# Enzymatic Activity in the Anaerobic Co-Digestion of Cavitated Coffee Waste and Sewage Sludge
###### Abstract  
Hydrodynamic cavitation (HDC) as a pre-treatment method is innovative and has potential for wide-scale industrial applications. The novelty of this research involves evaluating the enzymatic activity in the anaerobic co-digestion (AcD) of hydrodynamically cavitated coffee waste (CW) and municipal sewage sludge (SS) as well as its influence on the AcD performance. The effectiveness of AcD was assessed on the basis of changes in the physico-chemical composition of the feedstock and digestate as well as the biogas/methane yield, and attention was paid to the effect of coffee waste on enzyme activity, including that of \(\beta\)-Glucosidases (\(\beta\)-Glu), protease (PR), urease (URE), phosphomonoesterases acid (ACP) and alkaline (ALP). Moreover, the changes in the heavy metal content after the AcD of CW and SS were investigated. Comparing the enzymatic activity of the feedstock and digestate, we observed that the URE, ACP and ALP activities were 4.5 to 11 times higher for the feedstock than the enzyme activities in the digestate. Moreover, when using CW cavitated for 30 min, the highest enzymatic activity in both the feedstock and digestate occurred. The results indicated that the relationship between the \(\beta\)-Glu activity and biogas yield showed the strongest positive correlation (r = 0.98 at \(p\leq 0.05\)). At the same time, a positive correlation between the PAC, PAL, URE and PR activity and methane yield and methane content at \(p\leq 0.05\) was observed. The obtained results allow us to conclude that, in the future, such a digestate could be used as a bio-fertilizer to improve degraded soil to activate microbial populations. s 2025 18 1  
Therefore, coffee grounds are increasingly used commercially in the pharmaceutical, food, cosmetic, and/or spirits industries [4]. They also have an additional application which is the use of this type of bio-waste for bioremediation or absorbent production [2; 5]. In addition, data in the literature report a growing interest in the use of CW in anaerobic digestion (AnD), which might be an alternative for involving this bio-waste with energy and nutrient recovery [6]. However, CW indicated a very complex matrix with potential AnD inhibitors, such as caffeine, tannins, terpen, polyphenols, and terpenoids [7; 8]. As noted by [PERSON] et al. [9] the use of coffee grounds in mono-digestion may result in low process efficiency. Therefore, pre-processing this waste may reduce the negative effects associated with the presence of inhibitors as well as destroy the complex lignocellulosic structure of CW, consequently resulting in increased methane production [6]. [PERSON] et al. [10] reports that among pre-treatment methods, hydrodynamic cavitation (HDC) has several advantages, such as low cost, easy scale-up and high efficiency. Furthermore, research conducted using HDC indicates that this method is effective in fragmenting down lignocellulosic fibers, thus ensuring an increase in available surface area, the occurrence of subsequent hydrolysis, and the formation of a low quantity of AnD inhibitors [11]. Despite several studies on HDC, there is still a need to conduct detailed research for each new waste type individually, mainly due to the possibility of generating the toxic by-products within HDC that might adversely influence the subsequent anaerobic digestion process [12]. AnD is known to be one of the complex biological processes that involves the degradation, by different biological reactions of various groups of microorganisms, of a complex organic matrix [13]. One of the effects of microbial activity is the production of enzymes involved in the transformation of organic compounds and release into the circulation of components, such as carbon, nitrogen and phosphorus, which are also food for the microflora in the bioreactor. The properly performed anaerobic decomposition of organic matter determines the high efficiency of biogas production [14].