Particle size distribution As noted above, TSS is a lumped parameter. In an effort to understand more nature of the particles that omprise the Tss in wastewater, measurement of particle size is undertaken and lysis of the distribution of particle sizes is conducted Information on particle size is of importance in assessing the effectiveness of treatment processes(e.g. secondary sedimentation, effluent filtration and effluent disinfection) Because the effectiveness of both chlorine and disinfection is dependent on particle size, the determination of particle size has become more important, especially with the move toward greater effluent reuse Information on the size of the biodegradable organic particles is significant from a treatment standpoint, as the biological conversion rate of these particles is dependent on size. The methods can be divided into two general categories: (I)methods based on observation and measurement and (2)methods based separation analysis techniques. The methods used most commonly to study and quantify the paticles in wastewater are: (1)serial filtration, (2)electronic particle counting, and microscopic observation(See Tab ab 2-4 Analytical techniques application to particle size analysis of wastewater contaminants Typical size range, um Observation and measurement Microscopy 0.2->100 Transmisson electron 0.2-100 Scanning electron 0.002-50 Image analysis 0.2->10 Particle counters Conductivity difference Equivalent light scattering 0.005->100 0.2->100 Centrifugation 0.08->100 Field flow fractionation 0.09->100 Gel filtration chromatography <0.000l->100 Sedimentation 0.05->100 0.0001-1 Pore size Serial Filtration. In the serial filtration method, a tewater sample is passed sequentially through series of membrane filters(see Fig 2-3)with k circular openings of known diameter(typically 12, 8, 5, 3, 1. and 0. I um), and the amount of solids interesting to note is the amount of colloidal material found between 0. I and 1.0 um. If a 0. I-Hm filter had (略) instead of a filter with a nominal pore size equal to or greater than 1.0 um(2.0 ugm as specified in Standard Methods for the TSS test), more than 20 g/L of additional tss would have been measured Fig. 2-3 Definition sketch for the determination of the particle size distribution using serial filtration with membrane filters Ithough some information is gained on the size and distribution of the particles in the wastewater sample little information is gained on the nature of the individual particles. This method is useful in assessing the effectiveness of treatment methods(e. g, microfiltration) for the removal of residual TSS 2-6
2-6 Particle Size Distribution As noted above, TSS is a lumped parameter. In an effort to understand more nature of the particles that comprise the TSS in wastewater, measurement of particle size is undertaken and an analysis of the distribution of particle sizes is conducted. Information on particle size is of importance in assessing the effectiveness of treatment processes (e.g., secondary sedimentation, effluent filtration and effluent disinfection). Because the effectiveness of both chlorine and disinfection is dependent on particle size, the determination of particle size has become more important, especially with the move toward greater effluent reuse. Information on the size of the biodegradable organic particles is significant from a treatment standpoint, as the biological conversion rate of these particles is dependent on size. The methods can be divided into two general categories: (1)methods based on observation and measurement and (2) methods based on separation analysis techniques. The methods used most commonly to study and quantify the paticles in wastewater are: (1) serial filtration, (2) electronic particle counting, and microscopic observation(See Tab 2-4). Tab 2-4 Analytical techniques application to particle size analysis of wastewater contaminants Technique Typical size range,μm Observation and measurement Microscopy Light Transmissoin electron Scanning electron Image analysis Particle counters Conductivity difference Equivalent light scattering Light blockage 0.2- >100 0.2- 100 0.002- 50 0.2- >100 0.2- >100 0.005- >100 0.2- >100 Separation and analysis Centrifugation Field flow fractionation Gel filtration chromatography Sedimentation Membrane filtration 0.08- >100 0.09->100 <0.0001- >100 0.05- >100 0.0001-1 Serial Filtration. In the serial filtration method, a wastewater sample is passed sequentially through a series of membrane filters (see Fig. 2-3) with circular openings of known diameter (typically 12, 8, 5, 3, 1. and 0. l μm), and the amount of solids retained in each filter is measured. What is interesting to note is the amount of colloidal material found between 0.1 and 1.0 μm. If a 0. l-μm filter had been used to determine TSS for the treated effluent instead of a filter with a nominal pore size equal to or greater than 1.0 μm (2.0 μgm as specified in Standard Methods for the TSS test), more than 20 mg/L of additional TSS would have been measured. Fig. 2-3 Definition sketch for the determination of the particle size distribution using serial filtration with membrane filters Although some information is gained on the size and distribution of the particles in the wastewater sample, little information is gained on the nature of the individual particles. This method is useful in assessing the effectiveness of treatment methods (e.g., microfiltration) for the removal of residual TSS
Electronic Particle Size Counting. In electronic particle size counting, particles in wastewater are counted by diluting a sample and then passing the diluted sample through a calibrated orifice or past laser presence of the particle. The conductivity is correlated to the size of an equivalent sphere. In a sIml beams. As the particles pass through the orifice, the conductivity of the fluid changes, owing to th fashion, as a particle passes by a laser beam, it reduces the intensity of the laser because of light scattering The reduced intensity is correlated to the diameter of the particle. The particles that are counted are grouped into particle size ranges(e.g,, 0.5 to 2, 2 to 5, 5 to 20 um, etc). In turn, the volume fraction corresponding to each particle size range can be computed Typical effluent volume fraction data from two activated slu treatment plant are reported on Fig. 2-5. As shown, the particle size data for small particles are the same for both treatment plants However, the particle size data for the large particles are quite different owing primarily to the design and operation of the secondary clarification. Particle size information such as that shown on Fig. 2-5, is useful in assessing the performance of condary sedimentation facilities, effluent filtration, and the potential for chlorine and ultraviolet radiation disinfection Fig. 2-4 Imhoff cone used to determine settleable solids in wastewater Solids that accumulate in the bottom of the cone after 60 min are reported as mL/L Microscopic Observation. Particles in wastewater can also be enumerated microscopically by placing a small sample in a particle counting chamber and counting the individual particles. To aid in differentiating different types of particles, various types of stains can be used. In general, microscopic counting of particles is impractical on a routine basis. Nevertheless, this method can be used to qualitatively assess the nature and size of the particles in wastewater. A quantitative assessment of wastewater particles can be btained with a microscope by means of a process called optical imaging A small sample of wastewater is placed microscope slide. The images of the o Plant 1-3.4 m(11 ft)side water depth Plant 2-5.5 m(18 ft) side water depth wastewater particles are collected with a video camera attached to a microscope and transmitted to a computer where various measurements of the wastewater particles 5 Volume fraction of particle found in the effluent from two activated-sludge plants with clarifiers having diferent side water depths Log of particle diameter, dp um The types of measurements that can be obtained are dependent on the computer software but typically include the mean, minimum, and maximum diameter, the aspect ratio(length to width ratio), the circumference, the surface area, the volume, and the centroid of various particles. Particle imaging greatly reduces the time required to measure various characteristics of wastewater particles, but the cost of the software and equipment is often prohibitive for many small laboratories Turbidity Turbidity a measure of the light-transmitting properties of water. is another test used to indicat quality of waste discharges and natural waters with respect to colloidal and residual suspended matter. measurement of turbidity is based on comparison of the intensity of light scattered by a sample to the light 2-7
2-7 Electronic Particle Size Counting. In electronic particle size counting, particles in wastewater are counted by diluting a sample and then passing the diluted sample through a calibrated orifice or past laser beams. As the particles pass through the orifice, the conductivity of the fluid changes, owing to the presence of the particle. The conductivity is correlated to the size of an equivalent sphere. In a similar fashion, as a particle passes by a laser beam, it reduces the intensity of the laser because of light scattering . The reduced intensity is correlated to the diameter of the particle. The particles that are counted are grouped into particle size ranges (e.g., 0.5 to 2, 2 to 5, 5 to 20 μm, etc). In turn, the volume fraction corresponding to each particle size range can be computed. Typical effluent volume fraction data from two activated sludge treatment plant are reported on Fig. 2-5. As shown, the particle size data for small particles are the same for both treatment plants. However, the particle size data for the large particles are quite different, owing primarily to the design and operation of the secondary clarification . Particle size information, such as that shown on Fig. 2-5 , is useful in assessing the performance of secondary sedimentation facilities, effluent filtration, and the potential for chlorine and ultraviolet radiation disinfection. Fig. 2-4 Imhoff cone used to determine settleable solids in wastewater.Solids that accumulate in the bottom of the cone after 60 min are reported as mL/L Microscopic Observation. Particles in wastewater can also be enumerated microscopically by placing a small sample in a particle counting chamber and counting the individual particles. To aid in differentiating different types of particles, various types of stains can be used. In general, microscopic counting of particles is impractical on a routine basis. Nevertheless, this method can be used to qualitatively assess the nature and size of the particles in wastewater. A quantitative assessment of wastewater particles can be obtained with a microscope by means of a process called optical imaging. A small sample of wastewater is placed on a microscope slide. The images of the wastewater particles are collected with a video camera attached to a microscope and transmitted to a computer where various measurements of the wastewater particles can be assessed. Fig. 2-5 Volume fraction of particle sizes found in the effluent from two activated-sludge plants with clarifiers having different side water depths The types of measurements that can be obtained are dependent on the computer software but typically include the mean, minimum, and maximum diameter, the aspect ratio (length to width ratio), the circumference, the surface area, the volume, and the centroid of various particles. Particle imaging greatly reduces the time required to measure various characteristics of wastewater particles, but the cost of the software and equipment is often prohibitive for many small laboratories. Turbidity Turbidity, a measure of the light-transmitting properties of water, is another test used to indicate the quality of waste discharges and natural waters with respect to colloidal and residual suspended matter. The measurement of turbidity is based on comparison of the intensity of light scattered by a sample to the light
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 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, nm
2-8 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