An economic conversion from aerobic to anaerobic sludge stabilisation does not only require the economically best design of the digester, but also the optimal method of organic carbon removal from the biological treatment stage. Regarding the removal of organic carbon from the biological treatment stage, the HUBER Drum Screen LIQUID is the most economic option compared to a traditional primary clarifier (sedimentation tank), as numerous feasibility studies and implemented concepts have shown.
The economic efficiency of a sludge digestion system depends mainly on the type of digester construction. In general, a distinction is made between concrete digesters, screwed steel silos with a bottom chamber, screwed steel silos with a concrete hopper and welded steel silos. However, it is not only the type of construction that influences economic efficiency, but also the type of sludge digestion process used.
Compared to the usual fully mixed digester, in the "2-phase digestion" process the digester is operated with hydraulic displacement or stacking from the bottom to the top. This mode of operation means that there is no fresh substrate slip, and an increase in gas yield of up to 20% is achieved compared to conventional permanently mixed digesters. Another advantage of hydraulic displacement compared to a digester with mixing (agitator) is that a saving of 3-10 W/m³ can be achieved. Due to this process management without mixing, an enrichment and concentration of the necessary microorganisms is possible, which contributes to an increase in gas production.
Economic process integration:
Decisive for the economic efficiency of a possible process change from aerobic to anaerobic sludge stabilisation are both the electricity costs and the level of disposal costs. For wastewater treatment plants with an expansion stage of 15,000 to 25,000 p.e., there should be at least 200,000 euros per year in electricity and disposal costs. Furthermore, gas yield curves are used to draw conclusions about biogas production and sludge dewaterability. If a comparison of the original configuration of the wastewater treatment system with the system changed to anaerobic sludge stabilisation shows as a result a saving of at least 50,000 euros, a recommendation is made to continue the planning process.