Greywater and Irrigation Water: Setting Up Reuse That Actually Works

Reusing water for landscape irrigation needs the right source, the right treatment and a separate distribution that can never be mistaken for drinking water.

Greywater reuse means collecting water from showers, basins and laundry, treating it to a standard suitable for irrigation, and distributing it through a system that is physically separate and unmistakably marked. The engineering is well understood. Most failures come from choosing the wrong source, underestimating the treatment and storage required, or leaving a cross connection that nobody finds until somebody becomes ill.

Before stage one

This is worth doing when you have a landscape with a genuine and continuing irrigation demand, a source of greywater that roughly matches that demand in volume and timing, and space for treatment and storage. A villa with a small planted area and a large family may generate far more greywater than the garden can absorb, which turns the system into a disposal problem rather than a saving.

• Estimate the daily irrigation demand of the landscape by season, not as an annual average.
• Estimate the greywater volume available from the chosen fixtures, excluding kitchen and toilet waste.
• Confirm space for the treatment unit, the storage tank and safe access for servicing.
• Check whether drainage is already separated or whether the plumbing must be modified.
• Confirm the approvals and quality requirements that apply with the relevant authority before design.

Stage 1: Match the source to the demand

Compare the greywater generated with what the landscape can use, month by month. Irrigation demand peaks in hot months while greywater production is relatively steady, so there will be periods of surplus and periods of shortfall. Decide now what happens to surplus water, which usually means a discharge to the foul drain, and what supplements a shortfall, which usually means the mains supply through an air gap.

• Deliverable: A monthly balance of greywater available against irrigation demand.
• Owner: Designer, using fixture counts and a landscape water requirement.
• Approval gate: An agreed overflow and top up strategy before anything is sized.

Stage 2: Select the sources and separate the drainage

Showers, baths and hand basins are the practical sources. Kitchen waste carries fats and food solids that foul treatment quickly, and laundry contributes detergents and lint that need specific handling. In an existing building, separating these drains can be the largest part of the cost, since it means opening floors or running new pipework on walls and routing it to a collection point.

• Deliverable: A drainage plan showing which fixtures feed the system and the collection route.
• Owner: Plumbing contractor under the designer specification.
• Approval gate: Confirmation that no foul drainage can enter the greywater collection.

Stage 3: Specify treatment to the required quality

Treatment normally includes screening, settlement or filtration, biological treatment where the water will be stored, and disinfection. The required standard depends on how the water is used: subsurface drip irrigation demands less than spray irrigation, which can create aerosols near people. Choose the irrigation method first, because it sets the treatment standard and therefore most of the cost.

• Deliverable: A treatment specification referenced to the irrigation method and the required quality.
• Owner: Specialist supplier, reviewed by an independent engineer.
• Approval gate: Written confirmation that the specified process meets the applicable quality requirement.

Stage 4: Design storage with a short retention time

Untreated or partially treated greywater deteriorates quickly in warm conditions, developing odour within a day or two. Storage should therefore be sized for short retention and the system should be designed to use the water soon after it is produced. Tanks need cleaning access, overflow to the foul drain and protection from heat and light.

• Deliverable: Tank sizing calculations with the maximum retention period stated.
• Owner: Designer, with the landscape contractor confirming the demand pattern.
• Approval gate: Retention period and odour control strategy accepted before the tank is ordered.

Stage 5: Install a distinctly separate distribution

Reuse pipework must be visually distinguishable, marked along its length and at every outlet, and fitted with outlets that cannot accept a normal hose connection. Any top up from the mains must be through an air gap rather than a valve, so that a failure cannot push reused water back into the potable supply. These requirements are not a formality; they are the whole safety basis of the system.

• Deliverable: Installed marked pipework with the mains top up arrangement photographed.
• Owner: Plumbing contractor, inspected independently.
• Approval gate: A documented cross connection test before the system is filled.

Stage 6: Commission and set the irrigation controls

Commissioning covers treatment performance, disinfection, the behaviour of the system at overflow and at shortfall, and the irrigation schedule. Watering should run at times when nobody is nearby and when evaporation is lowest. Soil moisture sensing avoids irrigating after rain or when the ground is already wet, which is the easiest way to lose the saving the system was built to deliver.

• Deliverable: A commissioning report with water quality results and the irrigation schedule set.
• Owner: Supplier, witnessed by the owner or facilities manager.
• Approval gate: Water quality results within the required limits before handover is accepted.

Stage 7: Operate, monitor and keep records

The system needs filter cleaning, disinfection consumables, periodic water quality checks and someone watching for odour or wet patches that signal a failure. Keep records of every quality test and every service. If the property is sold or let, the new occupier must be told how the system works and warned clearly that the marked outlets are not drinking water.

• Deliverable: A maintenance log and a quality testing record held in the building file.
• Owner: Facilities manager or the household member responsible.
• Approval gate: A quality test result on file for every required interval.

Handovers between stages

The dangerous joint is between the plumbing contractor and the landscape contractor, where a hose is connected to the nearest available outlet during planting and nobody records it. Insist on a cross connection test after the landscape works are finished, not only after the plumbing is installed. The second joint is at sale or at a change of tenant, where the system is handed over without explanation and someone eventually drinks from a marked outlet or disconnects the treatment to stop a service cost.

How you know it is finished

The system is working when treated water quality meets the required limits on a repeated test, the landscape is irrigated without mains water in the months the balance predicted, no odour is reported, and the cross connection test is documented. Requirements for treated water reuse and the approvals needed are set by the relevant authorities and are updated as policy develops, so confirm the current position before design and again before commissioning, and take advice from a licensed adviser if the property is let, since responsibility for water quality in a tenanted building needs to be clear in writing.

Domira works on buildings that already exist: surveying what is installed, specifying the systems that have to be replaced, and running maintenance and refurbishment work around the people using the property. Tell Domira which asset you are responsible for and we will say what we would inspect first.