Selection of the type of fermentation chamber in relation to the characteristics of the treated wastewater 

The development of industrial wastewater treatment technology by means of methane fermentation resulted in the introduction of various types of anaerobic reactors into operation. In the 1970s, the first type of fermenter operating in the UASB system was designed. In the following years, the companies introduced further technical solutions, especially regarding the method of mixing the reactor contents. Reactors operating in the contact system have been introduced to anaerobic installations, and recently in the IC system with its various modifications. This raises the question of whether each fermenter is suitable for treating each type of wastewater. Well, it is not. This is due in particular to the type and characteristics of the wastewater composition, and then to the assumed treatment effect.

Primary UASB fermenters have been used to treat sugar wastewater, containing mainly carbohydrates and a minimum content of suspensions. The nature of this effluent corresponded to the UASB system, as the heated raw effluent was distributed without major problems through the bottom grate system over the entire surface of the bottom of the fermenter. The laminar flow of sewage through the layer of anaerobic sludge ensured the appropriate time of contact of anaerobic bacteria with dissolved compounds contained in sewage. The main mixing agent in the fermenter in this system is the biogas that is released. A limitation in the operation of this type of fermenters is the hydraulic load, because the greater it is, the shorter the contact time of the sludge with the incoming sewage. In addition, with intensive flow in the treated sewage in the outflow, there are increased amounts of fermentation sludge suspensions. Optimum cleaning effects are obtained when the fermenter is loaded with COD max. up to 10 kg COD/m3 • d. It was originally assumed that the outgoing digested wastewater contained negligible amounts of anaerobic sludge suspensions and did not need to be clarified in the settling tank. Unfortunately, since we do not have the possibility to regulate the concentration of suspended solids in digested wastewater, in many cases it turned out to be necessary to use post-fermenter UASB settling tanks. A problem in operating conditions over a longer period of time is the obstruction of holes in the sewage distribution grate, which in turn may reduce the efficiency of sewage treatment.

The advantage of contact reactors is the reliability of operation and full control over the process, i.e. the intensity of mixing, maintaining the concentration of anaerobic sludge at an optimal level, much better mixing of substrates with fermentation sludge, and thus maintaining pH in the reactor at an optimal level. This results in the possibility of achieving very high treatment effects, even when fermenting wastewater with a high concentration of suspensions and fats or containing toxic compounds. There is a need to build a settling tank after the fermentation chamber for the separation of anaerobic sludge from digested sewage and its recirculation to the reactor. Because the mixing of raw wastewater with anaerobic sludge is at the maximum level, i.e. the entire amount of biomass is working, therefore the reactor is very resistant to overloading with COD and pH drop below 6.5. For this reason, there is no need to artificially raise the pH by dosing sodium hydroxide.

IC reactors have been put into operation relatively recently. The innovation of the treatment is the introduction of heated wastewater into a cone-shaped tank separated at the bottom of the fermenter with holes in the side wall, through which flows raw wastewater, further mixed and diluted with treated wastewater recycled at the top of the fermenter. They are characterized by a high height-to-diameter ratio and high COD load of up to 30 kg COD/m3x d. In this type of fermenters, as a rule, only granulated sludge is used, which is retained in two separator sections. Due to the use of granulated sludge, wastewater with a high concentration of slurries and the presence of fats should not be treated.