This thesis describes the crystallization of biologically produced elemental sulfur (S8) in a biological gas desulfurization (BD) system. The studied system is commercially applied by Paques B.V. as THIOPAQ® for biogas desulfurization and by Paqell B.V. as THIOPAQ O&G for natural gas desulfurization (hereafter called: Thiopaq). Typically, the BD process operates at elevated salt concentration (0.8-1.5 M Na+) and pH (8-9.5). In the process, absorbed hydrogen sulfide (H2S) is oxidized by sulfide oxidizing bacteria (SOB) to elemental sulfur. The sulfur is recovered from the process to prevent accumulation in the reactor system. Although the Thiopaq process has been applied in 290 installations worldwide and has been researched for over 30 years, the formation, growth and separation of the formed elemental sulfur was not well understood. Therefore, a collaboration between the department of Environmental Technology of Wageningen University, where the process was originally discovered, Paques B.V., where the process was commercialized, and Paqell B.V., where the process is currently further developed, was started in 2016. The aim of the collaboration was to i) investigate the properties of sulfur formed in BD reactors, ii) to improve our understanding of the underlying mechanism of sulfur particle formation and growth and iii) to find a way to control the sulfur particle size and settleability. This thesis is the result of the research efforts enabled by this collaboration.Sulfur particle removal is essential for optimal process performance. If the sulfur is not separated from the process liquid, it accumulates in the reactor. High sulfur concentrations lead to foaming, clogging and eventually discontinuation of the process. All have a negative impact on the process economics. Especially in high total sulfur load systems, which are more common ever since the process is applied for natural gas desulfurization, continuous and effective sulfur removal is vital. The produced elemental sulfur can be removed by sedimentation in a settler or by a decanter centrifuge depending on the total sulfur load. Even when a decanter centrifuge is applied, it is economically relevant to concentrate the sulfur feed to the centrifuge with conical reactor bottom or settler. This allows for the design of a smaller decanter centrifuge. We described that sulfur crystallization can be explained by the general crystallization theory of supersaturation, nucleation and growth. Furthermore, we postulate that the crystal growth proceeds mainly by crystallization by particle attachment. This type of crystallization is referred to as ‘non-classical crystallization’. While this crystallization theory shows great overlap with classical crystallization, the main difference is that crystal growth takes place by the addition of particles instead of monomers.Biologically produced sulfur particles have various shapes and sizes. A common feature is the typical bipyramidal shape for elemental sulfur. The bipyramids were found as single crystals or in agglomerates. The agglomerates showed better settleability than single crystals (91% settled compared to 78.3% settled after 2 hours), unless the crystal grew to sizes comparable to the agglomerates (88.9% settled after 2 hours). Crystal growth is undesirable for BD reactors, as this can only happen under low S-formation rate and long sulfur particle retention time, i.e., low volumetric conversion capacity and thus requirement of large bioreactors.Recently, a new process configuration for the BD process was introduced by Paqell B.V. The new process configuration has an additional reactor between the absorber and aerated bioreactor. This added reactor is anoxic and contains dissolved sulfide originating from the absorber. The added reactor was found to increase the selectivity of the process from 75.6 to 96.6% by suppressing sulfate formation. Unpublished observations from these experiments showed that the sulfur particles produced in the new process configuration appeared to have better sedimentation capacity.As a result of these unpublished observations, we performed experiments with this new process line up. In these experiments we showed that polysulfides formed in the new line up changed the properties of the sulfur particles drastically. Instead of single crystals, agglomerates of crystals with a larger particle size were produced which had superior settleability (51 compared to 26% settled after 30 min). Moreover, the reaction of bisulfide (HS-) with elemental sulfur (S8) removed the smallest particles by completely dissolving them to polysulfides (Sx2-). The agglomerates were created by the rapid S8 formation upon oxidation of Sx2- in the aerated reactor. As the Sx2- already contains a chain of zero valent sulfur (S0), S8 formation was explained to go at least 5 times faster from Sx2- than from single HS- (due to the average approximate chain Sx2- length of x≈5 under typical operational conditions). Moreover, polysulfide formation was shown to change the surface characteristics of the sulfur particles, making it visibly rougher and more porous. Likely, polysulfide formation also decreased the hydrophilicity of the sulfur particles by disturbing the hydrophilic layer of absorbed organic molecules. Furthermore, a mechanism was proposed for the sulfur crystal growth and agglomeration. The final shape and size of the sulfur particles depends on the sulfur formation rate and the formed polysulfide concentration. These are varied by changing the H2S loading rate and the polysulfide formation time, by for example adding an extra anoxic, sulfidic reactor. The first solid form of elemental sulfur that is produced is a globule. Globules are produced at or near the surface of the SOB and released into the solution. The globule may then either grow to or become a part of a bipyramidal crystal at a low sulfur formation rate or agglomerate at moderate to high sulfur formation rate. At moderate sulfur formation rate the globules may still form the typical bipyramidal shape while being part of an agglomerate, but at high sulfur formation rate this morphology is completely absent. The proposed mechanism was furthermore extended with a type of growth; oriented attachment. Hollow bipyramidal skeletons were found, consisting of small globules (~50 nm). In conclusion, by studying the biocrystallization mechanism of biologically formed sulfur, the settleability of the formed crystals was improved. The recovered sulfur has many applications, such as reuse in agriculture as fertilizer or fungicide, but also is suitable to apply in industrial processes.