Can you garden over a domestic sewage treatment plant and drainage field?

Gardening over your domestic sewage treatment plant (STP) may not feel like the most romantic pastime, but with the right plant selection, you can create a space that is both beautiful and functional. This guide will provide you with tips and advice for landscaping, ensuring a healthy and thriving garden without causing damage to the system as a whole.

But first some definitions:

What’s the difference between a Sewage Treatment Plant (STP) and a Septic Tank?

A septic tank is a simple underground system that uses gravity to separate solid and liquid waste in the tanks. The solid waste settles to the bottom of the tank and is broken down by aerobic bacteria with the water phase discharging to a sub-surface drainage field. 

A small sewage treatment plant, on the other hand uses a more sophisticated process that treats the wastewater either using aerated technology (air blowers) or mechanical components to introduce aerobic bacteria, to breakdown the waste. 

Sewage treatment plants can also discharge into watercourses such as rivers and streams, whereas a septic tank cannot.  This is due to the lower quality of effluent from a septic tank.

What is the difference between a Drainage field and Soakaway?

British Water advises that drainage fields also known as infiltration systems, are an important component of a packaged sewage treatment system. A drainage field has two main purposes:

• To allow infiltration of the partially treated effluent into the ground at a controlled rate. 
• To allow further natural biological treatment of the partially treated effluent before it reaches the underlying groundwater. This treatment takes place in the aerated layers of the soil. 

A suitable percolation test will ensure that the receiving ground is suitable for the installation of a drainage field.

It is essential that the owner of a packaged sewage treatment plant or septic tank does not inadvertently agree to have a surface water soakaway installed, which should only be used with surface water runoff, not with wastewater.

So, can I plant a garden over my sewage treatment tank and drainage field?

Yes, of course! But there are certain rules you should follow to have a healthy, flourishing outdoor space, while avoiding damaging the system that is dealing with your domestic wastewater.

Does it just have to be a lawn?

Not at all. But you should select plants with shallow, non-invasive root systems to prevent damage to the pipes and other components of your system. Shrubs can provide a buffer zone between the drainage field and adjacent properties, and can also help to absorb excess nutrients. However, roots of trees and shrubs planted too close to a drainage field can enter the drainage pipe work and block the pipes causing extensive damage. Root ingress into drainage fields is the cause of many systems blocking and as a result flooding of the system. Avoid shrubs, or plants with deep roots.

If you do decide to plant near a drainage field it’s best to stick to shallow-rooted plants that will not encroach on the drainage field, such as herbaceous perrenials, annuals, bedding plants and lawn grass. Suitable plants for a drainage field should be able to thrive in nutrient-rich, moist soil conditions.

Here are some plants that are commonly used:=

• Grasses: Grasses are a popular choice for drainage fields as they can absorb excess nutrients and provide erosion control. Species such as fescue, rye, and bermuda grass can all work.
• Wildflowers: Wildflowers are another option for drainage fields as they can absorb excess nutrients and provide colour and interest to the area. Some examples include black-eyed susan, coneflower, and purple coneflower.

Planting in this way will help reduce erosion of the drainage field.

Can I have a vegetable plot on my drainage field?

It’s not ideal.  The soil will eventually treat/kill the bacteria and viruses but in very dry, sandy soil these pathogens can travel several feet before they are filtered out.

Over time there is a risk a drainage field may become blocked with detritus (waste or debris). There is a risk of some bacteria filtering into the ground and vegetables growing on the drainage filed may then be able to reach the water and take up pathogens.  

Can I build over my STP and drainage field?

Don’t install paths or greenhouses over the STP or drainage field as these may contravene Building Regulations.

Our 5-step guide to problem-free gardening on an STP drainage field system

1. Locate your plant and drainage field:

Before you begin gardening, locate the boundaries of your drainage field if you don’t already know them. Keep a safe distance from the sewage treatment plant itself. View your property plans or consult with a British Water-accredited service engineer to help you identify the exact location before you make your design decisions.

2. Soil Preparation:

Do not dig deeply or till the soil over your system as this can damage the pipes and disrupt the system's functionality. Instead, gently loosen the top few inches of soil and mix in organic matter, such as compost, to improve soil structure and fertility. 

3. Planting and Maintenance:

Plant your chosen plants according to their specific needs, being careful not to disturb the underlying sewage system components. Water sparingly and avoid overwatering, as excessive moisture can interfere with the drainage fields' ability to disperse wastewater. Apply a layer of organic mulch to help retain moisture and suppress weeds. And don’t forget to wear gloves when weeding, digging or working over the soil - just in case!

4. Avoid Heavy Structures and Equipment:

Refrain from installing heavy structures like patios, decks, or driveways over the drainage field. Additionally, avoid using heavy equipment, such as tractors or lawnmowers, directly over the system, as the weight can cause compaction and damage to the pipes. 

5. Regular Inspections and Maintenance:

Schedule regular inspections and maintenance of your system with an accredited British Water service engineer to ensure it’s functioning properly. This includes checking for any signs of damage or leaks. Address any issues promptly to prevent further damage and protect the environment.

Don’t forget to consult the regulations

Before you begin replacing sewage treatment plant or thinking about landscaping the system, don’t forget to check out the latest regulations and consider all your options for safe and environmentally friendly domestic sewage disposal.

When done correctly, gardening over a sewage treatment system can enhance a property aesthetically while ensuring the all-important functioning of the wastewater system. By choosing appropriate plants, maintaining a safe distance from the tanks, and following the guidelines provided in this blog you can enjoy a thriving garden without compromising the functioning of your system.