A hybrid dry-fermentation and membrane contactor system: Enhanced volatile fatty acid (VFA) production and recovery from organic solid wastes

Highlights

• Hybrid system was developed for volatile fatty acid (VFA) production and recovery.

• Hybrid fermentation and membrane system achieved 42% enhancement of VFA production.

• Continuous separation of VFAs might have decreased inhibition in the leach bed reactor (LBR).

• Approximately, 60% of the produced VFAs were recovered via the membrane system.

Abstract

Anaerobic dry-fermentation of food wastes can be utilized for the production of volatile fatty acids (VFA). However, especially for high load fermentation systems, accumulation of VFAs may result in inhibition of fermentation process. In this study, separation of VFAs from synthetic mixtures via a vapor permeation membrane contactor (VPMC) system with an air-filled polytetrafluoroethylene (PTFE) membrane was assessed at various temperatures and permeate solution concentrations. In addition, a pioneering integrated leach-bed fermentation and membrane separation system was operated with undefined mixed culture for the purpose of enhanced VFA production along with its recovery. Hybrid system resulted in 42% enhancement in total VFA production and 60% of total VFAs were recovered through the VPMC system. The results of this study revealed that integrated system can be exploited as a means of increasing organic loading to fermentation systems and increasing the value of VFA production.

Introduction

Majority of the commodity chemicals are currently synthesized through fossil fuel-based technologies. However, considering the concerns about climate change due to excessive use of fossil fuel resources, scientific research interest has shifted from the improvement of existing fossil fuel-based processes to the recovery of valuable chemicals from waste streams (Zacharof and Lovitt 2013; Agler et al., 2011; Da Ros et al. 2020). Anaerobic digestion of various waste streams using undefined mixed microbial cultures can yield valuable products such as methane, hydrogen, and carboxylates (short- and medium chain volatile fatty acids (VFAs) (Agler et al., 2011). Among all these valuable products; fermentation of complex organic wastes to carboxylates, also known as the carboxylate platform, has gained considerable attention lately (Agler et al., 2011; Holtzapple et al., 2015; Holtzapple and Granda 2009; Fernandez-Dominguez et al., 2020; Atasoy et al., 2020). Basically, through mixed culture fermentation, short-chain volatile fatty acids such as acetic (ethanoic) and propionic (propanoic) acids and medium-chain volatile fatty acids such as butyric (butanoic), valeric (pentanoic), and caproic (hexanoic) acids are formed through the carboxylate platform (Holtzapple et al., 2015) Produced VFAs through carboxylate platform may then be used as intermediate chemicals in the fields of food and beverage, pharmaceutical, and chemical industries (Zacharof and Lovitt 2013; Atasoy et al., 2018). Especially the utilization of VFAs for microbial protein production has gained significant attention lately (Sakarika et al., 2020; Andersen et al., 2014). However, continuous VFA production via anaerobic fermentation of wastes can be difficult since undissociated VFAs can easily penetrate through the cell membrane and inhibit microbial processes in anaerobic digestion (Cheng et al., 2010, Zoetemeyer et al., 1982, Silva et al., 2013). VFA accumulation can also lower the local pH and suppress the intracellular enzyme activity, thus deteriorating hydrolytic and acidogenic processes (Cheng et al., 2010, Zoetemeyer et al., 1982, Arora et al., 2007, Pind et al., 2003). It has been reported that for systems with high VFA concentrations, VFA concentration of 17 g COD/L results in cessation of VFA production during fermentation of pretreated waste activated sludge at operational pH values of 6.0 to 6.6 (Pratt et al., 2012). Whereas, Siegert and Banks (2005) suggests that VFA concentrations as low as 4 g/L results in slight inhibition of glucose fermentation. Therefore, it is essential to constantly remove VFAs during anaerobic fermentation in order to optimize VFA production and to possibly recover VFAs. Concentration gradient driven vapor permeation membrane contactors (VPMC) has been proven to successfully remove VFAs from synthetic feeds and actual waste streams. (Rodríguez et al., 1997; Aydin et al., 2018; Tugtas 2014). In VPMC systems, two aqueous phases are separated from each other via a membrane to prevent dispersion of one phase into the other (Han et al., 2005; Albrecht et al., 2005) VPMC systems are different from the microfiltration and nanofiltration processes as the system is not pressurized and the separation principle does not base on size exclusion (Aydin et al., 2018; Tugtas 2014). Only gaseous species can pass through the membrane and there is no convective flow through the pores of microporous hydrophobic membranes (Aydin et al., 2018; Tugtas 2014). Gaseous forms of VFAs desorb from the acidic feed solution, diffuse through the hydrophobic membrane pores, and subsequently are absorbed by the alkaline permeate solution, where VFAs rapidly dissociate into their ionic forms. Consequently, partial pressure of VFAs or other volatile species of interest in the feed solution is always higher than in the permeate solution, therefore, a constant concentration gradient can be created across the membrane and back diffusion of VFAs can be prevented (Tugtas 2014; Han et al., 2005).

Anaerobic fermenters can be classified based on total solids (TS) content of organic waste to be fermented as wet (< 10% TS), semi-dry (10–20% TS), or dry (>20%) (Karthikeyan and Visvanathan 2013). Dry fermentation reactors can be operated with high organic loading, therefore organic fractions of municipal solid wastes, animal wastes, or lignocellulosic biomass can be treated through dry fermentation/digestion processes (Karthikeyan and Visvanathan 2013; Li et al., 2011). Although continuous systems have also been used for the fermentation of wastes with high TS values, operational simplicity and minimum capital cost of batch systems make them favorable for the dry fermentation processes (Karthikeyan and Visvanathan 2013; Li et al., 2011) Leach-bed reactors (LBRs) are a batch type systems and mainly used for dry fermentation of wastes. LBRs do not require mixing, solid and liquid separation occurs within the reactor, and wetting of the solid waste is obtained through leachate recycle from the bottom to the top of the tank (Bayrakdar et al., 2018). Compared to other reactor types, liquid volume within the LBR can be kept low, resulting in leachate production with more concentrated VFA content, which is preferred for obtaining higher yields through VFA chemical platform.

Anaerobic fermentation of organic fraction of municipal solid wastes for the purpose of VFA production have been reported in the literature (Silva et al., 2013; Dogan and Demirer 2009; Cheah et al., 2019; Cavdar et al., 2011; Yesil et al., 2014). Recovery of produced VFAs having two to six carbon atoms from an anaerobic fermenter is very important in terms of; recovering valuable chemicals from organic-rich wastes and preventing inhibition in high-load anaerobic fermentation systems. In addition, continuous removal of VFAs from fermentation systems may enhance the VFA production within the reactor. Integration of VPMC system to an anaerobic fermenter treating organic rich solid wastes (SW) would enable constant removal of VFAs which then can increase the VFA yield and enable recovery of valuable commodity chemicals. The objectives of this study were to: (a) investigate VFA recovery from synthetic VFA mixtures using air-filled polytetrafluoroethylene (PTFE) membranes through VPMC system and (b) determine VFA production and separation efficiency in integrated fermentation and VPMC system treating organic fraction of municipal solid wastes (OFMSW). This is the first study investigating VFA production and separation through high solids (dry) fermentation of OFMSW through a pioneering integrated (hybrid) leach-bed reactor fermentation and VPMC system.