Water Purification Using Plants:

Wetland or Lagooning Methods  

Water purification techniques using plants are generally favoured by environmentalists. Unfortunately, the bucolic view of plants as THE water management solution hides a darker reality.

Humus and Dejecta

At the base of all terrestrial life, we find humus. It is a brown organic substance of great complexity whose presence in the soil makes all the difference between fertile land and desert. Humus formation requires the combined simultaneous presence of nitrogen-rich animal biomass (raw animal manure, urine, human dejecta) and nitrogen-poor plant biomass (cellulose, lignin).

Animal and human excreta are not waste that must be eliminated. They are part of the ecosystems that produce our food. As food comes from the earth, our excreta must inevitably return to the earth in form of stabilized humus to complete the natural cycle loop. This process does not occur when animal or human excreta are discharged in water: such shameful wastage becomes absolute, and irreversible. All excreta (animal or human) discharged in water remove precious organic nitrogenous matter from humus formation and produce, in fine, water pollution by nitrates.

Water treatment / purification, even when using plants, destroys and even deconstructs nitrogenous organic matter contained in dejecta. The attempt to have plants absorb the inorganic nitrogen resulting form this deconstruction short-circuits an annual cycle in the forming of humus, and eliminates the essential animal component in the composting process.

For further reading on the importance of humus, go to text on Ecological Sanitation page.

The Purification Efficiency of Plants

Fundamentally, in nature, aquatic and other plants do play an important role in the purification and filtering of surface waters. More precisely, the bacteria that attach to or live in symbiosis with plants’ root systems are the ones that do most of the work. Plants absorb nitrates and phosphates from water. They can fix a whole series of pollutants, and even some heavy metals. Purification using plants is therefore a reproduction of a natural process, whereas it is used to treat our wastewater.

When considering domestic waste water management from the viewpoint of an «environment-friendly» sanitation engineer, purification using plants appears like a panacea that eludes the problems of conventional sanitation systems. However, this (incorrect) view is based on the same principles that guide conventional treatment systems, just as they do for systems using plants: purify as best as can be done, without other considerations or concerns for the environment. In both cases, the initial criteria are the same: to assess the quantitative pollutant load with respect to a notion of inhabitant-equivalent [1], and to target purification efficiency.

When a purification system with plants is used (just as with conventional treatment systems), pollution reduction at the source is left to users’ discretion and good will.

The Disadvantages of Treatment Using Plants

Purification using plants is, in a sense, one and the same as conventional water treatment, especially for an observer who prioritizes minimum environmental impact over maximum purification efficiency. The main difference: conventional water treatment generates less water loss by evaporation.

For my part, I believe the major disadvantage of these systems is that they set aside concerns about preventing pollution at its source. When black water is removed from the equation (i.e. not produced), the notion of inhabitant-equivalent [2] loses all meaning. Pollution prevention is a predominant objective in reducing environmental impacts. A purification system using plants provides a clear conscience to those who don’t want to change or question their bad habits. In fact, plant systems are always designed to treat mixed wastewater (black and grey). Now, when a flushable WC is NOT used, purification using plants is no longer necessary; at this point, the required set-up is really nothing compared to what a full-fledged lagooning treatment system would represent.

Many environmentalists who live in suburbs regret not having enough space to install a wetland purification or lagooning system. In reality, the simple suppression of flushable WC’s from their home would open up the possibility of implementing selective grey water treatment that even a small yard could accommodate.

A lagooning system can give the illusion that plants take charge of the water polluted by excreta, and in so doing, re-establish the nitrogen cycle. This assertion, while partly founded, is overall incorrect. The reality is much more complex.

As already mentioned, when dejecta are discharged in water, the humus forming process is stopped, irreversibly. In water, there is no plant cellulose carbon to stop the enzymatic mineralization process of organic nitrogen. This is particularly true of urine which represents up to 80% of our excreta’s nitrogen content. This explains, for example, the extremely pollutant nature of liquid pig manure as agricultural fertilizer. The transformation of urea into ammonium ions removes the nitrogen from humus formation. Ammonium and nitrate ions are very mobile and are largely NOT absorbed by plants. If plants were so efficient in water treatment systems, spreading pig manure on the land would not generate so much pollution. Such overrating of the plant purification process comes from the fact that the same «purification efficiency» criteria are applied, just as in conventional treatment, without regards to other environmental impact.

Finally, the reinsertion of our dejecta's nitrogen and phosphorus into nature’s cycles - by way of plant purification - involves much loss for the biosphere, as it requires an additional solar cycle (i.e. an extra year) for their actual reinsertion into the environment. When compared to direct and efficient composting of dry toilet effluent, purification using plants is shameful environmental wastage [3].

Environmental Impact

Treated water by way of plant purification is essentially of the same quality as that which comes out of a conventional mechanical system (with equipment for nitrogen and phosphorus removal). They still contain too much nitrates and phosphates to be harmlessly discharged in a naturally clean river [4]. 

Plants present another advantage, with regards to the sludge produced. There is less of it, and it is of a better quality than that of conventional mechanical systems, due to part of the pollution being absorbed by the plants.

Water loss due to evaporation and infiltration is another factor. This is especially important in hotter and drier climate zones. In North Africa and southern Europe for example, such water loss can attain 60 and even 80% of the initial water entering a system. That is unfortunate, in areas where agricultural yields are directly proportional to the available water for irrigation. Regrettably, as soon as wastewater contains black water, irrigation of food crops becomes problematical, even after proper treatment. The sanitation risks remain.

Persistence in using plants for water purification instead of adopting the use of proper dry toilets is all the more regrettable whereby eliminating WC’s, we can effectively eliminate all sanitation risk (fecal content conveyed in water), reduce water needs by 25 to 35%, and reclaim all grey water produced by the household for agricultural / gardening purposes. In addition, soil improvement obtained from direct composting of our excreta greatly increases the water holding capacities of land, thus reducing the need for irrigation, chemical fertilizers and pesticides.

In this context, one can only be astounded to find lagooning systems in the middle of the desert, where above-mentioned water loss attains 80 %. Even their promoters acknowledge that the residual 20% of treated water is not altogether safe for agricultural reuse, due to the presence of intestinal parasites. Against all common sense, these systems are implemented in Africa (Sahara) by internationally renowned specialists.

If we were to evaluate direct and indirect impacts of a generalized use of WC’s, we would conclude that the flush-tnak-toilet is an invention that has outlived its usefulness. In the march towards a sustainable world, the WC should no longer exist. Research must now focus on developing socially acceptable dry toilets.

Remember that the use of plants to purify and treat water can only be justified when flushable WC’s are used.

The area occupied by a lagooning or wetland system [5] is enormous when compared to selective grey water treatment and even to conventional sewage treatment. Its installation costs and maintenance and operations costs are equally important. A set-up for a family of 5 will require about 100 m² of terrain with a very specific topography. On the other hand, the same family which adopts selective grey water treatment and a proper dry toilet will need a compost area of about 5 m², a 3 m³ anaerobic batch reactor (a septic tank) and an underground dispersal drain or trench of a few meters length. The installation costs are easily 10 times less than a wetland and the pollution is 50 to 100 times less.

Maintenance of a lagooning treatment system requires annual cutting back of plants, their subsequent composting, regular removal and elimination of sludge deposit in the basins, plant replacement after 5 to 10 years. When replacing these (such as macrophytes), the system ceases to operate for many months, until the plants are mature.

In contrast, maintenance of the grey water treatment system is nil: you put it in place, and put it out of your mind. The main task is to regularly carry dry toilet effluent to the yard and add it to the compost bin. This work is minimal for those who already compost their garden wastes.

For further reading, go to page on Conventional sanitation: a Major Environmental Hazard?

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[1] An inhabitant-equivalent (or IH), defined here as the amount of substance released by one person, is legally defined (in Belgium) as representing 60 g/day of BOD_5[ (biochemical oxygen demand) contained in 180 litres of wastewater. This value is the mean value measured in wastewater containing black water (dejecta) and grey water (soapy).

[2] As defined by law.

[3] One can easily understand the scale of animal (i.e. human) nitrogen loss for the planet. A lagooning system produces plants that, after waterweed cutting, but before composting, will have a very high carbon/nitrogen ratio. Without animal-based nitrogen, this carbon matter will «burn» during composting, producing large quantities of heat and CO₂. Human excreta mixed-in with plant based litter is composted directly, and in so doing, transforms animal-based nitrogen directly into humic acid.

[4] A good lagooning system still discharges about 10 milligrams of nitrogen (nitric N) per litre into the receiving milieu. This is obviously little compared to the organic nitrogen that initially entered the system: the purification efficiency of the system is therefore good. Unfortunately, the discharge of even such a minimal amount of nitrogen in a river that has not suffered any domestic pollution (e.g. mountain streams in sensitive natural areas) generates an eutrophication process that can ultimately asphyxiate the stream. In such sensitive areas, a combined use of BLT’s (biolitter toilets) and selective grey water treatment is the only way of efficiently protecting the waterway. Therefore, purification of black water using plants is an illusion.

[5] The wetland system referred to in this chapter is quite unlike the constructed wetland specified on the TRAISELECT page. Here, we are talking about a lagooning system, or wastewater treatment system, whereas the complete TRAISELECT system’s wetland is a grey water filtering system, the finishing-off stage, where daylight is the primary bio-filtration catalyst).