It can be considered with some caution.

It should not be used for activities where direct human touch is involved; irrigation of edibles, etc. in garden; gardening activities if levels of soaps, pharmaceuticals and personal care products concentrations are very high.

I would be cautious for this option if activities are directly related to humans!

Greywater, if kept for long, turns worse - comparable to blackwater. if treated, it can be stored according to level of treatment and can be used for irrigation. some nutrients can be recycled in the garden soil (subsurface) for indirect use. should also be mindful of what goes into the greywater stream, as there may not be foolproof checking possible at home.

Greywater can be treated reasonably easy with biological treatment followed by UV disinfection and then used for toilet flushing.

It fits nicely in average households in the western world that slightly more grey water is produced than is used for toilet flushing. Such a system can save about 30 % of the water consumption. In apartment blocks there the treatment system is shared it can be quite economical then the water price is as high as it is in Denmark.

We studied personal care products in such a system some years ago.

Article Estrogenic personal care products in a greywater reuse system

Henrick,

Accordingly Bioremediation treatment is economical viable however UV disinfection requires more cost of treatment.

In curing fresh water will be used, this is also the problem that fresh water will be wasted in the curing. So for this curing purpose in construction work we can utilize the grey water

More research needs for biodegradable personal care products by which use of grey water may useful.

Grey water can be correlated with carbon credit because 55% of water use for bathing and washing. This water can be use for flushing after some treatment. So we can reduce 30% of fresh water consumption for flushing.

In arid areas or areas with very seasonal rains it may be valuable to consider greywater re-use  but in temperate areas with good rainfall I think it is a waste of resources to put efforts into greywater when so much rainwater is still available, not utilised, and allowed to become a problem when urban  catchments are increasingly made impervious. As for the quality issues of greywater they are certainly not insurmountable but many of the potential problems highlighted by others need to be addressed with care.

All that the other speakers said is very true. Greywater can be adopted for garden , thus reducing mains water use for irrigation. Careful attention is required of the storage time (in Australia untreated greywater is recommended to be used within 24h  due to pathogen proliferation). In addition, households that reuse greywater are better off adopting detergents and other products that have low salts to reduce the impact of greywater on plants and soil salininization.  Regarding internal reuse of greywater, typically treatment and disinfection is recommended and then use for toilet flushing. This then provide you with a non-seasonal saving of mains water. However viability and cost effectiveness of adopting greywater over other sources for water conservation will be also determined by your availability of other water sources, the cost of greywater treatment (some on-site treatment technologies can be high energy intensive).  Check out the work of Robert Patterson, from Lanfax, Australia on greywater (just google it).    

Prof. Newman,

We have enough rain in Denmark that a one family house get enough rainwater on the roof to match the use for toilet flushing by a typical family.

For block of flats the roof surface is less per person so rain contributes relative little. In those cases grey water reuse for toilet flushing can be environmentally and economical sound.

Additionally much of the treatment system costs are almost independent of size (controllers, pumps, UV-disinfection) from single household to a few 100 people so a system for a block of flats is relative inexpensive.

Professor Andersen makes a good point with respect to high rise developments. In hindsight my comment was a little more negative towards greywater re-use than it should have been.

Recycling and reuse of grey water is very phenomenal important in current scenario.

Article Biomass Production Through Grey Water Fertigation in Eucalyp...

Article Grey water pollutant loads in residential colony and its eco...

We have been looking into this at Victoria Park in Poland which is designed to be built as a Zero Net Energy Project. We own the mother project to BedZed's gasification unit and we have upgraded the technology with our Cold Plasma [GATO]. Since typically one days wet biomass consumption is around 5 tons we have more than 1 ton of water which after GATO is drinkable. Unfortunately we are not blessed with so much rain like Denmark but we are hoping the two together will suffice    

We've done some work in this regard that is on RG: Jacobs and Van Staden (2008), Direct on-site grey water reuse – an illicit or illustrious option? In South Africa people use grey water on home gardens without any treatment or disinfection. Plants love this, but a colleague of mine from the Medical faculty (Community Health) pointed out that, "You can never research an epidemic". Be careful. Also look at the paper in WaterSA journal, "The use and disposal of greywater in the non-sewered areas of South Africa: Part 1 – Quantifying the greywater generated and assessing its quality" by Carden, Armitage, Winter, Sichone, Rivett1 and Kahonde; and also Part2. In the ISI-listed WaterSA Journal you will also find useful publications by N Rodda in reply to your questions.

Yes of course the recycling and reuse of grey water is very phenomenal important in current scenario. You may see the following papers in this regards.

Article Biomass Production Through Grey Water Fertigation in Eucalyp...

Article Grey water pollutant loads in residential colony and its eco...