About Constructed Wetlands

Construction and method of operation

Constructed wetlands are biologically highly active wastewater treatment plants. They consist of plant beds with a sealed base, through which the mechanically cleansed and therefore solids-free wastewater is led.

There are several manuals regarding the dimensioning of constructed wetlands. Through years of experience with the construction and operation of constructed wetlands there are guidelines (e.g. ATV) available today which contain generally accepted rules as well as the state of the art.

In constructed wetlands the prepurified wastewater is led through a piece of ground stocked with tall water and swamp plants (reed, bulrush, mace etc.). The purification effect is based upon microbiological, physical-chemical and plantphysiological processes in the plant - ground system. A differentiation of the different methods results from

the ground composition and
the percolation and manner of infeed

Illustration No. 1: Horizontal filter

In horizontal filters the wastewater flows through the constructed wetland from one side to the other, in other words horizontally. The wastewater continually is led into into the constructed wetland from the prepurification container.

Vertical filters

Illustration No. 2: Vertical filter

In a vertical filter the wastewater seeps through the constructed wetland from the top to the bottom, in other words vertical. The intermittend infeed of prepurified wastewater into the constructed wetland is regulated several times per day by a pump.

The ground in a constructed wetland is made up of a fixed mixture of sand and gravel of various grainages.

The water is purified through disintegration processes with the soil particles during its percolation process through the ground. In this way the water has attained to the desired quality when it has reached the outlet.

Chemical researches of wastewater prove that vertical as well as horizontal filters with a ground composition of sand and gravel face less hydraulic problems than systems with a ground composition of silt and clay (e.g. rooted ground systems). In consequence constructed wetlands using a mixture of sand and gravel also achieve significantly superior flow off rates and purification results.

The nutritiant removal through the plants – usually some sort of reed – as well as the oxygen charge through the gas channels of the swamp plant′s roots is relatively insignificant. Nevertheless the plants improve the total system through their isolating function during the winter as well as the rootation (and resulting improvement of the hydraulics) of the ground.

Generally a prepurification container is installed in front of the actual constructed wetland, through which the water is mechanically cleansed. After a while the gathered up sludge has to be removed by suction and disposed of seperately.

Requirements for systems with enlargement figures exceeding 50 inhabitants are formulated in the ATV work paper A 262, in which case the purification pit can be replaced by other prepurification levels (e.g. a settling pond). The maximum economically reasonable enlargement figure for plant wastewater purification plants is limited by ist acreage demand and ranges roughly from 3000 - 5000 inhabitants.

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Purification ability of constructed wetlands

As in technical wastewater treatment plants the goal of purifying wastewater with a constructed wetland is first of all the removal of carbon compounds, nitrogen compounds and phosphor. The water is also supposed to be cleansed of pathogenic as well as non-pathogenic microorganisms.

It takes no special measures for constructed wetlands to eliminate a considerable portion of the infed nutrients and to even exceed the required basic cleansing significantly.

The living soil provides favorable circumstances for the bacteria to convert the organically attached nitrogen to ammonium, nitrate and finally atmospheric nitrogen and thus to extract it from the water.

The phosphor retained in a constructed wetland accumulates in the soil.

A small surface (2 - 5 m²/inhabitant) usually provides a high purification performance due to the generally intermittent method of operation. Vertical filters are known to achieve high CSB disintegration results and an almost complete nitrification.

A series of authors (amongst others Kunst and Flasche 1995, Bahlo 1997, Platzer 1997) prove that vertical filters are more productive than horizontal filters. The average nitrogen concentration in the total flow off of vertical filters with an average of 67 mg/l total nitrate exceeds the average concentration in horizontal filters, which achieve flow off figures of 52 mg/l total nitrate (among others Geller 1997).

Purification performance of vertical and horizontal filters

Illustration No. 3: Purification performance of vertical and horizontal filters in comparison

An examination of the spacial distribution of the microbacterial activity yielded results suggesting that at least 80 % of the CSB disintegration takes place within a ground depth of 20 cm. With extraordinarily good oxygen supplementation the CSB elimination even reached 90 % within that same ground depth (Kunst and Flasche 1995, Platzer 1997).

The nitrification takes place in this area almost excusively as well. The nitrification of wastewater based-NH₄-N nitrogen reaches 95 % within a ground depth of 20 cm if favorable oxygen conditions are provided. The first 5 cm ground offer the highest potential for nitrification. The nitrification area expands to lower ground levels in response to an increasing hydraulic and organic load.

In a vertical filter made up of sand and gravel more than 80 % of the total nitrogen at 50 cm ground depth consists of nitrate, whereas only roughly 10 % consist of ammonium. The total nitrogen disintegration is low due to the fact that denitrification is impeded or stopped through the aeration of the especially active top ground levels.

These research findings lead to the following conclusions:

Intermittendly infed vertical filters are capable of very high nitrification performances.
The factor limiting nitrogen elimination is the empeded denitrification.
Higher CSB concentrations in the wastewater relocate the nitrification level to lower ground levels (> 50 cm).

Optimization starting points:

Through relinquishment of cohesive ground materials the constructed wetland can already be operated on a long term basis bearing a load of 3 - 5 m²/inhabitant without overdammage. The minimum requirements for BSB₅ and CSB can be met without problems. The resulting CSB surface loading according to more recent research by Platzer 1998 and Kunst/Flasche 1995²∗ d amounts to 20 - 25 g CSB/m.

With a surface loading of less than 20 g/m^2∗d and a nitrogen surface loading of less than 2 - 3 g/m^2∗d nitrification is completed and denitrification to 90 % complete. The CSB concentrations are usually below 40 mg/l (Fehr 1990). The above mentioned surface loading leads to an acreage demand of 5 - 7 m²/inhabitant.

The nitrification and consequently the denitrification performance of the systems can be significantly improved through an intermittend infeed (2-6 intervals per day).

Similarly a long term increase of phosphat compounds exceeding the elimination performance of pond systems can be achieved through the enrichment of the ground material with iron chips. Phosphate bondings in waters with a high carbonate hardness can be achieved even over decades through precipitation with calcium (Aulenbach 1988).

A vegetation period is necessary to get the plant level started and operating. The plants are especially necessary to maintain the denitrification and wastewater purification as well as preventing the system from clogging up.

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Operating safety

Vegetated constructed wetlands are marked by their high operating safety during all seasons. This has been shown in many test plants.

Of course there are certain performance losses to be expected during the colder months due to the slower metabolic processes. Nevertheless a stabil operation and observation of the required flow off rates is generally achieved through the stabil wastewater temperature as well as the insulating effect of the plants.

The rate of flow will decrease during the summer months due to the increased evaporation. In some cases the flowoff may cease completely. The shade provided by the plants counters a desiccation of the system.

The already high operating safety can evenbe increased by regular maintenance.

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Long-term operation

^{Figures relating to the long-term operation of} ^{constructed wetland}^{s (>>}^{10 years) are currently available for a period of 15 years since corresponding plants have only been in operation for this period of time.}

Vegetated ^{constructed wetland}^{s have produced favorable results in the treatment of domestic waste waters during continuous operation. Chemical wastewater research proves that plants utilizing a non-cohesive, permeable sand and gravel ground composition produce significantly better flowoff rates and therefore purification performances than root bed plants utilizing a cohesive silt and clay ground composition. Not only can the requirements for wastewater treatment plants of size 1
(<}^{1000 inhabitants, small purification plants), but also of size 3 and under certain circumstances of sizes 4 and 5 be met.}

Illustration No. 4: Flowoff rates of vegetated constructed wetlands after 5-10 years of continuous operation

^{The organic components of the wastewater are micro-biologically decomposed, evaporate or enrich the soil as humus. The grounds permeableness is not adversely affected by the humus, the purification performance however is enhanced by it. There is no time limit concerning the performance relating to the decomposition of organic waste particles and nitrogen.
The growth of the plants and the formation of well-rooted ground structures generally guarantee a sufficient hydraulic function even after many years when using non-cohesive, permeable ground material.
Continuous operation of the different individual processes for elimination of the wastewater contents without a aimed oxygen infeed can lead to colmation
(stoppage) as well as slud and material accumulation in the soil.
Research for wastewater relevant individual substances or sum parameters leads to the conclusion that wastewater contents are accumulated during long-term operation through transformation, filter and sorption processes as well as precipation reactions. Dependant on the plant these either concentrate in the area immediately surrounding the infeed, individual layers or over the whole plant. A high material pollution can be found in root bed plants (illustration No.5),

^{HG
- background figure

("12345Nullprobe")

Ah - resting level

oyYr - gravel

(basalt,}^{shell lime)

Y - silt}

Illustration No.5: Plant Schwalmtal-Hopfgarten, Zn content
(solid material)

whereas no noticable material accumulations can be found in

^{constructed wetland}^{s with a sand and gravel ground composition even after an operation period of up to 15 years (illustration No.6, table No. 1).}

^{Ah - resting level

Y -}^{Sand / gravel}

Illustration No.6: Plant Germerswang, Zn- content
(Solid material)

The highest pollutant concentration is found in areas where filterable substances are fed in or where a humus layer has developed over a period of several years through dying biological mass. TOC, AOX, Zn, Cu or nutrients could be used as a guide (HAGENDORF, 1999). In comparison to limit values and approximate values they partially become subject to obligatory disposal.}

	constructed wetland Germerswang	constructed wetland See	constructed wetland Mannersdorf	Waste and Wastewater sludge directive	normal content in the ground ∗
all figures are given in mg/kg dry substance
Lead	7,4	29	0,35	900	2-60
Cadmium	0,05	n.e.	0,01	10	<0,5
Chrome	11,6	42	0,24	900	5-100
Copper	7	20	0,73	800	14642
Nickel	8,7	29	0,33	200	18384
Mercury	0,01	n.e.	n.e.	n.e.	<0,5
Zinc	52,4	104	1,59	2500	29495
AOX	17	8	n.e.	500
Dioxins	0,12	n.e.	n.e.	100	0,7-5

n.e.: not determined
∗ Source Scheffer/Schachtschnabel 1989
Geller/Thum 1999

Chart No.1: Pollutant content after 5 or respectively 10 years of operation

Plant	Schwalmtal Hopfgarten			Höhenberg						See				Germerswang
Bed / area	1			1			2			1		2.1 / 2.2		1			2

Ground level of the sample extraction	Ah	oyYr	Yr	Ah	Yor	Yr	Ah	Yor	Yr	Fr/Ah	Yr	Yr	Yo/Yr	Ah	Yor	Yr	Ah	Yor	Yr

Sample total (n)	9	10	6	1	9	3	1	6	3	6	5	8	8	9	11	12	13	13	13

TA domestic waste
TOC (mg/l) > 20	8 (89%)	4 (40%)		1 (100%)						5 (83%)				9 (100%)			13 (100%)
NH4N (mg/l) >4,1	7 (78%)	1 (10%)		1 (100%)						6 (100%)	3 (60%)	4 (50%)	4 (50%)	2 (22%)			7 (54%)	4 (31%)	8 (62%)

Purification method
Zn(mg/kg TS) > 2500	1 (11%)
Cu (mg/kg TS) >800

Total sample percentage: n (%)

Source: U. Hagendorf, UBA, 2000

Table 2: Percentual comparison of ground sample counts with test results exceeding the requirements of the TA domestic waste and wastewater sludge order.

The efficiency of the decomposition of organic pollutants and nitrogen is not limited by time.

^{The operating duration of systems with an adapted preclarification, which are sufficiently large for the planned work-load, is mainly determined by the technical periphery.}

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Microbiology and hygienics

Vegetated constructed wetlands can effectively reduce pathogens.

Domestic wastewaters contain a large variety of pathogenic microorganisms. Depending on their type and amount they can present a health hazard for humans and they therefore need to be reduced as part of the wastewater treatment process. It can be assumed that pathogenic germs of many infectuous diseases can be found in wastewaters.

The surfaces settled by microorganisms (soil particles, roots) are immensly important for the vegetated constructed wetlands′ efficiency regarding the biologic degeneration of wastewater components as well as the elimination of pathogenic germs. These biological films are generally found in all areas of constructed wetlands affected by the wastewater seepage, whereas the composition, cell-density and activity of the different microorganisms vary in the individual constructed wetland sections and layers.

Antagonistic interactions enable the microorganisms to eliminate pathogenic germs and virals and thus especially improve the wastewater′s hygienics. The operation of biological wastewater treatment procedures causes a dependence of the elimination rate on different factors, such as original pollutant content, method and oxygen infeed of different microbiological cultures,

The evaluation of the pathogenic microorganism elimination performance of vegetated constructed wetlands up to this point in time was mainly based upon investigations (illustration) researching the reactions of indicator organisms (for example E.coli, faecal streptococcus, coliform germs). The especially high performance of this type of wastewater purification plantis confirmed by its high elimination rates ranging up to 5 decimal powers. Research of a possible reduction of pathogenic germs ( for example campylobacter, clostridiums, salmonella, yersinia) was only performed isolated. The results suggest a possible high efficiency under certain conditions as well.

Illustration No.7: Reduction of e.coli in different constructed wetland plants


Illustration No.8:Reduction of faecal streptococcus in different constructed wetland plants

These outstanding elimination rates are especially significant for the operation of private wastewater purification plants (small purification plants on private properties).

Picture of a private wastewater purification plant

Illustration No.9: Picture of a private wastewater purification plant

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Costs

In rural areas constructed wetlands are a cost-efficient and environmentally friendly alternative to wastewater disposal if the connection to a conventional wastewater purification plant by wastewater channels is to expensive.

constructed wetlands can be kept fully functioning by compliance with optimal planning and handling requirements as well as necessary maintenance and preventive maintenance measures.

Construction costs depending on the plant′s size

Illustration No.10: Construction costs of constructed wetlands depending on the plant′s size

Vegetated constructed wetlands require comparitively little maintenance and preventive maintenance labour.

The needed amount of technical equipment is very small, some of the purification plants function completely without electrical equipment. If a plant does use a pump it is usually only in operation for a few minutes every day. Highly complicated controlling devices have not been utilized so far.

constructed wetland plants do not produce wastewater sludge from the biological level (excess sludge), as conventional wastewater purification plats do. The sludge resulting from the pre-clarification (raw sludge) needs to be treated and disposed of as with other wastewater purification plants as well.

Illustration No.11: Operation costs depending on the population size to be served