Effect Of Biochar Addition On Vermicomposting From Sewage Sludge

Sewage sludge production is growing rapidly in China recently,only 30% of which can reasonably be handled,resulting in the pollution of the environment.Traditional treatment processes include landfill,incineration and sludge reclamation.Landfill is characterized by long period,large occupying area and bad smell.Incineration leads to the production of harmful gases,resulting in air pollution.Sludge reclamation pollutes environment of sea and harmful substances easily enter the material cycle through the food chain,thus do harm to animals,plants and even human beings.However,sewage sludge contains a lot of organic matter,which can be used as fertilizer after treatment.Vermicomposting is a processing method using earthworms and microorganisms to decompose organic waste and convert it to humus.Some studies have confirmed that earthworm composting can enrich heavy metals in sludge and produce organic fertilizer with rich nutrients.However,the leachate produced by sludge is not only toxic to earthworms,but also forms an anaerobic environment which makes it difficult for survival of earthworms.By adding various passivators,the physical-chemical properties of sewage sludge can be adjusted and changed to fit survival of earthworms.As an environment-friendly material,rice husk-based biochar can not only adsorb and purify harmful substances in soil,but also loosen the soil to prevent soil from hardening.It has the characteristics of high carbon content,good water permeability and large porosity,which can theoretically be used as a harmless passivator for vermicomposting.However,there are few studies on vermicomposting treatment of sewage sludge by rice husk-based biochar nowadays.In this paper,rice husk-based biochar was used as a passivator to study the effect of biochar combined with vermicomposting on sewage sludge,providing new ideas and methods for sewage sludge reduction and recycling.The results showed that the sludge environment was significantly improved after vermicomposting,and the p H of the vermicomposts stabilized around the neutral value 7.0,which is suitable for plant growth.The conductivity value decreased significantly.The content of total organic carbon(TOC)and dissolved organic matter(DOM)of composting products showed a decreasing trend.The ratio of ammonium nitrogen to nitrate nitrogen decreased with the increase of biochar,and the ratio of ammonium nitrogen to nitrate nitrogen in Group S2 with 1% biochar content reached below 0.16 firstly,indicating that biochar could effectively promote the vermicomposting.The content of heavy metals Cd,Mn,Fe,Cr,Cu,Pb and Zn decreased after vermicomposting,and the reduction ratio ranged from the least 1.62 to the most 46.76%.The addition of biochar promoted the reduction of seven heavy metals in sewage sludge by vermicomposting.The vermicomposting experiment showed that the addition of biochar significantly affected the fraction changes of heavy metals in sewage sludge.The addition of certain amount of biochar can promote the transformation of heavy metals to the residual fraction with low bioavailability.In the adsorption experiment of Cu(II)with different vermicomposts as adsorbents,the adsorption capacity of Group S5 vermicompost was the largest(35.58mg/g).Adsorption isotherm experiments proved that the Cu(II)adsorption process of vermicomposts conformed to the Langmuir isotherm adsorption model,and adsorption kinetics experiments proved that the Cu(II)adsorption process of vermicomposts conformed to the pseudo-second-order model.These indicated the adsorption process is mainly chemical adsorption and the single molecular layer adsorption is dominant.It is proved that the addition of biochar can effectively promote the adsorption of Cu(II)by vermicomposting adsorbents.In addition,the most suitable conditions for adsorption process were listed:temperature(35℃)、adsorbent dosage(0.15～0.20g),Cu(II)concentration of initial wastewater containing(200mg/L),p H(4～5).