A system for testing a processor. The apparatus includes N nonlinear elements NL1-NLN which receive N reception signals R split to N branch lines L1-LN, and output nonlinear output signals NLO1-NLON; N delay elements D1-DN which delay the nonlinear output signals by different times, respectively; and a combiner which combines delay signals DS1-DSN output from the delay elements D1-DN. Acidic salt solutions are utilized in a sewage treatment plant.