Physicochemical Properties Of Cyanobacterial Sludge And Its Filtrate From Filter Press Regulating The Dewatering Of Municipal Sludge

Cyanobacterial blooms have been one of the most serious environmental problems in the Taihu Lake region in recent decades.Cyanobacterial sludge is the main pollutant produced in the process of cyanobacteria bloom treatment.Deep dewatering is the main way to reduce the volume of cyanobacterial sludge,followed by drying and incineration,and finally achieve the harmless treatment of cyanobacterial sludge.However,the process has problems such as large dosage of flocculant,high content of inorganic components in the filter cake,and low calorific value of combustion,which is not conducive to the economic treatment and resource recycling of cyanobacterial sludge.A new process of deep dewatering by thermal filter press and pyrolysis of its filter cake to prepare magnetic biochar was constructed by our laboratory in the previous work,which provides a new option for deep dewatering and resource recycling of cyanobacterial sludge.But there is still space for further compression in processing costs.In this paper,the influence of storage time of cyanobacterial sludge on its physical and chemical properties and dewatering performance was investigated,and the key factors affecting the deep dewatering were analyzed.The processing difficulty of the filtrate and the comprehensive cost of cyanobacterial sludge could be reduced by reusing the filtrate of thermal filter press to the municipal sludge treatment.The present works could provide a technical basis for further improving the resource level of cyanobacterial sludge.The main research results are as follows:（1）The effects of storage time of cyanobacterial sludge on its physicochemical properties and dewatering performance were investigated.It was shown that the SCOD resulted from cyanobacterial sludge decreased from 3727 mg/L to 1864 mg/L,and the ammonia nitrogen increased from 488 mg/L to 1234 mg/L with the prolongation of the storage time.The pH increased from 6.64 to 7.36,and the particle size remained unchanged.Due to the transformation of tightly-bound extracellular polymer substances（TB-EPS）to soluble extracellular polymer substances（S-EPS）and loosely-bound extracellular polymer substances（LB-EPS）,the content of S-EPS and LB-EPS increased and the TB-EPS content decreased.The specific resistances of the filter cake from both the conventional deep dewatering（7%FeCl₃、50%CaO）and the thermal filtration deep dewatering（25%FeCl₃、80℃）gradually increased.The specific resistance of the filter cake from the conventional deep dewatering slowly increased to 5.0×10¹² m/kg within 5 days,and significantly increased to more than 2.0×10¹³m/kg after 5 days.Meanwhile,the specific resistance of the filter cake from the thermal filtration deep dewatering slowly increased to 5.0×10¹² m/kg within 10 days,and significantly increased to more than 2.0×10¹³ m/kg after 10 days.（2）The influence of binding morphology and molecular composition of extracellular polymer in the cyanobacterial sludge on its deep dewatering performance was investigated.The EPS with various binding tightness was isolated by deconstructing the binding morphology of extracellular polymers.It was shown that the S-EPS,LB-EPS and TB-EPS could all detrimental to the dewatering performance of cyanobacterial sludge,and the S-EPS had the most significant impact on the dewatering performance of cyanobacterial sludge.The extracellular protein was separated and removal by the Savage method,and the influence of extracellular polymer composition on the dewatering performance of cyanobacterial sludge was further investigated.Although the extracellular polysaccharide content was much less than that of the extracellular protein,the influence of extracellular polysaccharides on its dewatering performance was more significant than that of extracellular proteins,which was the key factor determining the specific resistance of cyanobacterial sludge.（3）The process conditions and treatment performance of collaborative dewatering of municipal sludge and filtrate from thermal filter press of cyanobacterial sludge were investigated.The technical and economic analysis of conventional deep dewatering process and collaborative dewatering process was carried out.It was shown that the best conditions for recycling the filtrate from the thermal filter press of cyanobacterial sludge to the municipal sludge was as follows:adjust the pH of the filtrate from thermal filter press to 7.0 with calcium oxide,and then mix the filtrate with the municipal sludge and transfer it to the filter press.The final volume of filtrate produced was 40.8%less than that of the conventional deep dewatering,and the concentration of pollutants in the filtrate was also lower.The total dosage of chemicals added was 68.4%less than that of the conventional deep dewatering,and the organic solid content of the filter cake was about 25%higher than that of the conventional deep dewatering.The collaborative treatment process provides a new choice for the resource recycling of cyanobacterial sludge.The results of the economic feasibility analysis showed that the cost per unit of the collaborative treatment process was 38.7%lower than that of the conventional deep dewatering process.