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Understanding Coagulation & Flocculation in Water Treatment: Stability & Mechanisms, Summaries of Civil Engineering Systems

Insights into the concepts of coagulation and flocculation in water treatment. It explains the meaning of stable water, the role of London van der Waals attraction force, and the impact of pH on silica particles. Additionally, it introduces the Zetasizer WT, a device used for real-time monitoring of flocculation process performance, and discusses the fundamental problem that coagulation/flocculation solves in a water treatment plant.

Typology: Summaries

2021/2022

Uploaded on 05/09/2022

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tanoogna-mallarapu 🇮🇳

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HW #2 (Download this page from Canvas & type your answers under each question). Due 10 pm, to
Canvas, on Monday April 11 (pre-check due Thursday April 7 at 4pm). The Week 2 lecture notes are
the basis for this HW, but I have uploaded a PDF from Zeta-Meter called “Everything you want to know
about Coagulation & Flocculation” which is an excellent easy to read primer on the topic. For those of
you who intend to continue in the water treatment field, I recommend you read the whole document.
1. To a water treatment engineer, what does it mean when water is called “stable” with respect to
particles and what causes this stability?
A: In wastewater treatment, the untreated water is known to be “stable” because most of the particles
in the water are negatively charged, attracting positive/counter ion charges towards their surface.
Hence, these particles in the water take time to get dispersed, hindering and slowing down the
treatment process.
2. If you can neutralize the usual negative surface charge that exists on most particles in natural
waters and then slowly mixes the water to promote gentle collisions, a ubiquitous short-range
attractive force will result in particles attaching to one another to form flocs (groups of
individual particles attached to one another). This is the essence of coagulation/flocculation.
What is this force called? What other force in nature is similarly attracting two MUCH larger
“particles” together in our universe?
A: The force which results in particles attaching to one another forming flocs is called “London van
der wall’s attraction force”.
“Nuclear force” attracts much larger particles together in our universe.
3. One of the most ubiquitous particles present in California waters is silica. At a pH of ~3, silica
particles have a neutral surface charge. Given the pH of most potable waters, is the surface
charge of silica particles in water likely to be negative, neutral, or positive and why?
A: The pH of potable water ranges from 6.5 to 8.5. Hence, the pH of silica needs to be increased. The
surface charge of silica particles is likely to be negative. (Due to the escae of Hydrogen ions).
4. Researchers studying particles might measure the particle surface charge using electrophoresis
where the velocity of an individual particle is observed in a cell exposed to an electric field
under a microscope. Dividing the particle velocity by the field strength determines the
electrophoretic mobility, which is an indication of the surface charge on the particle. Clearly, a
particle with a greater surface charge will move faster in an electric field than a particle with
lesser or no charge. This measurement is time-consuming and tedious (manual observations of
individual particles) so commercial devices that can be placed on pipelines and can detect and
quantify the streaming currents that develop when charged particles are present in a flowing
stream have been developed. Find one such device by searching online and provide a short
description (no more than 100 words needed) of the device, its capabilities, cost (only if
available online, usually not available online, you do NOT need to call to find cost) and the web
link so we can check your work. Searching the internet for commercial instrumentation is a
common preliminary task in the consulting field.
A: The Zetasizer WT is a fully automated machine that allows for real-time monitoring of
flocculation process performance and effective charge neutralization. The Zetasizer WT combines
industry-leading Electrophoretic Light Scattering technology from Malvern Panalytical with in-
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Download Understanding Coagulation & Flocculation in Water Treatment: Stability & Mechanisms and more Summaries Civil Engineering Systems in PDF only on Docsity!

HW #2 (Download this page from Canvas & type your answers under each question). Due 10 pm, to Canvas, on Monday April 11 (pre-check due Thursday April 7 at 4pm). The Week 2 lecture notes are the basis for this HW, but I have uploaded a PDF from Zeta-Meter called “Everything you want to know about Coagulation & Flocculation” which is an excellent easy to read primer on the topic. For those of you who intend to continue in the water treatment field, I recommend you read the whole document.

  1. To a water treatment engineer, what does it mean when water is called “stable” with respect to particles and what causes this stability? A: In wastewater treatment, the untreated water is known to be “stable” because most of the particles in the water are negatively charged, attracting positive/counter ion charges towards their surface. Hence, these particles in the water take time to get dispersed, hindering and slowing down the treatment process.
  2. If you can neutralize the usual negative surface charge that exists on most particles in natural waters and then slowly mixes the water to promote gentle collisions, a ubiquitous short-range attractive force will result in particles attaching to one another to form flocs (groups of individual particles attached to one another). This is the essence of coagulation/flocculation. What is this force called? What other force in nature is similarly attracting two MUCH larger “particles” together in our universe? A: The force which results in particles attaching to one another forming flocs is called “London van der wall’s attraction force”. “Nuclear force” attracts much larger particles together in our universe.
  3. One of the most ubiquitous particles present in California waters is silica. At a pH of ~3, silica particles have a neutral surface charge. Given the pH of most potable waters, is the surface charge of silica particles in water likely to be negative, neutral, or positive and why? A: The pH of potable water ranges from 6.5 to 8.5. Hence, the pH of silica needs to be increased. The surface charge of silica particles is likely to be negative. (Due to the escae of Hydrogen ions).
  4. Researchers studying particles might measure the particle surface charge using electrophoresis where the velocity of an individual particle is observed in a cell exposed to an electric field under a microscope. Dividing the particle velocity by the field strength determines the electrophoretic mobility, which is an indication of the surface charge on the particle. Clearly, a particle with a greater surface charge will move faster in an electric field than a particle with lesser or no charge. This measurement is time-consuming and tedious (manual observations of individual particles) so commercial devices that can be placed on pipelines and can detect and quantify the streaming currents that develop when charged particles are present in a flowing stream have been developed. Find one such device by searching online and provide a short description (no more than 100 words needed) of the device, its capabilities, cost (only if available online, usually not available online, you do NOT need to call to find cost) and the web link so we can check your work. Searching the internet for commercial instrumentation is a common preliminary task in the consulting field. A: The Zetasizer WT is a fully automated machine that allows for real-time monitoring of flocculation process performance and effective charge neutralization. The Zetasizer WT combines industry-leading Electrophoretic Light Scattering technology from Malvern Panalytical with in-

process data to provide a specialized simple with online zeta potential analyzer for water treatment plants. Zeta potential analysis is completely automated and produces results in minutes whose maintenance is also minimal. The measurement of grab samples with temperature and sample flow sensors. Water treatment, petroleum emulsions, mineral flotation, nanoparticles, cement, and clay are a few of the applications. Laval Lab Inc.

  1. What is the fundamental problem that coagulation/flocculation solves in a water treatment plant (WTP)? A: “Turbidity” is the fundamental problem that coagulation/flocculation solves in a water treatment plant.
  2. Name the three distinctly different mechanisms by which the problem in #5 can be and typically are solved in an actual WTP: a. Reverse osmosis b. Coagulation and filtration c. Settling and decanting d. Filtration Are all of these impacted in some way by pH? No Which of these produces the strongest floc? Coagulation and flocculation Which one of these produces the most sludge to be disposed of? Filtration aa
  3. Jar tests are often conducted to determine the optimal dose of coagulant or polymer needed to effectively reduce the turbidity of a water or to determine the impact of pH (which inevitably impacts coagulation in some way). Turbidity is a measure of the amount of light scattered by a water; turbidity is higher when there are more particles suspended in the water. If a coagulant or polymer dose is optimal, the particles will settle rapidly, and the turbidity will be lower. Jar tests are typically conducted with a series of 5 different doses, plus a zero dose control. One series of results is provided in the table below. Assume the regulatory limit at a WTP for turbidity is 2 NTU. Find the least cost ($ per day) to coagulate a flow of 2000 m^3 /d. You can assume the cost of polymer X is $1.50/kg and the cost of the alum coagulant is $0.25/kg. Show your calculations in a professional manner that someone else could readily repeat with different numbers. Notice that polymer is much more expensive than conventional coagulants but polymers provide additional strength to flocs and are less sensitive to pH changes so a combination of metal coagulants and polymers are often used in practice. Alum concentration used (mg/L)* Turbidity after settling tank (NTU) 0 30 2 10

Alum 25 0.14 =0.14 * 25 =3. FeCl 3 20 0.20 =0.20 * 20 = 4 Alum & polymer

FeCl 3 & polymer

b) The different factors to be considered for selecting a coagulant/polymer are:  Sludge considerations  Effectiveness  Environmental effects  Reliability of supply  Compatibility with other treatment processes  Labor and equipment  Regulations

  1. This question is meant to provide you with incentive to carefully read section II of the lecture notes and pull out relevant information. Fill out the form below. Identify by name each of the four distinct mechanisms of particle destabilization described in the lecture notes. State four significant and different features of each mechanism. Try to be clear but concise. Mechanism: Compression of the double layer
  2. The concentration of counterions rises, reducing the volume required to establish electroneutrality around the particle.
  3. Weakening of the repulsive forces takes place while maintaining and dominating the Van der wall attraction.
  4. The effectiveness of coagulation increases with the charge of the counter ion.
  5. Not cost-efficient Mechanism: Adsorption and charge neutralization
  6. Opposite charges act as effective coagulants b reducing net surface charges.
  7. Optimum dose = F (Concentration, pH, charge, type of coagulant)
  1. Overdosing = Re-stabilization and More particle surface area = Requirement of higher optimal dose
  2. Efficient mechanism Mechanism: Enmeshment in a precipitate
  3. As coagulant is added, pH decreases, and calculated alkalinity must be added.
  4. Adding coagulant – X10 times the solubility limit leads to the formation of a precipitate. (The particles act as nucleus sites for the initiation of precipitation)
  5. Overdosing will lead to more sludge and the rapid formation of a precipitate.
  6. Precipitates formed are dependent on the type of coagulant used. Mechanism: Adsorption and inter-particle bridging
  7. Polymer added – attaches to surface of two or more colloids by forming a bridge and making the particles into a larger floc.
  8. Overdosing = Re-stabilization
  9. Long carbon chain polymers need to be used.
  10. Formation of less sludge and less water content in sludge. Does not impact pH.