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Landfill
leachate treatment
Planted soil filter Hamburg-Havighorst
root and rhizome meshwork of reed
(Phragmites communis)
| Operator: |
Freie und Hansestadt Hamburg
(City of Hamburg)
Umweltbehörde, Amt für Umweltschutz (environmental
authority)
Fachamt für Gewässer- und Bodenschutz
Hermannstr. 40
D-20095 Hamburg |
| Author: |
Dipl.
-Ing. M. Blumberg
Ingenieurbüro Blumberg (Blumberg-Engineers)
Gänsemarkt 10
D-37120 Bovenden
Tel. 05593 937750
Germany |
Planted
soil filter Hamburg-Havighorst
In the south-east of Hamburg, on the area of the depleted moor of
Havighorst, surveyors in 1978 detected that major quantities
of
contaminated seepage water were leaking from the south-western boundary
of a former construction waste dumping site (18 ha), which had been
closed down and recultivated. Due to the bad smell it emanated, the
seepage also had a negative impact on the local recreational area. In a
move to solve this inherited problem, the leaking waters were captured
and a constructed wetland system was built using the root zone method.
This close-to-nature disposal concept, at the heart of which
is a reed bed, was put into practise by the city of Hamburg in 1986.
Description
of the water treatment procedure
The constructed wetland of Hamburg-Havighorst consists of a synthetic
membrane sealed reed bed which has a surface area of 2500 m2 and a
depth of 60 cm. The collected seepage is supplied to the reed bed via a
gravity pipe; discharge into the neighbouring circular ditch is
provided by a wind power-operated lifting facility and an electrical
pump.
| 1 |
Influent pipe |
4 |
HD-PE membrane
(2.5 mm) |
7 |
Control and
measurement shaft |
10 |
Fence |
| 2 |
Inlet distribution ditch |
5 |
Reed (Phragmites communis) |
8 |
Drain pipe |
11 |
Draining ditch |
| 3 |
Ground-water level |
6 |
Outlet ditch |
9 |
Windmill pump |
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In a root zone system the sewage is purified through an interaction of
plants, soil, and the microorganisms living in the soil. Important
sub-processes are microbial decomposition, attachment to soil
constituents and outgassing of volatile components.
Previous
removal rates
The effluent of the Havighorst waste landfill differs from domestic
sewage in many respects. For example, the biochemical (BOD5) and
chemical (COD) oxygen demand is relatively low, and further reduced by
root zone filtering by 60 - 70 % and 15 - 20 %, respectively. The low
feed concentrations of nitrogen and
phosphorus are also subject to significant further reduction (approx.
40 % of the nitrogen load and 25 % of the total phosphorus load).
The main problem substances in the landfill leachate are a number
of chlorinated hydrocarbons (chlorbenzenes and chlorphenols)
as well as specific aromatic hydrocarbons, in particular benzene,
ethylene benzene and naphthalene. The conspicuously high concentrations
of these persistent harmful substances are reduced by 94 - 100 % as
they pass through the root zone.
The most troublesome substance in the heavy-metal fraction is arsenic
which is present at a mean concentration of 29 µg/l (limit
value for drinking water = 10 µg/l) in the landfill leachate.
The mean value measured at the outlet of the planted soil filter is as
low as 8 µg/l. Likewise, the mercury concentration of 0.9
µg/l (limit value for drinking water = 1 µg/l) has
been reduced to 0.6 µg/l.
Treatment
results - mean decomposition rates of selected parameters
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|
Feed
pipe |
Drain
pipe |
| Chlorinated hydrocarbons |
| Chlorbenzenes |
(µg/l)(1) |
47.2 |
1.6 |
| Chlorphenols |
(µg/l) |
25.5 |
1.6 |
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| Aromatic hydrocarbons |
| Benzene |
(µg/l) |
62 |
1.35 |
| Ethylene
benzene |
(µg/l) |
22 |
n.d.
(2) |
| Naphthalene |
(µg/l) |
31 |
0.02 |
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| Heavy metals |
| Arsenic |
(µg/l) |
29 |
8 |
| Mercury |
(µg/l) |
0.9 |
0.6 |
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| Sum
parameters of oxygen depletion and nutrients |
| COD |
(mg/l) |
104 |
87 |
| BOD5 |
(mg/l) |
26 |
9 |
| total
N (3) |
(mg/l) |
26 |
15 |
| total
P (4) |
(mg/l) |
1.0 |
0.8 |
(
(1) µg/l – microgram per
liters = 0.000001 g/l;
(2) n.d. - not detectable;
(3) N – nitrogen;
(4) P
– phosphorus) |
Operational
experience
The soil filled in, consisting of a clay-peat mixture from local
sources, has so far performed very well in terms of percolation
capacity (hydraulic conductivity) and pollutant removal rates (root
zone process). Specific design-related features have resulted in a
non-uniform flow pattern where some areas have quantitative loads that
are significantly higher than those of others.
The introduced reed vegetation (Phragmites communis) has fully
displaced the strong competing local plant, reed mannagrass (Glyceria
ssp.), and formed a dense reed bed.
The inflow volume, largely influenced by precipitation events,
fluctuate between 0.2 and 3.0 l per second.
It still remains a mystery why the feed concentrations (effluents of
the landfill) do not follow a decay curve but have instead remained
almost unchanged during the previous times of service.
After 25 years of operation this close-to-nature landfill leachate
treatment plant (constructed wetland) is still operating at a
continuously high quality level. In addition, the building and
operating costs have been much lower than those of conventional
technical procedures.
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