Volatile fatty acids (VFA) build-up and its effect on E. coli inactivation during excreta digestion in single-stage and two-stage systems
This Chapter is based on a paper: Rinngu J., Ronteltap, M., van Lier, J.B. 2018. Volatile fatty acids (VFA) build-up and its effect on E. coli inactivation dining excreta digestion in single-stage and two- stage systems. Journal of Water Sanitation and Hygiene for Development. 10.2166/vashdev.2018.160.
Abstract
Digestion and co-digestion of faecal matter collected from Urine Diverting Dehydrating Toilet Faeces (UDDT-FS) and mixed Organic Market Waste (OMW) was studied in single stage pilot scale mesophilic plug-flow anaerobic reactors at UDDT-FS:OMW ratios 4:1 and 1:0. Escherichia coli (E. coli) inactivation and Volatile Fatty Acids (VFA) build-up was monitored at sampling points located along the reactor profile. E. coli inactivation achieved in digestion of UDDT-FS: OMW ratio of 4:1, 12% TS was 2.3 log times higher than that achieved iu UDDT-FS: OMW ratio of 1:0.
In subsequent trials, a two-stage reactor was studied, applying a UDDT-FS:OMW ratio of 4:1 and 10% or 12% TS slurry concentrations. Highest VFA concentrations of 16.3±1.3 g/'l were obtained at a pH of 4.9 in the hydrolysis/acidogenesis reactor, applying a UDDT- FS:OMW ratio of 4:1 and 12% TS, corresponding to a non-dissociated (ND)-VFA concentration of 6.9±2.0 g/1. The corresponding decay rate reached a value of 1.6 /d. In the subsequent methanogenic plug-flow reactor, a decay rate of 1.1/d was attained within the first third part of the reactor length, which declined to 0.6/d within the last third part of the reactor length. Results show that a two-stage system is an efficient way to enhance pathogen inactivation duiing anaerobic digestion.
Introduction
Ecological sanitation concepts have been developed due to the growing need for improved onsite sanitation systems aiming at the protection of human and environmental health (Esrey, 2001; Niwagaba et al, 2009a). Urine Diverting Dehydrating Toilets (UDDTs) fit well into this concept, especially in densely populated, low lying settlements (Katukiza et al, 2012; Niwagaba et al., 2009a; Schouten & Mathenge, 2010). The technology has been adopted by Sanergy (Nairobi, Kenya), a company working on sanitation in LIHDS. Currently, from Mukuru Kwa Njenga and Mukuru Kwa Reuben LIHDS, approximately seven tonnes UDDT- faeces (UDDT-FS) are delivered per day to the central treatment plant, located 50 km from the city centre. Key concern is digestion and sanitisation of the waste, as the addition of ash and sawdust after toilet use is insufficient for pathogen inactivation (Niwagaba et al., 2009a).
Anaerobic digestion (AD) provides a cost effective and energy saving alternative for waste treatment (Avery et al., 2014; Fonoll et al., 2015; Nallathambi Gunaseelan, 1997; Romero-Giiiza et al, 2014). Anaerobic systems can be applied at any scale and almost any place, whereas the effluent is stabilised with a good fertiliser value (Pabon-Pereira et al, 2014; Van Lier et al, 2008). Key reported drawback, however, is insufficient pathogen inactivation with solid and liquid digestate containing high levels of pathogenic organisms such as Salmonella, Shigella, Campylobacter jejuni, Clostridium perfringens, Enterococcus species and Vibrio cholera (Chaggu, 2004; Chen et al, 2012; Fagbohungbe et al, 2015; Horan et al, 2004; Kunte et al., 2000; Masse et al, 2011), As such, the poor microbial quality of the digested solids may lead to transmission of enteric diseases when applied to agricultural land (Pennington, 2001; Smith et al, 2005).
During anaerobic digestion, temperature and time play a key role in pathogen inactivation (Gibbs et al, 1995; Olsen et al., 1985; Olsen & Larsen, 1987; Smith et al, 2005), as does reactor configuration (Kearney et al, 1993; Olsen et al, 1985). In addition, pH and VFA concentration in the reactor broth are an indication for bacterial survival (Abdul & Lloyd, 1985; Farrah & Bitton, 1983; Sahlstrom, 2003). At a low reactor pH, the same amount of VFAs lead to a higher fraction of non-dissociated VFAs (ND-VFAs), which may result in higher microbial decay: ND-VFAs pass freely bacterial cell walls by passive diffusion and affect the internal pH (Jiang et al, 2013; Riungu et al, 2018b; Wang et al, 2014a). However, during the digestion of sewage sludge the high buffer capacity limits pH changes (Fonoll et al, 2015; Franke-Whittle et al, 2014; Gallert et al, 1998; Murto et al, 2004) and hence reduces the options of using ND-VFAs for pathogen inactivation. By со-digesting human waste (UDDT- FS) with mixed organic market waste (OMW), acid formation is enhanced, since OMW is carbohydrate rich and easily hydrolysable (Gomez et al, 2006; Lim et al, 2008).
Enhanced build-up of total VFA (TWA) concentrations during co-digestion of sewage sludge and other organic waste can be achieved by inhibition of methanogenesis (Wang et al, 2014), through use of a two-stage reactor system, where hydrolysis/acidogenesis and methanogenesis are separated. The different species of micro-organisms involved in the AD process can be divided into two main groups of bacteria, namely organic acid producing and organic acid consuming or methane forming microorganisms (Rincon et al., 2008). They operate under different pH conditions: whereas the optimal pH for acidogenic bacteria activity ranges between 5 and 7 (Fang & Liu, 2002; Guo et al, 2010; Liu et al., 2006; Noike et al., 2005), methanogenic activity requires a minimum pH of 6.5 (Wang et al., 2014b; Yuan et al., 2006). Key drawback in the two-stage reactor is the high VFA concentration in the acidogenic reactor, which requires pH collection for stable methanogenesis (Zuo et al., 2014). Yet, the low pH and high VFA concentrations create very good pathogen inactivating conditions. Hence, an optimum must be found between good pathogen removal and well-functioning methanogenic digestion. In practice, the latter can be achieved by recycling part of the digestate upfront to be mixed with the acidified UDDT-FS-OMW.
In Chapter 3, we evaluated the effect of UDDT-FS and OMW mix ratios on VFA buildup and E. coli inactivation in laboratory scale batch anaerobic reactors, within a retention time of four days. E. coli inactivation was a function of the OMW fraction in the substrate, increasing as the fraction increased (Riungu et al, 2018a). The ratio appropriateness depends on the required degree of sanitisation, final pH values in the final digestate, and obviously, the availability of OMW.
This study evaluated the potential for pathogen inactivation in anaerobic digestion, codigesting UDDT-FS and OMW, using pilot scale plug-flow reactors, hi particular, study results give a comparison of E. coli inactivation from single and two-stage plug-flow reactors.
