Civil Engineering Reference
In-Depth Information
Photocell
Lens
Scattered Light
Reflected Light
Light Beam
Lamp
Turbidimeter Body
Over-Flowing
Sample
Refracted Light
Sample In
Drain
Fig. 10-16.
Schematic diagram of surface scatter turbidimeter (Courtesy of Hach Co.)
15-degree angle. Part of the beam is reflected by the water surface and escapes to a
light trap. The remaining portion enters the water at approximately a 45-degree angle.
If particles of turbidity are present, light scattering will take place and some of the
scattered light will reach the photocell. With this ''surface scatter'' design, there is
virtually no upper limit for turbidity measurement. This type of turbidity meter can be
calibrated by using a Jackson candle turbidimeter and measuring the turbidity of the
same water that flows through both instruments. The in-line turbidimeter is adjusted
so that its output reading corresponds to the Jackson candle reading. Ranges as high
as 0 to 5,000 are available, and this instrument can be used for measuring the turbidity
of raw water or the settling basin effluent. A 0.25-0.50-gpm (0.016 to 0.032 L / s)
sample stream is required.
ENHANCED COAGULATION PROCESSES
In addition to particulate matter, coagulation is also capable of removing soluble con-
taminants, such as arsenic and true color, from the waters. The majority of true color
is imparted by natural organic matter (NOM). Since NOM molecules are very com-
plicated and do not have a definite structure, total organic carbon (TOC) concentration
is a surrogate commonly used to quantify NOM. While NOM itself causes aesthetic
problems (such as taste and odor) rather than health concern, the reaction between
NOM and chlorine (during the chlorination process) could generate carcinogenic dis-
infection by-products (DBPs). The Stage I D / DBP Rule promulgated in 1999 set lower
