Wastewater treatment: how to prevent sludge settling in digesters
In municipal wastewater treatment plants, where large quantities of wastewater are treated and purified every day, anaerobic digestion is a key process for the sustainable reuse of stabilised sludge and its valorisation for energy purposes.
In fact, anaerobic sludge digestion helps large wastewater treatment plants to:
- Stabilise the sludge and further reduce the pollutant load
- Valorise the organic fraction of sludge from an energy point of view
- Further recover the water contained in the digested sludge.
Intimate mixing of the sludge as a key factor in maximising digestion
A municipal sewage treatment plant must therefore always have a high efficiency anaerobic digestion process in order to guarantee a high and constant degree of pollutant removal at the end of the treatment process.
One of the key factors that ensures that digestion runs smoothly is the mixing of the sludge in the digesters. The fresh sludge fed into the fermenters must be mixed as intimately as possible with the sludge already being digested to avoid settling. In fact, accumulations of fresh sludge at the bottom of the digester not only reduce the volume of the tanks and make mixing more difficult, but due to their tendency to enter acidic fermentation processes, they disturb the alkalinity of the sludge, which is one of the main factors in ensuring maximum efficiency of the digestion process.
SewaTOR in recirculation transfers the benefits of controlled cavitation with the best energy efficiency
In this context, SewaTOR installed in recirculation on the anaerobic fermenter is the optimal solution to prevent the accumulation of fresh material at the bottom of the tank. With this type of installation, approved by BioBANG in the pre-treatment of biomass in more than 160 agricultural and industrial biogas and biomethane plants, the physical energy of controlled cavitation becomes the optimum means of keeping the fresh sludge and the sludge being digested in close contact at all times, while keeping the viscosity in the tanks under active control. In this specific application the suction point of SewaTOR is installed at the bottom of the tank where the sludge tends to accumulate and the exit point of the cavitated product is recirculated to the top, opposite to the intake point. In this way SewaTOR works directly on 15-30% of the total sludge present in the digester which tends to stratify at the bottom, bringing it back to the top of the digester ready to be intimately mixed with the rest of the product. The remaining sludge fraction, which makes up approximately 80-70% of the material in the digesters that does not stratify and immediately enters the digestion processes, is therefore not treated with controlled cavitation, ensuring that SewaTOR maximises its energy efficiency because it does not compete with the bacteria.
SewaTOR recirculates, defragments and solubilises the organic sludge received in the digester, breaking up the floccules into multiple particles, increasing homogeneity and reducing viscosity. The shock waves generated by the implosion of the cavitation bubbles are transmitted directly into the fluid and act by destroying the bacterial cell walls and membranes, releasing intracellular and extracellular matter and accelerating the digestion process. This greatly increases the efficiency of the digester, decreasing processing and sludge retention times, producing a much higher yield of biogas and a lower final quantity of stabilised sludge to be disposed of.