Citation: Klasinc, A.; Zupanˇ ciˇ c, G.D.; Hriberšek, M.; Hribernik, A.; Lobnik, A.; Zadravec, M. Experimental and Mathematical Investigation of Anaerobic Granulate Density via Settling Velocity. Processes 2022, 10, 1833. https://doi.org/10.3390/ pr10091833 Academic Editors: Pietro Bartocci, Qing Yang and Francesco Fantozzi Received: 8 August 2022 Accepted: 9 September 2022 Published: 12 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). processes Article Experimental and Mathematical Investigation of Anaerobic Granulate Density via Settling Velocity Aljaž Klasinc 1 , Gregor Drago Zupanˇ ciˇ c 2 , Matjaž Hriberšek 3 , Aleš Hribernik 3 , Aleksandra Lobnik 3 and Matej Zadravec 3, * 1 Institute for Environmental Protection and Sensors, Beloruska 7, SI-2000 Maribor, Slovenia 2 Croteh Ltd., Avenue Dubrovnik 15, HR-10020 Zagreb, Croatia 3 Faculty of Mechanical Engineering, University of Maribor, Smetanova Ulica 17, SI-2000 Maribor, Slovenia * Correspondence: matej.zadravec@um.si Abstract: The objective of this study was to determine the density of anaerobic granules on different heights of a full-scale Upflow Anaerobic Sludge Bed reactor. The density was defined through the settling velocities of anaerobic granules, measured in a full-scale Upflow Anaerobic Sludge Bed reactor. In this study, granular density was calculated with the measured settling velocities and developed mathematical model. The developed mathematical model is based on the Stokes model. In the experiment, granules were taken from different heights of an Upflow Anaerobic Sludge Bed reactor, from 0.6 to 7.6 m. The granules’ diameters varied between 1 and 5 mm. The granules were taken from six different heights through the reactor. The settling velocity of the active granules (with gas in the granule pore and on the surface of the granule) was measured first. After the active granules’ settling velocity measurement, the granules were placed in a vortex to obtain degassed granules (granules without any gas in the pores or on the surface), for which the settling velocities were also measured later. It is shown that granules’ densities at different heights are independent of the reactor height. Keywords: Upflow Anaerobic Sludge Bed reactor; anaerobic granular sludge; settling velocity; density 1. Introduction Industrial wastewater treatment is becoming more important every day [1–3]. The Upflow Anaerobic Sludge Bed (UASB) reactor is currently the most common reactor for treatment of industrial wastewaters [4]. The investigation of the density of anaerobic granules via settling velocity is presented in this paper. For optimal operation of UASB reactors a strong and active granular sludge bed is very important [5]. For optimal operation of a UASB reactor a well-developed granular sludge with high physical strength and settling velocity is necessary [6]. The advantages of anaerobic granular sludge are its high physical strength, resistance to shocks and toxins, and high liquid flows due to high settling velocity [7]. Through the height of a reactor the concentration of granules varies, and can usually be divided into three zones: (1) a dense sludge bed consisting of biomass aggregates in the bottom section of the reactor; (2) a sludge blanket containing finely suspended flocs or aggregates in the middle of the reactor; and (3) a zone of clarified water containing almost no solids on the top of reactor, as shown in Figure 1 [8]. Due to the high settling velocity of granules, a UASB reactor has the capacity for high liquid flows. One of the most important parameters in the initial granulation and development of granular sludge is the settling velocity [9,10]. The reported settling velocity of anaerobic granules is in the range of 18 to 100 m/h [11]. Anaerobic granules have good settling capability, which is very important for separating the solid and liquid states. Anaerobic granular sludge provides a high and stable rate of metabolism, and granules Processes 2022, 10, 1833. https://doi.org/10.3390/pr10091833 https://www.mdpi.com/journal/processes