Photocatalytic pretreatment of dairy wastewater and benefits of the photocatalyst as an enhancer of anaerobic digestion

High-fat wastes are difficult to degrade and mineralize under biological methods. It is imperative to identify suitable combinations of physicochemical and biological processes. This study analyzed the prospects of combining photocatalysis and anaerobic digestion of dairy wastewater using a nano-catalyst made of reduced graphene oxide (RGO) and nano-zerovalent iron (NZVI). The operational parameters of ammonium persulfate (APS) assisted photocatalytic pretreatment, initial pH (4–9), organic loading (2200–17,900 mg L−1) and catalyst dosage (0.1–0.5 g) were optimized. The maximum solubilization was found at an initial pH 5 until 4 h of pretreatment with an increase of 38.77 ± 0.85 % soluble chemical oxygen demand (SCOD) and 39.05 ± 1.3 % dissolved organic carbon (DOC). The anaerobic digestion of pretreated wastewater containing spent catalyst was carried out with slight amendment of fresh catalyst (D1) and without any fresh supplement (D2). The total biogas produced in D1 and D2 in 28 days was 31.69 ± 1.05 % and 7.14 ± 0.15 % more than that obtained from the control digester (CD) respectively. Also, D1 and D2 exhibited better CH4 production, which was 81.01 ± 3.24 % and 38.93 ± 1.1 % more than CD. D1 exhibited better removal of SCOD with 72.99 ± 2.84 % efficiency, whereas, D2 and CD showed only 67.72 ± 2.91 % and 61.08 ± 1.89 % respectively. According to the modified Gompertz model, the lag phase time (λ) for CD is more than D1 and D2. [Display omitted] •Photocatalytic pretreatment technique improved the solubility and biodegradation.•Photon-induced electrons from RGO-NZVI aided formation of reactive sulfate radicals.•The fresh catalyst enhanced the biodegradation compared to the spent catalyst alone.•Pretreatment enhanced methanogenesis by reducing the load on hydrolytic microbes.