| Sewage sludge is the product of the wastewater treatment. With morewastewater treatment plants and better capacity of wastewater treatment,larger sewage sludge production exists. Meanwhile, the treatment ofsewage sludges which has large quantity of water and pollutants is moreand more difficult. To some seaside countries and regions, ocean-dumpingwas a convenient and economical approach to treat sewage sludges. Butthis method was finally banned because pollutants in sewage sludges coulddo harm to marine ecosystem. The leaching of pollutants from sewagesludges in marine environment is affected by different factors, butcomprehensive and basic researches in this field are scarce. Therefore, it isnecessary to study influencing factors which affect the leaching ofpollutants in sewage sludges and it has some ecological value.This paper was designed to study the effect of five influencing factors,respectively pH, temperature, salinity, microbial activities and redoxpotential, on the leaching of pollutants from seven sewage sludges. Thiscan supplement basic researches in this field and complete relevant data ofthe leaching of pollutants from sewage sludges. Besides, this will provide theoretical basis for protecting marine ecosystem. The main results were asfollows.(1) The moisture contents of seven sewage sludge samples were between68.5%and81.7%. The variation ranges of TN and TP were from3.80%to6.84%and from0.052%to0.19%respectively. The contents of TOCwere between12.2%and21.8%, and that of S2was the highest.VSS/SS ranged from47%to70%. The contents of AVS were between30.2mg·kg-1and3760mg·kg-1, and compared with results of others,those were very high.(2) The contents of Zn and Cu were high in all sludge samples, followed byPb, As, Cd and Hg.The total amounts of heavy metals in S2, S5and S4were higher than the other four samples, followed by S6, S3and S7, andthat of S1was the lowest. High proportion of industrial wastewater ofreceived wastewater was the main reason of the high amounts of heavymetals in sewage sludges.(3) Cd showed the greatest degree of mobility, as seen from the highproportion (70%) of the metal extracted in the water soluble,exchangeable, carbonate-precipitated and oxide-bound fractionsrepresenting high mobility and potential bioavailability. The unstablespeciations of Zn accounted for20%-50%. Cu, which mainly presentedin the sulfide and organic bound form and residual fration, showed the poor mobility. The heavy metals of worst mobility were Pb, Hg and Asfor most of them were recovered in the residual fraction.(4) The effect of pH on the leching of pollutants from sewage sludges wasstudied according to CEN/TS14429:2005. The results showed that theleached amounts of Zn, Cd and As were high under acid and alkalineconditions, but low under neutral condition. Alkaline condition waspreferred to acid condition in the Cu leaching process. Hg and Pb wereleached obviously under acid and alkaline conditions respectively. Withthe pH increasing, the leached amount of DOC increased. The obviousleaching of DS was observed only from the sample S7with the highestlevel of AVS, and with the pH increasing, the leached amount of DSdecreased.(5) The effect of temperature on the leching of pollutants from sewagesludges was studied by improved HJ557-2010.The results showed thatthe leached amounts of Zn and Hg increased with temperatureincreasing, but the leached amount of Hg changed just a little. Withtemperature increasing, the leached amounts of Cu, Cd, Pb and As hadsmall changes and were higher slightly at14.5℃or21℃. The leachedamount of DOC was lowest at21℃, and DS had a higher leachedamount at21℃and30℃.(6) The effect of salinity on the leching of pollutants from sewage sludges was studied by improved HJ557-2010. The results showed that theleached amount of Zn decreased with salinity increasing. Cu and Pb,whose leached amounts changed just a little, had higher leachedamounts at salinity5.5and10.5, then with salinity increasing, theiramounts decreased to equilibrium, and at last, slight higher amountsoccurred at salinity30.5. For Cd, it was only leached out at salinity5.5and30.5. As and Hg, whose leached amounts also changed just a little,had higher amounts at salinity20.6and15.2respectively. The leachedamount of DOC increased first and decreased later, showing the highestamount at salinity20.6and the lowest at salinity30.5. The leachedamount of DS increased with salinity increasing, and that of S3was thehighest of the seven sludge samples.(7) According to experimental method of C. Lors, the effect of microbialactivities on the leching of pollutants from sewage sludges was studiedby improved HJ557-2010and continuous oscillation. The resultsshowed that the leached amounts of Zn, Cu and Hg were generallyperformed as sterilization higher than non-sterilization, while to Pb andCd, that was opposite. To non-sterilized S1and sterilized S1, S5and S6,As had leached amount and was performed as sterilization higher thannon-sterilization. The leached amounts Of DOC and DS (except S4)showed sterilization higher than non-sterilization. (8) According to experimental method of P. Kelderman, the effect of redoxpotential on the speciations of heavy metals in sewage sludges wasmainly studied. The results showed that with continuous oxygen, Ehvalues of mixtures of sludge and seawater would not increase all thetime, they would decrease and even decrease below the initial values.Except Cd and Pb, the changes in speciations of the other four heavymetals were not obvious with Eh increasing. Besides, the change inspeciation of Cd was performed as S1to S3similar and S4to S6similar,and the same was true in Pb. With Eh increasing, the water soluble andexchangeable frations of Cd and Pb increased.From what has been discussed above, on the conditions ofsterilized sewage sludge, elevated Eh and no matter what the pHenvironment, season and the salinity of sea area sewage sludge is in,sewage sludge will pose a threat to the marine ecosystem. This shouldbe paid attention to. |
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