scattered by a reference suspension under the same conditions. Formazin suspensions are used as the primary reference standard. The results of turbidity measurements are reported as nephelometric turbidity units(NTU Colloidal matter will scatter or absorb light and thus prevent its transmission. It should be noted that the presence of air bubbles in the fluid will cause erroneous turbidity readings. In general. there wastewater. There is. however. a reasonable relationship between turbidity and total suspended solids for the settled and filtered secondary effluent from the activated sludge process. The specific value of the conversion factor will vary for each treatment, depending primarily on the operation of the biological treatment process. The conservation factors for settled secondary effluent and for secondary effluent filtered with a granular-medium depth filter will typically vary from 2. 3 to 2. 4 and 1.3 to 1.6, respectively One of the problems with the measurement of turbidity(especially low values in filtered effluent) is the high degree of variability observed, depending on the light source(incandescent light versus light-emitting diodes) and the method of measurement (reflected versus transmitted light). Another problem often encountered is the light-absorbing properties of the suspended material. However, turbidity readings at a given facility can be used for process control. Some on line turbidity meters used o monitor performance of microfiltration units are affected by the air used to clean the membranes Color Historically, the term "condition"was used along with composition and concentration to describe wastewater. Condition refers to the age of the wastewater, which is determined qualitatively by its color and odor. Fresh wastewater is usually a brownish-gray color. however. as the travel time in the collection svstem increases, and more anaerobic conditions develop. the color of the wastewater changes from gray to dark gray and ultimately to black e color of the wastew he wastewater is often described as septic. Some industrial wastewaters may also add color to domestic wastewater. In most cases. the gray dark gray and black color of the wastewater is due to the formation of metallic sulfides, which form as the sulfide produced under anaerobic conditions reacts with the metals in the wastewater Absorption/Transmittance The absorbance of a solution is a measure of the amount of light, of a specified wave-length, that is absorbed by the constituents in a solution. Absorbance, measured using a spectrophotometer and a fixed path length(usually 1.0 cm), is given by following relationship Where A-absorbence absorbence unit, a u /cm lo-Initial detector reading for the blank (i.e. distilled water) after passing through a solution of 1- Final detector reading for the blank (i.e. distilled water)after passing through Absorbance is measured with a spectrophotometer using a specified wavelength, typically 254 nm Typical absorbance values for various wastewater at 254 nm are 1. Primary: 0.5 to 0.8/cm 2. Secondary: 0.3 to 0.5/cm 3. Nitrified secondary: 0.25 to 0.45/cm 4. Filtered secondary: 0.02 to 0.40/cm Transmittance T, %=(o)X100 Activated sludge goon G50 Fig. 2-6 Transmittance 20 240 280290300310 Wavelength, nm2-8 scattered by a reference suspension under the same conditions. Formazin suspensions are used as the primary reference standard. The results of turbidity measurements are reported as nephelometric turbidity units (NTU). Colloidal matter will scatter or absorb light and thus prevent its transmission. It should be noted that the presence of air bubbles in the fluid will cause erroneous turbidity readings. In general, there is no relationship between turbidity and the concentration of total suspended solids in untreated wastewater. There is, however, a reasonable relationship between turbidity and total suspended solids for the settled and filtered secondary effluent from the activated sludge process. The specific value of the conversion factor will vary for each treatment, depending primarily on the operation of the biological treatment process. The conservation factors for settled secondary effluent and for secondary effluent filtered with a granular-medium depth filter will typically vary from 2.3 to 2.4 and 1.3 to 1.6, respectively. One of the problems with the measurement of turbidity (especially low values in filtered effluent) is the high degree of variability observed, depending on the light source (incandescent light versus light-emitting diodes) and the method of measurement (reflected versus transmitted light). Another problem often encountered is the light-absorbing properties of the suspended material. However, turbidity readings at a given facility can be used for process control. Some on line turbidity meters used to monitor the performance of microfiltration units are affected by the air used to clean the membranes. Color Historically, the term "condition" was used along with composition and concentration to describe wastewater. Condition refers to the age of the wastewater, which is determined qualitatively by its color and odor. Fresh wastewater is usually a brownish-gray color. However, as the travel time in the collection system increases, and more anaerobic conditions develop, the color of the wastewater changes sequentially from gray to dark gray, and ultimately to black. When the color of the wastewater is black, the wastewater is often described as septic. Some industrial wastewaters may also add color to domestic wastewater. In most cases, the gray, dark gray, and black color of the wastewater is due to the formation of metallic sulfides, which form as the sulfide produced under anaerobic conditions reacts with the metals in the wastewater. Absorption/Transmittance The absorbance of a solution is a measure of the amount of light, of a specified wave-length, that is absorbed by the constituents in a solution. Absorbance, measured using a spectrophotometer and a fixed path length (usually 1.0 cm), is given by following relationship: A = log(I0/I) Where A-absorbence, absorbence unit, a.u./cm I0-Initial detector reading for the blank (i.e. distilled water) after passing through a solution of known depth I- Final detector reading for the blank (i.e. distilled water) after passing through solution containing constituents of interest Absorbance is measured with a spectrophotometer using a specified wavelength, typically 254 nm. Typical absorbance values for various wastewater at 254 nm are: 1. Primary:0.5 to 0.8/cm 2. Secondary:0.3 to 0.5/cm 3. Nitrified secondary:0.25 to 0.45/cm 4. Filtered secondary:0.02 to 0.40/cm Transmitttance T, % = (I/Io)×100 Fig. 2-6 Transmittance measured at various wavelengths for