ARTICLE

WETPARKS - not a new way to purify water

By Folke Günther, Holon ecosystems consultant, Sweden

http://www.holon.se/folke

Introduction

The water purification capacity of the hyporheic* zone is well known to ecological engineers. It lies behind the construction of wetlands, reed beds and the like for water purification. However, there are some drawbacks:

1. In cold climates, it can be maintained that the effect of the wetland will be diminished during the winter, although the production of wastewater will continue.
2. The N/P ration in ordinary wastewater is rather high, around 7. This ratio is well suited for plant uptake. However, during the passage of the water through the wetland, nitrogen will be lost to the air, but phosphorus will stay. This will lower the N/P ratio, eventually to levels where it is hard or impossible for the plants to take up phosphorus. This might lead to a low phosphorus uptake in the system

To address these problems a biological water purification system, the wetpark was developed (Günther, 2000). Some of the main properties of these are:

1. The turnover of water in the system is about one year. Thus, during the passage of the system, all the water will spend at least one summer in it.
2. The water that is let into the system is as low polluted as possible, i.e. grey water. Urine and faeces are collected by means of source separating toilets. This means that the phosphorus level is initially low.
3. To avoid the problem with a low N/P ratio (which is typical for grey water), nitrogen-fixating plants (e.g. Alnus, Hipphophae) are introduced in the shore-zones. By their extraction of nitrogen from the air, the can maintain a higher N/P ratio in their root zone, and thus take up phosphorus.

The name 'wetpark' is chosen for the ornamental qualities (wet-park) of this constructed wetland to be used in densely built-up areas.. At the same time, they increase the biodiversity of the area by creating new habitats for plants and animals.

Figure 1: The water flows very slowly from left to right. The turnover time is about one year. The total water volume of the wetpark is equal to the annual water flow. By that, the water entering the park will always spend a summer in the system, regardless of the time it is inserted.

The purification capacity of the shore zones in the pond system depends on the chain:

1. Microbial and plant activity in the hyporheic water flow
2. Plant uptake of the decomposed material
3. Continuous biomass removal. by harvesting plant material (and fishing crayfish or fish).

Using some sort of back pumping from the end of the park to the beginning, the actual size of the plant can be diminished in relation to the pumping capacity. This method is used in the smaller Rönås system ( http://www.holon.se/folke/projects/vatpark/ronas/ronas.shtml), as well as in the Gudmundsparken storm water purification plant (http://www.holon.se/folke/projects/Eng/gudmeng.shtml).

In order to contain the water in the system, the bottom of the wetpark is sealed. This can be done by the use of puddled clay, and/or bentonite mats and/or an EPDM rubber mat, depending on available time and money in the construction. The sealing must be done very carefully, since a leaking wetpark will lose a lot of its capacity of water recycling.

In order to maintain a large and continuous flow of water in the wetpark, and to make up for evaporation during hot spells, rainwater is collected from roofs and put into the system. Normally, this constitutes an excess of purified water. This give rarely rise to any problems. In the Kalmar wetpark (http://www.holon.se/folke/projects/vatpark/Eng/wetpark.shtml), the calculated excess is about 700 m³ a year, In Rönås it is estimated to about 75 m³ per year. The purified water thus produced can be 'given away' to anybody needing it.

Figure 2: A smaller wetpark, adapted for installation in an ordinary garden, The plants along the paths absorbs nutrients as well as the plants in the shore-zones of the pond. The water is slowly pumped back to the beginning of the system (with a very simple pump) to improve purification capacity.

With source-separating toilets and local purification of grey water, the household will be freed of the need for external sewage system and water services. Urine and faeces is used as a nutrient source for the local agriculture, and the water can be recycled.

N.B! The use of source separating toilets may facilitate a circulation of nutrients, but it should be noted that this doesn't constitute a circulation of nutrients unless the people involved are mainly fed from the products of the farmland. In that case, the nutrients put on farmland could be converted into food and recycled. See Günther, 2002

Figure 3: Situation plan. The arrows indicate water and nutrient flow.

Figure 4: The construction of the wetpark at Rönås. Water is let in from the tank (Fig. 5), passes through the system and is pumped back with a wind pump (not yet installed)

The water flows from the inlet to the right, through the first, second and third shore zone until it is taken out in the reception well. It is continually pumped back to the inlet with a small wind pump. By this, the water will recycle through the system several times, thus increasing the system's capacity to take up nutrients. The inlet tank (Figure 5) is constructed so the purified water can be taken back to the house(s) for domestic (graywater) use.

Figure 5: The tank at the beginning of the system collects the purified water for use in the house. The overflow mixes with the primary grey water for another passage through the system.

Effects on the ecosystem

The extra vegetation together with the water mirrors will make a new refuge for wildlife animals, insects and plants. They will all participate in the water purification in some way.

By this, the fulfilment of a human need (clean water) will also enhance the ecosystem, a requirement for a sustainable relationship with the rest of the nature.

This plant was constructed with a large contribution of the owner's own work. The cost for construction therefore amounted to less than SEK 25,000 ($ 3200 or EUR 2800). The price of the source-separating toilet system is less than SEK 15,000, so you are still well below the connection fee to the grid, which in Sweden is on the average SEK 60,000

• Günther, F. 2000. Waste water treatment by source separation. Ecological Engineering Vol 15 Iss 1-2, p.139-141
• Günther, F. 2002 Ruralisation - Integrating Settlements and Agriculture to Provide Sustainability. Proceedings from the NJF-seminar No. 327 Urban Areas - Rural Areas and Recycling - The organic way forward? Copenhagen, Denmark 20-21 August 2001.Darcof Report no. 3, Copenhagen August 2001

* The hyporheic zone : Hyporheic zone is the area under a stream channel or floodplain that contributes water to the stream. Hyporheic flow, also called interstitial flow, is water/subsurface flow between the water table and surface water flow. The source of hyporheic flow can be from the channel itself.

The water flow in this zone is rather large. The water volume in the hyporheic zone can even be as large as the stream itself. In this zone, a lot of the organic material is consumed and the nutrients are converted to inorganic ions as NO₃^-1 or PO₄^-3 . Plants that extend their roots into the hyporheic zone thus have an excellent source of nutrients.

Thus, the hyporheic zone is important to the removal of nutrients from the water body. The method of using biological groundwater activities to purify water is definitively not a new one. It is the 'normal' way for water to be sufficiently clean to use. The difference in this case is that the system is consciously created and maintained by humans.