The Study On Detoxification Of Heavy Metals In Municipal Sewage Sludge By Enhanced Thermal Hydrolysis Method

With the acceleration of urbanization,the production of municipal sewage sludge will increase year by year.We urgently need to develop a simple,reasonable and efficient method to deal with heavy metals of sewage sludge.Thermal hydrolysis is one of common pretreatment methods of sewage sludge.It is of great significance to study the migration and stability of heavy metals of sewage sludge with different alkaline thermal hydrolysis conditions and microwave thermal hydrolysis conditions.Firstly,three kinds of municipal sewage sludge were treated by alkaline thermal hydrolysis to investigate the influences of parameters（such as temperature,reaction time,liquid-solid ratio,the kind of alkaline and alkali dosage）.The quality of solid residue and the migration of heavy metals（Cr、Co、Ni、Cu、Zn、Cd、Pb）are tested.The results show that the quality of sewage sludge is significantly reduced after alkaline thermal hydrolysis treatment.The decrease of solid residue led by addition of NaOH is higher than that led by addition of Ca（OH）₂.The increase of temperature is favorable for stabling heavy metals in solid residue.But increasing time and liquid-solid ratio promoted dissolving heavy metals into the liquid residue.With the increase of Ca（OH）₂,the transfer rate of heavy metals drops,which is opposite with adding NaOH.Secondly,the solid products of alkaline thermal hydrolysis were treated by the horizontal vibration method to study leaching characteristic of heavy metals.Results show that the leaching concentration and rate of heavy metals in sewage sludge obviously reduce after alkaline thermal hydrolysis.The increase of temperature,the reaction time and liquid-solid ratio can lead to the reduction of leaching concentration and rate.The addition of Ca（OH）₂ will enhance stabilization of heavy metals,but the effect of NaOH is opposite.Finally,microwave assisted thermal hydrolysis was used to treat several kinds of sewage sludge.The influence of microwave temperature and time on migration and leaching ability was investigated.Results show that the quality of sewage sludge significantly reduces after microwave assisted thermal hydrolysis.The increase of temperature and time promotes the dissolution of solid residue.The transfer rate of most heavy metals drops with the increase of temperature.When the reaction time is 30min,the transfer rate is smaller.With the increase of temperature and time,the leaching concentration and rate of heavy metals in solid residue products decrease gradually.