An optical biosensor based on attenuation of the light intensity during multiple reflections in a planar waveguide has been developed for water pollution monitoring. The planar waveguide consists of a 190 nanometer thick silicon nitride (Si3N4) core layer sandwiched between 1.5 micrometer thick silicon dioxide (SiO2) cladding layers. Composite polyelectrolyte self-assembled membranes containing Cyclotetrachromotropylene (CTCT) as an indicator and enzymes, such as Urease or Acetylcholine Esterase (AChE) were deposited on top of silicon nitride core layer within a 4 × 6 mm sensing window. Experimental studies on the light propagation through the planar waveguide show the advantages of this method over conventional UV-visible absorption spectroscopy. It was found that the planar waveguide sensitivity is higher by several orders of magnitude than that for UV-visible absorption spectroscopy. The respective enzyme reactions as well as their inhibition by heavy metal ions were studied by monitoring the light intensity in the planar waveguide. Cadmium (Cd2+) and lead (Pb2+) ions were registered in very low concentrations down to 1 ppb with the planar waveguide transducer. The enzymes used were inhibited differently by the above pollutants, which is promising for the development of enzyme sensor arrays.