Treatment Efficiency

Wastewater pumped on the bed surface gradually percolates down through the bed and it allows air to refill the bed. This kind of feeding provides good oxygen transfer and hence the ability to nitrify. The treated water is collected in an aerated system of drainage pipes placed in 0.2 m layer of coarse gravel in the bottom of the filter. In order to achieve denitrification about a half of the effluent water from the filter is recycled to the first chamber of the sedimentation tank or to the pumping well (Figure 24). It also stabilizes treatment performance of the system.

VF CWs provide a good removal of organics, suspended solids, and ammonia (Tables 4 and 5) but on the other hand, they provide little room for denitrifica-tion, and therefore, ammonia-N is usually only converted to nitrate-N. As a consequence, removal of total nitrogen is low, usually lower than in HF CWs. Removal of phosphorus is also low unless special filtration material with high sorption capacity is used. Such medium is, for example, Leca (light-expanded clay aggregates) which is successfully used, for example, in Norway, Estonia, or Portugal and can remove more than 90% of the phosphorus from the sewage. However, the sorption capacity is saturable and, therefore, the filtration material must be either replaced when saturated or the volume of the bed must be very high in order to maintain the sorption capacity over a long period of time. Large volumes of special filtration material and relatively high cost of these materials make these systems very expensive as compared to VF CWs with 'conventional' filtration materials such as sand or gravel.

Table 4 Treatment efficiency of VF CWs from operational systems around the world

Parameter

Inflow (mgl 1)

Outflow (mgl-1)

Efficiency (%)

N

BOD5

166

19.8

88.1

37

COD

313

56

82.1

34

TSS

128

24

81.2

26

TP

7.6

4.5

40.8

35

TN

70

36

48.6

24

NH4-N

41.4

10.4

74.9

41

NO3-N

1.1

26.1

27

Table 5 Loading of constructed wetlands with vertical subsurface flow

- worldwide experience

Parameter

Inflow"

OutfloW

Removed

Efficiency (%)

N

BOD5

73.3

6.5

66.8

91.1

25

COD

126

34

92

73.0

31

TSS

92

12.7

79.3

82.8

14

TP

114

52

62

54.4

29

TN

1102

537

565

50.4

11

nh4-n

605

202

403

66.6

27

no3-n

26

339

8

aInflow for BOD5,

COD, and TSS is expressed in kg ha

31 day31

and for TP, TN, NH4-N, and NO3-N in g m32

yr-1.

bOutflow for BOD

5, COD, and TSS is expressed in kg ha 1 day"

-1 and for TP, TN, NH4-N, and NO3-N in g m

-2yr-1.

VF CWs are very often used either as single house on-site treatment systems (Figure 27) or for treatment of municipal sewage from small communities (Figure 28).

It is possible to consider all the factors that play a role for the efficiency of a CW by the use of models. Figures 29-31 show such a model that has been applied for design of subsurface wetlands. The model considers the oxidation of organic nitrogen to ammonium to nitrate, the uptake of ammonium and nitrate by plants, decomposition of organic matter measured as BOD in this case and the adsorption of phosphorus to the gravel and the uptake by plants. All the processes are temperature dependent.

Figure 27 HF CW for a single house at Morke, Denmark. The previous year's Phragmites stems are still standing upright while the senescent leaves had been blown away and new shoots are sprouting. Photo by J. Vymazal.
Figure 28 The first stage of VF CW in Roussillon, France for 800 (winter) to 1250 (summer) PE. The systems consist of three beds of 350 m2 in the first stage followed by two beds of 250 m2 in the second stage (total area 1550 m2). Photo by J. Vymazal.

BOD5

bod5

BOD5-

BOD5

BOD5-

bod5

ORMD-A DENI-

ORMD-B DENI-

ORMD-C DENI-

ORMD-D DENI-

ORMD-E DENI-

Figure 29 The BOD5 submodel (UNET-IETC). DENI* 1.97, decomposition of organic matter by; ORMD, decomposition of organic matter by.

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