WASTEWATER
By Rick Lallish
With proper conditioning, what suspended solids concentrations can you expect from a gravity belt thickener?
A. 1%-3%
B. 4%-6%
C. 8%-12%
D. 14%-28%
Answer: B. Gravity belt thickeners are excellent means of solids thickening, provided the operator has good control over the various operation parameters. Proper conditioning with polymer and proper belt speed and hydraulic loadings can affect the quality of thickening. When operated within the parameters, you can expect to thicken sludge at 0.3% to 0.6% solids to 4%-6% solids. More information may be found in OWP CSU-Sacramento textbook: Operation of Wastewater Treatment Plants, Volume 2, Eighth Edition, Chapter 4.
DRINKING WATER
By Ron Trygar
The passage of a liquid containing dissolved solids and salts through a semipermeable membrane at elevated pressure to become a less concentrated solution is known as which process?
A. Ultrafiltration
B. Osmosis
C. Reverse osmosis
D. Gravity filtration
Answer: C. Reverse osmosis relies on higher than atmospheric pressure to separate dissolved salts such as sodium and chloride ions from a saline feedwater. The amount of pressure applied across the semipermeable membrane is predetermined and depends on factors such as membrane material and thickness, the feedwater characteristics such as pH, salinity, turbidity, TDS, temperature and silt density index. The desired product water (permeate) characteristics are also a determining factor. Is the ultimate goal to remove divalent ions and provide softened water, to remove natural dissolved organic matter for reduction of disinfection byproducts and/or color removal? If so, nanofiltration at lower pressure can be used. If there is need for desalination to remove dissolved sodium and chloride, then higher pressure will be needed.
About the authors
Rick Lallish is water pollution control program director at the Environmental Resources Training Center of Southern Illinois University Edwardsville. Ron Trygar is a certified environmental trainer and water and wastewater program manager at the University of Florida’s Training, Research and Education for Environmental Occupations Center.
