Everything you need to know about septic tanks in 2025

If you’re building a new home, upgrading an old system, or simply want to stay compliant with your existing on-site wastewater treatment system, understanding how septic tanks and packaged wastewater treatment systems work is essential. From EPA site suitability assessments and percolation areas to planning permission, certification, and ongoing maintenance — there’s a lot to consider before choosing or maintaining the right solution for your property. This guide brings together everything you need to know about septic tanks and on-site wastewater treatment systems in Ireland.

A septic tank is an underground container that collects wastewater generated in your home or commercial facility. Using natural processes, a septic tank removes suspended solids from the wastewater — this is considered “primary wastewater treatment.”

As there is limited anaerobic digestion (a process by which microorganisms break down organic matter in the absence of oxygen), the majority of the “secondary wastewater treatment” is carried out in a percolation area — a network of sub-surface perforated pipes in stone-filled trenches that distributes final wastewater into the soil for further treatment.

Wastewater from your home or commercial facility — from toilets, showers, baths, sinks, washing machines, and dishwashers — flows by gravity into the septic tank through an inlet pipe.

Heavy solids sink to the bottom of the tank, forming sludge, while oils, fats, and greases create a scum layer on top. The remaining “effluent” undergoes anaerobic digestion.

This partially treated effluent then flows to your percolation area. Here, an organic layer (biomat) forms at the base of the trenches and enhances the removal of pathogens before the treated water is discharged into groundwater.

Solids, fats, and greases must be periodically removed by a licensed de-sludging company to ensure the system continues to function properly.

If you're in an area that does not have access to a mains sewer connection, you'll need to install an on-site wastewater treatment system — such as a septic tank — to safely process your household wastewater. Untreated wastewater can contaminate local water supplies and pollute the environment with excess phosphorus, nitrogen, bacteria, and viruses.

While septic tanks rely on large percolation areas to carry out the secondary treatment of wastewater, packaged wastewater treatment systems provide more advanced, controlled processing within the unit itself. This allows for several key advantages, especially on sites with limited space or difficult ground conditions.

One major benefit is that wastewater treatment systems typically require a much smaller percolation area compared to traditional septic tanks. This makes them a better fit for sites where space is tight or where achieving minimum separation distances from wells, buildings, or water tables would otherwise be impossible with a standard septic tank.

Additionally, wastewater treatment systems can be designed to pump treated effluent to a higher elevation if needed, providing greater flexibility for challenging site layouts or sloped land.

They can also be paired with an advanced tertiary treatment filter, which further polishes the effluent before it is discharged. This level of treatment is ideal for environmentally sensitive locations, such as areas near lakes, rivers, or protected habitats, where extra protection is required to meet regulatory standards.

An EPA Site Suitability Assessment is required during planning for a new build or renovation. This assessment determines if the ground conditions and space are appropriate for a septic tank and percolation area.

For example, a 4-bedroom house (6-person equivalent) would require 6 trenches at 18 metres each with 2.5m spacing. That’s a 270m² footprint — plus required separation distances from the property, wells, roads, water table, and more.

If your site doesn’t meet these conditions, you may need a packaged secondary wastewater treatment system instead, which requires less space and handles more challenging ground conditions.

A percolation area is an essential part of the treatment process for both septic tanks and wastewater treatment systems. It’s where the treated effluent is discharged into the ground on your site, allowing it to safely filter through the soil for final treatment before re-entering the environment. Without a properly functioning percolation area, wastewater can pose risks to groundwater, nearby wells, and the surrounding environment.

You can learn more from these resources:

• EPA Code Of Practice 2021
• S.R. 66:2015
• Building Regulations Technical Guidance Document H

These outline the legal and environmental guidelines for installing, operating, and certifying domestic wastewater treatment systems.

Yes. In Ireland, septic tanks must be tested and certified under S.R. 66:2015 and EN 12566 Part 1. A full list of certified systems is available from PIA website.

Installation cost: Between €5,000 and €12,000 depending on contractor and site location.

Emptying cost: Typically €250–€450, depending on tank size and location.

Available grants: Up to 85% (max €12,000) under:

• National Inspection Plan (triggered by local authority inspection)
• High-Status Objective Catchment Areas (based on EIRCODE eligibility)
• Priority Areas for Action (designated by the Local Authority Waters Programme)

Note: Routine maintenance or de-sludging is not covered by grants.

While it’s possible to install a septic tank yourself, it requires specific expertise, machinery, the right materials, and a clear understanding of the installation guide provided by the manufacturer. It’s also essential to ensure you’ve selected the right system for your site conditions, including soil type, available space, and groundwater levels. Installation errors — or choosing an unsuitable system — can lead to costly failures, environmental contamination, or planning permission issues. If you’re considering self-installation, you’ll still need support from the system manufacturer and possibly a certified engineer to ensure the system is installed correctly, tested, and fully compliant with local regulations.

Yes, in most cases — but it depends on the scope of the work. If you’re replacing an existing system with one of equal or equivalent capacity in the same location, you typically do not need planning permission. However, if you’re building a new home, renovating a property, or making changes that require increased capacity or a new drainage layout, then planning permission is required. Your EPA site suitability assessment, along with local planning guidelines, will help determine which type of system is suitable for your site and whether permission is needed.

Registration: All domestic wastewater systems must be registered. You can do this on the Protect Our Water website.

Inspection: Systems may be inspected by EPA-authorised inspectors. You’ll receive 10 working days' notice. A passed inspection gives you a certificate; failed inspections result in an advisory notice and a set timeline to address the issue — grant funding may be available.

Every 12–18 months depending on usage, household size, and tank capacity. Failing to de-sludge regularly risks clogging the percolation area and polluting the environment.

Rainwater should never enter your septic tank. These systems are specifically sized to handle the volume of wastewater generated inside your property — from toilets, sinks, showers, and appliances — not the unpredictable and often excessive flow of rainwater.

Allowing rainwater in can overload the tank, disrupt the treatment process, and push solids into the percolation area, leading to blockages, flooding, and environmental contamination. All stormwater drains, gutters, and surface water should be kept completely separate from your wastewater treatment system.

If you want to make use of rainwater you can install a separate rainwater harvesting system or you can install a stormwater soakaway using plastic crates.

Limit water use immediately. Get the system inspected and potentially de-sludged. Continuous flooding suggests underlying issues with site conditions or system design — speak to a qualified wastewater professional for a permanent solution.

As a general rule, only substances that resemble typical domestic wastewater should enter your septic tank or wastewater treatment system. Introducing materials that are toxic, biologically incompatible, non-degradable, or heavily contaminated can disrupt the treatment process and potentially damage your system. The following must not enter your system:

• Rainwater or surface water (should be managed separately)
• Livestock residues (solid or liquid manure)
• Commercial or agricultural wastewater
• Chemicals, solvents, mineral oils, or pharmaceuticals
• Food waste, plastics, sanitary products, wet wipes, or other household items
• Milk and dairy products
• Excessive amounts of blood

If your household produces large volumes of fats or vegetable oils, we strongly recommend installing a grease trap to capture greasy waste before it enters the system. Ideally, dispose of fats and oils in a sealed container and place them in your general waste bin — not down the sink.

A sewage smell in your garden is often a sign that something isn’t working properly in your on-site wastewater treatment system. Common causes include a blocked or failing percolation area, an overloaded tank, or untreated effluent surfacing due to poor drainage or saturated ground.

It can also result from insufficient or blocked venting — both on the tank itself and on your property. Proper venting allows gases like methane and hydrogen sulfide to safely escape above roof level; if vents are missing, damaged, or clogged, these odours can linger around the ground.

Structural damage, like cracked pipes or lids, can also cause odours to escape. If you notice persistent smells, it’s important to have your system inspected by a wastewater professional to identify the root cause and prevent environmental or health risks.

Licensed de-sludging companies are widely available throughout Ireland, and in most cases, you’ll find a local provider operating within your community. These companies are authorised to safely remove and dispose of waste from on-site wastewater treatment systems, including septic tanks.

If you’re unsure who to contact or need help finding a reliable provider in your area, we’d be happy to make a recommendation. Just get in touch with us for more information or assistance.

Not usually. A healthy septic tank does not require additives. Most systems operate optimally without them. Some additives may be harmful or unnecessary.

• Unsuitable site conditions (bedrock, high water table, poor separation distances)
• Lack of maintenance (leading to blocked or saturated percolation areas)
• Rainwater ingress (from gutters or drains connected to the tank)
• Structural damage (cracked tanks, leaking pipes, groundwater infiltration)
• Incorrect installation (wrong tank size, wrong location)

If you’re facing recurring issues, schedule a professional septic tank inspection to identify root causes and build a path to long-term reliability.

Yes, it can — especially if the system isn’t properly maintained. A well-functioning, compliant on-site wastewater treatment system is unlikely to negatively impact your property’s value. However, if the system is failing, unregistered, or in poor condition, it could lead to costly repairs or full replacement, which may reduce buyer confidence. Depending on the type of system required, replacement costs can range from €4,000 to €12,000, so staying on top of maintenance is key to protecting your investment.

• Always get a certified EPA Site Suitability Assessment before you begin.
• Use S.R. 66:2015 certified products — it’s the national standard.
• Hire licensed, experienced contractors — cutting corners on installation leads to long-term pain.
• Regular de-sludging (every 12–18 months) is essential for system health.
• Don’t install on unsuitable sites — poor drainage, bedrock, or a high water table = major issues.
• Avoid connecting roof drains/rainwater pipes to your tank — this overloads the system.

Whether you’re installing a new system or upgrading an existing one, your choice of on-site wastewater treatment system plays a major role in protecting your property, the environment, and your long-term investment.

We created this guide to make sure you have all the information needed to navigate the process clearly — from selecting between a septic tank or wastewater treatment system to understanding site suitability, regulations, maintenance, and common pitfalls to avoid.

If you’re unsure what’s right for your site or just want expert advice, don’t hesitate to speak to a member of our team. Making the right decision now will save you time, money, and stress in the future.