In advanced wastewater treatment, tertiary sewage treatment is specifically designed to remove which pair of nutrients to prevent eutrophication of natural water bodies?

Introduction / Context:
This environmental chemistry and biology question concerns the purpose of tertiary sewage treatment. Primary and secondary stages remove solids and much of the organic matter, but tertiary treatment targets specific dissolved pollutants. Two nutrients, phosphorus and nitrogen, are major contributors to eutrophication, which is the excessive growth of algae and plants in natural water bodies. Knowing that tertiary treatment focuses on these nutrients helps learners understand water quality management and pollution control.

Given Data / Assumptions:
- Several pairs of elements are listed, including combinations with calcium, phosphorus, and nitrogen.
- The question asks which pair is targeted in tertiary treatment to prevent eutrophication.
- We assume typical municipal wastewater treatment plant design and standard environmental science terminology.

Concept / Approach:
Eutrophication occurs when lakes, rivers, or coastal waters receive excess nutrients, especially phosphates and nitrates, leading to dense algal blooms. When these algae die and decompose, they deplete dissolved oxygen, harming aquatic life. Primary and secondary treatment remove most suspended solids and biodegradable organic matter but may leave high levels of dissolved nitrogen compounds and phosphates. Tertiary treatment includes specific chemical, biological, or physical processes to remove these nutrients. Calcium is not generally a target of tertiary nutrient removal in this context. Therefore, the pair phosphorus and nitrogen is the correct answer.

Step-by-Step Solution:
Step 1: Identify the main nutrients that cause eutrophication. These are typically nitrogen in forms such as nitrate and ammonia and phosphorus in the form of phosphate.
Step 2: Recall that primary treatment removes large solids and some suspended matter, while secondary treatment uses biological processes to break down organic matter.
Step 3: Understand that tertiary treatment is an additional step used when higher water quality is needed. It may employ chemical precipitation, advanced biological processes, or filtration to remove remaining nutrients, particularly nitrogen and phosphorus.
Step 4: Examine the options. Only the pair phosphorus and nitrogen directly corresponds to the key eutrophication causing nutrients.
Step 5: Recognise that calcium is not considered a pollutant contributing to eutrophication and is not the focus of tertiary nutrient removal.

Verification / Alternative check:
Environmental science sources often describe tertiary treatments such as phosphorus removal by chemical precipitation and nitrogen removal by nitrification and denitrification processes. Case studies of upgraded sewage plants frequently mention reductions in total nitrogen and total phosphorus to meet environmental standards. Calcium, on the other hand, may influence hardness but is not a major eutrophication driver. This confirms that phosphorus and nitrogen are the targets of tertiary nutrient removal.

Why Other Options Are Wrong:
- Calcium and Phosphorus: While phosphorus is a key nutrient pollutant, calcium is not a significant eutrophication driver. This pair does not capture the full set of nutrients targeted.

Why Other Options Are Wrong (continued):
- Nitrogen and Calcium: Nitrogen is important, but calcium is not the second critical nutrient causing eutrophication, so this option is incomplete and incorrect.
- Calcium and Nitrogen: Again, calcium is not a key nutrient pollutant in this context, so this pair does not match the objective of tertiary treatment.

Common Pitfalls:
Learners might confuse water hardness, which involves calcium and magnesium, with nutrient pollution. Hardness can be a water quality concern but is not typically addressed by tertiary nutrient removal. Another mistake is to focus only on nitrogen and forget the equally important role of phosphorus in algal growth. Remember that environmental regulations often set limits for both total nitrogen and total phosphorus, highlighting that both must be controlled.

Final Answer:
Phosphorus and Nitrogen are the two nutrients that tertiary sewage treatment is designed to remove in order to reduce eutrophication of natural water bodies.