Septic Tank Design and Construction | How to Calculate Septic Tank Size
Designing and constructing a septic tank is a critical process that ensures the efficient disposal of wastewater in areas without a centralized sewage system. A well-designed septic tank system is essential to prevent environmental contamination and public health hazards.
Understanding the Basics of Septic Tank Design
What is a Septic Tank?
A septic tank is an underground chamber made of concrete, fiberglass, or plastic through which domestic wastewater flows for basic treatment. Its primary function is to separate solids from liquids in the wastewater and initiate the breakdown of organic matter. The clarified liquid is then discharged into a drain field, where it undergoes further purification.
Components of a Septic Tank System
A typical septic tank system comprises the following components:
- Inlet Pipe: This pipe connects the household plumbing to the septic tank, allowing wastewater to enter the tank.
- Septic Tank: The main chamber where the separation of solids and liquids occurs. The tank is designed with compartments to enhance the efficiency of the separation process.
- Outlet Pipe: The outlet pipe leads the treated effluent from the tank to the drain field.
- Drain Field (Leach Field): A network of perforated pipes laid in gravel-filled trenches, which allows the effluent to percolate into the soil for further treatment.
- Vent Pipe: Ensures the release of gases produced by anaerobic bacteria within the tank.
Importance of Proper Septic Tank Design
The design of a septic tank is crucial for its functionality. An improperly designed tank can lead to frequent failures, environmental pollution, and health risks. Key factors to consider in septic tank design include the size of the tank, the material used, and the placement of the tank and drain field.
Calculating Septic Tank Size
Factors Influencing Septic Tank Size
The size of a septic tank depends on various factors, including:
- Number of Bedrooms: The number of bedrooms in a household typically determines the volume of wastewater generated.
- Daily Water Usage: Average daily water consumption per person plays a significant role in determining the tank size.
- Soil Permeability: The soil's ability to absorb and treat effluent affects the drain field size and, consequently, the septic tank size.
- Local Regulations: Building codes and health regulations may dictate minimum tank sizes based on local conditions.
Step-by-Step Guide to Septic Tank Sizing
1. Determine Household Size and Water Usage
Calculate the total number of people living in the house. Typically, it is assumed that each person uses about 100-150 gallons of water per day. For a household of four people, the daily water usage would be approximately 400-600 gallons.
2. Estimate Daily Wastewater Flow
Multiply the number of occupants by the per capita water usage to estimate the daily wastewater flow. For example, for a family of four using 150 gallons per person, the daily flow would be 600 gallons.
3. Consider a Safety Factor
It's wise to include a safety factor in your calculations to account for peak usage times and potential future expansion. A 25-50% safety factor is common.
4. Calculate the Septic Tank Capacity
A general rule of thumb is that the septic tank capacity should be about three times the daily wastewater flow. For example, if the daily flow is 600 gallons, the septic tank should have a capacity of 1,800 gallons. However, if local regulations specify a different multiplier, that should be used.
5. Account for Soil Permeability
The soil's ability to absorb wastewater influences the drain field design and, by extension, the septic tank size. Percolation tests are conducted to measure soil permeability, and the results are factored into the final tank sizing.
Septic Tank Construction Best Practices
Choosing the Right Materials
Septic tanks can be constructed from various materials, each with its own advantages and disadvantages:
- Concrete: Durable and long-lasting, concrete is a popular choice, although it can be prone to cracking if not installed correctly.
- Fiberglass: Lightweight and resistant to corrosion, fiberglass tanks are easier to install but can be more expensive.
- Plastic: Plastic tanks are affordable and easy to transport, but they may be more susceptible to damage during installation.
Site Selection and Preparation
The location of the septic tank and drain field is critical. The site should be:
- Away from Water Sources: The tank should be located at least 50-100 feet away from wells, streams, or other water sources to prevent contamination.
- On Level Ground: To avoid structural issues, the tank should be installed on level ground with adequate drainage.
- Accessible for Maintenance: Ensure that the tank location is easily accessible for regular maintenance and pumping.
Excavation and Installation
The excavation for the septic tank should be done with precision, ensuring that the hole is slightly larger than the tank itself to allow for backfilling. Once the tank is in place, it should be level and firmly seated to prevent shifting. Backfill the area around the tank with suitable soil, compacting it carefully to avoid settling.
Connection and Testing
After installation, connect the inlet and outlet pipes, ensuring that they are securely attached and at the correct slope. Once the connections are made, the system should be tested by running water through the household plumbing to check for leaks or blockages.
Maintenance and Longevity of Septic Systems
Regular Inspections and Pumping
Regular inspections are crucial for the long-term performance of a septic tank. The tank should be pumped every 3-5 years, depending on usage and tank size. During inspections, check for signs of leaks, cracks, or excessive sludge accumulation.
Avoiding System Overload
To prevent overloading the system, reduce water usage by fixing leaks, installing low-flow fixtures, and spreading out laundry loads throughout the week. Avoid flushing non-biodegradable items, chemicals, or grease down the drains, as these can cause blockages and damage the tank.
Protecting the Drain Field
The drain field is a vital part of the septic system, and it should be protected from damage. Avoid planting trees or shrubs near the drain field, as roots can infiltrate the pipes. Additionally, do not drive or park vehicles on the drain field, as the weight can compact the soil and reduce its effectiveness.
Conclusion
Proper septic tank design, construction, and maintenance are essential for the safe and efficient treatment of wastewater. By accurately calculating the size of the septic tank, selecting the right materials, and adhering to best practices during construction, you can ensure a long-lasting and trouble-free septic system.