OPTIMIZATION OF MULTIPLE HEARTH FURNACE OPERATIONS TO MINIMIZE CYANIDE FORMATION WHEN COMBUSTING DEWATERED SEWAGE SLUDGE

The Central Contra Costa Sanitary District (CCCSD) was facing the probable reduction of wastewater treatment plant final effluent limit for cyanide compounds (CN) from 25 micrograms per liter (μg/l) to a new regulatory limit of 10 μg/l. The major CN source was the multiple hearth furnaces (MHFs) where CN compounds are formed as a byproduct of the biosolids combustion process and captured in the scrubber water that is discharged to the Headworks of the treatment plant. Five (5) days of testing were conducted on one of CCCSD's eleven hearth MHFs, where hearth 1 is the afterburner and sludge is fed to hearth 2. The tested ranges of the key MHF operating variables resulted in six (6) test mode combinations of oxygen (3 ranges) and afterburning (AB) temperature levels (2 ranges) which were used to classify the sample test cases. The medium oxygen — high AB temperature operating mode achieved the best overall performance rating of the two modes particularly on fuel use rate and the emission test variables. This comprehensive test project confirmed and verified previous test findings from other facilities about the effects of AB temperature and oxygen level on MHF scrubber water CN levels. Concurrent measurements of furnace and emission performance variables revealed the overall benefits of increasing AB temperature level. An AB temperature of ∼ 1300°F was determined to be the primary control technique for reducing scrubber water CN levels to minimize the negative impacts associated with operating at higher oxygen levels.