You wrote "effluent", that ...
Published by John Salonich, Vice President / Managing Director
You wrote "effluent", that means post MBBR treatment, if there are sulfides in the effluent that means the desulfivibrio bacteria are not being inhibited from forming sulfides. An adjustment to increase ORP to >-50 mV (the quantifiable value at which inhibition starts) can be achieved by adding alkalinity or by adding aeration. If this is truly effluent--aerate the effluent to convert H2S into SO4.
Related article in TPO magazine on using ORP as a treatment predictive analytic:
http://www.tpomag.com/editorial/2011/10/its_not_black_magic