DIY Guide: Constructing a Small Septic System for Off-Grid Living

Living off-grid or in rural areas often means managing your own wastewater. While professional septic system installations are ideal, they can be expensive. For smaller homes, cabins, or workshops, a DIY small septic system can be a viable and cost-effective option, provided you adhere to local codes and regulations. This comprehensive guide will walk you through the process of designing and constructing a basic, small-scale septic system, focusing on components and step-by-step instructions. Please note: This guide is for informational purposes only. Always consult local authorities and obtain necessary permits before beginning any construction.

Understanding the Basics of a Septic System

Before diving into the construction process, it’s essential to understand how a septic system works. A typical system has two main components:

1. Septic Tank: This is a watertight container, usually made of concrete or plastic, where wastewater from your home enters. In the tank, solids settle to the bottom (forming sludge), and lighter materials, like grease and oil, float to the top (forming scum). The partially clarified wastewater (effluent) then flows out of the tank.
2. Drainfield (Leach Field): The effluent from the septic tank is discharged into the drainfield, which consists of a series of trenches or beds containing gravel and perforated pipes. The effluent slowly filters through the soil, where bacteria break down remaining pollutants, cleaning the water before it returns to the groundwater.

This guide focuses on a small, gravity-fed septic system suitable for low-flow scenarios. We will be excluding complex systems that require pumps or alternative treatment methods. The system described here is designed for minimal daily water usage and must be properly sized for the number of occupants and their water consumption. Oversizing the system is crucial for its longevity and effectiveness.

Planning and Design: Essential Considerations

Proper planning is the most critical aspect of any septic system installation. Before you lift a shovel, you need to address these important factors:

1. Local Regulations and Permits: This is the most critical step. Contact your local health department or building department. Regulations vary widely by location. They will guide you on:
• Permitting requirements
• Minimum setbacks from wells, property lines, and water bodies
• Soil testing requirements
• Required tank size and drainfield dimensions
• Approved materials
2. Soil Testing (Percolation Test): The soil’s ability to absorb water (percolation rate) is paramount. A perc test determines how quickly water drains through your soil. This affects the required size of your drainfield. Typically, a qualified professional (engineer or soil scientist) performs this test. They will dig a hole, fill it with water, and measure the time it takes for the water to drain. The results are crucial for drainfield design.
3. Water Usage Estimation: Estimate your average daily water consumption. This will help determine the required size of your septic tank. A standard formula is 100-150 gallons per day for a single person. Multiple this by the number of occupants for an estimate. Consider future growth as well.
4. Site Selection: Choose a location for your septic system that adheres to all setback requirements. Avoid areas that are:
   • Too close to wells or water bodies
   • Located in low-lying, frequently flooded areas
   • Areas with poor soil drainage
   • Near tree roots that can damage the system
   • On steep slopes (gravity systems usually require gentle slopes)
5. System Design: Based on your water usage and soil conditions, plan your septic tank and drainfield sizes. For a small cabin or workshop with limited water usage (1-2 people), a 750-1000 gallon tank, and a smaller, linear drainfield may suffice.

Materials and Tools You’ll Need

Gathering all necessary materials and tools before starting will streamline the construction process. Here’s a comprehensive list:

Materials:

• Septic Tank: Choose a plastic or concrete tank with the appropriate capacity.
• Inlet and Outlet Fittings: Typically 4-inch PVC fittings to connect sewer lines.
• Effluent Filter: Optional, but highly recommended to prevent solids from entering the drainfield.
• Distribution Box (D-Box): Directs the effluent evenly into the drainfield.
• Perforated Drainfield Pipe: 4-inch schedule 40 PVC or specialized drainfield piping.
• Gravel: Washed gravel, usually 1-2 inch diameter (amount dependent on drainfield size).
• Non-Woven Geotextile Fabric: Separates the gravel from the soil, preventing soil intrusion.
• Sewer Pipe: 4-inch PVC or ABS pipe for connecting the house to the septic tank.
• Cleanout: A capped pipe providing access to the sewer line for maintenance.
• Marking Paint or Stakes: To mark the layout of the system.
• Backfill Material: Soil for filling the excavations.
• Warning Tape: Buried above the system as a marker.

Tools:

• Excavator or Backhoe: To dig the trenches and the hole for the septic tank (can be rented).
• Shovels: For digging and backfilling.
• Level: For ensuring proper pipe grading.
• Measuring Tape: For accurate layout and trench dimensions.
• Saw: For cutting pipes.
• PVC Cement and Primer: For connecting PVC pipes.
• Compactor (optional): For compacting backfill material.
• Safety Glasses, Gloves, and Hard Hat: For personal safety.

Step-by-Step Construction Guide

Once you have all your materials, permits, and a solid plan, you’re ready to begin the construction process. Remember to take your time and ensure each step is done correctly.

Step 1: Site Preparation and Layout

1. Mark the Boundaries: Using stakes and marking paint, mark out the areas for the septic tank and drainfield according to your approved plan.
2. Clear the Area: Remove all vegetation, rocks, and debris from the marked areas.
3. Determine Slope: Ensure there is a gentle downward slope from the septic tank to the drainfield to allow for gravity flow.

Step 2: Excavating the Septic Tank Hole

1. Dig the Hole: Use the excavator or backhoe to dig a hole slightly larger than the septic tank dimensions. The bottom of the hole should be level and firm.
2. Ensure Proper Depth: The top of the tank should be buried approximately 6-12 inches below ground level, depending on your local code requirements and frost line depth. Check with your local authority for specific requirements.
3. Gravel Bed: If required by your local codes, add a bed of gravel 4-6 inches thick to the bottom of the excavated hole to provide a level and stable base for the tank.

Step 3: Installing the Septic Tank

1. Position the Tank: Carefully lower the septic tank into the excavated hole. Ensure it is level.
2. Connect Inlet Pipe: Connect the sewer pipe from your house to the inlet fitting of the septic tank. Ensure the pipe is at a minimum downward slope of 1/8 inch per foot. Use PVC primer and cement for all pipe joints.
3. Install Outlet Fitting and Effluent Filter: Install the outlet fitting and an effluent filter if you are using one. The outlet should be positioned lower than the inlet to allow for proper gravity flow. The effluent filter will help prevent solids from entering the drainfield.
4. Backfill: Carefully backfill around the septic tank in lifts (layers) of 6-8 inches, tamping down each layer firmly to prevent settling. Use caution around the inlet and outlet pipes, ensuring they are supported. Fill the tank with water to prevent it from moving during the backfilling.

Step 4: Excavating the Drainfield Trenches

1. Dig the Trenches: Excavate the drainfield trenches according to your approved plan. The trenches should be 18-36 inches wide and approximately 24-36 inches deep, with flat bottoms. Maintain proper spacing between trenches, typically a minimum of 6 feet, but check with local code. The trench depth depends on your percolation test results and local regulations, consult your local code.
2. Ensure Level Bottoms: Use a level to ensure the bottoms of the trenches are level and at the proper depth.
3. Slope: Maintain a very slight slope for each trench from the distribution box to the end of the trench, no less than 1/16 inch per foot, but check with local code.

Step 5: Installing the Distribution Box (D-Box)

1. Place the D-Box: Position the distribution box at the start of the drainfield, typically at the end of the outlet pipe from the septic tank. Ensure it is level.
2. Connect Outlet Pipe: Connect the outlet pipe from the septic tank to the inlet of the distribution box. Use PVC primer and cement for all pipe joints.
3. Connect to Drainfield Lines: Connect the distribution box outlets to the drainfield pipes going into each trench.

Step 6: Laying the Drainfield

1. Lay Geotextile Fabric: Lay a layer of geotextile fabric over the bottom and sides of each trench. This fabric will help prevent soil from clogging the gravel. Ensure it extends up the sides of the trenches.
2. Place Gravel: Place a 4-6 inch layer of washed gravel in the bottom of each trench.
3. Install Drainfield Pipe: Lay the perforated drainfield pipes on top of the gravel. Ensure the perforations are facing down.
4. Cover with Gravel: Cover the drainfield pipes with another 8-12 inches of gravel.
5. Fold Over Geotextile Fabric: Fold the excess geotextile fabric over the top of the gravel layer.

Step 7: Backfilling and Finishing

1. Backfill: Carefully backfill the trenches with the remaining excavated soil. You may want to compact the backfill material in layers to prevent settling.
2. Install Cleanout: Install a cleanout pipe above the sewer line leading to the septic tank. Make sure it is accessible for maintenance.
3. Install Warning Tape: Place warning tape above the trenches and tank, approximately 12 inches below ground level, to indicate the location of the septic system.
4. Grade and Seed: Grade the surface to ensure proper drainage and seed the area with grass to prevent erosion.

Important Notes and Considerations

• Professional Inspection: Once the system is installed, have it inspected by the local authorities to ensure it complies with all regulations and obtain final approval.
• Regular Maintenance: Septic systems require regular maintenance. The tank should be pumped out every 3-5 years, or more often depending on usage. Regularly check the drainfield for any signs of issues such as saturated or pooling areas.
• Water Conservation: Conserving water reduces the load on your septic system, leading to a longer lifespan. Use low-flow toilets, showerheads, and faucets.
• Avoid Disposing of Grease, Chemicals, and Non-Biodegradable Items: Never pour grease, chemicals, paints, or non-biodegradable items down the drain, as they can harm the system.
• Plant Selection: Choose plants for your landscape that have shallow root systems to avoid damaging the septic system, this especially applies to trees.
• DIY Limitations: DIY septic system construction should be approached with caution. If you are uncomfortable with any aspect of this guide, consider hiring a professional.

Conclusion

Constructing a small septic system is a significant project. This guide provides a basic framework, but it’s important to adapt the process based on your specific needs, local codes, and regulations. Always prioritize safety, accuracy, and proper planning. Proper installation, combined with regular maintenance, will allow your septic system to function effectively and reliably for many years. Remember that this DIY approach is for simple, low-flow situations. If your needs are more complex, always seek professional assistance.