Water Treatment > Quality Water Solutions > Coagulation Control > Particle Measurement

The added particulate monitoring requirements that are part of the Partnership for Safe Water, D/DBPR and ESWTR depend on the ability to count particles of various sizes accurately. These requirements require utilities to measure particle distribution in various size ranges. Plants can use a variety of monitors to alert operators to excessive amounts of particles between 2 and 100 mm range. Devices do this work in one of four ways: light obscuration, laser light scattering, electrical sensing and direct microscopic examination.

In light obscuring or blocking technology, a pressurized sample flows through a light path. The particles essentially cast a shadow onto a sensor (photodiode). The area of the shadow is converted to a particle size, and the number of signals in each range provides the "count." The unit is blind to clear (translucent) particles and fibrous materials that cast long, thin shadows. These detectors respond relatively rapidly to particles from less than 2 mm to more than 100 mm.

In laser light scattering, the sample passes through a laser beam. Light scattered by particles in the forward direction is sensed by a detector. The intensity of the signal corresponds to particle size. These instruments are extremely sensitive and can measure particles as small as 0.005 mm. Lower cost laser counters exist that allow utilities on tight budgets to place them after each filter and at other key points in a facility, while using a more sophisticated laser device in the lab.

In electrical zone sensing the sample is diluted with an electrolyte of known inductance, usually by a normal saline solution (0.85% concentration), and pumped through an orifice of known size and charged to a specific voltage. The particles disrupt the electric field in proportion to particle volume. This signal is converted to size ranges and counts. The main drawbacks of these units are the need to change orifice sizes for different size ranges and the need for an electrolyte.

Direct optical microscopic evaluation is the only way to identify the particles present. All other methods assume that the particles are neat spheres and do not differentiate animal, vegetable or mineral, e.g., algae, protozoa, zooplankton, debris and mineral fragments. Direct examination is slow and cumbersome, requires analysts skilled in the art of identification, and does not allow on-line measurement. Even so, this method is a valuable tool in assessing water quality variations.