What is “grey water”? Recovery facilities


For several years now, there has been much talk in Poland of a worsening water shortage. Poland is one of the countries with the smallest water resources in the entire European Union. Poland has an average of 1 600 m3 of water per inhabitant per year, but during periods of drought this average drops to 1 000 m3 of water per inhabitant. So then we have 37.5% less water. The average amount of water for a European is 4500 m3/year per inhabitant.

If we take a closer look at the water consumption characteristics of an average Polish household, it emerges that on average we use 6% of water for cleaning our homes, washing dishes consumes 10%, laundry 15%, personal hygiene 36%. Together, these categories of our activities consume 67% of the water supplied to our households. Water that we can completely recycle. Despite the fact that the issue of water shortages is often raised, the slogan ‘water recycling’ is still not very popular in our country. In a nutshell, it simply refers to the reuse of water used in the household. But how is it possible to reuse used water when all wastewater is treated in the same way and flows down a single pipe into the sewage system? In such cases it is impossible, so if we want to take care of one of the most precious natural resources on the planet, we need to think and design in such a way that we anticipate the possibility of recovering as much used water as possible from the waste water stream. Popularising the term ‘grey water’ can help us to do this, since this is what we produce most when we use drinking water for a variety of purposes.

What is ‘grey water’?

Grey water is the waste water that is produced during domestic activities such as bathing, cleaning, washing or washing dishes. The name of this type of waste water comes from the peculiar colour that water contaminated with detergents and carried impurities takes on. At this point, it is important to point out that grey water is not drinkable, but also does not contain contaminants from the toilet (we call such contaminated water ‘black water’). In order to be able to recycle grey water, we first need to be able to separate it from the waste water stream and separate it from the biological contaminants from household toilets. To this end, it is advisable to design a separate sewage system for the disposal of grey water in suitable storage tanks as early as the planning stage of the house. This is where two types of grey water capture and reuse systems come to the rescue.

Internal systems for the capture and reuse of grey water

Indoor systems are designed for installation in the basement, garage or boiler room of a residential building. Their dimensions are small, so the installation of such a system does not pose any major problems. The entire system consists of a grey water storage tank with a capacity of 200 to 400 litres. Already in this tank, the purification process is initiated. The contaminated water is pre-filtered to remove any solids (e.g. sand or organic matter). The pre-treated grey water is then subjected to ultrafiltration, usually using specialised membranes. The filtered water, already clean, is stored in a separate tank (also with a capacity of 200 to 400 litres). This tank is equipped with a control panel that manages the re-introduction of purified water into the water supply system inside the building and the diversion of the recovered water to appliances such as the washing machine or toilet tank. The treated grey water can also act as technical water for cleaning the house. Another task of the control panel is to replenish the clean mains water tank in case the purified water runs out to supply the connected appliances. The capacity of such an internal system is approximately 200 litres/day.

Outdoor systems for grey water

In this category we can separate two types of systems. Those designed to supply the garden with recycled water from the grey waste stream, and those more advanced for the use of recycled water both at home and in the garden. The first category includes simple installations based on the use of biological processes to completely purify the water, which is ultimately ideal for irrigating green spaces, filling ponds and swimming pools. The grey water leaving the building flows through several shallow reservoirs filled with a porous medium (gravel, expanded clay, pumice, sand, etc.) in which suitably selected aquatic and marsh plants are planted. In such a system, any pollutants are consumed by micro-organisms that live on the roots of the plants and thrive on the porous structure of the materials from which the so-called biological beds are made. The advantage of these systems is the simplicity and low investment costs associated with their installation. The disadvantage is that they only work 100% during the growing season. In winter, their efficiency drops by up to 40%. A simple solution to this problem could be to place such a biological bed in a greenhouse, which extends the growing season of the plants. One must also bear in mind the need to periodically remove the excess biomass resulting from the huge annual plant growth. But the biomass thus obtained can be an excellent input for compost heaps or even an alternative fuel for heating after drying and processing into briquettes.

More sophisticated grey water recycling systems omit the use of plants and revert to ‘hard technologies’ based on multi-stage filtration. Their design and principle of operation is almost identical to that of indoor systems, except that the collection of grey water and recycled water takes place in large tanks buried in the ground next to the dwelling. There are also variants of the above systems that work in conjunction with rainwater collection systems and its treatment for economic re-use.

What should you look out for when recycling grey water?

To use grey water safely and cost-effectively, there are a few simple rules to keep in mind. It is advisable to make a separate network of pipes and risers at the construction stage of the house, so that the installation will cost much less than if such a system were to be installed in an existing building. Of course, older buildings can also be fitted with such a system, but the costs will certainly be higher. Another important aspect is to ensure that the grey water supply system and the water supply system are completely isolated from each other, so that the treated grey water does not come into contact with the tap water. Grey water can be used to irrigate green areas around the building, for cleaning and washing cars, for flushing toilets and even for laundry. According to estimates, grey water consumption per person can be around 60 litres per day. By recycling grey water, we can reduce the need for both drinking water and waste water generation, and this translates into significant savings. The basis of a well-designed grey water recycling plant is, as usual, quantification and well-executed calculations. Based on these, it can be determined how much grey water we are able to separate from the overall waste water stream. On this basis, we can select appropriately sized storage tanks and pumps to transport the water to the parts of the plant where it will be reused. This is equally important when it comes to designing the entire piping system of a building, choosing the right filtration method, or taking into account the coupling of the grey water recycling system with the tap water and rainwater recovery system.

Author: Łukasz Nowacki

The text was written as part of the project „Hydrozagadka – jak wygrać z suszą?”