Process behavior of UASB reactor treating a wastewater containing high strength sulfate

Two UASB reactors were in parallel operated over two years, the one receiving a sulfate-rich wastewater (2000 mg COD l^-1 versus 1000 mg SO₄^2--Sl^-1), and the other receiving a sulfate-poor wastewater (2000 mg COD l^-1 versus 33 mg SO₄^2--Sl^-1), in order to compare the process behaviors with each other. Vial activity tests revealed that undissociated hydrogen sulfide caused an inhibitory effect more seriously on methane producing bacteria, rather than on sulfate reducing bacteria. In order to alleviate sulfide inhibition, a sulfide-stripping device was incorporated in the sulfate-rich UASB reactor. As a result, the sulfate-rich UASB accommodated successfully a sulfate loading of 1.3 kg SO₄^2--S m^-3 d^-1 with 90% COD removal. The COD elimination carried out in the sulfate-rich UASB was associated with no methanogenesis, but was thoroughly performed by sulfate reducing bacteria solely. Hydrogen-utilizing methanogens were completely excluded from the sulfate-rich sludge consortium by proliferation of hydrogen-utilizing sulfate reducers.