Application of integrated biochemical processes for the valorisation of sheep dairy bioresidues in the framework of waste biorefinery concept

The recovery of materials/energy from biowastes needs to be intensified and the development of integrated waste biorefineries could significantly contribute to the transition towards a new sustainable bio-based economic model. The biorefinery concept is not new, and its traditional meaning has evolved driven mainly by two pivotal needs, environmental and economic sustainability. The integration of different processes aimed at producing a mix of biofuels and bioproducts supports economic sustainability, since it makes possible hitting the market with an appropriate mix of products characterised either by significant market size or high added value. Such flexible integration also has high environmental value, as the number of usable and marketable outputs increases, this would logically correspond to less waste production, thus approaching the zero-waste concept. The improvement in environmental sustainability is the main element governing the desirable transition towards the deployment of waste biorefineries. Indeed, the use of residual biomass would entail further environmental and economic benefits, such as the environmentally sound management of residues through valorisation and the reduction of production costs, since waste biomass is a widely available and inexpensive feedstock. Furthermore, the use of residual waste biomass would also contribute to the reduction of CO2 emissions, considering that it is a renewable source for biofuel and bioproducts production, in contrast with fossil sources. The present PhD thesis presents and describes a study which, framed in the above-described context and consistent with it, aims at a multi-step valorisation of sheep cheese whey (SCW), the primary biowaste of the sheep dairy supply chain. The proposed valorisation scheme is based on the high SCW lactose content, which is well suited to be converted through dark fermentation (DF) or anaerobic digestion (AD) into marketable gaseous, such as bio-H2 and bioCH4, and soluble products, such as organic acids (OA). DF represents the core of the multi-stage process since can convert the lactose into shares of bio-H2 and OA as a function of the process operating parameters adopted. In the framework of the present thesis, a maximum lactic acid yield was attained of 69 gHLa LSCW-1 when the operating pH was set at 6.0 and the fermentation time at 45 h. A maximum yield of 5 LH2 LSCW-1 was observed by adopting the same value of operating pH, but extending the fermentation time up to 168 h. The pool of organic acids obtainable through CW DF also proved to be suitable precursors for PHA production, even in the absence of specific inoculum and extra nutrients. The high nutrient content of SCW made possible the selection of PHA-storing biomass without extra nitrogen supply, but on the other hand, it could also represent a limiting factor for PHA accumulation. An overall yield 11-19 gPHA LSCW-1 was obtained in function of the adopted pH in the fermentation stage. The adopted pH also affected, besides H2 and VFAs yields, the quality of the biopolymer produced in terms of HV fraction. DF is known to be suited to be performed as the first step in a two-stage CW methanization process, though the overall specific energy recovery observed in the present study resulted in being slightly lower than what obtained through single-stage AD, 0.81 vs 0.91 MJ Lscw-1 respectively. However, the two-stage approach may still be attractive in terms of process stability . The results attained during the 3 years activity are promising and showed the inherent potential of the dairy waste to produce high-value products through a waste biorefinery approach. The implementation of such integrated systems aimed at energy and material recovery from dairy wastes could support the dairy supply chain promoting environmentally sound practices, implementing circular bioeconomy concepts and creating new economic opportunities in rural areas.