Primary clarification is the first step of WWT in a classical WWTP for the removal of about 50-60% of TSS and 30-40% of BOD. The hydraulic retention time in primary clarifiers typically varies between 1.5-2.5h and they occupy a significant fraction of the footprint of conventional WWTPs and often limit the potential for expansion of the hydraulic capacity of existing plants if there is no free space available. The solid content of primary sludge is between 3-5%, and after dewatering can be increased up to20%. However, as no primary clarification is practiced in extended aeration systems, the wastewater, after the headwork, enters directly into the aeration tank. It has a dramatic impact on the energy consumption by the aeration process, as TSS that could have been removed by clarification is digested by aerobic microorganisms. Thus, extensive aeration is energy intensive and it is likely that it will be gradually abandoned due to the high cost of aeration energy. An alternative process for the removal of the primary solids is the use of a fine mesh sieve (microscreen), which can produce biosolids with solid content up to 45%. The high solids content of sludge removed by the microscreen makes it much more suitable to be used as gasifier feed stock, compared to the primary sludge (which even after dewatering contains at max. 20% of solids). Microscreen requires 1/20 the area of an equivalent capacity primary clarifier, while, their operation can follow the inlet flowrate (e.g. microscreens can be turned off during reduced flowrates, while this is not an option for primary clarifiers).

The projected dryer, bases its operation on the agitation of humid material in the flow of hot air produced by the gasifier as exhaust gases. Special conveying and simultaneous continuous agitation augers, ensures the material’s permanence of the material in the hot air flow, throughout the 12 meter path (2 mod X 6 meters each). Before the end of each module, humid air is sucked in from the suction circuit. In each module is introduced hot and dry air by the hot air distribution system. The suction circuit is discharged into a cyclone to catch volatile and light particles. The output of the second module that would be the final output of the treated material, is mixed with the material captured by the cyclone, and then conducted to the gasifier.

The gaseous product of gasification (main compounds: CO, H2 and small amounts of CH4), known as syngas, can be used as fuel for the production of heat and electric energy. Heat energy could be recovered and used to partially dry the biosolids prior to gasification. The biosolids produced from the microscreen have high potential for gasification and production of syngas. Complete biosolids drying is not essential, as the optimum moisture content for biosolids gasification is between 15-20%. Biosolids remaining mixture is needed to fix the C/O ratio close to 1/1. This ratio is in favor of carbon, which requires moisture to provide the extra oxygen.

Gasification is a significantly more efficient process for electrical energy production, as it yields approx. 190% of the energy that may be produced by the anaerobic digestion (AD) process, using almost 100% of the carbon in sludge, while AD can utilize up to 50-60%. Gasification is preferable when biosolids content is high (the case of LIFE B2E4sustainable-WWTP after the microscreen pre-treatment). AD is more favorable with wetter biosolids (80% moisture), but its residues contains a relatively large fraction of organic matter that requires further treatment.