thumb|310px|Muret spildevandskanal i Prag (nedgravet omkring år [[1900).]]
'''Spildevand''' er en betegnelse for brugt brugsvand fra husholdninger, industri og landbrug, når det udledes enten til organisk stof og andre faste partikler]] alle slags vandopløselige stoffer.

* Husholdningsspildevandet indeholder især organisk stof og næringsstofferne kvælstof (i både ammonium- og nitratform), fosfor (i fosfatform) og andre mineralioner.
*Industrispildevandet indeholder små mængder organisk stof og opløste rester af råstoffer, hjælpestoffer og affaldsstoffer - herunder mest problematisk: tungmetaller.
*Endelig indeholder spildevand fra landbrug og gartnerier ofte store mængder organisk stof, som er blandet med plantenæringsstoffer og forskellige kemiske stoffer.

Langt det meste spildevand i Danmark bliver renset i rensningsanlæg, og nu om stunder bliver vandet renset både mekanisk (sining og bundfældning af faste partikler), biologisk (bakteriel omsætning af organisk stof samt næringssaltene nitrogen og fosfat) og kemisk (udfældning af fosfat og tungmetaller). Efter rensning skal vandet være så rent, at det kan udledes til recipienten uden at volde skade på miljøet. Til gengæld står renseværkerne tilbage med slam fra bundfældning og udfældning. Denne rest indeholder stoffer, som man ikke gerne ser spredt i naturen, så slammet går ofte til forbrænding, hvorefter det indgår i den askerest, der er forbrændingsanlæggets restprodukt.

<!-- The following is cut and pasted verbatim from [http://www.ozpolitic.com/gardening/water.html]-->
'''Greywater''', sometimes spelled '''graywater''', '''grey water''' or '''gray water''' and also known as '''sullage''', is non-industrial wastewater generated from domestic processes such as dish washing, laundry and bathing. Greywater comprises 50-80% of residential blackwater in the amount and composition of its chemical and biological contaminants (from chemicals). Greywater gets its name from its cloudy appearance and from its status as being neither fresh (white water from blackwater.

In recent years concerns over dwindling reserves of groundwater and overloaded or costly sewage treatment plants has generated much interest in the reuse or recycling of greywater, both domestically and for use in commercial irrigation. However, concerns over potential health and environmental risks means that many jurisdictions demand such intensive treatment systems for legal reuse of greywater that the commercial cost is higher than for fresh water. Despite these obstacles, greywater is often reused for irrigation, illegally or not, in older rural construction, simple construction old and new, often consisting of nothing more than a "drain out back" (pipe pointed down the nearest hill). In droughtzones or areas hit by hose pipe bans (irrigation restrictions) greywater can be harvested informally by manual bucketing. In the third world, reuse of greywater is often unregulated and is common. At present, the recycling of greywater is poorly understood compared with elimination.

==Elimination of greywater==

Domestic wastewater is usually combined at the treated to limit pollution and health risks, before being returned to the environment at large. The majority of greywater ends up as effluent in rivers and oceans in this way. Despite treatment, this arguably results in greater contamination of natural waters, as the natural purification capacity of surface water is millions of times less than that of soil. Simply dumping greywater on the soil, from an ecological standpoint, is less damaging than sending highly treated greywater directly into natural waters.

Advocates assert that there have been no documented cases of greywater-transmitted illness in the U.S.<ref>[http://www.oasisdesign.net/greywater/misinfo/index.htm Common Greywater Mistakes and Preferred Practices<!-- Bot generated title -->]</ref> They say this suggests that the reuse of greywater could represent a safe way to conserve natural water supplies and keep natural freshwater free of contamination, and shows that the elimination of greywater is not the most efficient way to deal with it. They say pouring greywater onto the soil is a better alternative to deal with it rather than pouring it down the drain because the soil acts as a natural filtration system. There are other alternatives to eliminating greywater that allow for efficient use; using it to irrigate plants is a common practice. The plants use the alleged contaminants of greywater, such as food particles, as nutrients in their growth.[http://www.oasisdesign.net/greywater/misinfo/index.htm] Treating greywater before using it to irrigate plants is like treating water then pouring it into the drain; it's a pointless practice. The elimination of greywater in sewage treatment plants is a low priority to many environmental conservationists because of its many possible uses and alternatives to elimination are highly recommended by online sources.<ref>[http://www.oasisdesign.net/greywater/misinfo/index.htm Oasisd]</ref>

==Recycling of greywater==
Most greywaters are much easier to treat and recycle than blackwaters, due to their lower levels of contamination. However, entirely untreated greywater is still considered to be a potential health and pollution hazard, because studies have established the presence of the same micro-organisms within greywater as found in sewage (albeit in much lower concentrations). Nevertheless, while all greywater will contain micro-organisms the health hazards associated with greywater from a multiple dwelling source should be considered different from that of a single dwelling greywater source. Within single dwellings inhabitants and their clothing are mutually exposed to each other's greywater and their shared living arrangements will likewise expose them to the existing reservoir of micro-organisms within the dwelling, whereas greywater from multiple dwelling sources provides scope for exposure to a broader reservoir of micro-organisms thus increasing the risk of disease spread between dwelling unit inhabitants.

If collected using a separate plumbing system to blackwater, domestic greywater can be recycled directly within the home, garden or agricultural company and used either immediately or processed and stored. Recycled greywater of this kind is never clean enough to drink, but a number of stages of filtration and microbial digestion can be used to provide water for washing or flushing toilets; relatively clean greywater may be applied directly from the sink to the garden or container field, as it receives high level treatment from soil and plant roots. Given that greywater may contain nutrients (e.g. from food, fertiliser, ...), pathogens (e.g. from your skin), and is often discharged warm, it is very important not to store it before using it for irrigation purposes, unless it is treated first.

===Greywater recycling systems===

At present, several water recycling systems exist which can be used to
* recycle the water without purifying it -or-
* recycle the water while purifying or decontaminating it
Water recycling without purification is used in certain agricultural companies (e.g. tree nurseries, paprika producing companies) and rainwater) is already fairly clean to begin with and/or has not been polluted with non-degradable chemicals such as soaps (thus using natural cleaning products instead). Water purification/decontamination systems then again are used for applications where potable water ''is'' required (e.g. to allow drinking, and/or for other domestic tasks as washing, showering). Such websites as [http://peswiki.com/index.php/Directory:Home_Generation:Gray_Water_Recycling Pure Energy Systems] and the [http://www.dmoz.org/Science/Environment/Water_Resources/Wastewater/Household_Wastewater_Management/ Open Directory Project] have indexed these commercial and diy-systems used for these applications. 

====Water recycling systems without purification====
=====Water diversion systems=====
The simplest greywater system is to simply divert the water directly to the garden. Regulations change by country and region, but common [http://www.epa.vic.gov.au/water/Reuse/reuse.asp guidelines] for safe usage include not storing the greywater for more than 24 hours, ensuring it cannot pool or run off, and depositing it with subsurface irrigation. Greywater diversion systems can be both designed-in to new homes, or retrofitted to many existing homes. When systems are fully designed, manufactured and installed to relevant standards such as the Australian [http://www.watermark.standards.org.au/ Watermark] standards. Water diversion systems tend to be highly efficient, effective and safe for simple applications where potable water is not required.

Diversion systems can be as basic as running the outlet hose from a washing machine out a window to the garden, or can be designed in as a permananet part of the home plumbing. Fully engineered systems like the [http://www.waterwisesystems.com Waterwise] system incoporate a sump pump and surge tanks and deliver the water through sub-surface irrigation.

Greywater from the shower or bath is generally great quality water for the garden. The soap levels at the dilutions typical are actually good for the garden as they are a wetting agent. When laundry greywater is diverted to the garden then the laundry products must be chosen carefully to ensure phosphate and salt levels are low, and the pH balance is neutral. Independent reviews like [http://www.lanfaxlabs.com.au Lanfax Laboratories] provide detailed information on laundry products. Basic guidelines are also available from system suppliers like this [http://www.waterwisesystems.com/themes/default/pdf/laundry_fact_sheet.pdf guideline] from [http://www.waterwisesystems.com Waterwise Systems]in Australia. It is essential that greywater is diverted to sewer when garden-unfriendly products are being used.

====Water recycling with purification====
For filtering the water to become potable (or near-potable), there are numerous systems based on "soft" processes. These include natural biological principles such as
* mechanical systems (sand filtration, lava filter systems and systems based on UV-radiation)
* biological systems (plant systems as  activated sludge systems, aerobic and anaerobic biofilters, submerged aerated filters, biorolls <ref>[http://www.oost-vlaanderen.be/docs/nl/gc/35447.%20afvalwaterzuivering%20op%20landbouwbedrijven.pdf Overview of biological systems with pictures]</ref> 

Finally, also used for creating potable (or near-potable) water are the "hard", direct processes, such as distillation (evaporation) which need not necessarily be as energy intensive as they might initially appear.  There seem to be no commercially available "hard" greywater recovery devices suitable for on-site use in the individual household, even though a number of such technologies exist. 

In order to purify the water adequately, several of these systems are usually combined to work as a whole. Combination of the systems is done in 2 to 3 stages, knowingly a primary and a secondary purification. Sometimes a tertiary purification is also added. 

Some municipal sewerage systems recycle a certain amount of grey and black waters using a high standard of treatment, thus providing reclaimed water for irrigation and other uses.

==Application of recycled greywater==
===Irrigation===

Greywater typically breaks down faster than blackwater and has much less nitrogen and phosphorus.  However, all greywater must be assumed to have some blackwater-type components, including pathogens of various sorts. Greywater should be applied below the soil surface where possible (e.g. in mulch filled trenches) and not sprayed, as there is a danger of inhaling the water as an aerosol. 

However, long term research on greywater use on soil has not yet been done and it is possible that there may be negative impacts on soil productivity. If you are concerned about this, avoid using laundry powders; these often contain high levels of salt as a bulking agent, and this has the same effect on your soil as a drought.

===Indoor reuse===