Treated wastewater may occasionally require further treatment. Local discharge rules may pose strict limits that cannot be fulfilled by even the most effective process, or a higher quality of effluent may be requested for its reuse. When tertiary treatment is needed, various levels of filtration can be applied, including very fine filtration. Often, however, a final step of mixing and aeration is sufficient to achieve the target. When this is the case, Sulzer's submersible mixer types ABS XRW and RW can be used, along with submersible aerator type ABS TA–TAK and disc diffuser system types ABS PIK, DS and Sucoflow.