biologically based grey water purification

Proceedings from the
UNESCO symposium on Integrated Water Management in Urban Areas
Lund, 26-30 september 1995

Folke Günther
Department of Systems Ecology, Stockholm University, 106 91 Stockholm, Sweden

Key words:
biological purification, grey water, non-mixing system, nutrient use, phosphorus, regenerative cycle, water recycling.

The importance of recycling nutrients is discussed, both regarding the structure of natural systems and ecological adaptation. The conventional waste water management system is unable to purify the sewage water to a higher grade than the nutrient content of the grey water. Biological plants are not well adapted to the purification of a mixed sewage, but if source separating toilets are used, the urine and faeces could be used for agriculture, and the grey water could be efficiently purified with biological methods to a grade that it can be re-used in the settlement. An outline for a purification plant based on riparian ecotones and buffer ponds is provided. A plant of this type was built in Kalmar, Sweden in 1996.


Why recycle nutrients and water?

In the introduction, I will discuss why the lack of circulation of nutrients ultimately lead to detrimental effects. In ecosystem a lack of circulation leads to a cessation of its capacity to utilise solar exergy and therefore to support life. In urban settlements lack of circulation leads to an inevitable accumulation in the suburban area and subsequently to a non-point leakage from this area. Following that, I will discuss the currently common way of transport human waste products with water.

The demographic changes in the society during industrialisation have transformed the nutrient management system from a fairly circulating one to a mainly linear [5, 6, 7 ]. Phosphorus is an important nutrient in both animal and plant physiology. It is probably the element that is easiest to become limiting to living organisms. It is needed in fairly large amounts, in vertebrates as a structural material, and in all organisms as a vital element in the genetic material and in the intracellular energy transport system, yet it is fairly rare in the total biogeosphere compared to its abundance in biomass. It is also considered an important pollutant in coastal and inland waters [11]. The element is therefore a good example of waste as a resource in the wrong place, and it can therefore be concluded that the phosphorus management typical for a large part of the world is far from optimal.

The management of nutrients in ecosystems

In the ecosystem, crucial materials (e.g., water, nutrients, mineral salts) are reloaded with exergy by the green cells.. The combined action of the system's compartments (e.g., cells, individuals, populations etc.) enhances the system's capacity to absorb exergy, convert it to low exergy energy and improve its own structure, which in turn increases its capacity to improve its regenerative cycle [9, 7]. Tapping off the elements necessary for re-loading the system with exergy, will make the system unstable and eventually cease to exist.

The reconstruction takes place in the green plants receiving high exergy radiation from the sun. Presence of animals and other consumers in the system facilitates efficient exergy degradation. The organisms carry the genetic code providing information of the most efficient way to use exergy and to convert it into low exergy energy products. Such products are for example carbon dioxide, water and for the organism itself unusable products as faecal matter and urine containing nitrogen and phosphorus. In a terrestrial ecosystem nitrogen is seldom limiting, because it is conveyed by the air. Neither is carbon, and in many cases not even water. Phosphorus, however, is often limiting, since it has no (non-poisonous) gaseous phases. Therefore, regenerative cycling of phosphorus is common in an ecosystem. In an advanced ecosystem, as a tropical rain forest, the regenerative cycling of phosphorus is very efficient (e.g., [17]). As phosphorus is limiting the general exergy degrading activity of the system, this promotes self-organisation toward effective regenerative cycling of phosphorus. The components of the system would rather be called consumers, recyclers and reconstructors than the common denomination consumers, decomposers and producers, since it is a cyclic, not a linear system, which easily is inferred from the later set of terms. In this case, solar radiation, carbon dioxide and water are the substances connecting the ecosystem to the supersystem, the ecosphere.

To exist as a sustainable part of an ecosystem, it is therefore necessary to develop settlements that improve the retention of nutrients, especially phosphorus, and water, since these are often limiting substances for life. This is far from the way settlements, especially those in urban areas, are organised today. A large part of the responsibility for this is the attitude to waste water management discussed below.

The HEAP trap (Hampered Effluent Accumulation Process)

A consequence of urbanisation is that food production areas and residence areas are separated. An effect of this is that plant nutrients are exported with food from agricultural areas to urban areas, which lead to a lack of nutrients in the agriculture. Nutrients that leave a large part of the agriculture never come back but remain in the urban areas or are let out into lakes and seas. The important structural shift is that the flow of nutrients changed from being fairly circular and local to linear and global.

After digestion, food becomes a waste product. In settlement closely associated with agriculture, the waste products are re-used as plant nutrients. In urban areas, there is a large disproportion between the amount of inhabitants and the availability of agricultural areas. Thus, the human waste, i.e., urine and faeces, is considered a nuisance and disposed of. Water is often used as a transportation media in an increasingly elaborated piping system. It was soon recognised that the waste products was polluting when let out into recipient waters (e.g., [21]). This was not only due to its content of oxygen demanding compounds, but to a major extent to its content of plant nutrients that caused a secondary pollution by the growth of algae.

To counteract this point source pollution, advanced waste water treatment with phosphorus reduction was introduced. In a majority of the plants the phosphorus content of the water was reduced with about 92% [19]. The phosphorus in the waste water is converted to sludge.

However, phosphorus itself is not degradable since it is an element. Nor has it significant gaseous phases, as nitrogen. Therefore, it is accumulated in the area limited by the sludge transport facilities used. This area seems to be about 1/100 of the food production area [8]. If this accumulation process is continued, the leakage from the area will tend to increase. This increase will continue until a HEAP steady state is formed (Figure 1). When this is established, the former point-source leakage is converted to a non-point source leakage from the entire area.
The HEAP trap: An accumulation process caused by hampered effluent (HEAP) will ultimately give rise to an effluent corresponding to the import. The only way to check this process is by impeding the import.

Thus, by the import and accumulation of nutrients into a small end-user area, the normal circulation is broken and pollution is inevitable, especially if it is postponed by improved waste water treatment. Furthermore, there is a threat to the resources of raw material for fertilising, the life-time of which may be as low as a century (discussed further in [8]). In order to counteract this problem, it is necessary to re-establish a thorough circulation of nutrients, especially phosphorus as it is a rare element compared to its abundance in biological material.

The MIx-First-and-Separate-LAter (MIFSLA) attitude to waste water management

Besides the extensive funnelling, a major problem included in the water management system common to most developed countries of today, is that clean water is used as a transportation media for human waste. This heavily polluted water is mixed with the less polluted, 'grey' water from washing, dishing and bathing. The mixture is let into the sewage system. Unfortunately, into this system, there is an extensive uptake of drainage water, of about the same size as the amount of polluted water from the households (Table 1).

To avoid the pollution problems associated with the expel of human waste into the water bodies used as recipients, the mixture of waste water and drainage water is purified in the waste water plants. Extensive purification regarding BOD and phosphorus was introduced the last three decades, and, in Sweden, also large investments are directed to the reduction of nitrogen compounds in the expelled waste water.

However, as can be seen in Table 1, the composition of the grey water is almost equivalent to the water let out from the sewage plants. In the 'purified' water the content of nitrogen compounds is considerably higher, even after extended purification. Its content of phosphorus may be lower, but the figure in Table 1 is calculated from the total use of phosphorus in detergents in Sweden divided by the number of inhabitants.The figure will be about 0.05 kg/p*yr if the users conscientiously use phosphorus free detergents [18].

Thus, the MIFSLA attitude have lead to a waste water management system that works in the folloowing way:

The conclusion is that the actual function of the waste water plants is to remove urine and faeces from the grey water. The question emerges if there does not exist cheaper and more efficient methods to do this.

Table 1. The three different phases mixed in the waste water from one person living in Sweden compared with water obtained from the purification of waste water in an standard sewage plant. Note that the volume of water is doubled during the transport from the house to the sewage plant due to leakage of drainage water into the pipes and from other sources.

Per person:


Grey water
Sewage water after purification
Volume, l/p*yr
Nutrients, Kg/ year
N: 4.11

P: 0.59
N: 0.74

P: 0.11
N: 0.44

P: 0.14
N: 3.96

P: 0.06
N-conc. (mg/l)
P-conc. (mg/l)
N/P rate
High risk for pathogens. High dry mass
concentration of pollutants
Risk for viruses and nitrogen pollution

More details about this table.

Why wetlands doesn't do the job in a MIFSLA system

The wetland method for waste water management was introduced during the eighties in Denmark, Germany and also in Sweden. According to the MIFSLA philosophy, waste water containing both grey water, urine and faeces is let into the root zone of different wetland plants [3]. The financial investments in this method is rather low, and the purification effect is high, both in bacterial, nitrogen and phosphorus reduction, at least initially. In this method nitrogen is either denitrified or incorporated in biomass and phosphorus is either accumulated in soil particles or incorporated in biomass. The N/P ratio of the water penetrating the system is therefore continuously changing. If this ratio is below 10, there is an increasing risk for a lower degree of phosphorus reduction (For the elements to be fully incorporated in plant biomass, the N/P ratio should be about 10). especially when the system is growing older and the soil is becoming saturated why excess phosphorus can not adhere to soil particles. This is probably the reason why the rate of phosphorus reduction in such systems is lower than should be expected (Figure 2).


Figure 2. A tentative model to explain the relatively low phosphorus reduction rate in conventional wetland systems applied for purification of mixed waste water. When the nitrogen is used up, reduction of phosphorus can only take place by adsorption to soil particles, a process of diminishing importance during saturation of possible sites for adsorption.

Another way of doing it: Source separation of waste water

In the following I will discuss the effect of the introduction of source separating (or rather: non-mixing) toilets in buildings. Toilets of that type are available under different trade marks in Sweden. They all transfer urine, with about 80% of the nutrients, to a tank for subsequent transport to agricultural land. This phase is easy to re-use as plant nutrient solution in agriculture.

Regarding the treatment of faeces, they can be classified into two different types. One type collects the faeces in a composting chamber for the elimination of pathogens during a six month composting process. The other type use water for the transport of faeces to a tank or a subsequent separation of water and faecal matter. The latter type is in a part a MIFSLA system, and the problem with the contaminated water establish themselves immediately.

The main advantage of the source separating toilets is however, unregarding the type, that the main bulk of the waste water, the grey water, remains uncontaminated. As can be seen in Table 1, the grey water is of a quality equivalent to the water expelled from a waste water plant after purification, i.e., by the standards regarded as clean. This phase can be handled in a much simple way than the MIFSLA waste water.

Biological treatment of grey water

The main problem with the grey water is its large bulk, in Sweden 70,000 - 80,000 litres per person per year. A large advantage is that its nutrient content is comparable to waters that by different standards are regarded as 'clean' (Table 2).

Table 2. A comparison of nutrient content in different types of water.

Nutrient (mg/l)
US major rivers [1]
Cropland runoff [1]
Average grey water content in Sweden (Table 1)
Discharged water from sewage plant (Table 1)
0.09 (as nitrate)
27 (15)
N/P rate
67.5 (37.5)

x After the increased nitrogen purification in the making in Sweden

The N/P ratio of the grey water is below 10, which means that its uptake in plant biomass is nitrogen limited. This problem may be eliminated by the introduction of nitrogen fixing plants, as alder (Alnus) in a biological grey water purification plant. Nitrogen fixing plants normally invade places where such water occur, as lake or river shores and places with cropland runoff.

Two ways to avoid the winter

To avoid the problems supposed to occur during wintertime, i.e., low biological activity and decreased water conductivity, there are two main routes to chose. One is to place the entire system beneath a glass roof and heat it in wintertime, which may lead to very high construction and maintenance costs. The other method is to accept the occurrence of winters and prolong the turnover time for water in the system to a year. By that, even water entering the system in mid-autumn will spend a summer in the system. A decreased conductivity of the system due to frost may be counteracted by a larger depth of the beds.

However, a interrupted biological activity of the plant roots due to winter frosts seem hardly to be the case. Mander & al. [13] compared the nutrient reduction capacity of vegetated bioponds during the vegetation period and in winter. They found that the reduction capacity only was reduced with 88-65% during the winter period for phosphorus, nitrogen and BOD5.

A system outline of a triplicate soil layer infiltration-wetland-pond system for grey water purification

General principle

To reduce incoming bacteria, BOD5 and nutrients as much as possible, the processes occurring when ground water flows under the root zone of plants on a lake or a stream shore are imitated and encouraged as much as possible. (Figure 3). Before the introduction to the shore, the water passes a section filled with lime-gravel in order to increase the surface for aerobic bacteria ant to buffer pH.


Figure 3. The root-zone processes of shore plants are utilised and encouraged in the grey water purification system.

In order to keep the water in the system even on grounds with large infiltration capacity, a waterproof secluding layer is placed under the purification plant. For longevity bentonite mats are chosen. This material is used under roads built over water supplying areas and under waste deposition plants. It has a very low water conductivity.

After the shore zone, a pond is placed that collects the purified water. In order to increase the purification capacity of the system, another shore-zone is fed with the water from the pond. This water is collected in a new pond, where the process is still repeated another time. The total system thus consists of three subsequent shore-ponds. After the last pond, the water is let into a sand filter system and is collected in a well. (Figure 4)


Figure 4. An outline of a triplicate shore-pond system for grey water purification

Plant composition

The plants that are chosen in the system are those prevailing in a normal lake shore in the region. Plants with nitrogen fixing root nodules, as Alnus, are introduced because of their capacity to extract phosphorus from solutions with a low N/P ratio. Otherwise, Salix species and the Aegopodium podagraria - Filipendula ulmaria - Cirsium oleraceum plant community are chosen because of their capacity to extract nutrients and accept wetland conditions. In the area close to the ponds, plants as Phragmites communis and Typha latifolia are chosen because of their capacity to transport air in their aerenchyme down to their roots, and by that create conditions favourable for denitrification, should it be needed.

The plants are supposed to be continuously harvested and composted, in order to remove nutrients from the system. In the ponds, also some fishes and cray-fishes are introduced, in order to control insect larvae and digest leaf litter falling into the dams.


Practical results for system efficiency are yet to be measured in a R&D programme for the plant planned in Gävle (se below), but calculations assuming winter efficiency of the system indicate that the residual nutrient content of the water would be about 0.06 mg nitrogen /l and 0.02 mg phosphorus /l. Due to the long turnover time, the reduction of bacteria and viruses emitted with the grey water would be complete.

Planned projects

In Engeshöjden, Gävle (200 km N Stockholm), building start is planned to Jan. 1996 for a group of 22 houses with source separating toilets of the 'dry' type. The grey water from the houses, about 12 m3 a day, is planned to be purified in a system of the above described type. This water is to be re-used in the houses, both as grey water and as drinking water. Rain water from half of the house roofs is collected and combined with the grey water from the houses in order to make up for losses from the system (urine, garden plant watering, car-wash). The extra water is calculated to exceed the losses, why the house group may be a net exporter of clean water. The area requirements for the plant is less than 3,000 m2, or about 40 m2 per person connected. It will serve 66 persons.

Construction cost

The construction cost for the grey water purification system (the name of which is proposed: Wetpark) is about 5,000 SEK (about 700 US$) per person, including buffer tanks and pumps. To this come the maintenance cost, calculated to 300 SEK per person per year. This cost includes also the maintenance of the source separating toilets, i.e., the emptying of urine tanks and a trimonthly change and composting of faeces containers. The cost of conventional systems in Sweden is about 60,000 SEK for the connection to the municipal system and about 1,500 SEK/person annually.


The easiest way to achieve clean water is avoiding to pollute it. The abandoning of the MIFSLA attitude to water management leads to a much simpler method. (Figure 5).


Figure 5. The main idea of a non-MIFSLA system is to process the different phases of waste according to their individual qualities.

The method is not only cheaper than the conventional, it has also recreational and psychological values. If you see the ponds where your waste water is treated, and if you know that you will have it back in your tub after one year, you will be very careful about it!

Apart from the aesthetical values of the system, it will also provide habitats for birds and increase the diversity of the plant communities in the area.

It can be said that this is not a method for dense urban settlements, because of its area requirements (about 40 m2 per person). However, land use is a matter of preferences. Forty square meters is about the same area as three parking lots.


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Updated 1996-02-14 for the WWT-conference
Folke Günther/Jan Johanson

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