The Research On The Treatment Of Swine Wastewater By A Zero Valent Iron-based Anaerobic System

With the accelerated development trend of the pig farming industry in recent years, large amount of swine waste products and wastewater have had a tremendous impact on the surrounding ecological environment. Although anaerobic digestion is widely used in swine wastewater treatment due to its availability, reliability, and economy, several problems exist regarding its practical operation, including high COD concentrations in the effluent, low phosphorus removal efficiency and methane production. Zero-valent iron(ZVI) has been proven to provide electron donors for microorganisms and improve the anaerobic environment, thus facilitating the biodegradation of organics such as chlorinated aliphatic compounds, nitrobenzenes and organic dyes. In order to solve the problems of COD removal and CH4 generation in swine wastewater, the ZVI-anaerobic sludge combined system was been employed to treat swine wastewater. This stu’dy is of great significance to explore the new treatment technique for piggery wastewater. The main results are as follows:1、Batch experiments explored the effects of iron types, initial pH value and ZVI dosage on the combined system treating swine wastewater. The research of iron types indicated that both types of iron powder and iron scrap addition could increase the COD removal efficiency by 8.63% and 4.32%, the COD removal rate by 32.47% and 20.98% as well as the methane production by 24.39% and 13.04%, respectively. It was shown that the strengthening effects induced by iron powder was more significant than that by iron scrap. The research of pH indicated that the COD removal efficiency and removal rate of the combined system increased from 76.83% and 14.52 mgCOD/(gVSS·h) to 83.45% and 18.54 mgCOD/(gVSS·h) with the initial pH increasing from 5 to 8. The largest improvement extent induced by ZVI was obtained at the initial pH of 6. The ZVI dosage was expressed as the mass ratio of iron and sludge(rZVI/VSS).When the rZVI/VSS ranged from 0 to 2.63, the COD removal efficiency and removal rate increased from 75.14% and 14.20 mgCOD/(gVSS·h) to 90.64% and 19.81 mgCOD/(gVSS·h). The methane production was also raised up from 3.02 mmol to 6.07 mmol. However, further increasing of rZVI/VSS to 5.26 showed no more facilitating effect. Under this ZVI dosage, microbial cells appeard damaged and low metabolic activity. The enhancement efficiency of per unit ZVI reached the maximum at the rZVI/VSS of 0.74. Based on the processing requirements and economic costs, it was advised to choose the optimum rZVI/VSS between 0.74 and 2.63.2、Continuous flowing experiments explored the operating performance of ZVI-UASB reactor treating swine wastewater comparing with the control reactor. Results indicated that ZVI could improve the COD removal efficiency, phosphate removal efficiency and methane production by 8.5%,27.8% and 50% respectively, thus enhancing the capacity of resistance to loading shock and influent pH fluctuation. Furthermore, the presence of ZVI was conducive to the secretion of microbial extracellular polymeric substances(EPS), which increased by 53.24%. Of these, the protein content increased greater than the polysaccharide content. Compared to the granular sludge of control reactor, the ZVI-granular sludge had a more compact structure and a better specific methanogenic activity, which was 44% higher than the control. With the operation of the reactor, ZVI corroded gradually and the effluent Fe2+ concentration decreased from 9.02 mg/Lto 4.51 mg/L. In addition, some ZVI particle was encapsulated by the sludge and involved in the process of sludge granulation. Some ZVI reacted with the anion, generating the iron precipitates and Fe3O4 at the surface.3、qPCR and high-throughput sequencing experiments explored the effect of ZVI on microorganism community structure of anaerobic sludge. The qPCR results showed that the abundance of methane mcrA gene which was associated with methanogenesis was 1.6 times higher than that of control group, indicating that the presence of ZVI enhanced the methanogenesis potential of ZVI-UASB reactor. In the high-throughput sequencing analysis of archaea, the results show that the addition of ZVI made the abundance of Methanosaeta and Methanobacterium increase and the abundance of MiscellaneousEuryarchaeotic_Group (MEG) and Euryarchaeota_unclassified decrease. In the view of bacteria, the addition of ZVI increased the abundance of syntrophic acetogens such as Syntrophomonas, Syntrophus and Pelotomaculum as well as the abundance of homoacetogens such as Clostridium, Ruminococcus and Treponema. In addition, the abundance of VC2.1_Bac22_norank, Lutispora, Bacteriovoracaceae_uncultured decreased as ZVI added. It was indicated that ZVI had selective effects on the microorganisms and could enrich the special microbial populations.