Effect Of Heavy Metals On The Performances And Methanogenic Community Shifts Of UASB Process During The Treatment Of Swine Wastewater

With the rapid development of pig breeding industry in China, the increasing amount of swine wastewater has become a major source of agricultural pollution in our country, causing critical impair on water, soil and air environment. Up-flow Anaerobic Sludge Blanket(UASB) has been widely applied in the swine wastewater treatment for many years. However, the produced swine wastewater is now heavily loaded with excessive amount of heavy metals such as Zn2+. These heavy metals are considered to be inhibitors for the anaerobic microbial populations in UASB, which present a high challenge for the traditional UASB operation experiences and result in the weak practical performances of UASB in treating swine wastewater.In this work, two identical UASB reactors (R1, R2) were set up under the same operation environment to treating swine wastewater using the same parameters. All the experiments are divided into two parts. In part one, R1and R2are set as parallel to each other and operated by absolutely the same way. In this part, the effects of heavy metals accumulation on the UASB performance from the start-up time to the steady-state period were explored. In part two, Zn2+was chosen to be the core of the research according to the results of part one. In this part, R1was set as a control without added Zn2+and R2reactor was dosed with extra Zn2+. The effect of Zn2+on the COD removal, methane production and the16S rRNA gene concentration of different methanogens were detected and the relationships among them were analyzed. More importantly, reveal the radical factor and gradual process about how Zn2+affects methanogens, and then the changes in methanogens impact the UASB performance indicators:COD removal and methane production. Thereby, quantitative foundation would be provided according to the mechanism to adjust the effect of Zn2+on the UASB performance. The main conclusions are as follows:(1) In treating swine wastewater, UASB reactors present stable in their ability of TP removal, with an average removal amount of20mg/L-30mg/L. And the reactors were invalid in the removal of ammonia-N of the swine wastewater. During the steady-state, the COD removal efficiencies were high, reaching70%or more. If measured by the performance of COD removal, UASB could be considered as a viable technology for the treatment of swine wastewater.(2) During the whole operation, Zn2+accumulated fastest in the anaerobic sludge, but Cu2+and Cr3+increased slower in a same pace. The final Zn2+content of0.95mg/g was15times more than those of the Cu2+and Cr3+. Among the heavy metals, Zn2+was of the most significant influences on the UASB performances. Zn2+content of more than1.22mg/g was the main inhibitive factor hindering the start-up of UASB reactor treating swine wastewater.(3) Zn2+of5mg/L and10mg/L were stimulative for the COD removal and methane production in treating swine wastewater by the UASB reactor, meaning these amounts of Zn2+promoted the UASB performance. The dosage of15mg/L, presenting in a trend of inhibition, began to cause decline in the COD removal rates and methane production.(4) Before Zn2+was added, in UASB reactor Methanomicrobiales (MMB) was the most abundant one (60%), Methanosarcinales (MSL) formed a small population (10%), Methanobacteriales (MBT) was in the middle amount. When Zn2+of5mg/L was added, these three methanogens were all promoted. When the dosed Zn2+reached10mg/L, MSL was presented in a trend of inhibition, while, MMB and MBT were still in promotion. However, Zn2+dosage exceeding15mg/L would restrain all the methanogens. Among the three methanogens, MMB got the most significant influence. During the whole operation, Methanococcales (MCC) was not detected, probably due to the lack of living conditions in the UASB reactor treating swine wastewater.(5) Quantitative changes in methanogenic community structure were related to the changes in COD removal and methane production. MMB should be the most responsible methanogen for the performance of UASB reactor with respected to the Zn2+dosage. Total amount of Zn2+less than17.8mg/L would be advisable for stable and efficient treatment of swine wastewater by UASB reactors with HRT48h at35℃. And the performance of UASB treating swine wastewater would be promoted when the contaminated Zn2+is in an appropriate level or the Zn2+tolerant MMB communities are cultivated.