Quality of septic-tank effluent: Which statement best describes the characteristics of effluent leaving a septic tank?

Introduction / Context:
Septic tanks provide primary treatment via sedimentation and anaerobic digestion. They remove settleable solids and some floatables but do not provide complete biochemical stabilization. Understanding the effluent quality is crucial for safe disposal in soak pits, leach fields, or subsequent treatment units.

Given Data / Assumptions:

• Septic tank effluent is being discharged after primary treatment only.
• No additional secondary or tertiary treatment is assumed.

Concept / Approach:
Primary treatment reduces suspended solids but leaves significant dissolved organics (BOD, COD) and fine suspended particles. Therefore, effluent remains “foul” and must be infiltrated into soil or treated further to protect public health and receiving waters.

Step-by-Step Solution:
1) Identify treatment level: septic tank = primary treatment.2) Recognize residual pollutants: dissolved organics, pathogens, fine suspended solids.3) Conclude that effluent requires subsurface disposal (soil absorption) or additional treatment.4) Select the option describing this state: foul and still containing dissolved and suspended solids.

Verification / Alternative check:
Regulatory frameworks typically prohibit direct surface discharge of septic tank effluent without further treatment, confirming the characterization of the effluent as inadequate for direct discharge.

Why Other Options Are Wrong:
Option A: Direct discharge into open drains is unsafe and usually illegal.Option C: Equating septic effluent with complete treatment is incorrect.Options D and E: Effluent is neither sterile nor free of organics; pathogens and BOD remain significant.

Common Pitfalls:

• Overestimating the performance of septic tanks.
• Ignoring pathogen risks in partially treated effluent.

Final Answer:
Foul effluent containing dissolved and suspended solids, requiring soil absorption or further treatment