First, I would like to do a some form of extraction to only get the mercury, then followed by a titration to identify the concentration or the presence of mercury.
The potentiometric titration of mercury with dithiooxamide has been successfully applied after preliminary separation of mercury by reduction (Comprehensive Water Analysis, Volume 2, pp.186-187, Elsevier).
Selective identification of mercury in waste water can be accomplished by fluorescence enhancement chemosensors (SENSOR LETTERS 2010, 8, 1–6). In this recent paper, the ﬂuorescent emission intensity of the probe is enhanced upon binding to mercury ions. The ﬂuorescence enhancement is attributed to a 1:1 complex formation between the probe and Hg(II) ion. The chemosensor can be applied for the quantiﬁcation of mercury; it shows a high selectivity toward Hg(II) ions in comparison with common metal ions.