Research On Drying Characteristics And Technology Of Sewage Sludge

With the development of urbanization in China,more and more municipal wastewater as well as sewage sludge is produced.Sewage sludge formed during wastewater treatment has high water content and large volume as well as various toxic substances. Any improper disposal of the sewage sludge will result in secondary environmental pollution and influence the operation of sewage treatment plant.How to dispose of it economically and efficiently has become a newly worldwide problem,and is being sought after both in China and abroad.There are four general sludge disposal methods:land application,landfill,drying and incineration.Drying has become an important step in sludge disposal.Drying characteristics of sludge granules were studied,and a project drying sludge by waste heat from flue gas was summarized.Based on these,improvement schemes of sludge drying technique were proposed,and a new sludge drying technique was established.Four different weight grades of sludge granules were dried at constant temperature of 100～200℃.Results showed that sludge drying process included four periods:preheating period,constant rate drying period,the first falling rate drying period and the second falling rate drying period.Sludge drying process was a weak intensity drying process at 100～120℃,critical moisture content was constant.It was a high intensity drying process at 140～200℃,and critical moisture content increased with the drying temperature or granule size increasing.The drying rate was affected with not only drying temperature but also the quantity of water evaporating heat in a unit surface area.Reduction of granule size can increase specific surface area and drying rate in a unit surface area.There were a quadratic nonlinear relationship between drying time and temperature,and a linear relationship between drying time and sludge surface area.Sludge drying process at 200～500℃included three periods:preheating period, constant-like rate drying period,falling rate drying period.Drying rate during constant-like rate drying period was not constant,but fluctuant.It took 70%of total drying time and total loss.Cracks were produced,which became larger in earlier stage then became smaller in later stage,it was because of water loss imbalance in sludge surface and inner.The particle size of dried sludge were mainly sand,silty sand and clay.Dust produced during sludge drying could be collected by water film dust collector.On the basis of thermogravimetric analysis,it was suitable to dry sludge at 100～300℃.Main organic matters in condensate at this temperature were chain alkane,aromatic hydrocarbon and nitrogenous heterocyclic compounds,and main pollutants were benzene,o-xylene and phenol.There were no PAHs in consendate. On the summarize of Jiangyin Sludge Drying Project and experiment conclusion, several techniques as low temperature drying,nature drying,solar drying and waste heat recovery,also some advanced equipments as heat pipe and heat pump,could be used in sludge drying technique.Heat utilization efficiency of improved technique could be enhanced from 44.62%to 64.60%,and emission of CO₂ and SO₂ was reduced significantly.