Microbial digestion of organic wastes of sewage is called primary/secondary/tertiary treatment.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Sewage Treatment**: Sewage treatment involves a series of processes to remove contaminants from wastewater. It is typically divided into three main stages: primary, secondary, and tertiary treatment.

2. **Primary Treatment**: In the primary treatment stage, solid waste is removed from the sewage. This is primarily a physical process where large particles, sand, and grit are filtered out. The solid waste settles at the bottom, and the liquid portion (supernatant) remains on top.

3. **Secondary Treatment**: After primary treatment, the supernatant undergoes secondary treatment. This stage focuses on the removal of organic waste. Here, microorganisms such as bacteria and fungi are introduced to the wastewater. These microorganisms digest the organic matter, breaking it down into smaller, less harmful compounds. This process is also known as biological treatment.

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