# Wastewater Treatment with Bacterial Representatives of the _Thiothrix_ Morphotype
## 5 Factors Affecting _Thiothrix_ Proliferation  
In wastewater treatment systems, due to the significant role of bacteria similar in morphotype to _Thiothrix_, it is important to consider factors that can lead to their increased growth and methods to control it.  
Bacteria of the _Thiothrix_ morphotype are able to dominate in activated sludge in the presence of thiosulfate, which can be related to the ability of these bacteria to grow mixotrophically using reduced sulfur compounds as an energy source [26]. Consistent with this phenomenon, the core genome of the genus _Thiothrix_ was found to contain genes encoding sulfur metabolism complexes: the SQR and FCSD complexes, participating  
Figure 7: Nitrification–denitrification reactions. In the processes of the transformation of nitrogenous compounds leading to nitrogen removal from wastewater, representatives of the genus _Thiothrix_ are able to perform the denitrification step.  
in the oxidation of sulfide to polysulfide/sulfur; the Sox complex (_soxAXBYZ_), which participates in the branched pathway of thiosulfate oxidation to sulfur and sulfate; and the rDsr system (_dsrABCEFHEMPKJOL_), which participates in the subsequent oxidation of elemental sulfur [4].  
It was shown that the presence of sulfide in wastewater treatment systems can evoke the active growth of the _Thiodthrix_ morphotype [44; 47; 85]. High levels of dissolved oxygen (DO) are detrimental to bacteria similar in morphotype to _Thiodthrix_ and inhibit their growth, while, conversely, low DO values favor the growth of these bacteria [21]. This is due to the fact that they belong to the group of colorless sulfur-oxidizing bacteria, which are known to be catalase-negative and prefer growth in narrow microaerobic eco-niches where fewer toxic reactive oxygen species are produced. At low DO levels and low nitrate concentrations, bacteria of the _Thiodthrix_ morphotype gain a competitive advantage over other microorganisms due to the ability to accumulate nitrates intracellularly, which leads to the explosive growth of these bacteria and activated sludge bulking [129]. This phenomenon is related to the ability of bacteria of the _Thiodthrix_ morphotype to utilize nitrate as an alternative electron acceptor. The appearance of bisphenol A in wastewater also leads to an increase in the abundance of the _Thiodthrix_ morphotype [33].  
To suppress the growth of _Thiodthrix_, the addition of polyaluminum chloride and a decrease in VFAs were tested, but these approaches were ineffective in resolving the problem conclusively [34]. Although a correlation was observed between the propionic acid content and the Type 021N-mediated bulking of sludge, this relationship was, in reality, an indirect effect of the inhibition of flocculating microorganisms, which gave filamentous bacteria a competitive advantage in growth [29].  
Several methods have been demonstrated to reduce the abundance of bacteria similar in morphotype to _Thiodthrix_ in wastewater. These include the use of intermittent fasting [34], moderate oxygen concentrations with low aerobic periods in the surrounding habitat [130] and the addition of 1% hydrogen peroxide [80], the backwashing of membrane reactors with NaClO in wastewater systems [65] and even exposure to a magnetic field [79].  
Regarding the growth rate of bacteria currently assigned to the genus _Thiodthra_, it was demonstrated to have a statistically significant correlation with the food/mass ratio (daily activated sludge load, defined as the ratio of the daily amount of incoming organic pollutants F to ash-free substance of activated sludge M (F:M)) and the ammonium ion concentration in the primary effluent, as well as an inverse correlation with the dissolved oxygen concentration in the microaerobic selector [14].