Grey water - Rain water

Grey Water and Rainwater Recycling: A Smarter Way to Use Water

Water is one of the most valuable resources in every home, hotel, commercial building, and industrial facility. Yet a large percentage of the water used every day does not need to be drinking-quality water.

Toilet flushing, garden irrigation, floor cleaning, vehicle washing, certain laundry applications, cooling processes, and many other uses can often be supplied with properly treated grey water or collected rainwater instead of fresh mains water.

GREHA develops complete water-reuse systems designed to collect, treat, store, monitor, and redistribute water safely and efficiently. Our solutions are available for residential properties, hotels, commercial buildings, and industrial facilities, with each system configured according to the available water sources, the required reuse applications, and the daily treatment capacity.

Our goal is simple: to reduce unnecessary freshwater consumption without reducing comfort, functionality, or reliability.

Live Green. Save Blue.

What Is Grey Water?

Grey water is wastewater generated from sources such as:
Showers Bathtubs Bathroom washbasins Handwashing areas Selected laundry processes Other approved low-contamination sources
Unlike black water, grey water does not normally include wastewater from toilets or highly contaminated kitchen waste streams.

Grey water may appear relatively clean, but it can still contain:
Soap and shampoo residues Hair and fibres Detergents Skin particles Oils and cosmetic products Suspended solids Organic matter Microorganisms Residual chemicals
For this reason, grey water should not simply be collected and reused without treatment. It requires a carefully designed process that removes solids, reduces organic contamination, controls odour, improves clarity, and provides the appropriate level of disinfection.

Once properly treated, grey water can be redirected to applications where drinking-quality water is not required.

What Is Rainwater Harvesting?

Rainwater harvesting is the process of collecting rainfall from suitable surfaces, usually roofs, before storing and treating it for future use.

A rainwater system may collect water from:
Residential roofs Hotel buildings Commercial properties Warehouses Industrial roofs Covered parking areas Other approved collection surfaces
Although rainwater is naturally soft and usually contains fewer dissolved salts than mains water, it is not automatically clean.

As rainwater passes across a roof and through gutters, it may collect:
Dust Leaves Pollen Sand Bird droppings Organic matter Airborne pollutants Metal particles Microorganisms Debris from gutters and pipes
A properly designed rainwater system therefore includes collection management, first-flush separation, filtration, storage control, treatment, disinfection, and automatic redistribution.

The required treatment level depends on the final application. Water used for irrigation may require a different treatment process from water used for laundry, cleaning, toilet flushing, or industrial processes.

The Difference Between Grey Water and Rainwater

Grey water and rainwater are both valuable alternative water sources, but they are not identical.

Grey water is generated continuously when a building is occupied. Every shower, bath, or approved washbasin use can create a relatively predictable amount of reusable water.

Rainwater availability depends on:
Local rainfall Season Roof collection area Rain intensity Storage capacity Building design Climate conditions
Grey water can provide a more consistent daily supply, while rainwater can provide larger quantities during rainfall periods.

The most effective solution is often a combined system that uses both sources. When grey water production is sufficient, the system can rely primarily on treated grey water. When rainwater is available, it can supplement the water supply and reduce the demand for mains water even further.

This combined approach can improve water availability, system flexibility, and overall freshwater savings.

What GREHA Offers

GREHA provides complete solutions rather than individual filters or isolated components.

Our systems can include:
Grey water collection Rainwater collection Pre-filtration Solids separation Biological treatment Membrane filtration Fine filtration Activated carbon treatment Disinfection Treated-water storage Pumps and pressure control Automatic mains-water backup Water-level monitoring Overflow management Remote monitoring PLC-based automation HMI control screens Alarm notifications Flow measurement Water-quality monitoring Stainless-steel or coated-metal frames Protective enclosures Custom pipework and hydraulic integration
Every project begins with an evaluation of the building or facility.

We examine where the water comes from, how much water is available, how it will be reused, what space is available for the equipment, and what level of treatment is required.

The result is a complete system designed around the actual needs of the project.

How a Grey Water Recycling System Works

A grey water recycling system operates through a series of controlled treatment stages.

The exact process depends on the project, but a typical GREHA system may include the following steps.

1. Grey Water Collection

Grey water is collected through a dedicated drainage network.

Only approved sources should be connected to the system. Toilet wastewater and unsuitable industrial or kitchen waste streams must remain separate unless the system has been specifically designed for them.

Correct separation at the plumbing stage is essential.

Grey water from showers, baths, and approved washbasins is directed to the treatment system instead of being discharged directly to the sewer.

2. Initial Screening

The first treatment stage removes larger particles such as:
Hair Fibres Soap particles Lint Plastic fragments Other visible debris
Screening protects pumps, valves, membranes, and downstream filters.

Depending on the application, the screening system may be manual, automatic, self-cleaning, or integrated into a removable filter basket.

3. Equalisation and Buffer Storage

Grey water production is not constant.

A hotel may produce a large quantity of grey water in the morning and evening, while production may be lower during the rest of the day. A residential property may experience short periods of heavy flow during showers and laundry use.

An equalisation tank balances these variations.

It provides a controlled water supply to the treatment process and prevents sudden flow peaks from affecting system performance.

The tank may include:
Level sensors Mixing Aeration Overflow protection Emergency bypass Low-level protection Automatic cleaning functions

4. Biological Treatment

Grey water contains dissolved organic matter that cannot always be removed through simple filtration.

Biological treatment uses controlled microorganisms to break down organic contamination.

Depending on the project, GREHA systems may use technologies such as:
MBR MBBR Aerobic biological treatment Hybrid biological processes Other project-specific treatment methods
The biological stage helps reduce:
BOD COD Odour-producing compounds Dissolved organic material Residual soaps and biodegradable contamination
Correct aeration, hydraulic retention time, and biological stability are important for reliable performance.

5. Membrane or Fine Filtration

After biological treatment, the water passes through a fine filtration stage.

Membrane technology can provide a highly effective barrier against:
Suspended solids Fine particles Turbidity Biological flocs Many microorganisms
Depending on the selected technology, filtration may include:
Ultrafiltration Membrane bioreactor filtration Cartridge filtration Multimedia filtration Bag filtration Fine screen filtration
The system is selected according to the required water quality and daily capacity.

6. Activated Carbon Treatment

Activated carbon may be used as a polishing stage.

It can help reduce:
Residual odour Colour Organic traces Certain detergent residues Compounds affecting water appearance
This stage is particularly useful when the treated water will be used in visible applications or where improved aesthetic quality is required.

7. Disinfection

Disinfection is one of the most important treatment stages.

Depending on the project, GREHA systems may use:
Ultraviolet disinfection Controlled chlorination Combined UV and chlorine treatment Alternative approved disinfection methods
UV treatment provides rapid disinfection without adding chemicals to the water.

Chlorination can provide residual protection inside treated-water tanks and distribution networks.

The exact disinfection strategy depends on:
Storage time Water temperature Final application Distribution network Risk of aerosol formation Local regulatory requirements

8. Treated-Water Storage

Once treated, the water is stored in a dedicated clean-water tank.

This tank must remain completely separate from untreated grey water and drinking-water systems.

The treated-water tank may include:
Level control Recirculation Residual disinfection Overflow protection Tank ventilation Cleaning access Low-level alarms Automatic flushing Sampling points
The size of the storage tank is calculated according to daily production, daily demand, peak consumption, and available installation space.

9. Redistribution

The treated water is pumped through a dedicated non-potable water network.

Potential uses include:
Toilet flushing Urinal flushing Garden irrigation Landscape irrigation Floor cleaning Outdoor cleaning Vehicle washing Selected laundry applications Cooling or process applications Other approved non-potable uses
The water is not supplied to drinking-water taps, showers, or food preparation points.

Correct pipe identification and system separation are essential.

10. Automatic Mains-Water Backup

Water production and water demand are not always equal.

For example, a hotel may require water for toilets even when grey water production is temporarily low.

GREHA systems can include automatic mains-water backup. When treated-water levels fall below a defined point, the system can introduce the minimum required amount of mains water.

This ensures continuous operation without requiring manual intervention.

Backflow protection and physical separation are incorporated according to the project requirements.

How a Rainwater System Works

A rainwater harvesting system follows a different treatment process.

1. Roof Collection

Rainwater is collected from approved roof surfaces through gutters and downpipes.

The available daily and annual water volume depends on:
Roof area Local rainfall Collection efficiency Seasonal weather patterns Storage volume
A water balance study can help determine the most suitable tank capacity.

2. First-Flush Separation

The first rainfall after a dry period often contains the highest level of contamination.

Dust, dirt, pollen, bird droppings, and other debris may have accumulated on the roof.

A first-flush system diverts the initial contaminated rainwater away from the main storage tank.

After the roof has been rinsed, cleaner rainwater is directed into the collection system.

3. Pre-Filtration

Before storage, rainwater passes through filters that remove:
Leaves Sand Insects Organic material Roof debris Larger suspended solids
Different filter designs can be used depending on roof area and expected flow.

4. Rainwater Storage

The filtered rainwater is stored in an above-ground or underground tank.

A properly designed tank may include:
Calmed inlet Floating suction Overflow siphon Level sensors Ventilation Inspection access Cleaning points Rodent and insect protection Emergency overflow
The tank must be protected from sunlight as much as possible to limit algae growth and water deterioration.

5. Treatment According to Use

Rainwater treatment is selected according to the required application.

For basic irrigation, treatment may include screening and filtration.

For toilet flushing or cleaning, additional fine filtration and disinfection may be required.

For laundry or more sensitive uses, the system may include:
Fine filtration Activated carbon UV disinfection Controlled chlorination Additional water-quality monitoring
GREHA does not apply one treatment process to every project. Each system is designed around its intended use.

Combined Grey Water and Rainwater Systems

A combined GREHA system can manage grey water and rainwater within one integrated water-reuse strategy.

The system can automatically prioritise available water sources.

A typical operating sequence may be:
Use treated grey water when available. Supplement with treated rainwater. Use mains water only when stored recycled water is insufficient.
This approach can maximise freshwater savings while maintaining continuous supply.

The system may include separate untreated tanks, independent treatment lines, or a shared polishing and distribution stage, depending on the project requirements.

Combining grey water and rainwater is particularly effective for:
Hotels Apartment buildings Villas Resorts Sports facilities Schools Office buildings Commercial centres Industrial facilities

Residential Grey Water and Rainwater Systems

In a home, a significant amount of water is used for activities that do not require drinking-water quality.

A residential GREHA system can collect water from showers and baths and reuse it for:
Toilet flushing Laundry Garden irrigation Outdoor cleaning
Rainwater can also be collected from the roof and added to the non-potable water supply.

Residential systems are designed to be:
Compact Quiet Automated Easy to operate Suitable for indoor or outdoor installation Integrated with the building’s plumbing Configured for low maintenance
The system works automatically in the background. Residents continue using showers, toilets, washing machines, and irrigation systems normally.

The difference is that part of the water is used more than once before leaving the property.

Hotel Grey Water Recycling

Hotels are among the most suitable buildings for grey water recycling.

Every day, significant quantities of water are used in:
Guest showers Bathtubs Bathroom washbasins Laundry Toilet flushing Landscape irrigation Cleaning operations
This creates both a large source of grey water and a large demand for non-potable water.

A properly designed hotel system can collect water from guest rooms, treat it centrally, and redistribute it to toilet flushing, irrigation, cleaning, or other approved uses.

Benefits for hotels may include:
Lower freshwater consumption Reduced operating costs Improved environmental performance More efficient use of existing water resources Support for sustainability targets Better resilience during water restrictions Reduced dependence on mains-water availability Stronger environmental positioning for guests and partners
GREHA hotel systems are available in different capacities, including systems designed to process approximately 2,000 litres per day and larger custom solutions.

System capacity is calculated according to:
Number of rooms Expected occupancy Water consumption per guest Available grey water Toilet and irrigation demand Seasonal operation Laundry requirements Future expansion

Industrial Water-Reuse Systems

Industrial water use differs significantly from residential and hotel use.

Every industrial facility may have different:
Contaminants Flow rates Operating hours Water-quality requirements Temperature conditions Chemical loads Reuse targets Space limitations
For this reason, GREHA industrial systems are custom made.

The treatment line may be configured for:
Process-water recovery Equipment washing Floor cleaning Vehicle washing Cooling-system supply Utility-water applications Rinse-water reuse Other approved industrial applications
Industrial systems can include multiple treatment technologies such as:
Screening Oil separation pH adjustment Chemical dosing Biological treatment MBR MBBR Ultrafiltration Activated carbon UV disinfection Chlorination Automated backwashing Sludge management Remote monitoring
Before designing an industrial system, a water analysis is normally required.

The analysis helps identify parameters such as:
pH Conductivity TDS TSS BOD COD Oils and grease Detergents Metals Chlorides Sulphates Nitrogen compounds Microbiological contamination Process-specific chemicals
The system is then engineered around the actual wastewater characteristics and the quality required for reuse.

Water Savings

The percentage of freshwater savings depends on the building, climate, water consumption profile, available grey water, rainwater collection area, storage capacity, and final reuse applications.

Depending on the project, a well-designed combined grey water and rainwater system may help reduce mains-water consumption by approximately 30% to 80%.

This range is not a guaranteed result for every installation.

Actual savings depend on factors such as:
Number of users Shower frequency Toilet demand Laundry demand Garden size Irrigation season Local rainfall Roof area Storage capacity Hotel occupancy Industrial production schedule Water-reuse restrictions System operating conditions
GREHA evaluates these factors before recommending a system.

A project-specific water balance can estimate:
Daily water production Daily reusable-water demand Required treatment capacity Recommended tank size Expected mains-water reduction Potential overflow Mains-water backup requirements

Water Quality and Treatment Targets

The required treated-water quality depends on the intended use and applicable regulations.

A GREHA system may be designed to target parameters such as:
pH BOD₅ COD Total suspended solids Turbidity Ammoniacal nitrogen Total nitrogen Detergents Conductivity Dissolved solids Residual disinfectant E. coli Total coliforms Enterococci Legionella, where relevant
Target values are established during the design stage.

They are selected according to:
Source-water quality Final application Treatment technology Local regulations Client requirements Project risk assessment
Performance values should be presented as design targets unless they have been confirmed through sampling and analysis by an accredited laboratory under defined operating conditions.

This distinction is important because water quality can be affected by changes in incoming water, detergents, user behaviour, temperature, maintenance, hydraulic loading, and system operation.

Monitoring and Automation

GREHA systems can be equipped with automatic monitoring and control.

Depending on the project, the control system may monitor:
Tank levels Flow rates Pump operation Filter condition Pressure Turbidity pH Conductivity Disinfection status UV operation Chlorine levels Membrane pressure Equipment faults Overflow conditions Water production Water consumption
PLC-based control allows the system to operate automatically.

An HMI screen can display:
System status Tank levels Active pumps Treatment stages Alarm history Water production Maintenance reminders Operating hours Manual controls
Remote monitoring can also be integrated, allowing authorised users or technicians to review system performance and receive alarm notifications.

Safety and Separation

Recycled water systems must be designed with clear separation between drinking water and non-potable water.

Important safety measures may include:
Dedicated recycled-water pipework Clear pipe identification Backflow protection Air gaps Separate storage tanks Controlled mains-water backup Warning labels Restricted access Sampling points Emergency bypass Overflow connection Automatic fault response
Recycled water should only be used for the applications for which the system has been designed.

It should not be presented as drinking water unless a completely different treatment and certification framework has been specifically applied.

Maintenance

All water-treatment systems require maintenance.

A properly maintained system can provide stable performance, better water quality, and longer equipment life.

Maintenance may include:
 Cleaning screens Replacing filters Inspecting pumps Checking valves Cleaning tanks Inspecting membranes Monitoring biological treatment Checking UV lamps Refilling dosing chemicals Calibrating sensors Sampling treated water Reviewing alarm history Inspecting the pipe network 
Maintenance frequency depends on:
 Water quality System size Daily flow Technology Operating environment Final application 
GREHA systems can be designed with easy access to service points and removable components to simplify maintenance.

Custom Engineering

Not every building can use a standard system.

GREHA provides custom engineering for projects with:
 Limited installation space Underground plant rooms Rooftop installation requirements High daily capacity Variable water flow Multiple buildings Seasonal operation Industrial contaminants Special water-quality requirements Existing tanks Existing plumbing networks Expansion plans 
The system can be supplied as:
 A compact enclosed unit A skid-mounted system A modular treatment plant A containerised unit A plant-room installation A custom metal-frame assembly A multi-stage industrial system 
Capacity, treatment stages, automation, tanks, pumps, and enclosure design are configured for each project.

System Construction and Documentation

GREHA systems may be manufactured using industrial-grade components selected for durability and serviceability.

Depending on the system, construction may include:
 Metal frame Powder-coated protective panels Stainless-steel components Chemical-resistant pipework Industrial pumps Automated valves Electrical control panel PLC HMI Sensors Membranes Filters Dosing equipment UV systems Storage tanks 
Applicable equipment and electrical components can be selected with the appropriate conformity documentation.

Where legally applicable, the complete system is supplied with the required technical documentation and conformity assessment according to its final configuration, installation, and intended use.

Why Water Reuse Matters

Water reuse is not only an environmental decision.

It is also a practical response to:
 Increasing water demand Higher utility costs Drought Water restrictions Urban growth Seasonal tourism Climate uncertainty Industrial expansion Pressure on water infrastructure 
Using drinking water for every application is inefficient.

A modern building should distinguish between water that must be potable and water that can safely be supplied from an alternative treated source.

Grey water and rainwater systems create this distinction.

They allow water to remain useful for longer and reduce the amount of fresh water required for everyday operation.

The GREHA Approach

GREHA does not simply supply a tank and a filter.

We develop complete water-reuse solutions.

Our approach includes:
 Understanding the application. Identifying available water sources. Estimating daily water production. Calculating reusable-water demand. Reviewing water quality. Selecting the treatment process. Designing tanks and hydraulic systems. Integrating pumps and automation. Defining treatment targets. Supporting installation and commissioning. 
Every project is different.

A private residence requires a different system from a hotel. A hotel requires a different operating strategy from an industrial facility. A building in a dry climate has different rainwater potential from a building in a high-rainfall area.

Our systems are therefore designed around real operating conditions rather than a one-size-fits-all approach.

A More Efficient Future for Water

Grey water and rainwater should not be viewed as waste.

They are alternative water resources that can be collected, treated, monitored, and reused.

For homes, this can mean using shower water for toilet flushing and irrigation.

For hotels, it can mean recycling thousands of litres every day from guest bathrooms.

For industrial facilities, it can mean recovering water from selected processes and returning it to cleaning, cooling, or production-support applications.

The technology already exists.

The next step is to integrate it intelligently into the way buildings and facilities operate.

GREHA designs systems that make water reuse practical, automated, and scalable—from compact residential units to custom-built hotel and industrial installations.

Live Green. Save Blue.