Correct Answer - Option 3 : Calcium
Sewage sludge:
- Sludge is a residual, semi-solid by-product left from industrial or refining processes.
- It is a separated solid suspended in a liquid, typically containing major quantities of interstitial water between its solid particles.
- This material can be dried to cut its volume and reduce most of the moisture content of the biosolids present in the sludge.
- Dried or treated sludge offers a wide range of applications, mainly including agricultural land application.
- When the drying is completed to produce biosolids, these biosolids act as a fertilizer for crop harvesting.
- Calcium, the organic nitrogen and phosphorous found in biosolids are used quite efficiently by crops as these plant nutrients are released slowly throughout the growing season.
- This allows the crop to absorb these nutrients as the crop grows.
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Calcium is typically the most abundant in dried sewage sludge.
Chemical composition of dried sewage sludge:
| Element |
Wt % |
| Silicon |
36.7 |
| Iron |
16.3 |
| Aluminium |
14.7 |
| Calcium |
13.1 |
| Phosphorus |
11.8 |
| Potassium |
3.66 |
| Titanium |
1.00 |
| Zinc |
0.81 |
| Magnesium |
0.77 |
| Sulphur |
0.36 |
| Nitrogen |
0.17 |
| Mangan |
0.16 |
| Copper |
0.13 |
- Fresh sewage consists of a higher per cent of total nitrogen and phosphorus but during the treatment process, these nutrients are removed from the sewage using various biological and chemical processes.
- This is done to remove the nutritional load in the sewage before disposal into a water body to avoid algal growth.
- More, importantly we must consider that the question is asked for dried sewage sludge (DSS) and not fresh sewage.
Therefore, Calcium is the correct answer.
