Advanced Purification Of Domestic Wastewater By Microalgal Biofilm And Subsequent Anaerobic Co-digestion With Pig Manure

Water resources shortage and energy crisis are the two main challenges faced by our country's economy sustainable development. Advanced purification of secondary effluent could curb the eutrophication of water body from the source and achieve the recycle use of wastewater. The technology for wastewater treatment by algae not only can effectively remove nitrogen and phosphorous, but also can get the algae biomass which is a clean and renewable energy resource to replace fossil fuel.Three kinds of algal biofilms(Anabaena flos-aquae, Scenedesmus obliquus, Oscillatoria tenuis) for advanced purification of secondary effluent were investigated in this paper. The optimum algal strain with high purification efficiency for wastewater and stable algal biofilm was selected and was employed to study the influence of aeration rate on the removing efficiency. After that the collected algae biomass were mixed with pig manure to conduct anaerobic co-digestion research. The main conclusions obtained in experiments were as follows:(1) Anabaena flos-aquae reached the highest removal rate of NH4+-N 98.9 %, TP 99.2 %, respectively. While for Oscillatoria tenuis was 96.2 %, 90.5%, respectively. During the course of experiment, Oscillatoria tenuis was easier to be adsorbed on the carrier and much more stable. Therefore, Oscillatoria tenuis was selected as the optimal algal strain.(2) Aeration could improve the growth of algae and the efficiency of removing nitrogen and phosphorous. The optimal aeration rate was 1.0 L/min, at this time Oscillatoria tenuis reached the highest removal rate of NH4+-N 98.9 %, TP 95.5 %.(3) The proportion of pig manure and algae biomass has a great effect on the biogas productivity and methane productivity produced by anaerobic co-digestion of pig manure and algae biomass. When the proportion was 2.0, the nutrients in the mixed substrate reached the optimal balance state and the efficiency of biogas productivity by anaerobic co –digestion also realized the highest value. After the Biochemical Methane Potential(BMP) tests, the methane production process was highly correlated to the Cheynoweth's equation and the R2 was higher than 0.97 under all the experimental condition, which indicated that the BMP of pig manure and algae biomass could be described using this model.(4) After acid, alkali and thermal pretreatment, the SCOD concentration of algae were increased, thermal pretreatment for 20 min made the solubilisation reached the highest value of 20.4%. Three kinds of pretreatment methods could inordinately improve the biogas production and methane content by anaerobic co-digestion of pig manure and algae biomass. The maximum biogas production and methane production were 4235 m L,2772 m L produced after thermal pretreatment, and increased 31%,43%,respectvely, compared to not treated experimental sample, and the methane productivity also reached the maximum value of 256 m L/g VS. It can be concluded that thermal pretreatment method showed the best effect among these three methods.