The Effect Of Sodium Lignosulfonate On Changes Of Nitrogen,Phosphorus And Heavy Metals During Sludge Composting

The treatment and resource utilization of municipal sludge are the difficult and key problems in environmental protection in China.Sludge is a by-product of sewage treatment,and the output is huge.It’s predicted that the amount of sewage sludge will be over 60 million tons（by 80%moisture content）form 2020 to 2025.The method of“Composting and land utilization”is one of major methods in sludge treatment at home and abroad.However,the nutrient elements and heavy metals will affect the quality of compost products and the feasibility of its land utilization.Therefore,it is important to study how to maintain nutrients and control heavy metals to improve the bioavailability of compost products.In this study,dehydrated sludge from a sewage treatment plant in Guangzhou,Guangdong Province was mixed with wood chips,and different proportions of sodium lignosulfonate were added for composting experiment.Firstly,the physical-chemical properties of temperature,moisture content,pH and humic substance（HS）were measured,meanwhile HS was analyzed by UV-VIS spectrum and 3D-EEM spectrum to explore the effect of sodium lignosulfonate on the humification process of compost,and the plant toxicity of different piles after composting was verified by seed germination index.Then,the total nitrogen,available phosphorus and the total amount of Cd,Cu and Zn in each pile and the content of available state extracted by DTPA were determined to research the effects of sodium lignosulfonate on the changes of nitrogen and phosphorus in compost and the availability of Cd,Cu and Zn.Finally,the effect mechanism of sodium lignosulfonate on the changes of nitrogen,phosphorus and heavy metals in compost was discussed through the correlation analysis between the availability of total nitrogen,available phosphorus and heavy metals and the physical-chemical properties and HS of the compost.The results are shown below:Sodium lignosulfonate increased the maximum temperature of sludge compost and the duration of high temperature period.The maximum temperature of T9 reached 59.5℃,and the high temperature period lasted for 6 days.The HS content in T3,T6 and T9 was higher than CK.Sodium lignosulfonate increased the molecular weight and aromatization degree of HS,accelerated the transformation of quinone substances,increased humic acid content,promoted the humification of sludge,and got a more stable compost product.The GI values of CK,T3,T6 and T9 were 85.89%,92.30%,90.49%and 96.35%,respectively,indicating that 9%sodium lignosulfonate achieved the best composting effect.Sodium lignosulfonate reduced the loss of nitrogen and phosphorus during composting.Compared to CK,the TN content and available phosphorus content of T9 increased by 10.27%and 9.66%after composting,which improved the fertilizer efficiency of composting products.After composting,the total content of Cd,Cu and Zn in CK,T3,T6 and T9 increased to varying degrees,however,the content of available states decreased.Sodium lignosulfonate had a significant effect on the stabilization of Cd and Cu.The content of available Cd in T9 was reduced by 35%（22%lower than CK）,and the content of available Cu was reduced by 66%（16%lower than CK）.Sodium lignosulfonate was also beneficial to the reduction of the content of available Zn.The content of available Zn in CK,T3,T6 and T9 was reduced by 27.91%,36.98%,35.71%and 32.76%,respectively,and the best effect was achieved when the addition ratio was 3%.The effect of sodium lignosulfonate on reducing loss of nitrogen in composting was mainly related to its effect on C/N and moisture content of the compost.Sodium lignosulfonate increased the C/N of the compost and reduced the moisture content,which inhibited the volatilization of NH₃ and N₂O.Organic acids such as HA and FA in HS increase the content of available phosphorus in the reactor through various ways,and the complexation with heavy metal ions is one of them.Sodium lignosulfonate promotes the complexation between Cd and HA,Zn and FA,and further increases the content of available phosphorus.HA has a significant effect on the stability of Cd,Cu and Zn,and sodium lignosulfonate can promote the synthesis of HA,thus reducing the availability of heavy metals.In addition,FA has a great correlation with the reduction of available Zn.