you may follow either the APHA / AOAC to estimate the arsenic from waste waters.

Try following:

[1] Computer Distributed Control and Intelligent Environmental Monitoring (CDCIEM), 2011 International Conference pp: 385-88{Pls. find abstract below)

The trace concentrations of arsenic after concentrating by JM-33 resin reacted with disulfoantipyl methane to form a 1:2 complex which the maximum absorption wave lied at 325nm with an apparent molar absorptivity of 1.74×1041/(mol•cm). When the concentration of arsenic was between 0.02-0.5mg/l, the reaction conformed to Beer's law. The effects of some important factors were also investigated, which showed the method was simple, selective and with high precision and accuracy.

[1II]Indian Journal of Chemical Technology, 13 (2006)36-40(Should work)

[IV] Pillai A., Sunita, G. and Gupta, V.K., Anal.Chim.Acta., (2000)111.