Macrophyte Ecology within Experimental Reed Beds Applied for Heavy Metal Removal

by Miklas Scholz, University of Bradford, School of Engineering, Environmental Water Engineering Research Group, West Yorkshire BD7 1DP, UK; E-mail: m.scholz@bradford.ac.uk

Background

Wetlands can be applied for passive treatment of diffuse pollution including mine wastewater drainage (Kadlec and Knight, 1995). The functions of macrophytes in terms of their physical effect on wetlands have been reviewed extensively (Brix, 1994). The biology of Phragmites australis was reviewed in ‘Biological Flora of the British Isles' (e.g.; Haslam 1972). However, the role of macrophytes within complex reed bed ecosystems treating heavy metal pollution has not yet been fully reported. The aim of this paper is to compare experimental wetland filters of different composition.

Materials and Methods

Wetland habitats were simulated on a laboratory scale with six vertical-flow wetland buckets. The empty bucket volume was 59.2 dm³. Table 1 indicates the packing order of filter media and plant roots in January 2000.

The experiment ran continuously with modified inflow water taken from a nearby beck. In order to simulate metal contamination such as may be found in process water from mining, copper sulfate and lead sulfate were added to the inflow water to give concentrations of 1.000 and 1.277 mg dm^-3, respectively.

The range of the hydraulic load per filter bucket was between 1.35 and 2.02 cm d^-1 (mean: 1.91 cm d^-1). In June 2000, water evaporation accounted for approx. 0.08 cm d^-1, Phragmites australis evapotranspiration for approx. 0.15 cm d^-1 and Typha latifolia evapotranspiration for a value between 0.12 and 0.17 cm d^-1.

TABLE 1. Packing order of vertical-flow filter buckets simulating wetlands.

Height (cm)	Filter 1	Filter 2	Filter 3	Filter 4	Filter 5	Filter 6
56-58	(Water/air)	(Water/air)	(Water/air)	(Water/air)	(Water/air)	(Water/air)
49-55	Water + C	Water + C	Water + C	Water + C	Water + C	Water + C
47-48	6	6 + A	6 + A + B	8 + A + B	8 + A + B	9+A+B+Fs
41-46	6	6 + A	6 + A + B	8 + A + B	8 + A + B	8 + A + B
37-40	5	5 + A	5 + A + B	6 + A + B	7 + A + B	7 + A + B
35-36	5	5	5 + B	6 + B	7 + B	7 + B
33-34	4	4	4 + B	6 + B	6 + B	6 + B
29-32	4	4	4	5	6	6
25-28	4	4	4	5	5	5
21-24	3	3	3	4	5	5
17-20	3	3	3	4	4	4
15-16	3	3	3	3	4	4
9-14	2	2	2	3	3	3
0-8	1 + 2	1 + 2	1 + 2	1 + 2	1 + 2	1 + 2

1 = cobblestones; 2 = coarse gravel; 3 = fine gravel; 4 = pea-gravel; 5 = coarse sand;
6 = fine sand; 7 = Filtralite; 8 = activated carbon; 9 = charcoal; A = Phragmites australis;
B = Typha latifolia; C = marginal, floating and submerged plants; Fs = Osmocote fertilizer

Discussion and Conclusions

Standardized set-up cost ratios in England (Spring 2000) for Filters 1 to 6 (Table 1) are 1 : 2 : 3 : 37 : 41 : 42, respectively. However, the overall reduction performance of all filters in terms of lead, copper, biochemical oxygen demand (BOD), suspended solids, turbidity and bacteria was substantially great and similar for all filters during the first five months of operation (Table 2).

TABLE 2. Filter efficiencies: reduction of parameters for Filters 1 to 6.

Performance variables (outflow water)	Inflow Water		Reduction (%) per wetland filter
	Mean	Unit	1	2	3	4	5	6
Lead reduction	1.4	mg dm-3	98	99	99	99	99	99
Copper reduction	1.0	mg dm-3	96	98	97	99	98	99
BOD	2.2	mg dm-3	60	57	41	45	53	41
SS reduction	17.0	mg dm-3	55	42	50	53	51	33
Turbidity reduction	2.3	NTU	95	87	68	80	97	99
DO reduction	8.5	mg dm-3	46	68	74	77	72	78
THB reduction	2948	number per ml	88	98	92	94	91	88
TC reduction	368	number per ml	100	98	69	89	98	96

BOD = biochemical oxygen demand; SS = suspended solids; DO = dissolved oxygen;
THB = total heterotrophic bacteria; TC = total coliforms

Table 3 presents a summary of the performance parameter for Phragmites australis. Filter 3 showed a relative poor performance (Tables 2 and 3) which may have resulted from a high level of plant decay indicated by mid leaf color transformation (Pavey, 1978). Shoot density was high, stem diameters were sufficiently large and leaf/stem ratios were low (Table 3). These are indicators of good general performance (defined by Haslam, 1972). The strong normal plant diameter distribution shows that the physical strength and growth performance of Phragmites is independent of filter media and fertilizer application. However, shading decreased the stem diameters (Haslam, 1972) of Phragmites growing in fertilized filter media (Table 3).

The filters containing macrophytes contributed artificially to the inflow BOD. The real inflow BOD to the filter media was, therefore, the sum of the natural inflow BOD (10 - 40%) and the BOD resulting from plant decay (60 - 90%). BOD resulting from plant decay was greatest for filters containing Typha. The addition of fertilizer (Filter 6 only) increased the degradation rate.

TABLE 3. Performance parameter of Phragmites australis for Filters 2 to 6.

Performance Parameter	Filter
	2	3	4	5	6
Total plant number	54	34	48	39	72
Mean plant height (cm)	46.7	47.6	54.7	45.5	46.0
Median plant height (cm)	46.0	47.5	55.5	41.0	46.0
Leaf/stem ratio	4.66	3.62	5.65	4.44	5.68
Average node number	2.04	2.00	2.90	2.33	2.14
Average stem diameter (mm)	2.65	2.8	2.61	2.63	2.55
Average stem and branch number	1.11	1.21	1.40	1.23	1.51
Growth density (number per m2)	434	273	386	314	579
Cluster density (number per m2)	30	16	32	24	48
Color: plate; green variations	28.6	28.7	28.6	28.8	28.5
Color: column; intensity E (%)	30	10	30	30	20
Color: column; intensity F (%)	70	90	70	70	80
Color: row; darkness	6.3	5.5	6.6	6.6	7.0

The presence of Phragmites (dominant stands) and Typha in all reed beds does not lead to an overall increase of the wetland performance in laboratory scale experiments. Plant decay within all reed beds resulted in increases in biochemical oxygen demand and bacteria numbers within the water layer on top of the litter zone.

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