Efficiency Studies On The Treatment Of Sludge And Leachate In Constructed Wetlands With Different Combinations

Sewage sludge is an important product of the wastewater treatment process,if it is not treated properly,it will cause environmental pollution and waste of resources.The high cost of traditional sludge treatment and disposal approach is an important issue for sewage treatment plants in small towns.Therefore,it is of great significance for small scale sewage treatment plants to develop low investment and low operating cost technology for sludge treatment.As an ecological sludge treatment technology,sludge treatment wetland?STWs?technology has the advantages of low investment and low energy consumption,and has attracted more and more attention around the world.This study investigated three combinations of STWs system,the influence of plant presence,plant species and earthworm addition in the STWs were analyzed,optimum condition for sludge treatment in STWs were concluded,the feasibility for sludge land use after the STWs was evaluated as well.The results showed that plants can improve the treatment performance of sludge and leachate.Compared with the unplanted STWs,the total solids?TS?content of sludge in the planted STWs increased by 10.5% and the ratio of volatile solid and total solids?VS/TS?content decreased by 9.6%;meanwhile,the removal efficiencies of chemical oxygen demand?COD?,ammonia nitrogen?NH₄⁺-N?,total nitrogen?TN?and total phosphorus?TP?in leachate increased by 2.5%,182.7%,56.1% and 52.4%,respectively.Different plant species have different ability to handle sludge,however,the effect of reed and Typha on leachate treatment was similar.Compared with the Typha STWs,the TS content of sludge in the reed STWs increased by 25.1% and the VS/TS content decreased by 5.9%.Earthworms can promote the sludge dewatering and stabilization,but have little influence on the removal of pollutants from leachate.Compared with the earthworm free STWs,the TS content of sludge in the earthworm STWs increased by 2% and VS/TS content decreased by 2.6%.The best loading rate and feeding frequency for sludge and leachate treatment in planted and unplanted STWs were 10L/4d,12L/4d,respectively.The best loading rate and feeding frequency for sludge and leachate treatment with different plant species in STWs was 3L/d.The best earthworm density for sludge and leachate treatment in earthworm addition STWs was 10.8 kg/m3.Under the optimal conditions of these different STWs,the removal efficiencies of COD,NH₄⁺-N,TN and TP in leachate can reach more than 90%,and the concentrations of heavy metals in leachate fulfilled the comprehensive sewage discharge standard in China?GB8978-1996?.The presence of earthworms and reeds was the best combination of sludge dewatering and stabilization.Under the best condition,the contents of TS,VS/TS and DHA of the stabilized sludge were 73.5%,21.1% and 1.0 g?TPF?/g?sludge?,respectively.After the treatment by STWs,the sludge contents of heavy metals can fulfill a series of Chinese standards on sludge land?e.g.GB-18918-2002,CJT-362-2011,CJT-291-2008?.All the analyzed heavy metals may not be a threat to plants and soil based on their speciation,except Cd.However,the bioavailability of Cd will decrease with the addition of earthworm in the STW.Hygiene pollution indicators indicate that the removal efficiency for E.coli is above 90% in the accumulated sludge.The lowest volatile fatty acids were found in the Earthworm + Reed system,and the structure of the bottom sludge was more loose with higher porosity than the surface sludge.Through agricultural risk assessment of the treated sludge,it can be concluded that the content of heavy metals in vegetables cannot meet the food safety standards?GB 2762-2012?,and there was a certain risk for agricultural utilization of the treated sludge.