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-
