Regulation of Selective Acidogenic Fermentation of Organic Wastewater and the Products Recovery

有機廢水選擇性厭氧產酸發酵過程的調控與產物回收新技術

Student thesis: Doctoral Thesis

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Award date12 Dec 2018

Abstract

Organic wastewater is produced in many industrial and agricultural sectors.  There are growing interests to valorize such wastewater through recovering energy and resources, for which two-step anaerobic bioconversion offers an attractive route.In such processes, complicated organic matter in wastewater is converted into simple molecules, including volatile fatty acids (VFAs). VFAs can either bedirectly recovered or in-situ utilized for further production of higher-value products. However, the selectivity and conversion rate of acidogenic products are still too low to be economically practical.

This work reports a series of improved anaerobic fermentation processes to enhance the production selectivity and efficiency. First, we developed an acid-stimulated continuous, selective system for the production of acetate, with concomitant generation of methane (CH4) as the gaseous product, during anaerobic treatment of organic wastewater. At acidic pH (5.0), the system produced a stable liquid stream rich in acetate (accounting for 66% of the liquid-phase products) and a CH4-rich gas stream accounting for about 41% of the gaseous products. No hydrogen (H2) was detected in the gaseous  phase. The microbial community analysis showed that Clostridium, Bacteroides and hydrogenotrophic methanogens were enriched under the low pH condition. The enhanced H2 consumption created a low H2 partial pressure environment that is favorable for selective production of acetate. This work demonstrates the feasibility of diverting the anaerobic fermentation pathway towards selective co-production of acetate and CH4 by simply controlling the wastewater pH.

After improving the selectivity of VFA production, the next issue to be addressed is how to efficiently separate and recover the produced VFAs from wastewater. Here, we designed an electrodialysis (ED) to concentrate and recover the VFAs in situ. By using an acidogenesis-ED integrated system with 20.0 V applied voltage, the acetate was concentrated 4-fold and the propionate and butyrate were concentrated over 3-fold in the integrated system after 528 hours of operation. This work demonstrates the feasibility of the acidogenesis-ED integrated reactor for wastewater valorization and discusses the remaining challenges and opportunities.

Although short-chain VFAs are useful raw materials for many industries, it is more economically practical if higher-value products can be separated and obtained from wastewater. Here, we attempted to recover medium-chain fatty acids (MCFAs), e.g., caproate, from organic wastewater by in-situ utilization of the produced acetate and ethanol through chain elongation. An anaerobic membrane bioreactor(AnMBR) was adopted to enhance biomass retention and improve the activity of MCFA-related bacteria. High selective production of caproate was achieved in the AnMBR. Less oxidation of ethanol made the pH stable at neutral. The utilization of ethanol could be improved by the retention of a functional microbiome. The microbial community difference between seed and acclimated sludge was found to be small, meaning that the substrate just changed the metabolic pathways. Hydrogenotrophic methanogens survived in the seed and fermentation sludge and produced CH4 during the fermentation process.

The caproate production with high-strength wastewater fermentation effluent as the substrate might be affected by the accompanying biogas. Our results show that CH4 has no effect on caproate production, while CO2 could promote the caproate production rate and yield. H2 suppressed the chain elongation process completely, which could be alleviated when the H2 was consumed by hydrogenotrophic methanogens under coexistence of CO2. Regulation of H2 and CO2 could alleviate the inhibition and avoid excessive ethanol oxidation.

The findings in this work may provide implications for the development of more sustainable anaerobic wastewater treatment processes.

    Research areas

  • Organic wastewater, Acidogenic fermentation, VFAs, H2 partial pressure, Electrodialysis, Caproate, Chain elongation