| Sewage sludge is the secondary resources as it contains abundant nutrients,however, the high moisture content and heavy metals restrict the agricultural application. It will cause the risk of heavy metals contamination if sewage sludge is fertilized into soil indirectly. It has put forward an indirect agricultural application of sewage sludge in which the sewage sludge would be recovered after agricultural use.This paper evaluates the effects of indirect application of sewage sludge, in which the sludge was contained in a mesh bag, on the growth of and heavy metals contents in the dwarf banana, in comparison with no sludge control and direct application of sewage sludge. Changes of physico-chemical properties of soil under different treatments were monitored. Changes of nutrients and heavy metals in the sludge were also investigated.These data permit to estimate the safety of direc and indiret land application of municipal sewage sludge. In addition, reducing the leaching of heavy metals from sewage sludge by applying the limestone powder was tested, the amount of heavy metals leached from sewage sludge was analyzed under different addition mode of limestone powder.Further more, indirect application of sewage sludge into the ditches nearby the hybrid giant napier were studied. In comparison, sludge was covered with plastic film in a treatment,together with no sludge control and no plant control. The growth, nutrient content and heavy metal content of plant and the changes in physico-chemical properties of sewage sludge were measured.The main results of this work were as follows:(1) All treatments can provide rather sufficient nutrients to the dwarf banana there were no significant difference between different treatments, in major nutrients contents in stem and leaves of the plant. The Pb, Cd contents in banana fruit was below the limits of food standards(GB 2762-2012), the Pb, Cd content in banana caulo and leaves of all treatments were conform to the standard of organic fertilizer standards(NY 525-2011).(2) The sludge direct application improved the physical properties of soil, the soil porosity was higher than that of control and mesh bag treatment. The sludge direct application also increased N, P, organic matter, CEC content in soil, which was 2.56, 3.08,2.45 and 1.55 times higher than the control respectively two years after sludge applications.However, K content in soil of all treatments decreased two years later. The heavy metals in soil of different treatments increased. The Cu, Zn, Pb, Cd contents in soil of sludge direct application were 2.77, 1.74, 1.12, and 3.87 times higher than mesh bag treatment, and the Zn, Cd contents in soil were higher than the tolerance limits of the soil environment quality standards(GB 156-1995). The sludge direct application increased significantly the DTPA extractable Cu, Zn, Cd in soil, The DTPA Zn in soil of mesh bag treatment was higher than the control, however, there was no significant difference of DTPA Cu, Cdand Pb between the control and the mesh bag treatment.(3) The longer the sewage sludge applied, the lower the organic matter, N content in the sludge in mesh bag. Therecovered rate of N in sewage sludge was 78% 8 months later,and 35% 24 months later. P,K contents in sewage sludge were were increased with time.The recovered rate of P, in sewage sludge was 96% 8 months later, and 83% 24 months later; meanwhile the recovered rate of K were 89% and 77% respectively. The recovered rate of heavy metals in sewage sludge varied among Cu, Zn, Pb, Cd. The recovered rate of Cu was 99% 8 months later, and 83% 24 months later; meanwhile Zn were 93% and 51%,Pb were 99% and 73%, Cd were 73% and 53%, respectively.(4) There were 755 kg sewage sludge applied with mesh bag and 337.3kg recovered two years later. The longer the sewage sludge was applied, the higher the sludge was reduced..(5) The p H of leachates increased with addition of limestone powder and fluctuated between 5.3~6.9. There was no significant difference between different positions where the limestone powder was added. The p H of leachates of control was lower than that treated with limestone powder by 1~2 p H unit. The amount of Cu, Pb in leachates from sludge of the control were 23 mg and 186 ug. The position of the addition of limestone powder did not affect significantly the amount of Cu, Pb. The amount of Zn, Cd in leachates of the control was 364 mg and 735 ug, which was 2.97~6.03 and 5.45~6.65 times higher thanthe limestone powder treatments respectively. It indicats that the leaching of Cd and Zn from sewage sludge were effectively controlled by addition of the limestone powder.(6) The hybrid giant napier biomass of the two sludge treatments in the second half year of 2014 were 56.72 t·hm-2and 86.66 t·hm-2, which were 3.84~5.87 times of the control;the biomass of the sludge treatments in the first half of 2015 were 95.16 t·hm-2, 94.62 t·hm-2,which were 3.45~3.48 times of the control; and in the second half of 2015 they were 95.52t·hm-2, 106.31 t·hm-2, which were 3.26~3.62 times of the control.(7) The content of Pb, Cd in the plant of all treatment were below the limit of heavy metals according to the Chinese standards of feeds(GB13078-2001). The hybrid giant napier contained abundant nutrients, can be utilized as organic fertilizer or animal feeds.(8) The nutrient in sewage sludge changed slowly for a half year after the application into ditches. The recovered rates of N were > 80%, P 97%~98%, K >90%, respectively.The recovered rate of Cu in sewage sludge was about 95%, Pb >95%. The recovered rate of Zn under plants treatments was 93%, and for the control without plant, it was 88.99%. The recovered rate of Cd of the control was 90.5%, and under plants treatments, they were97%. The plastic film covering of sludge reduced the heavy metals leaching.(10) The sludge was significantly dried and reduced in fresh weight. The water content in sludge under different treatments decreased to less than 50% in the second half of 2014,the reduction of sludge in the planted ditches with and without covering of plastic film reached 69.98% and 62.45% respectively; That in the first half of 2015 reached 41.94% and39.88%, and in the second half of 2015 reached 41.66% and 41.49% respectively. |
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