What is the main problem found when aerobically digesting primary sludge?
The answer to the question is B, to maintain DO levels, more air is required. Primary sludge may be used in aerobic digesters, although secondary sludge is more commonly used. The use of primary sludge, due to an increase of organics (food) will initially increase the microorganism population until this food source is consumed, then the endogenous respiration will begin. The length of time that aerobic digestion requires will still allow for this to occur. But, due to the growth phase increased amounts of air will be required to maintain the DO levels.
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 listed below?
The answer is C, reverse osmosis. Reverse osmosis relies on higher than atmospheric pressure to separate dissolved salts such as sodium and chloride ions from the 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 (SDI). 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.
