Introduction:
Sludge refers to the residual semi-solid material that is produced during wastewater treatment processes. It consists of solids that settle out from the wastewater and various microorganisms, including bacteria. Pathogenic bacteria are harmful bacteria that can cause diseases in humans and animals. In wastewater treatment, it is important to eliminate or reduce pathogenic bacteria to ensure the safety of the treated sludge. The form of sludge that is free from pathogenic bacteria is digested sludge.

Digested Sludge:
- Digested sludge is the form of sludge that has undergone a process called anaerobic digestion.
- Anaerobic digestion is a biological process in which microorganisms break down organic matter in the absence of oxygen.
- During anaerobic digestion, pathogenic bacteria are significantly reduced or eliminated due to the high temperatures and the action of anaerobic microorganisms.
- The high temperatures, typically around 35-55°C (95-131°F), reached during anaerobic digestion help in destroying or inactivating pathogenic bacteria.
- Anaerobic microorganisms, such as methanogens, bacteria, and archaea, outcompete and suppress the growth of pathogenic bacteria.
- The digestion process also helps in breaking down complex organic compounds into simpler compounds, making the sludge more stable and less odorous.
- Digested sludge has a lower concentration of pathogenic bacteria compared to undigested sludge.

Other Forms of Sludge:
- Undigested sludge refers to sludge that has not undergone anaerobic digestion. It contains a higher concentration of pathogenic bacteria compared to digested sludge.
- Primary sludge is the sludge that is collected from the primary settling tanks in wastewater treatment plants. It contains a mixture of organic and inorganic solids, including pathogenic bacteria.
- Supernatant liquor refers to the liquid portion that separates from the sludge during settling or centrifugation. It does not contain significant amounts of pathogenic bacteria as they mainly reside in the solid fraction.

Conclusion:
Digested sludge is the form of sludge that is free from pathogenic bacteria. Through the process of anaerobic digestion, pathogenic bacteria are reduced or eliminated due to high temperatures and the action of anaerobic microorganisms. The digestion process also helps in stabilizing the sludge and breaking down organic compounds. Undigested sludge, primary sludge, and supernatant liquor may still contain pathogenic bacteria, although their concentrations vary.