Complex substrates, like proteins, carbohydrates, and lipids, are major components of domestic wastewater, and yet their degradation in biofilm-based wastewater treatment technologies, such as aerobic granular sludge (AGS), is not well understood. Hydrolysis is considered the rate-limiting step in the bioconversion of complex substrates, and as such, it will impact the utilization of a large wastewater COD (chemical oxygen demand) fraction by the biofilms or granules. To study the hydrolysis of complex substrates within these types of biomass, this paper investigates the anaerobic activity of major hydrolytic enzymes in the different sludge fractions of a full-scale AGS reactor. Chromogenic substrates were used under fully mixed anaerobic conditions to determine lipase, protease, α-glucosidase, and β-glucosidase activities in large granules (>1 mm in diameter), small granules (0.2–1 mm), flocculent sludge (0.045–0.2 mm), and bulk liquid. Furthermore, composition and hydrolytic activity of influent wastewater samples were determined. Our results showed an overcapacity of the sludge to hydrolyze wastewater soluble and colloidal polymeric substrates. The highest specific hydrolytic activity was associated with the flocculent sludge fraction (1.5–7.5 times that of large and smaller granules), in agreement with its large available surface area. However, the biomass in the full-scale reactor consisted of 84% large granules, making the large granules account for 55–68% of the total hydrolytic activity potential in the reactor. These observations shine a new light on the contribution of large granules to the conversion of polymeric COD and suggest that large granules can hydrolyze a significant amount of this influent fraction. The anaerobic removal of polymeric soluble and colloidal substrates could clarify the stable granule formation that is observed in full-scale installations, even when those are fed with complex wastewaters.

Volatile fatty acids (VFA) may serve as building blocks for chemicals and polymers. A technology enabling high-rate VFA production from carbohydrate-rich water is the anaerobic granular sludge process. In this study the characteristics of an anaerobic granular sludge enrichment was evaluated at different solid retention times (SRT). A lab-scale anaerobic sequencing batch reactor fed with 6 g·L-1 glucose was operated at a pH of 5.5 and at various SRT of 1-2, 10-20, and 40-50 d. A low sludge volume index (SVI) of 11-44 mL·gTSS-1 allowed a high volatile suspended solid (VSS) concentration that reached 59 gVSS·L-1 . This high VSS concentration enabled a glucose consumption rates in the range of 1100 gCOD·L-1·d-1. Two product spectra were obtained: (i) an propionate:acetate mixture with a ratio of 2.05:1 (molpropionate:molacetate) produced at an SRT of 40-50 d; (ii) an acetate dominated product spectrum was obtained at 1-2 d and 10-20 d SRT (0.71-0.75 molacetate·molVFA-1). Overall a high VFA yield between 0.77-0.79 was obtained throughout all enrichments. This work demonstrates that high rate VFA production combining high yields and low solid concentrations in the effluent can be achieved. This works contributes to the implementation of waste-based production of VFA using anaerobic granular sludge.