Production of polyhydroxyalkanoates in an open mixed culture from fermented hydrolyzed sludge under high organic loading rates

In this paper the production of polyhydroxyalkanoate (PHA) in open mixed cultures with a not previous used substrate, fermented high temperature and pressure hydrolysed activated sludge (FHS), was studied. In the selection for PHA storing biomass two different organic loading rates (OLRs) (12 g COD/L/day and 6 g COD/L/day) were used. DO-controlled fed-batch reactors were used for the accumulation of PHA in the selected biomass. The FHS showed a potential as a substrate for PHA production and a PHAs storing biomass was selected. The highest PHA yield (0.46 C-mol PHA/C-mol VFA) was obtained when the lower OLR selected biomass was used in the batch accumulations. The lower OLR selected biomass gave higher PHAs storage yields (0.25-0.46 C-mol/C-mol) when the FHS was used as substrate in the batch accumulation compared to the higher OLR selected biomass (0.04-0.10 C-mol/C-mol). When a synthetic substrate was used a substrate in the batch accumulations the difference between the higher OLR (0.18-0.32 C-mo/C-mol) and the lower OLR (0.32-0.39 Cmol/C-mol) selected biomasses were small. The non-VFA COD and the high nutrients concentration in the FHS did most likely cause the low PHA yield when the higher OLR selected biomass was used in the batch accumulations with FHS as substrate. INTRODUCTION PHAs is a group of biodegradable polymers, with properties similar to polypropylene and polyethylene (Salehizadeh et al. 2004). Due to their physical properties, PHA can replace conventional plastics in many applications, such as plastic bottles for food, water resistant surface on cardboard (Philip et al. 2007) and also as carriers for the controlled release of medicine and hormones (Reddy et al. 2003). However, the current pure culture production is expensive; the high cost of PHA has so far caused PHA to play a marginal role in the world’s plastic market (Albuquerque et al. 2007). PHA is an energy and carbon storage molecule produced in bacteria during growth limiting conditions (Lee 1996; Salehizadeh et al. 2004). To decrease the production cost of these biopolymers, open mixed cultures can be used for PHA production (Albuquerque et al. 2007). A suggested three step process for PHA production in open mixed cultures consist of: 1) acidogenic fermentation of the waste stream to produce VFAs, 2) selection of PHA storing organisms, 3) accumulation of PHA in the selected biomass (Dionisi et al. 2005b). The acidogenic fermentation is needed because storage of carbohydrates as glycogen is more energetic favorable than storage as PHA in open mixed cultures (Dircks et al. 2001). VFAs can, however, be stored as PHA by bacteria in opened mixed cultures. In the selection for PHA storing organisms, the feast-famine approach is most commonly used (Dionisi et al. 2005a; Albuquerque et al. 2007; Bengtsson et al. 2008). In the feast-famine selection, the substrate is fed in pulses and the substrate concentration alternates from high (feast) to low (famine) (Dionisi et al. 2004). A final accumulation step is used to saturate the biomass with PHA (Dionisi et al. 2004). So far most of the PHA production in the literature has been conducted with synthetically produced VFAs; only a few complex substrates as sugar cane molasses (Albuquerque et al. 2007), olive mill effluents (Dionisi et al. 2005b) and paper mill wastewater (Bengtsson et al. 2008) have been used. In this article a new substrate, FHS, was used as substrate for PHA production. MATERIAL AND METHODS System setup The PHA production system consisted of three lab-scale steps: 1. Fermentation of the hydrolyzed sludge in an anaerobic continuously stirred tank reactor, (previously conducted by AnoxKaldnes Biopolymer). 2. The selection of PHA storing organisms, conducted in aerated SBRs. 3. The accumulation of PHA by selected biomass in DOoperated fed-batch reactors. Fermentation of hydrolysed sludge The FHS substrate had previously been fermented at AnoxKaldnes Biopolymer AB. Sludge was collected from the outlet of the Cambi process in Fredericia Central Wastewater Treatment Plant (Fredericia, Denmark) and fermented in two parallel semi-continuous reactors (V=3 L, T=42°C and 55°C, HRT=SRT=1-6 days and pH=6.1-6.6). The FHS was centrifuged to remove the solids and stored frozen before use in the selection SBRs. Before the FHS were used in selection for PHA accumulating organisms all the effluent batches was mixed to get a similar composition in the FHS. The FHS was rich in