Study Of Improving Sludge Dewatering Efficiency In Sewage Treatment Plant Of Mudanjiang With Frozen-thaw Method

With the improvement of sewage treatment in China, sludge production isgrowing up at a rate of10%~15%each year, while sewage sludge treatment anddisposal are of low efficiency and high cost. A lot of untreated sludge caused greatpressure on the environment with the secondary pollution. Sludge dewateringcontains much organic matter, and moisture content is also as high as80%,all ofthese make the sludge treatment difficult. Conventional disposal methods of sludgeare sludge incineration, sludge pyrolysis, etc, but all kinds of methods have acommon problem： high moisture in the sludge,which makes sludge disposaldifficult.And the cost is raising. Looking for a method to reduce the cost to low themoisture content of sludge has a practical significance.This subject explored the mechanism of freezing method to improve sludgedewatering properties and the main influence factors to change the sludgedewatering properties of freezing method. Considering the actual application of thefreezing method, we selecte the following three factors as the main research objects:sludge freezing time, sludge frozen rate, sludge frozen final temperature. Thisexperiment takes sludge moisture content,dehydration rate, sludge settling curve, thesludge capillary absorption time CST as the sludge dewatering properties changingstandards before and after freezing. With the particle size distribution apparatus todiscuss the mechanism and draw a corresponding conclusion.In order to imitate the process of sludge in frozen field，we use self-madecontainers to ensure the sludge froze from the surface to the bottom and have noinfluence around the sludge. Through studying the sludge freezing time、sludgefrozen rate and the final sludge frozen temperature，we found that as the freezingtime is longer, the sludge dewatering properties becomes better, for example:whenthe frozen time is2h, the CST is79.9s,;when the frozen time56h, the CST is19.2s.The slower frozen rate is, the greater the increased of sludge dewatering is. Such aswhen we put the sludge directly into-15degrees centigrade, the CST is81.5s; Whenthe sludge directly is put into-23degrees centigrade, the CST is104.8s.The lowerthe frozen final temperature is, the greater the increased performance of sludgedewatering is. When the sludge is put into-20degrees Celsius for6h,thetemperature of sludge is-5degree，and its CST was38.4s.When the sludge is put for20h，the temperature of sludge is-27degree，and its CST is22.9s, they have adifference of more than2times. The results suggested that if it is wanted to becommercialized, the sludge must be as thin as possible, with slower frozen rate andmore time. This experiment studies how the rate of thaw influences sludge dewateringproperties and find that the influence of the rate of thaw to sludge settling curve、CST、water content is very little,and we can ignore it,so the process of thawproduces little influence on the dehydration property.The density of EPS has a biginfluence on sludge dewatering property.When the density of EPS is47.2mg/L SS,CST is least,so the dehydration property is best.If we increase thedensity of EPS, the dehydration property will descends. After the sludge is frozen,the sludge supernate’s pH drops,from6.58to6.16.SCOD rises,from127.93mg/l to4044.17mg/L, and TS rises with VS keep invariant.Through studying the mechanism of freezing method to improve the sludgedewatering properties we found that when freeze sewage sludge, the sludge particlesof water around began to freeze. If the water contained impurities, the watermolecules in front begin to freeze crowing out the impurity and make impurity forma concentrated volume. When the sludge particles of water molecules around thecrystallization freeze faster than water molecules flowing into the rate of tissuelayers, the sludge particles would be frozen in ice. The surface water in the sludge islittle, accounts for less than one percent of the total water. However, the freezing ofthe water for sludge dewatering properties has a significant influence, that’s becausethe water blocks the sludge particles mutual bond together. So the freeze-thawmethod make sludge shape bigger particles and better dehydration property. Throughthe determination of particle size of sludge before and after sludge frozen, theconclusion was validated.