| Aquatic worm is often used in the wastewater treatment systems to minimize the sludge production, which has been proved an effective, economical and sustainable technology. Waste sludge especially industrial waste sludge is a complex mixture of water, bacteria, and organic and inorganic materials, containing phosphorus, nitrogen compounds and various pollutants such as heavy metals. If this new technology could be applied to reduce such kind of excess sludge reduction treatment, it will promote the development of the technology undoubtedly.Chromium salts, is one of the main series of products of inorganic chemicals industry. The “Twelfth Five-Year Guideline" emphasized that, stricter regulations should be put on hazardous waste, especially on the industries generating chromium salt. It is urgent to handle excess sludge containing chromium (Cr), efficiently and economically. Hexavalent chromium(Cr (Ⅵ)) was studied as a typical representative of heavy metal, and the interaction mechanisms between which and worms were investigated.The effects of pH, and dissolved oxygen (DO) concentration and temperature on the respiration rate of worms under the stress of Cr (Ⅵ) were investigated in this paper. The orthogonal tests showed that, the intergrated effect had an significant impact on the respiration rate of worms, and low concentrations of Cr (Ⅵ) could simulate the respiration, while high concentration of Cr (Ⅵ) could inhibit the respiration. When the Cr(Ⅵ) concentration was2.5mg/L, temperature was26℃, pH was6.0, DO concentration was6.0mg/L, the maximum respiration rate was achieved. The order of importance of these effects on the respiration rate was:Cr(Ⅵ) concentration> temperature> DO concentration> pH.Exposed to Cr (Ⅵ) solutions, the enzymatic activities of worms were different under aeration and non-aeration conditions. The enzyme activities of superoxide dismutase (SOD) and catalase (CAT) changed significantly compared to the control group (no Cr(Ⅵ) group). Under aeration condition, the activity of SOD was activated followed by inhibited, the maximum inhibition ratio was91.59%when the Cr(VI) concentration was0.8mg/L at48h, while the ratio was98.08%when the concentration was0.1mg/L at42h under anoxic condition. The activity of CAT was most active when exposed to Cr(Ⅵ) for6h, they were1.32and0.83times higher than the control group under aeration and anoxic condition, respectively. But for peroxidase (POD), the change was slight when exposed to aeration condition, while under non-aeration condition, it was inhibited for almost the whole exposure time. The results showed that, the DO concentration also had an effect on the enzyme activity of worms. As a biomarker of worms, SOD enzyme was the most sensitive to Cr(Ⅵ).The adsorption capacity of worms at different Cr(Ⅵ) concentrations, pH values, worm quantities, temperatures was investigated. Exposed to different concentrations of Cr(Ⅵ), the time to reach equilibrium of dead worms(2h or so) was much shorter than the live worms(30h or so), but for the adsorption rate, the living worms’was higher. When the concentration of Cr(Ⅵ) was2.5mg/L or below, the adsorption efficiency and amount per kilogram for live worms was higher than dead worms. When pH changed from6.0to8.0, the adsorption efficiency and amount per kilogram of live worms increased gradually, then tend to gentle with time prolong, they were56.45%and90.47mg/kg(dw) respectively when pH was8.0. While when pH shifted from6.0to10.0, the adsorption efficiency and amount per kilogram of dead worms increased firstly, then decreased, the maximum rate and amount were47.13%,75.53mg/kg(dw-dry weight) respectively, achieved at pH8.0. In a0.1L vessel, when the worms quantity increased from1.0g(ww-wet weight) to5.0g(ww), the adsorption rate of live worms increased from o.94μg/h to2.07μg/h, while the maximium rate of dead worms was only0.33μg/h, and adsorption efficiency and amount per kilogram of live worms were all higher than dead worms. Temperature had a great influence on the adsorption capacity. The adsorption efficiency and amount per kilogram of dead worms were almost half of live worms at different temperatures. Above all, Cr(Ⅵ) was adsorpted more by live worms than dead worms. If live and dead worms are in the same sludge reduction system, the temperature could be controlled at22～25℃, pH around7.5. After predation by the worms,65.66%of total Cr in sludge re-distributed to worms and worm faeces in worm-sludge reactor. Cr was bioaccumulated, and then large amount of them was biotransformed to faeces by worms, while29.51%of total Cr still remained in sludge. The Cr concentrations in worms, sludge and worm faeces increased firstly, then decreased along the exposure time. It showed that, the detoxification mechanism could be simulated when the amount of Cr(Ⅵ) was too high in worms. What’s more,63.27%of Cr(Ⅵ) was reduced to Cr(Ⅲ) by worms.Actually, the research results could provide technical support for optimizing the process conditions for sludge reduction by worms, especially for sludge containing Cr. It will undoubtedly promote the technology theory of sludge reduction by worms. |
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