Domestic water consumption and solutions to reduce it

The civil water sector in all countries of the western world coincides with the water utilities served by an aqueduct and sewage network. In the context of civil uses, only actual residential users are considered “domestic uses”, which on a national average amount to around 75% of civil uses. However, many “non-domestic” uses are similar to domestic ones: they are water intended for offices, shops, restaurants, hotels, stadiums, cinemas, etc. which perform the same functions for which we use them in homes (toilets, washing, cooking, irrigation, etc.), albeit with important differences in terms of quantities used. A very modest fraction of civil consumption (about 1-2%) is destined for actually particular uses, such as crafts, the fire service, etc.

Good practices to reduce consumption

If it’s yellow let it mellow, if it’s brown flush it down is one of the water-saving tips proposed by the former environmental mayor of London, Ken Livingstone, during one of the recent water crises in England. The Italian version of this verse – which, we must admit, does not sound as good as the original – was proposed in Nuova Ecologia of December 2007, provoking, in the following issue, an indignant letter from a reader who spoke of a “disgusting incipit”, emphasizing that much more would be expected from the “historical magazine of environmentalists”. Yet the theme raised by the nursery rhyme is not secondary: it arises from the simple observation that every time we pee, we literally throw “in the toilet” from 6 to 15 litres of precious drinking water! On the other hand, the mood of the reader is also understandable and indicates an evident fact: it is very difficult to change the lifestyles we have been accustomed to for a long time, especially when they have to do with very “private” like our poops and pee.

Ken Linvingstone’s advice is just one of many proposed by the information campaigns on “good practices” to contain domestic water consumption: one of the best were the 10 recommendations of the Bologna Area Agency for Public Services, listed below:

1- Taps Avoiding consuming water unnecessarily is the main source of savings. The faucet in your bathroom has a flow rate of over 10 litres per minute, if you leave it open while brushing your teeth, more than 30 litres of drinking water escape down the drain and arrive at the treatment plant without needing it. When you shave, collect water in the sink to rinse your razor, an open faucet does not increase the efficiency of your shave. While massaging your hair with shampoo or conditioner, turn off the tap, you will avoid unnecessary waste.

2- The drops At the rate of 90 drops per minute, 4,000 litres of water are wasted in a year. Checking if the faucets or the toilet tank have a leak is simple. During the night or during the day, when you’re at work, put a small container under the tap (be careful not to close the drain!), after a few hours you will be able to detect even the slightest leak. Before going to sleep, you can empty a bottle of food colouring into the toilet tank (it is washable and does no harm!). Any colouring of the walls of the toilet, or of the water on the bottom, will signal a leak. Proper maintenance of home faucets saves water and money.

3- The flush More than 30% of the water you consume at home comes out of your toilet. Every time you press the button you “drink” 10-12 litres of water, often just for a small piece of toilet paper. Installing a flush cistern equipped with a double button, or a flow regulator, which dispenses different quantities of water according to need, allows you to save tens of thousands of litres of water in a year. Before this intervention, even inserting a brick or a bottle full of water into the drain box, being careful not to obstruct the float and the drain mechanism, can save you several litres of water.

4- The washing machine These appliances consume a lot of water for each wash (80-120 litres), regardless of the load of clothes and dishes. Using them only when necessary and always fully loaded allows for significant savings in water and energy. Read their maximum capacity (kg of laundry and number of dishes) in the instruction booklet and adopt it as a rule for loading them. Reducing washing will improve your life and theirs will last longer.

5- The car 100 litres of water. When you can, reduce washing and always use a bucket instead of running water: wet the bodywork, soap the car and rinse it, you can do it and obtain an excellent result by wasting less water. If you go to a car wash, when you ask if they wash the mats or if they use natural wax, also remember to ask if the system has water recirculation or recovery. Having saved water will be your best result.

6- Plants Water the garden sparingly and always in the evening: when the sun has gone down, the water evaporates more slowly and is not wasted but absorbed by the earth. Add plenty of mulch, it will protect your plants from drought and heat. Collect rainwater when you can, still today, plants really like it. If you are proud of your green thumb, show yourself at the forefront: for your garden, choose plants that need less water (xerophilous plants) and install a “drip” irrigation system (those with black plastic pipes) that can be programmed with a timer, your plants will have their right ration of water and your bill will also benefit.

7- Tap aerator Modern tap aerators are simple devices which, through an innovative system, reduce the amount of water coming out of the tap without reducing the washing performance or comfort. They cost a few euros, can be purchased at hardware stores or through the internet, can be mounted in a few minutes on bathroom and kitchen taps: unscrew the water outlet terminal (the one that holds the net against the pieces of limestone) and insert this small plastic cylinder and then screw the terminal back on. A small effort that costs little but will save a lot.

8- The dishes If you have to wash the vegetables to prepare lunch, remember that a good washing is not done by letting a lot of water flow over them, but by filling a basin or other container, leaving the vegetables to soak so that the solid residues can soften and rubbing abundantly and vigorously each vegetable with your fingers. Similarly, when you wash the dishes, fill a basin with hot water (you could also use the one used to cook pasta), add the detergent, leave the dishes to soak for a while and remove the dirt with a sponge. Use running water only for rinsing. You will see the savings.

9- The shower Taking a nice bath is relaxing but requires over 150 litres of water. Letting yourself be caressed by the water flowing from the shower, scrubbing yourself vigorously with a horsehair glove – remembering to turn off the water while soaping yourself – is instead invigorating and revitalizing, but above all it requires much less water: on average between 40 and 50 litres. If you have also installed a flow reducer in your shower, the savings will be even greater. Keep your body and the planet in shape. Choose the shower and save water.

10- The water meter In the evening, before going to sleep, check that all the taps in the house are well closed and read the level of consumption reached on the water meter. As soon as you wake up in the morning, before starting the day, check your counter again. Even a small difference means that there is a leak (from the toilet flush, from the taps or, more likely, from the pipes) which not only wastes water unnecessarily – a one millimetre hole in a pipe leaks over 2,300 litres of drinking water per day – but it could cause worse damage to the structures of your home by damaging walls, attics and cladding.

What we use the water for in our homes

Conventionally, the water supply for domestic use is estimated at around 200 litres/day per inhabitant (although today the average supply probably does not exceed 180). But what is domestic consumption due to? Unfortunately, by analysing the available sources on the distribution of consumption within homes, it is not easy to obtain reliable data that can be applied to the Italian context. This is due to the lack of updated studies on our territory, but also to the considerable variability of individual consumption, which depends on many factors. The following figure shows an estimate based on the available data.

According to this estimate, and assuming an average per capita consumption of 200 litres per day, each of us uses approximately 70 litres for personal hygiene (a shower plus washing hands, teeth, etc.), 54 for toilet flushing (6-9 flushes per day), 24 litres for the washing machine, 30 for the kitchen and the dishwasher, 22 litres between house cleaning, watering and outdoor uses.

Breakdown of water consumption for domestic use

It is evident that the majority of water consumption concerns uses for which drinking water would not be necessary, clarified, odourless water, but not necessarily drinkable, would suffice. The uses that require truly potable water, if we want to be cautious, could be limited to the bathroom and personal hygiene (32%), food cooking (12%), dishwashers (3%): this is less than half of current domestic consumption which could be replaced with less valuable water such as rain water or treated greywater.

The collection and reuse of rainwater

Rainwater represents a renewable and local source and requires simple and economical treatments for limited use in certain applications. In general, the uses that lend themselves to the reuse of rainwater concern external uses, such as:

  • the irrigation of green areas, lawns, gardens, vegetable gardens;
  • washing outdoor areas (roads, squares, car parks, balconies) and cars;
  • technological uses (e.g. cooling water);
  • supply of fire networks;

and uses inside buildings, such as:

  • the supply of toilet flushing cisterns;
  • the power supply of washing machines;
  • related technological uses, such as passive/active air conditioning systems.

A modern rain collection system is simple; is basically based on three elements:

  • the network that collects the water from the drained surface and filters it before introducing it into the cistern;
  • the cistern;
  • the water lifting and distribution system for the intended uses.

Perhaps the most critical aspect of the design of a rain collection system is the estimate of the quantities of water obtainable according to the available collection surfaces and the volume necessary to store them, which depends on the average distribution of rainfall and on the variations in use in the different periods. In general, rainwater harvesting systems tend to collect water that does not risk being contaminated: therefore, they are limited to using the roofs or terraces of houses as collection surfaces.

Diagram of a rain collection system

The separation, treatment and reuse of greywater

Conventionally, discharges from a domestic dwelling are divided into greywater and blackwater. Greywater accounts for about 70% of domestic consumption and has chemical characteristics that allow for easier treatment: therefore, by collecting them separately and treating them, large quantities of reusable water are produced for almost all non-potable uses.

In reality, it is often convenient to combine the sewage also with the drains from the kitchen sink, which, although not particularly contaminated, contain a large quantity of solids (food and washing residues, coffee powder, etc.) and oils. In this way, the division between grey- and blackwater settles on a ratio of 60 to 40%.

Greywater reuse scheme

The typical diagram of a greywater separation and reuse system is shown in Figure 2. The water from showers and washbasins is collected, treated and sent, via a pump, to the reuse points: generally, the toilet discharge, the washing machine and some non-potable water taps to be used for washing floors, outdoor spaces, irrigation, etc.

The practice of local treatment and reuse of greywater is spreading quite rapidly in countries where the cost of water is higher. For this reason, some manufacturers have marketed extremely compact and automated treatment systems, which can also be easily installed in a cellar. Among the most interesting solutions there are certainly the systems that provide for treatment using phyto-purification systems, integrated into the green furnishings of the buildings. The advantage of phyto-purification lies in the extreme simplicity and stability of the treatment: a well-designed system requires minimum maintenance that does not necessitate specialized personnel and can last for tens of years.

Greywater and blackwater

Only a very small part of the water we use at home is actually “consumed”. The one we drink, the one we use to wash the house and evaporates after use, the one we use to water the plants in the apartment: of the 200 litres/inhabitant/day only 1-3 litres, are actually consumed, the rest we drain into the sewers after use. Well, the water that we discharge has very different characteristics depending on how we use it: the discharge that comes from the toilet, which contains human faeces and urine, will contain water with a different chemical and microbiological composition from the water discharged from the sinks and showers.

An important difference between greywater and black water consists in the different rate of degradation of the pollutants. One might think that greywater, which contains soaps and other residues of household hygiene products, is less biodegradable: in reality, the exact opposite occurs. Black waters contain organic substances that have undergone one of the most efficient degradation processes in nature: the one that takes place in our gastro-intestinal system. The organic substance that remains in the faeces after digestion is largely composed of cellulosic matter (the famous “fibres” that dieticians invite us to eat precisely to promote physiological functions), or the long organic molecule of which paper and wood are made, substances which – although of natural origin – we all know degrade very slowly at room temperature. Furthermore, most of the nitrogen is found in sewage – particularly in urine – which requires a long time and large quantities of oxygen to be eliminated. Some Swedish scholars have studied the biological degradation process of the organic substance contained in grey and black waters: in a five-day process, only 40% of the organic substance present in black waters undergoes complete mineralization, while in the case of greywaters it reaches in the same period a 90% reduction in organic matter. This rapid decay of the organic substance present in the greywater can be explained by the abundance of sugars, proteins and fats, easily available to the bacterial flora, characteristic of this type of waste water.

In conclusion, therefore, in our homes we produce about 60% of grey waste water, polluted by easily biodegradable substances, little contaminated by pathogenic bacteria and viruses – and therefore whose management does not involve particular health risks; the remaining 40% is black water, the treatment of which is more complex, both from a biochemical and microbiological point of view.

Currently in homes greywater and black water, which have such different characteristics, are normally mixed and discharged in the sewer. We can say that today we do with wastewater what we did with solid waste until a few decades ago. We still don’t understand the opportunities that arise from keeping our drains separated: greywater, in fact, can be used to cover a large part of domestic needs.

Rate of degradation of organic matter in grey and black waters