The key difference between primary sludge and secondary (biological) sludge is that
- APrimary sludge is more difficult to dewater than secondary sludge
- BSecondary sludge has a higher calorific value than primary sludge
- CPrimary sludge is predominantly inorganic, while secondary sludge is organic
- DPrimary sludge is produced from physical settling, while secondary sludge is produced from biological growthCorrect
Solution
Concept
Comparison of primary and secondary sewage sludge
Formula
NONE
Step-by-step solution
1. Primary sedimentation tanks rely entirely on physical gravity settling of raw solids. 2. Secondary clarifiers settle excess microbial biomass synthesized in biological reactors. 3. Primary sludge is coarser and easier to dewater than secondary sludge. 4. Therefore, the origin (physical settling vs biological synthesis) is the fundamental distinction. Answer: D
Explanation
Primary sludge consists of settleable raw particulate solids removed from wastewater via gravitational sedimentation (physical process). In contrast, secondary (biological or activated) sludge consists of microbial biomass and cellular synthesis products generated by microorganisms assimilating dissolved organic matter during biological treatment.
Why the other options are wrong
A) Secondary sludge is more difficult to dewater due to cellular bound water and fine floc structure. B) Primary sludge has higher volatile solids and higher calorific value than stabilized biological sludge. C) Both sludges are predominantly organic in domestic wastewater.
Shortcut
Primary sludge = Physical settling of raw solids; Secondary sludge = Biological growth of biomass.
Common mistake
Believing primary sludge is harder to dewater; secondary sludge is gelatinous with bound water, making it much harder to dewater than primary sludge.
Correct answer: D) Primary sludge is produced from physical settling, while secondary sludge is produced from biological growth