Background information Dairy production plays a crucial role in human nutrition but generates millions of tons of waste per year, including cheese whey (CW) and second cheese whey (SCW), both of which have a high environmental impact if not properly managed. SCW, produced by heating CW during ricotta cheese production, is rich in lactose (35–50 g L-1) and has high COD (50 g L-1) and BOD (80 g L-1) levels (Pires et al., 2021). In Italy, 15% of CW is used to produce ricotta cheese, producing about 1 million tons of SCW per year. Although often reused as animal feed, SCW contains valuable compounds such as proteins, peptides, and lactose, making it an excellent resource for biotechnological valorization. Highly versatile purple non sulfur bacteria (PNSB) are capable of producing Bio-H2 production and accumulate poly-β-hydroxybutyrate (PHB), efficiently utilizing various organic substrates, including agricultural and industrial waste. This study explores processes for the potential production of Bio-H2 and PHB from SCW, using PNSB, both in sequential two-stage processes (combining lactic and photofermentation) in 5L bioreactors, and in bioelectrochemical systems. Main results H2 was obtained in a two-stage process consisting of: i) conversion of the lactose contained by SCW to lactic acid by a co-inoculum of Lactococcus lactis MK L84 and Lacticaseibacillus paracasei MK L49 at pH 4.5-5.5 ii) photofermentation of the lactic acid produced in the first stage using Rhodopseudomonas palustris 42OL, yielding an average of 0.47 L of H2 per liter of substrate and 1.66% wPHB/wCDW, in a 5- liter tubular photobioreactor. Bioelectrochemical systems were also set up to demonstrate the feasibility of photo-cathodic H2 production using Rhodopseudomonas palustris 42OL cultures in H-shaped reactors. Conclusions The approaches here described successfully converted dairy waste into high-value products, promoting circular economy principles.