain:chemistry,plant abstraction and bioleaching.The bioleaching process, which causes acidification and solubilization of heavy metals, is recommended as an economical and effective process for the removal of heavy metal from sewage sludge.In this paper, through laboratory-scale experiment,the optimum parameters of bioleaching of sewage sludge which have much microbe and antibiotic with domesticated Thiobacilli are basically reviewed.These parameters contain energy source,pH,solids concentration,inoculum and so on. Based on their theoretical similarity and analogy, the reaction mechanism is studied.The significance of starting pH, the solids concentration and the energy source in bioleaching are evaluated in this paper.Results show that taking into account the experimental effect ,economy and environment impact, the best amount of S and the inoculum respectively was 5g/L and 2%.S as nutrition is economic and effective.Adjusting starting pH is unnecessary. Due to the buffering capacity of sludge solids, the rates of pH reduction and metal solubilization are reducing with the increase of the solids concentration. When the solids concentration is 4% (w/v), high efficiencies of metal solubilization are achieved.All the removing of cadmium in three species of sewage sludge have reached 60%.That result indicate that there are no evident difference between different sewage sludge.The adaptability of Thiobacilli to different sewage sludge is strong.The bioleaching temperature is about 25-35°C .The transformation among different heavy metals form during bioleaching could be described by continuous measurement, and the bioleaching mechanism could be indirectly presumed with variation of heavy metals forms. Cd at initial sludge mainly existed as sulfide and carbonate precipitate form, most of which turned to exchangeable form during bioleaching.pH and ORP are linear correlative well.After bioleaching, The content of Cd is 8.73mg/kg, decreased greatly. Though exceeded. Chinese agricultural using standard (GB 4284-84) ,it mostly exists with steady condition. Wherefore, agricultural using risk minishes far and away.Though the loss of nitrogen,phosphorous,potassium and organic materials from sludge after bioleaching was up to32.45%,55.51%,20.92% and 34.23% , respectively, the sludge still remained with satisfactory fertilizer value.The experiment showed that the bioleaching process improved markedly the ability of the settlement and dehydration for the bioleached sludge. The volume of the bioleached sludge decreased 59.2% after 12h settling, meanwhile the untreated sludge only reduced 29%. The bioleached sludge was easier to dewater than untreated sludge .As a result, the inorganic or organic flocculating agent was not required for dewatering the bioleached sludge. Low pH resulted from the sulfur element in sludge seems to be responsible for enhancing the ability of the settlement and dehydration.