A septic tank (Johkasou) is an on-site wastewater treatment system used when a building cannot connect to a public sewer system.
It plays an essential role in residential, commercial, hospitality, and food-service facilities.

This article explains the key points necessary for practical design and planning:

1. Where a septic tank is required / where it is not required
2. How a septic tank works
3. What BOD means — and the critical difference between “influent BOD” and “effluent BOD”
4. Why most jurisdictions require “one septic tank per lot” and how exceptions (separate installation) are handled

---

① Where Septic Tanks Are Required / Not Required

The first step is to determine whether the property is located in a public sewer service area.

---

■ When a septic tank is not required

(= areas with public sewer infrastructure)

• Public sewer service areas (where wastewater can be directly connected to the municipal sewer)

In these areas, septic tanks are not installed.
All wastewater is discharged directly into the public sewer system.

---

■ When a septic tank is required

A septic tank is necessary when the property cannot connect to a public sewer, such as:

• Non-sewered areas (rural, suburban, or remote locations)
• City planning “non-urbanized” or “undeveloped” districts
• Resorts, mountain areas, villages, or large sites with no public sewer access
• Any building where wastewater must be treated on-site

Food-service and lodging facilities often require special attention because their wastewater tends to have higher organic loads.

---

② How a Septic Tank Works

A typical septic tank treats wastewater through several stages:

1. Primary sedimentation – solids settle
2. Anaerobic treatment – microorganisms break down organic matter without oxygen
3. Aerobic treatment – air is supplied to enhance microbial decomposition
4. Secondary sedimentation – microorganisms settle out
5. Disinfection – treated water is chlorinated and discharged

Although designs vary, the essential flow is:

Anaerobic → Aerobic → Sedimentation → Disinfection

This process reduces pollutants significantly before water is released into a drainage channel or waterway.

---

**③ What Is BOD?

Understanding Influent vs Effluent BOD**

BOD (Biochemical Oxygen Demand) is one of the most important indicators in septic tank design and performance assessment.

---

■ What BOD Measures

It represents the amount of oxygen microorganisms need to break down organic matter in water.

• High BOD = high pollution / dirty water
• Low BOD = cleaner water

---

■ Septic tanks involve two different BOD values

This distinction is essential:

---

1. Influent BOD (IN)

The BOD of wastewater entering the septic tank.
This varies greatly depending on building use:

• Residential wastewater: 150–200 mg/L
• Lodging facilities: 200–300 mg/L
• Restaurants / food service: 500–2000 mg/L (very high)

Influent BOD is used to determine which septic tank model and capacity are appropriate.

---

2. Effluent BOD (OUT)

The BOD of treated water discharged after the septic tank.
This represents the tank’s performance.

Typical requirement:

Influent BOD 200 → Effluent BOD 20 mg/L or lower

(Approximately a 90% reduction)

This is the basis of the often-heard explanation:

“BOD200 comes in, and BOD20 goes out.”

---

■ What “BOD20 septic tank” means

This does not describe the incoming water.
It means:

“The septic tank can treat wastewater so that the effluent (discharge water) has BOD ≤ 20 mg/L.”

This distinction prevents major misunderstandings in planning and design.

---

④ The Principle of “One Septic Tank per Lot”

Most municipalities follow the principle:

One septic tank per lot (site).

---

Why this rule exists

• To manage wastewater consistently and safely
• To maintain stable effluent quality
• To simplify maintenance
• Because dividing wastewater among multiple small tanks complicates monitoring and control

Therefore, the ideal configuration is:

All wastewater on the lot → One septic tank → Final discharge

---

■ Summary

• Septic tanks are required only in areas without public sewer access
• In sewered areas, septic tanks are unnecessary
• Septic tanks use anaerobic → aerobic → sedimentation → disinfection processes
• BOD has two meanings:
  • Influent BOD = incoming pollution level
  • Effluent BOD = treated discharge water quality
• Typical performance: BOD200 → BOD20
• “BOD20 septic tank” refers to effluent performance
• The principle is one septic tank per lot

---

■ Disclaimer

This article provides general information. Actual requirements depend on local regulations, site conditions, and the judgment of municipal authorities. Always consult qualified professionals and local government offices for project-specific decisions.