Will grey water recycling become state of the art?

Fortunately, the market for water reuse and rainwater management has now changed due to regulatory requirements such as the EU Water Framework Directive, legal requirements for building regulations on greening and rainwater management, as well as investments in environmental technology (such as the EU Green Deal). With the introduction of the EU Taxonomy Regulation 2020/852, technologies can now be classified according to their environmental sustainability criteria, allowing access to favourable funding. But what are the barriers? Why is a water reuse system not at least included in every new build?

In many countries, e.g. United Arab Emirates, Ireland, etc., water is so heavily subsidised that water reuse is not economically viable for the operators. Another reason is the lack of rules and regulations. For example, there is no regulation for self-supply with rainwater in Germany, where the new risk-based approach in EN1717 can help. Water is often not implemented in building standards and is far behind issues such as energy and_CO2. In Germany, for example, we have the "connection and utilisation obligation" in the statutes. We are also hindered by contradictory rules and regulations, e.g. swimming in a lake is permitted under the EU Bathing Water Directive, but showering with the same water would not be allowed. Worldwide, there are big differences in the accepted water quality for different uses, there is no ISO standard. In Singapore you need ultra-filtered water to clean the floor and in South Africa you are lucky if you can use unfiltered rainwater to make tea. The quality standards lack "hygiene" - water quality for bathing and showering, for example. There is only drinking water, process water and irrigation water quality. In some countries, the requirements for service water quality are so high that it is not profitable to reuse water. One problem for the economic viability of rainwater utilisation systems is, for example, fee splitting. The operator of a rainwater utilisation system only saves on drinking water costs. This makes such systems largely unprofitable. However, many cities have now realised that the positive impact of rainwater cisterns, for example protection of the groundwater supply and retention effect, is so great that these systems are now at least exempt from the sealing fee (Berlin) or even financially subsidised. In almost all parts of Belgium, the installation of a cistern in new buildings has been mandatory for decades. Does grey water recycling then make more sense than rainwater harvesting?

Rainwater is collected from the roof, filtered and used for applications such as watering the garden, flushing toilets or washing machines via a separate service water pipe. It is relatively easy to collect and usually free of organic impurities, which makes treatment less complicated. In the case of green roofs, the amount of roof run-off water is greatly reduced while the demand for irrigation increases. Depending on the distribution of precipitation over the year, large cisterns are required. Grey water, on the other hand, comes from showers, bathtubs and washbasins, which is collected via a separate wastewater pipe. It contains soap residue and organic substances and therefore needs to be treated more intensively before it can be reused. After appropriate cleaning, it is also suitable for flushing toilets or watering gardens.

DIN EN 16941-2:2021-11

On-site systems for non-potable water - Part 2: Systems for the use of treated grey water; German version EN 16941-2:2021

This document specifies the principles of design, dimensioning, installation, marking, commissioning and maintenance of greywater utilisation systems for the on-site use of greywater. It is particularly applicable for the use of treated grey water for: WC flushing; garden irrigation; cleaning of laundry; cleaning of objects. This document also specifies the minimum requirements for grey water systems

In contrast to rainwater utilisation, drinking water and wastewater costs can always be saved. Although greywater systems are subject to notification in Germany, they do not require approval. In particular, grey water recycling is independent of precipitation and the connected roof areas. A grey water system only requires a daily volume as storage and can therefore be installed within a building. In order to increase the degree of self-sufficiency in the water supply, it can make sense to combine the two technologies.

The grey water from showers, bathtubs and washbasins is pre-filtered via a filter before it enters the bioreactor. The dirt collected in the pre-filter is regularly flushed back into the sewer using a backwash nozzle. In the centrepiece of the system - the bioreactor - continuous biological decomposition takes place using bacteria. These are supplied with oxygen by a pipe diffuser located at the bottom of the system. The bacteria colonise the packed beds, which are special floats with a large surface area. A sludge pump, also located at the bottom, periodically removes any sediment that forms. The membrane stations, which filter the grey water into the clear water tank using ultrafiltration, are also located in the bioreactor. Due to the small membrane pore size of 0.02µm, particles, bacteria and even viruses are retained. The membranes are periodically cleaned by backwashing and coarse air bubbles. Systems with a treatment volume of over 5,400 litres per day are equipped with a browser-enabled remote control and an automated chemical cleaning process. This enables even more economical and low-maintenance operation, especially for the larger systems. The clean, hygienically treated process water is pumped to the consumers by booster systems and used for toilet flushing, washing machines or irrigation. To maximise safety, the already ultra-filtered process water undergoes UV treatment as a final stage.

Thanks to the special treatment process with a membrane bioreactor, which includes both biological degradation and ultrafiltration with a pore size of 0.02 µm, the systems deliver outstanding water quality that meets the highest standards. Compared to other processes, even germs and viruses are safely retained here. The treated water was tested with only 1 E.coli / 100 ml water on average. It has very low turbidity and is odourless. Visually, the water is often almost indistinguishable from drinking water. The INTEWA systems are the first and so far only systems in the world to be certified according to the important NSF 350 "Class C" standard. The systems have also been certified in accordance with the British Standard for spray irrigation.

Greywater recycling systems are particularly suitable for sports facilities, schools, hotels and apartment buildings with increased water consumption due to showering and bathing and a corresponding service water requirement for toilet flushing, washing machines, irrigation or cooling, as the efficiency of the systems is optimised here. As a separate grey water and service water pipe system is required for grey water recycling systems, new buildings or renovation projects are generally suitable. In this case, the separate pipes cause reasonable additional costs. To ensure good accessibility and maintenance of the system, the system should be installed inside the building.

The dimensioning of a grey water system depends on the daily demand and daily yield. On average, approx. 40 - 50 litres per person per day can be expected. INTEWA now offers a range of ready-to-connect complete systems from 300 (up to 6 people) to 48,600 litres (up to > 1000 people) treatment volume per day. Larger systems can be expanded on a modular basis. The prefabricated systems can be planned very quickly and safely into a building and enable uncomplicated installation, which can be carried out quickly by any installer. As a rule of thumb, the installation area can be calculated as twice the daily volume in square metres. A grey water recycling system with a treatment volume of 5 m³ per day is therefore suitable for approx. 100 residents and only requires approx. 10 m² of installation space in the building.

In over 10 years, a wealth of international experience has been gained with several hundred grey water systems, so that the fundamental suitability of this technology in modern building services is no longer in question.

Alsdorf indoor swimming pool

The pool is used by an average of 350 people every day. The corresponding grey water from the showers and washbasins is treated in a 5000 litre internal tank and used to supply 11 toilet facilities

For this magnificent new museum in Los Angeles, California, rainwater is collected from the roofs and condensate is collected in summer for reuse in irrigation, adiabatic cooling, and toilet flushing. The AQUALOOP graywater system is designed for an NSF 350-certified average daily treatment capacity of 47,800 liters per day. The system also collects, filters, and stores the collected rainwater on a roof area of 89,000 m². Several PURAIN 200 and PURAIN 300 filters are installed to pre-filter the rainwater. The entire system is monitored and controlled by the I-CONNECT control system. Once completed, the museum will house all forms of visual storytelling, including painting, photography, sculpture, illustration, comic art, performance, and video. It is currently under construction in Los Angeles, California, and is scheduled to open in 2025.

The enormous significance and high future potential of graywater recycling for the water supply of buildings is now scientifically recognized. The ready-to-connect system series with certified technology enable safe use for planners, installers, and operators. Most of the barriers associated with rainwater harvesting do not apply to graywater recycling. However, the use of this technology is mostly limited to new construction or renovation. The biggest hurdle is the cost of installing separate networks for service water and wastewater. If these additional pipe systems are taken into account during the planning stage, the additional costs are reasonable. Since larger systems are now not only sustainable but also very economical, the door is open for new buildings in particular to make this technology state of the art./

Author: Oliver Ringelstein, INTEWA GmbH

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Further information:

Virtual tour of the 5,400-liter demo graywater system with 360° walkaround: https://building.intewa.net/de/360-ausstellung/grauwasser-recycling-komplettsystem

Basic knowledge of water treatment: https://wiki.intewa.net/index.php/Wasseraufbereitung_und_Grauwassernutzung

Free online planning tools: https://www.intewa.com/de/unternehmen/service-kontakt/planungshilfen/

Reference projects: https://www.intewa.com/de/referenzen/

Certifications: https://www.intewa.com/de/downloads/

Note: Modifications to the technical article are only permitted with the express permission of the author!