Research On The Mechanism Of Improving Sewage Sludge Dewaterability With Desulfurization Ash And Its Application As Cement Admixture

Separating the water from urban sewage sludge economically and efficiently is the critical issue for reducing the cost of sewage treatment and achieving the reduction and recycling of sludge.The large amounts of desulfurization ash existed in the chimneys of plants is gradually becoming a great threat to both industrial production and environmental safety.Hence,disposal of these waste products has been one of the challenge tasks in the current stage.Inspired by the idea of disposing waste with waste,the conditioning program of sewage treatment were optimized by combining the utilization of desulfurization ash with the improvement of the performance of sludge dewatering closely,basing on the study of the mechanism of desulfurization ash conditioning sludge.The object of our research is to try to use desulphurization ash instead of conventional sludge conditioner,and reduce the cost and improve the performance of sludge dewatering.Finally,by mixing conditioning dewatering sludge into the cement raw meal,the resource recycling of desulfurization ash and sludge is exploited.The influences of treating sludge with desulfurization ash alone and in combination with FeCl₃,O₃ and electrolysis on the performance of sludge dewatering were compared by using the coagulation dewatering test,the sludge settling height,water content of the sludge cake（Wc）,specific resistance to filtration（SRF）and capillary suction time（CST）as the effected parameters.The related mechanism was investigated through measuring the extracellular polymeric substances（EPS）content in different sludge layers,soluble chemical oxygen demand（SCOD）,ζ potential,and fourier transform infrared（FTIR）spectrometry.Its optimal amount and possible effects of the conditioning dewatering sludge in cement admixture were obtained by testing the burn-ability of raw cement,mortar strength,chloride ion content,as well as the amount of toxic leaching.The main results were demonstrated as follows:In the study of improving the sludge dewatering performance,the two aspects of speed and degree of dehydration should be considered.The bridging function of desulfurization ash could provide the channel for free water to filter out.The adsorption performance of desulfurization ash could pull a parts of EPS which into the supernatant of the sludge back to the solid surface,reduce soluble organic compounds and improve the speed of dewatering.Desulfurization ash,O₃,or electrolysis method could peel off tightly bound EPS,destroy the structure of cells and floc,make part of them converted to soluble and loosely bound EPS,other part was hydrolyzed into small molecules,such as carboxylic acids,carboxylate,etc.These processes leaded to the release of a small amount of surface adsorbed and inner bound water,and the improvement of the degree of dehydration.The electric charge neutralization of the CaO in desulfurization ash could change the surface characteristics of sludge particles,compress the electrical double layer,strengthen the cohesion of the flocculation,which induced the release of a small amount of surface adsorbed water.The CaO could improve not only the speed of dehydration,but also the degree of dehydration.The tightly bound EPS is the key factor of influencing the sludge dewatering performance.The desulfurization ash conditioning played a very important role in improving the sludge dewatering performance.The optimal stirring speed and duration for mixing and coagulation by desulfurization ash were 350 rpm for 30 seconds;and then,the optimal flocculation stirring speed and duration were 50 rpm for 15 minutes.When the sludge was treated by desulfurization ash at a dosage of 425 mg/g,its Wc was decreased to 79.34%,its SRF,CST and Wc was reduced by 76.43%,60.5%and 14.65%respectively.Under these conditions,it could turn the difficult dewatering sludge（SRF=1012-1013cm/g）into the medium difficult dewatering sludge（SRF=（0.5-0.9）×1012 cm/g）,when the amount of added desulfurization ash was less than 50%of the dry mass of sludge.The combined treatment of sludge by desulfurization ash with FeCl₃,O₃,and electrolysis method was more effective than that used alone.When the sludge was treated by desulfurization ash and FeCl₃ at a dosage of 300 and 60 mg/g,its Wc was decreased to 72.68%,its CST and Wc was reduced by 97.7%and 13.16%respectively.Correspondingly,with O3,desulfurization ash,and FeCl₃ at dosages of 85.72,400,and 60 mg/g,its Wc was down to 70.70%,and its CST and Wc was reduced by 83.5%and 15.41%respectively.In addition,with desulfurization ash and FeCl₃ at dosages of 300,and 60 mg/g and electrolysis for 40 minutes using Ti/RuO₂ as electrodes with 4 cm electrode spacing,at 20 volts,and with NaCl 0.100 mol/L,its Wc was down to 70.50%,the CST and Wc were reduced by 92.1%and 15.66%respectively.The dehydrated sludge could be used as an additive in cement.The optimal dosage of sludge was around 7.5%of total cement admixture.With this agent,the C3S crystallinity in the sludge was highly increased by accompanying with a large number of intermediate phases,and the calcining performance of cement clinkers was also improved.Meanwhile,the cement strength has no obvious change in the similar conditions,while more AFt crystals were observed in the cement hydration products.With this treatment,the amount of chloride ion and heavy metals in the clinkers were significantly lower than that of the national standard limiting value.The research indicates a new route for the reduction,stabilization,harmless treatment and resource utilization of desulfurization ash and sludge.It might provide a theoretical basis for the use of desulfurization ash to improve the sludge dewatering performance,and a useful reference for sewage treatment plant to reduce the cost of sludge dewatering.