Organic Sulfur/Nitrogen Transformation And Release Characteristics During Thermal Drying Process Of Sewage Sludge

Sewage sludge is a semi-solid or solid substance produced by sedimentation or other separation methods in the process of wastewater treatment.It has high moisture and high organic compounds content,with easy compressive and difficult dewater characteristics.Reducing water content has always been an important goal in the sludge treatment process,and the drying process is still an indispensable link before its disposal.However,the large amount of unstable organic substances in sludge would lead to serious release of sulfur-containing or nitrogen-containing odorous gases during the drying process,causing severe secondary pollution.Therefore,this paper aims to investigate i)the effect of conditioner addition on sulfur/nitrogen transformation during conventional conditioning-dewatering-thermal drying process,ii)the transformation path and influencing factors during hydrothermal drying process.The key scientific and theoretical issues involving: the control mechanism of alkali addition on the sulfur-containing gases release;the possibility of simultaneous control of sulfur and nitrogen gases;whether the existence of water affects the sulfur and nitrogen transformation path during hydrothermal drying process;how the hydrothermal treatment temperature affect the distribution of sulfur and nitrogen products.In order to solve the above scientific issues,this paper has carried out a comprehensive and in-depth study.The effect of alkali addition on sulfur transformation was studied on a horizontal fixed-bed direct-heating drying system.The sulfur-containing species speciation analysis and the sulfur-containing model compounds experimental verification were adopted.Confirmed that: the addition of alkali,such as Ca(OH)2,greatly reduces the emission of the sulfur-containing gases,with more significant effect for the stronger alkalis;the presence of alkali promotes the oxidation of aliphatic and aromatic sulfurs into more stable sulfoxides and sulphonic acid,suppressing the generation of H2 S from aliphatic and aromatic sulfur decomposition at low temperatures;also,the alkalis fix the sulfur in the form of inorganic sulfide and sulphate in char,further reducing the release of sulfur into gas phase.The key factor that influencing the organic sulfur transformation is the base strength of alkali.Stronger alkali can provide more OH radicals,thus leads to better oxidation and fixing effect on aliphatic and aromatic sulfur.On the basis of meeting the technology requirements of the treatment process such as dewater,choosing alkali conditioner can suppress the sulfur-containing odorous gases release in the subsequent thermal drying process.On the bases of conclusion of alkali on sulfur evolution,the alkaline acetates were selected for further study to avoid the promoting effect of alkali on NH3 releasing.Through the nitrogen balance analysis during the whole process,several important conclusions can be obtained: the addition of Ca acetate increases the nitrogen retention in char,mainly due to the inhibition of protein decomposition as well as the formation of stable N species,while Na acetate enhances the decomposition of nitrogen species in sludge such as proteinand inorganic-N;the effects of Ca and Na acetates on nitrogen transformation during sludge pyrolysis is due to their different catalytic effects on the decomposition of N species in sludge;Ca and Na acetates can catalyze direct cracking of amino acids and subsequent dehydration of amides intermediates to increase nitriles formation;the ring-opening reaction of pyrrole is also promoted by metal cations at high temperatures;the N species in tar are mainly in the form of amine-and heterocyclic-N,with less nitrile-N;the N species in tar is greatly suppressed by Ca acetate,due to its inhibition effect on the decomposition of protein;both Ca and Na acetates significantly reduce the emission of nitrogen-containing gases,as acetates can release acetone during thermal drying process which easily reacts with NH3 to produce binary clusters or amines in tar.On the basis of meeting the technology requirements of the treatment process such as regulating carbon/nitrogen ratio of biochemical pool,choosing alkaline acetates can suppress the nitrogen-containing odorous gases release in the subsequent thermal drying process.The migration and transformation path of sulfur compounds during hydrothermal treatment was studied in a batch reactor.The following conclusions can be generated: the sulfur-containing gases were mainly produced by decomposition of aliphatic and aromatic sulfur,mainly composed by H2 S,CH3SH and a small amount of SO2;the production of sulfur-containing gas increases with temperature increase;the organic sulfur compounds would react with water during hydrothermal treatment;the large amount of OH* radicals generated by water would act as electrophilic reagent to attack the sulfur atom in organic sulfur compound,add oxygen on to sulfur atom,oxidize aliphatic and aromatic sulfur compounds to the corresponding sulfoxides and sulphonic acid,even inorganic sulfate;the reaction rate increases with the increase of temperature;the formed sulfoxides and sulphonic acid has lower thermal stability than before oxidation,through C-S bond breaking,SO2 or SO3 in gas,and SO32-and SO42-in aqueous solution were generated.During the whole process,a large amount of water participate in organic sulfur decomposition and dissolve parts of products,such as sulphate and SO2,leading to major sulfur species migrate to the aqueous solution,reduce the sulfur content in the solid product and sulfur-containing gases release.Without considering residue content of total organic matters,the increase of hydrothermal treatment temperature is beneficial to produce solid products with lower sulfur content.For nitrogen,with hydrothermal treatment temperature increase,more nitrogen compounds convert to tar-N and solution-N.The nitrogen content in tar and solution are higher than char-N when temperature rise to 210 °C.Thus higher temperature is more conducive to the preparation of clean energy and the control of nitrogen residue in char.The main pathways can be described as: protein decomposition and hydrolysis,produce amine,water-soluble organic nitrogen,NH3 and NH4+ in turn;followed by the reaction of inorganic nitrogen and water,generate NH4+ and NO3-and other nitrogen-containing ions.The reaction rate increases with temperature rise.Nitrogen residues in char include undecomposed protein-N,heterocyclic-N such as pyrrole and pyridine,quaternary-N and a slight amount of nitrile-N.The nitrogen-containing species in tar are amine-N and heterocyclic-N without nitrile-N.In the aqueous solution,water-soluble organic nitrogen is the main part,followed by a large number of NH4+ ions.HCN is the dominant nitrogen-containing gas that released.In the comprehensive consideration of residue content of total organic matters,and residue sulfur,nitrogen content in the solid product,the hydrothermal treatment temperature is better to choose between 180 °C to 210 °C.To sum up,for direct thermal drying process,choose alkaline acetate,can create an alkaline environment to control sulfur-containing gases release,and can take advantage of its decomposition by-product to capture nitrogen-containing gas,realized the aim of simultaneous control of sulfur-and nitrogen-containing gases;for hydrothermal drying process,the transformation pathways of sulfur and nitrogen as well as the influence characteristics of the final treatment temperature are proposed,which provides a theoretical basis for the control of these two of pollutants.