Volatile fatty acids production using waste activated sludge and food waste to produce bioplastics

Abstract

Fossil fuel exhaustion, increasing greenhouse gases emissions and population growth, among many other issues, are leading to many environmental problems which require a transformation of the production system and waste management including wastewater treatment plants (WWTPs). Hence, the end-of-pipe processes for organic wastes treatment are being converted into resource recovery facilities that produce value-added products. Anaerobic biological processes using mixed cultures can handle the variability of organic wastes: for example, these wastes could be initially converted to volatile fatty acids (VFAs) through acidogenic fermentation and the remaining part (non-VFA organic matter) could be directed to anaerobic digestion to produce biogas. VFAs have multiple applications and one of them is its use as carbon source for polyhydroxyalkanoates (PHA) production. PHA are value-added products mainly composed by polyhydroxybutyrate (PHB) and/or polyhydroxyvalerate (PHV), that can be obtained using mixed microbial cultures in 4 phases: acidogenic fermentation, selection of PHA-storing microorganisms, accumulation of PHA using selected biomass and PHA extraction. In this Master thesis, the first 3 stages of the PHA production process using organic wastes (namely, the acidogenic fermentation, the selection of PHA-storing microorganisms and the PHA accumulation) are studied. The effect of pH on food waste (FW) and wasted activated sludge (WAS) co-fermentation was studied using batch tests and semi-continuous experiments at mesophilic conditions (35 ºC). The pH control in semi-continuous fermenters was difficulted by continuous foaming events. Moreover, PHA production using VFA-rich wastewater (simulating the effluent resulting from an acidogenic fermentation of the organic fraction of municipal solid wastes (OFMSW)) and different selection strategies (one using aerobic feast-famine with nitrogen depletion during feast and the other applying only aerobic feast-famine regime). Furthermore, the VFA profile obtained along the suitable operation of co-fermenters is compared with the synthetic stream used to assess PHA production. The acidogenic fermentation unit was monitored with VFA profile and distribution, pH, chemical oxygen demand and soluble chemical oxygen demand (COD and sCOD), total ammonium nitrogen (TAN) concentration, total and volatile solids (TS and VS) and alkalinity. Batch tests were performed in glass-bottles filled with 150 mL of fixed proportions of WAS and FW (75:25 on VS basis) and using a lab-scale semi-continuous fermenter’s effluent to test the influence of pH in substrates and an operating fermenter, respectively. Furthermore, 2 semi-continuous fermenters with 1.75 L of working volume were operated for 46 days using fixed proportions of WAS and FW (65:35 on VS basis), organic loading rate (OLR) (11 g VS kg-1 d-1), hydraulic retention time (HRT) (3 d) and mixing at 80 rpm. Main VFAs resulted from batch tests and fermenters were acetic acid (30%), butyric acid (30%) and propionic acid (20%). Higher pH showed an increased VFA yield as the solubilisation of organic matter (hydrolysis) was enhanced. Acetic acid consumption in fermenters was experimented and reduced by changing operational parameters (reduction in FW and HRT). Due to foam formation, pH control in semi-continuous operation could not been studied although different strategies were tested to minimise foam (better homogenisation, discharging foams using effluent tubes and lowering HRT and FW proportion). Regarding to lab-scale PHA production, the monitoring of cycles was performed by recording dissolved oxygen (DO) profile, TAN and VFA concentrations, total and volatile suspended solids (TSS and VSS) and pH. VFA-rich synthetic influent with a concentration of 3.5 g COD L-1 for both selection and accumulation phases was used. Biomass selection was performed with a selection sequential batch reactor (sSBR) at 35 ºC and 80 rpm agitation with aerobic conditions using diffusors connected to air pumps and net-air. Furthermore, for accumulation tests a 1 L capacity glass reactor at 35 ºC, agitation at 80 rpm and air supply system were used. Experimental results showed that if double selection strategy (aerobic feast-famine plus nitrogen decoupling in feast) higher PHA content in purge was obtained (30% on suspended solid (SS) basis) along with higher contents of PHA during accumulation (50% on SS basis) compared to a single selection strategy (only aerobic feast-famine regime) with PHA contents of 11% and 38% (SS basis), respectively. Similar PHA compositions were obtained through selection and accumulation phases with the 90% in PHB and 10% in PHV. To sum up, raising pH increases VFA yields in acidogenic fermentation as consequence of hydrolysis and organic matter solubilisation enhancement. Moreover, increasing pH earlier in fermentation batch tests derives in higher VFA production. Due to PHA production, although both strategies selected the biomass successfully, double selection results in higher PHA accumulation potential. It is expected a suitable PHA production if fermenter’s effluents are used (removing previously the nitrogen content) because of the higher ratios CODVFA sCOD-1 and the similar composition of VFAs.