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1 - Steen Nielsen
SLUDGE TREATMENT AND DRYING REED BED SYSTEMS
Steen Nielsen
Orbicon
DK-4000 Roskilde, Denmark E-mail [email protected]
ABSTRACT
Sludge Reed beds have been used for dewatering (draining and evapotranspiration) and mineralisation of
sludge in Denmark since 1988 when the first sludge processing system was introduced. Sludge from
wastewater treatment plants (2,500-125,000 PE) is treated in sludge reed bed systems with 1-18 basins with
loading rates of 25-2,200 tonnes dry solids/year for 10 years. In 2002, approx. 95 systems were in
operation. Dimensioning and design of reed bed systems depends on the sludge production (tonnes of dry
solids/year), sludge type, quality and regional climate.
The operation of a system may be divided into a number of phases related to different periods in the
lifetime of a system. A system generally runs for a total of at least 30 years; this period is divided into two
to three 8-12 year phases. Each phase consists of commissioning, full operation, emptying and re-
establishment of the system. In general, the sludge type is surplus activated sludge and surplus activated
sludge mixed with anaerobically digested (mesophillic) sludge.
The sludge loadings amount to a maximum of 60 and 50 kg dry solids/m²/year, respectively. Loading cycles
are related to the sludge type and the age of the sludge reed systems. The sludge residue will, after
approximately 10 years of operation, reach an approximate height of 1.2-1.5 metres with an approximate
dry solids content of 30-40%. Experience has shown that the quality of the final product with respect to
heavy metals, hazardous organic compounds and pathogen removal after 10 years of treatment make it
possible to recycle the biosolids to agriculture as an Advanced Treated product.
KEYWORDS
Sludge drying, dewatering, Reed beds, Dimensioning, Construction, Loading rate,
Loading cycles, Emptying, Quality of sludge residue, Heavy metals, Hazardous organic
compounds, Pathogen removal, Biosolids, Advanced treated, Enhanced treated.
INTRODUCTION
Reed beds have been used for sludge reduction in Denmark since 1988 when the first
sludge processing system was introduced (Nielsen, 1990; Nielsen et al 1992).
Long-term sludge reduction takes place in reed-planted basins, partly due to dewatering
(draining, evapotranspiration) and partly due to mineralisation of the organic matter in the
sludge. From waste-water treatment plants the sludge is pumped onto the basin
surface/sludge residue. The dewatering phase thus results in the dry matter content of the
sludge remaining on the basin surface as sludge residue, whereas the majority of its water
content continues to flow vertically through the sludge residue and filter layer. The sludge
residue water content is further reduced through evapotranspiration. In addition to
dewatering, the organic matter in the sludge is mineralised, thereby minimising the sludge
volume. Oxygen diffusion via filter aeration and through the cracked sludge surface, and
oxygen diffusion from the roots into the sludge residue enable e.g. aerobic
2 - Steen Nielsen
Figure 1. Kolding Sludge Reed Bed System (September 2000)
microorganisms to exist close to the roots and in the sludge residue (Figure 4). The
overall sludge volume reduction occurs without the use of chemicals and involves only a
very low level of energy consumption for pumping sludge and reject water. Experience
from the reference plants (Table 2) is that this type of system is capable of treating many
types of sludge with a dry matter content of approx. 0.5 to approx. 3-5%.
SYSTEMS IN DENMARK
Since the sludge reed bed treatment method was introduced it has spread to the whole
country. In recent years, several municipalities have established one or more sludge reed
bed systems. From 1988 to 1996, 27 systems were established. From 1997 to 2000, 56
systems were established. In 1999 alone, 14 systems were brought into operation. In
2002, approx. 95 systems were in operation. Interest in the systems remains high and in
2003, approx. 105 systems are expected to be in operation (Figure 2). The majority of the
systems (approx. 55%) have a capacity of up to 200 tons of dry matter per year. Within
the last couple of years, more systems have been established with a capacity of between
300 and 1,000 tons of dry matter. In 1998, the two largest systems in Skive and Kolding
(Figure 1) were brought into operation and now have a processing capacity of 2,000 and
2,168 tons of dry matter,
Table 2: Reference Plants (March 2003) in Denmark and Sweden
(S: Swedish plant)
Municipality City/town Established
Year
Number
of
Basins
Sludge
Tons
dm/yr Type
PE
(Approx.)
Skövde (S) Skövde 2003 10 1200
Activated sludge
and digested sludge
60,000
Nordborg Himmark 2003 10 350 Activated sludge
and digested 15,000
3 - Steen Nielsen
sludge
Skagen Skagen 2002/ 2003
Test plant (digestion tank)
fish industry
wastewater
Sorø Tuelsø 2003 10 300 Activated sludge 11,000
Tinglev Gårdeby 2001 8 250 Activated sludge 15,000
Rønnede Kongsted 2000 8 230 Activated sludge 15,000
Hadsten Hadsten 1999 8 300 Activated sludge 11,000
Greve Mosede 1999 10 1000 Activated sludge 60,000
Simrishamn (S) Simrishamn 1998 8 450 Activated sludge 25,000
Skive Skive 1998 18 1375 Activated sludge 123,000
Vallø Strøby Ladeplads 1998 8 300 Activated sludge 9,000
Kolding Kolding 1998 13 2000 Activated sludge and digested
sludge
125,000
Helsinge Helsinge 1996 10 630 Activated sludge 40,000
Høje-Taastrup Kallerup 2003 2 60 Activated sludge 9,000
Høje-Taastrup Kallerup 1996 8 240 Activated sludge
Tidaholm (S) Tidaholm 1995 8 360 Activated sludge 18,000
Samsø Ballen 1995 8 130 Activated sludge 5,000
Rudkøbing Rudkøbing 1992 8 232 Activated sludge 13,000
Svinninge Gislinge 1991 3 42 Activated sludge 3,000
Galten Galten 1990 6 152 Activated sludge 10,000
Skovby 1995 8 185 Activated sludge 12,000
Nakskov Nakskov 1990 10 870 Activated sludge 33,000
Præstø Allerslev 1988 2 14 Activated sludge 1,000
Jernløse
Regstrup 1988 4 25 Activated sludge 2,500
Regstrup 1992 2 40 Activated sludge 3,000
Undløse 1992 3 30 Activated sludge 2,500
respectively (Figure 3,4). The Kolding Sludge Reed Bed System (Table 2) was
established in connection with the Kolding Wastewater Treatment Plant which is
dimensioned to a load of 125,000 PE. The sludge production of 2,100 tons of dry matter
per year is comprised of surplus activated sludge with biological removal of phosphorous
(65%) and sludge from the sludge digester (35%). The sludge processing system that was
brought into operation in the summer of 1998. The system has 13 planted basins and
operation and control systems for sludge processing. The plant covers an area of approx.
6.2 hectares (62,000 m2).
Figure 2. A. Number of systems and year of establishment. B. Total number
(accumulated). (April, 2002) – (Nielsen, 2002).
2
0
3
5 5
2
5
23
11
9
14
11111111
0
2
4
6
8
10
12
14
16
1988
1990
1992
1994
1996
1998
2000
2002
2004
No.
1A
0
20
40
60
80
100
120
1988
1990
1992
1994
1996
1998
2000
2002
No.
1B2A 2B
4 - Steen Nielsen
Figure 3. Increase in processing capacity (tons of dry matter per year). A. Annual
incrrease in processing capacity. B. Total processing capacity (April, 2002) – (Nielsen,
2000).
Skive Sludge Reed Bed System, which was brought into operation in the autumn of 1998,
was built 6-7 km from the wastewater treatment plant and dimensioned to a load of
sludge from 123,000 PE. The sludge is pumped to a buffer tank before it is processed in
the sludge reed bed system that consists of 18 planted basins. The sludge reed bed system
is divided into 4 stages. Stages 1 and 2 are designed and dimensioned to be able to
process approx. 2,000 tons of dry matter. The following stages open up the possibility of
expanding the sludge reed bed system to process a total of approx. 2,850 tons of dry
matter per year. Then the system will cover a total area of approx. 9.9 hectares, thus
making it the largest system in Europe (Nielsen, 2002).
According to a statement by the Danish EPA (DEPA, 2000 a), sludge production
(excluding industrial sludge) from municipal wastewater treatment plants amounts to
approx. 155,000 tons of dry matter. By about 2003, approx. 30,000 tons of dry matter, or
19% of the total sludge quantity, will be processed in sludge reed bed systems (Figure
3B). The majority part of the sludge may be recycled on agricultural land according to
requirements applicable as of 2000 (DEPA, 2001).
DIMENSIONING
Dimensioning of the sludge reed bed systems is based on the following factors:
1. Sludge production
2. Sludge quality and sludge type
3. Climate
Periods of operation
A sludge reed bed system is in operation for an average of 10 years. The first period of
operation includes a commissioning period of two years (Nielsen, 1994). After
commissioning, the system runs at full capacity for subsequent 10-year periods of
operation, including periods of emptying. Normally, emptying is planned to start in year 8
0
2.000
4.000
6.000
8.000
10.000
12.000
1988
1990
1992
1994
1996
1998
2000
2002
2004
ton DS
2A
0
5.000
10.000
15.000
20.000
25.000
30.000
1988
1990
1992
1994
1996
1998
2000
2002
ton DS
2B3B 3A
5 - Steen Nielsen
and is completed in year 12 of each operation period. In accordance with the operation
plan, the order of succession in which basins are emptied during the period between year
8-12 must be established in years 6-7. In order to meet the requirements of capacity for a
10-year treatment period of operation, as well as dewatering of the sludge residue to a dry
matter content of approx. 30 - 40%, the following dimensioning standards are
recommended (Nielsen, 2002).
Sludge quality
The physical quality of the sludge changes at different stages of the dewatering process
(Nielsen, 1991). The content of fat and starch, etc. in the sludge, as well as the form of
production (e.g. low sludge age, concentration, pre-dewatering using polymer, mesophile
or thermophile digestion) are of importance to the sludge dewatering capacity and
consequently to the final dimensioning and number of basins.
Areal loading rate
The areal loading rate is determined in relation to the sludge type and climate and in
connection with emptying. Over a commissioning period of approx. two years, the
loading rate is increased to full capacity. With regard to loading of surplus activated
sludge, the areal loading rate is set to maximum 60 kg dry matter/m²/year after
commissioning. With regard to sludge types, e.g. from digesters (mesophile,
thermophile), sludge with a high fat content, or sludge with a low sludge age (< 20 days),
an areal loading rate of maximum 50 kg dry matter/m²/year is recommended.
It is necessary to operate the basins with alternating periods of loading and resting.
Regardless of sludge type and the size of sludge production, a minimum of 8 basins is
necessary, in order to achieve the required ratio between loading and resting periods.
Based on the ratio between periods of loading and resting. (Nielsen, 2002).
SYSTEM DESCRIPTION AND DESIGN
Sludge from wastewater treatment plants and buffer tanks
Sludge from the wastewater treatment plant the sludge may be pumped out from the
active sludge plant, final settling tanks, concentration tanks or digesters. Excess sludge is
pumped from the wastewater treatment plant to a buffer tank where it is stirred to a
homogeneous mixture and aerated, if necessary and then pumped in batches into the
basins. The buffer tank must have the capacity to mix excess sludge with reject water so
that the dry matter content of the sludge pumped into the sludge reed beds may be
regulated (Nielsen, 2002).
6 - Steen Nielsen
Figure 4. Schematic diagram of design and processes (Nielsen, 2002).
Filter design
Each basin forms a unit consisting of a membrane, filter, sludge loading system and reject
water and aeration system. The basins are designed with a tight membrane (Figure 4).
The filter consisting of several different layers of gravel, filter sand and the growth layer
at the top. The total filter height is approx. 0.55-0.60 m before sludge loading (Nielsen,
1990; Nielsen, et. al. 1992). Sludge residue heights in each basin are monitored according
to set scales. There are at least four scale poles in each basin (Nielsen, 2002).
The reeds (Phragmites australis)
The reeds are critical to the efficiency of the system and to the extent of reduction in the
sludge volume reduction (Nielsen, et. al. 1992). The reeds contribute to dewatering the
sludge via increased evapotranspiration from the sludge residue and by mechanically
influencing the sludge residue and filter. The movement of the stems in the wind and the
reeds' complex root system maintain the porosity of the sludge residue and the filter in
addition to transferring oxygen into the sludge residue and filter via their aerenchym. The
mechanical impact from shoots, roots and rhizome growth maintains drainage efficiency
and prevents clogging of the sludge phase.The subsequent decay of the roots creates a
fine and tight pore system which increases the run-off capacity of the water. Finally, the
presence of reeds contributes to the mineralisation (DEPA, 2000 B; Nielsen, 2003) of the
organic matter in the sludge, i.a. by improving dewatering and consequently the oxygen
diffusion via the sludge surface and the roots, enabling aerobic microorganisms to exist
close to the roots and in the sludge residue. In the vast majority of references (Figure 2),
reeds have been planted manually in the growth layer with four pots of reeds (0.5 l) per
m2 or one pot (1.5 l) per m
2.
Sludge loading system
Pressure pipes are installed from the valve and pump building to each basin, terminating
in a distribution system to distribute the sludge (Figure 4). The sludge loading system is
designed and dimensioned to a pumping capacity which ensures a uniform hydraulic and
dry matter load (kg dry matter/m2) across the entire basin surface, regardless of basin size
and distance to the sludge pump. (Nielsen, 2002).
Reject water and aeration system
The reject water system has two functions. The first is to collect and return the filtered
water back to the wastewater treatment plant. The second function is to aerate the filter
7 - Steen Nielsen
and the sludge residue. Drain pipes (reject water pipes) are placed on the membrane. The
reject water system, including the reject water pump, is dimensioned to a maximum run-
off of 1.4 l/min/m2. (Nielsen, 2002).
The reject water system is overdimensioned and primarily used for aeration. (Figure 4).
Air exchange in the filter and sludge residue may occur via the reeds, the reject water
system or enter the filter via the surface and the sludge layer (Nielsen, 1990; Nielsen et.
al. 1992).
LOADING – OPERATIONAL STRATEGY
Operation of reed bed systems may be divided into a number of periods relating to
periods of the lifetime of a system. A system generally runs for a total of at least 30 years;
this period is divided into two or three 8-12 year phases (Table 3). Each phase consists of
commissioning, normal loading, emptying and re-establishment of the system (Nielsen,
1994.
Commissioning The commissioning of each basin is commenced right after the plantation of the basin.
The loading is designed to have an intensity and scope - so that the rate and quantity
enables reed development to keep pace with the increasing sludge residue. The phase
comprises the planting season, the 1st growing season and the 2nd growing season (Table
3). The duration of the phase depends on the time of the planting. The purpose of the
commissioning is the following:
1. to create the best growing conditions for the reeds and to avoid weeds in the basins.
2. to adapt the loading to the development of the vegetation, so that replanting is
avoided.
3. to establish the variation of the sludge production at a yearly basis and to work out
operational plans for the final joint operation of the wastewater treatment plant and
sludge reed bed systems.
Table 3. A simplified schedule of long term operation for one basin. The entire plant is
fully operational during the entire period (Nielsen, 1994).
Year Phase Operation
1 Reeds planted
2 Commissioning 1st growing season
3 2nd growing season
4 1st full operation Basic operation
5
6
7 Long-term operation
8
9 Emptying Rest
8 - Steen Nielsen
10
11 Re-establishment 1st growing season
12
13 2nd full operation Basic operation
Periods of loading and dewatering (rest periods)
Full Operation following the commissioning in the 3th-4th year of the plant operations
means that the yearly loading is increased to the sludge production from the wastewater
treatment plant or to the amount of sludge corresponding to the maximum capacity (tons
dry matter/year) of the sludge reed bed system.
If the reeds are planted in May, the plant will have the capacity for full operation already
in the third year of operation. During the period from the 4th-8th year the operation is
divided into basic operation and a long-term operation (approx. 6-8 years). The ratio of
the basic operation to long-term operation can vary. During the basic operation each
square meter in all the basins of the sludge reed bed system is loaded with the same
amount of sludge (kg dry matter/m²/year) – (Nielsen, 1994; Nielsen, 2002).
The loading strategy involves assigning an individual quots to each individual basin. This
quota is a sludge volume which generally increases throughout the entire period of
operation until emptying, but it may also vary or even decrease to zero for periods. The
quota loading rate (kg dry matter/m2) is calculated based on the quota and dry matter
content of the sludge. The length of the loading periods and rest periods between loadings
depends on the age of the system/basin, the dry matter content and thickness of the sludge
residue and the intensity of partial loadings during the period of loading (Nielsen, 2002).
On a daily basis, the basins are subjected to a loading of 1-3 partial loadings of approx. 1
hour for a short period (from a few days during commissioning to maximum 2 weeks for
systems of 5-10 years) until the quota is used and loading switches to the next basin
(Figure 5). The loading period is followed by a rest period where the sludge residue in the
basins is dewatered through drainage and evaporation and aerated. The loading period
and dewatering period (rest period) ratio is regulated by changing the quota relative to the
need to pump excess sludge from the wastewater treatment plant (Nielsen, 1994).
Emptying prognosis
An emtying prognosis is a management tool to estimate when it would be appropriate to
empty a basin. Such a prognosis ensures that emtying is commenced in due time so that
the sludge reed bed system operates optimally during the emptying phase, and
overloading of the last basins to be emptied is prevented. An emptying prognosis is based
on the basis of registrations of the sludge residue height and accumulated areal-specific
loading rates (kg dry matter/ m2/year) after commissioning. Two basins selected for
emptying are excluded from the loading plan approx. ½-1 year before emptying.
After a basic operation phase, an emptying phase or cycle is planned so that the first
basins are emptied in approx. year 8-9, while the last basins are emptied in approx. year
11-12. Ideally, the emptying should take place between May and September.
9 - Steen Nielsen
Re-establishment The re-establishment phase begins after emptying, at the latest in September. The
operation of the sludge reed beds system during this period resembles the operation
during the commissioning. It is, however, expected that the vegetation will establish itself
more rapidly and therefore the loading during year 10 (the first year in a new operating
phase), year 11 (2nd
.) and year 12 (3rd
.) will probably be heavier than in the
corresponding years during the commissioning (Nielsen, 1994).
Final Disposal When the beds are emptied, the sludge residue has a consistency which makes it easy to
spread onto agricultural or forest land or to use as landfill cover. The final disposal of the
sludge residue from the basins will depend on whether the quality of the sludge residue
meets with the Danish standards (Table 4) in particular lead, cadmium, mercury, nickel
and hazardous organic compounds concentrations (DEPA, 2000 a).
OPERATION AND CONTROL
The sludge reed beds system is monitored from the wastewater treatment plant computer
and operated automatically. In general, systems have been manually controlled, but since
1995, they are increasingly automated (Table 2). The CRS system, including software,
consists of two main modules, an operation module and a data collection module. The
operation module contains graphics (Figure 5) illustrating what actually happens in the
system. All sludge processing systems (Table 2) are equipped with automatic control and
operation systems as standard. The sludge pumping is activated automatically in the event
of excess sludge production in the wastewater treatment plant, and pumping sludge into
the sludge reed bed system. Operation is optimised through monitoring and automation,
thus improving dewatering and mineralisation.
Figure 5. Overview (Nielsen, 2002)
10 - Steen Nielsen
The control systems ensure optimal and varied management of the sludge treatment. The
data collection module monitors and registers data in connection with sludge loading in
individual basins (Figure 5,9). Sludge flow and dry matter content are registered before
the sludge is directed into the basins for dewatering. The sludge is distributed to basins
with free capacity according to a loading plan. In this way dewatering is optimised,
thereby securing a high dry matter percentage and also prolonging the life of the sludge
reed bed system. The loading rate (m3, tons dry matter, kg dry matter/m
2/year) at system
and basin level is computed on an ongoing basis. (Figure 6). Increase in sludge residue
height (manual registration of sludge residue height from scale poles) in relation to
loading rate and dewatering efficiency in the form of drained volume per 24 hours and
area-specific run-off (l/sec./m2) are also recorded (Nielsen, 1994, Nielsen, 2002).
Figure 6. Overview of monthly loading – Greve Sludge Reed Bed System (second year
of commissioning) – (Nielsen, 2002).
These data are important control parameters used to monitor the system's function and
dewatering capacity and which provide a basis for future loading plans. Prognosiss are
prepared concerning the contribution of individual basins to the operation based on
whether dewatering is stable or if there is a tendency to reduced dewatering efficiency
(Figure 7).
Figure 7. Reject water run-off (l/sec./m2) A. Kolding Sludge Reed Bed System B.
Hadsten Sludge Reed Bed System (Nielsen, 2002).
Måned Total
tons TS 1 2 3 4 5 6 7 8 9 10 tons TS
Januar 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00
Februar 7,06 4,67 2,07 0,00 0,00 0,00 0,00 0,00 0,00 8,35 22,15
Marts 0,00 0,00 3,38 7,26 6,78 7,05 6,69 0,13 0,00 0,00 31,29
April 6,62 6,66 6,75 6,32 2,55 0,00 0,00 6,12 8,19 6,76 49,97
Maj 0,53 0,00 2,48 3,06 7,09 9,77 9,65 9,45 6,44 3,27 51,74
Juni 6,40 6,58 5,89 5,86 6,01 5,59 0,00 0,00 3,10 6,50 45,93
Juli 7,24 7,11 7,24 7,12 7,02 7,34 10,69 6,07 6,12 7,29 73,24
August 7,07 8,51 8,79 8,30 5,66 2,08 2,36 6,84 6,45 6,44 62,50
September 7,99 0,00 0,00 0,00 0,00 2,70 5,11 5,73 12,56 9,63 43,72
Oktober 2,12 8,31 9,60 10,32 10,47 10,80 6,03 0,00 0,00 0,00 57,65
November 9,30 4,88 0,00 0,00 0,00 0,00 5,00 10,60 13,17 9,95 52,90
December 0,00 4,28 8,88 8,33 8,00 0,33 0,00 0,00 0,00 0,00 29,82
tons TS/år 54,33 51,00 55,08 56,57 53,58 45,66 45,53 44,94 56,03 58,19 520,91
kg TS/m2/år 32,59 30,59 33,04 33,94 32,14 27,39 27,31 26,96 33,61 34,91
Bassin nr.
0,000
0,005
0,010
0,015
0,020
21-05-01 26-05-01 31-05-01 05-06-01
0,000
0,005
0,010
0,015
0,020
10-07-01 15-07-01 20-07-01 25-07-01
7 A 7 B
11 - Steen Nielsen
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
04-0
6-9
7
04-0
9-9
7
04-1
2-9
7
04-0
3-9
8
04-0
6-9
8
04-0
9-9
8
04-1
2-9
8
04-0
3-9
9
04-0
6-9
9
04-0
9-9
9
04-1
2-9
9
04-0
3-0
0
04-0
6-0
0
04-0
9-0
0
04-1
2-0
0
04-0
3-0
1
04-0
6-0
1
04-0
9-0
1
04-1
2-0
1
Meter
RESULTS
This paper presents experience, guidelines for dimensioning, operations and descriptions
and know-how from a 15-year period (1988-2002), primarily with references (Table 2)
from Denmark and Sweden.
Helsinge Sludge Reed Bed System - Loading
The Helsinge Sludge Reed Bed System was established and planted with reeds in October
1996 (Table 2). The sludge system has a capacity of 630 tons dry matter per year, 10
basins, each of an area of 1,050 m2
at the filter surface and a maximum area loading rate
of 60 kg dry matter/m2/ year. Sludge production from the Helsinge Wastewater Treatment
Plant consists of active sludge direct from activated sludge plants and sludge from final
settling tanks and constitutes approx. 50% of the loading of the sludge reed bed system.
The remaining 50% of the sludge production consists of concentrated activated sludge
from four smaller wastewater treatment plants. The two sludge types are mixed before
being added to the sludge system. Annual sludge production amounts to approx. 540 tons
dry matter.
Since 1998, individual basins, here represented by basin number 1, were subjected to an
average loading rate of approx. 55 tons dry matter per year after commissioning (Figure.
11A), resulting in an average area-specific loading rate of 52.4 kg dry matter/ m2/year
(Figure. 11B). The status of the sludge residue height in basin 1 in relation to time was
calculated on the basis of scale pole readings. The sludge residue height increase from
1998 to 2001 was 0.68 m, and the total sludge residue height at the end of 2001 was 0.79
m (Figure 9).
Figure 8. Helsinge Sludge Reed Bed System – Basin number 1. A. Annual loading rate
(tons dry matter). B. Area-specific loading rate (kg dry matter/m2/year) – (Nielsen, 2002).
Figure 9. Helsinge Sludge Reed
Bed System – Basin number 1.
Increase in the sludge residue
height (Nielsen, 2002).
0
10
20
30
40
50
60
1997 1998 1999 2000 2001
Ton DS
0
10
20
30
40
50
60
1997 1998 1999 2000 2001
Kg DS/m2
12 - Steen Nielsen
Rudkøbing Sludge Reed Bed System – Loading
The system was established in 1992 with 8 basins (Figure 10) and a capacity of 240 tons
dry matter per year. Each basin has a filter area of approx. 500 m2. The following
operational requirements were specified when dimensioning the sludge reed bed system:
A treatment period of 10 years.
An operation period of minimum 8 years before an emptying phase is required. Thus,
the typical 4-year emptying cycle for all the basins would occur between years 8 to 12
of operation.
A sludge residue which meets quality guidelines for final disposal (agricultural land).
Re-establishment of vegetation without re-planting.
Treatment capacity maintained in years when the basins are emptied and
subsequently re-established.
Figure 10 : Rudkøbing Sludge Reed Bed System.(1998)
Throughout the period of operation (1992-2002), the system was subjected to sludge from
the activated sludge plant on a weekly basis. From 1994, after commissioning, the system
was subjected to a loading rate of approx. 233 tons dry matter per year, resulting in an
average area loading rate of 59 kg dry matter/m2/year (Figure 11).
0
50
100
150
200
250
300
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
Ton DS
A B
Slamresthøjde (middel, 8 skalapæle)
0,00
0,20
0,40
0,60
0,80
1,00
1,20
1,40
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
Height of sludge residue (metre)
13 - Steen Nielsen
Figure 11. Rudkøbing Sludge Reed Bed System – Basin number 2. A. Development of
sludge residue height Annual loading. B. Annual loading (Nielsen, 2002)
Emptying Phase
The plan is to empty the Helsinge Sludge Reed Bed System over a 5-year period with 2 of
the 10 basins selected for emptying per year. According to the plan, emptying will
commence in 2004. Thus, all 10 basins in the system will be emptied after 5 years.
Capacity during the emptying period (5 years) is maintained at 630 tons of dry matter per
year during the emptying phase.
The Rudkøbing Sludge Reed Bed System has lived up to expectations regarding its
operation and efficiency. The eight basins in the Rudkøbing Sludge Reed Bed System are
emptied over a 4-year period where two basins per year are taken out of operation and
then emptied. With regard to the operation of the remaining basins (minimum 6),
experience has shown that it is best to empty only two basins per year. This maintains the
total capacity of the sludge reed bed system as a maximum of 25% of the basins (or two
basins) at a time are in the emptying phase. The treatment capacity at the Rudkøbing
Sludge Reed Bed System under normal operation was 230 tons of dry matter per year
distributed among the eight basins. During the emptying phase, the sludge load was
distributed to six basins.
It was originally planned that the emptying phase for the first two basins would
commence in 2000, however, this has been postponed and will not commence until
August 2002. Thus, the period of operation was increased from 8-12 years to 10-(14)
years. In September and October 2001, two basins were taken out of operation and will
rest until being emptied. For the remaining basins, the area-specific loading rate from
September 2001 to August 2002 was approx. 80 kg dry matter/m2/year. The loading rate
was not changed during the emptying period. The loading cycle continues to consist of a
loading period of 3 weeks followed by a 15-week rest period. Maintaining full capacity or
more than full capacity during emptying cycle has only been tested in Rudkøbing to a
limited extent since autumn 2001 up to a point and is only possible provided that the
basins are re-established after emptying with sufficient regeneration of vegetation and
provided that the loading rate is adapted to vegetation growth.
Quantity and quality of sludge residue - Rudkøbing Sludge Reed Bed System
The quality of the sludge residue in The Rudkøbing Sludge Reed Bed System meets the
criteria set out in the Statutory Order from the Ministry of the Environment on Sewage
Sludge and the final disposal of the residual sludge will be on agricultural land (DEPA,
2000 a).
During the entire ten-year period of operation, the sludge from the Rudkøbing
Wastewater Treatment Plant underwent treatment in the reed bed system in the form of
draining and evapotranspiration as well as partial mineralisation whereby the quantity of
sludge residue was reduced. In September 2002, the sludge residue had a height of 1.02
m. Since the reed bed system was brought into operation in 1992, the average height
increase was 0.10 m per year. After a 10-year period of operation, the sludge residue in
basins number 3 and 7 was a total of approximately 940 m3.
14 - Steen Nielsen
During the entire period, the quality of the sludge from the Rudkøbing Wastewater
Treatment Plant which was loaded in the sludge reed bed system met the criteria of the
Statutory Order (DEPA, 2000 a) regarding content of heavy metals. Analyses of the
sludge residue for heavy metals and hazardous organic compounds (DEPA, 2000b;
Nielsen, 2003) were performed prior to emptying the basins. The dry matter content in
the sludge residue was up to 40%. The nitrogen and phosphorus content were on the
order of 25,000 and 40,000 mg/kg dry matter, respectively. The quality of the sludge
residue after ten years of biological treatment in the sludge reed bed system met the valid
Statutory Order criteria for use on agricultural land (Table 4).
The emptying phase for two basins was first started in September 2002 and finished in
about one week. The vegetation was cut down prior to emptying the basins and the actual
emptying was undertaken with a backhoe machine. Metal plates were placed on the
surface of the sludge residue to support the excavation equipment. The excavation was
monitored with laser survey equipment to monitor the depth of excavation and to prevent
damage to the filter (Figure 12).
Table 4. Analysis results. Content of heavy metals (mg/kg dry matter) and phosphorus
(mg/kg total phosphorus) at Rudkøbing Sludge Reed Bed System after 10 years of
operation. The samples were taken in May – July 2002 and standards for heavy metals in
sludge products which are finally disposed of on agricultural land (DEPA, 2000 a)
Parameter Basin 3 Basin 7 Limit value Units
Lead 1,300 2,900 10,000 mg/kg P
Lead - - 120 mg/kg dm
Cadmium 15 34 100 mg/kg P
Cadmium - - 0.8 mg/kg dm
Mercury 26 95 200 mg/kg P
Mercury - - 0.8 mg/kg dm
Nickel 400 690 2,500 mg/kg P
Nickel - - 30 mg/kg dm
Chromium 39 99 100 mg/kg dm
Copper 260 470 1,000 mg/kg dm
Zinc 410 1,100 4,000 mg/kg dm
PAH 2.9 2.8 3 mg/kg dm
DEHP 3.4 3.7 50 mg/kg dm
NPE 3.1 1.5 10 mg/kg dm
LAS <50 <50 1,300 mg/kg dm
15 - Steen Nielsen
Figure 12. Rudkøbing Sludge Reed Bed System. Emptying of basins 3 and 7, September
2002.
The sludge residue from basins 3 and 7 was spread on agricultural land with a dung
spreader and then plowed under. The area required for spreading the residual sludge was
determined based on the Statutory Order criteria (DEPA, 2000 a) of maximum 90 kg
phosphorus per hectare resulting in approximately 9 tons/ha and 47 – 56 kg N/ha on a
total of approximately 100 ha. According to the Statutory Order, the land may first
receive sludge again in year 2005.
Loading of the emptied basins began in October 2002. The two emptied basins will
gradually come into operation. As the vegetation is again re-established, the basins will
gradually regain their treatment capacity, which will steadily decrease the total areal
loading. The re-growth was satisfactory (Figure 15) and it is not expected that replanting
will be necessary.
Infectius matter
Good reduction in infectious matter in the sludge means that the treated sludge complies
with the quality guidelines in the Statutory Order regarding controlled sanitation. The
sludge residue from the sludge reed beds system is cleaner and better suited for recycling
on agricultural land than mechanically dewatered sludge.
Wastewater sludge contains a large number of bacteria. Salmonella, Coli bacteria and
faecal Streptococci are found in wastewater sludge (raw and mesophilic-digested) in the
following numbers per ml: 10-100, 10,000-1,000,000 and 10,000-1,000,000, respectively.
As a general rule, pathogenic bacteria which are excreted and end in a foreign
environment, only live for a shorter period of time depending upon various environmental
factors and the bacteria’s own characteristics.
16 - Steen Nielsen
According to the Statutory Order from the Ministry of the Environment regarding sewage
sludge, sludge which is to be spread on agricultural land must meet the following quality
guidelines for controlled sanitised products:
Salmonella must not be detected.
The number of faecal Streptococci must be fewer than 100/g.
Analyses of the reduction in infectious matter in the sludge residue from Galten Sludge
Reed Bed Plant in sludge residue samples taken 6 – 9 months after the last loading
indicated that the content of infectious matter was reduced approximately 6 log units
based on drymatter to a level corresponding to the requirements for controlled sanitation
(Table 5).
Table 5. Concentration of infectious matter in sludge residue after treatment in sludge
reed beds for 6-9 months.
Salmonella Not detected
Faecal Streptococci Less than 100 pr. g
E. Coli Less than 20 pr. g
The results are in agreement with results reported by the Danish EPA for storage of
sludge (Environmental project number 351 regarding sanitation aspects during handing
and recycling of organic waste).
Mineralisation In the last fifteen years, there has been focus on the negative effects which may be
associated with sewage sludge, in particular addition of heavy metals to soil. In 1997,
stricter legislations, in the form of so called ”cut off values” were brought into effect to
regulate the content of hazardous organic compounds (PAH, DEHP, LAS, NPE) in
sewage sludge being spread on agricultural land.
Consequently there is increasing interest in finding methods which can reduce the content
of hazardous compounds in sludge so that it may continue to be used in agriculture.
In 1999, The Danish EPA and Hedeselskabet Environment and Energy A/S began a
research project to investigate the potential for mineralisation of LAS and NPE in sewage
sludge (The Danish EPA, 2000 b). The treatment of digested sludge involved three
separate three sludge treatment methods for a period of about nine months:
Sludge mineralisation in a full-scale reed bed system
Storing in storage containers
Storing in storage a pile periodically turned over mechanically
Mineralisation of LAS in a sludge reed bed
The concentration of LAS in the sludge was approximately 5,000 mg/kg dry matter
(Figure 13). During the period from day 35 (30.03.99) to day 150 (23.07.99), the
concentration of LAS was reduced by 90%. At the end of the experiment, the sludge had
a LAS content of approximately 100 mg/kg dry matter, which represents approximately
17 - Steen Nielsen
2% of the start concentration (Figure 13 A). The total mass of LAS added at the start of
the experiment was approximately 24.5 kg. After 9 months there was 0.4 kg remaining.
Figure 13. Mineralisation of LAS in the digested sewage sludge. Concentration (mg/kg
dry matter) as a function of time. A: sludge reed bed. B: container and sludge pile
(Danish EPA, 2000 b).
Mineralisation of NPE in the sludge reed beds
The concentration of NPE (total) at the start of the experiment (23.02.1999) was
approximately 54 mg/kg dry matter (Figure 14). The total mass of NPE added at the start
of the experiment was approximately 0.3 kg. After 9 months there was 0.02 kg remaining
(Danish EPA, 2000 b).
During the course of the experiment (284 days), the concentration of NPE (total) in the
sludge residue was reduced by a total of approximately 93 % to a total concentration of
approximately 4 mg/kg dry matter at the end of the experiment. Mineralisation of NPE in
the sludge pile resulted in a 43% reduction (Figure 14 and Table 6).
Figure 14. Mineralisation of NPE in digested sewage sludge. Concentration (mg/kg dry
matter) as a function of time with treatment in A: sludge reed bed. B: container and
sludge pile (Danish EPA, 2000 b).
0
10
20
30
40
50
60
70
0 50 100 150 200 250 300
days
NPE (sum)
(mg/kg DS) Container Sludge pile
0
10
20
30
40
50
60
70
0 50 100 150 200 250 300
days
NPE (sum)
(mg/kg DS) Sludge Reed BedA B
0
1000
2000
3000
4000
5000
6000
0 50 100 150 200 250 300
days
LAS
(mg/kg DS)Container Sludge pile
0
1000
2000
3000
4000
5000
6000
0 50 100 150 200 250 300
days
LAS
(mg/kg DS) Sludge Reed BedA B
18 - Steen Nielsen
In the sludge stored in containers a reduction of LAS and NPE of 41% and 10%,
respectively, was achieved due to mineralisation in the uppermost layer. The mechanical
turning of the sludge in the pile improved the oxygen influx to the sludge and in general
had a positive effect on the mineralisation of LAS and NPE.
The reduction of the LAS and NPE observed in a sludge reed bed system were in the
same degree of mineralisation as obtained with composting (Jørgensen, 1999) and from
mechanical aeration (Jørgensen et al.,1999) in a wastewater treatment plant (Table 6).
The duration of the treatments was the following: composting treatment - 18 weeks; full-
scale experiment with aeration in a wastewater treatment plant - approximately 12
Weeks; sludge mineralisation in a reed bed system – approximately 2-3 months in the 9
months investigations period.
Table 6. Reduction of LAS and NPE (%) achieved using various treatment methods
(Danish EPA, 2000 b).
Method NPE LAS
Long-term storage (0 –120 cm) ~10% ~41%
Long-term storage (0-20 cm) 61% 75%
Long-term storage (20-120 cm) ~0% ~0%
Mechanical turning 43% 90%
Sludge reed bed 93% 98%
Composting 78-95% 100%
Mechanical aeration 75-95% 95%
Based on the results of the three monitoring experiments (sludge reed bed, container and
pile storage) the following main conclusions may be made:
Oxygen is a crucial factor for mineralisation of LAS and NPE.
Mineralisation under anaerobic conditions was limited.
Temperature affects the rate at which mineralisation occurs.
19 - Steen Nielsen
Figure 15. Rudkøbing Sludge Reed Bed System. Re-establishment of reed growth in
basin 7, October 2002.
CONCLUSIONS
The sludge treatment in reed bed systems in Denmark are built to treat sludge for an
average period of 10 years. The total lifetime of a plant is at least 30 years. With regard
to the sludge quantity and type it is very important that the sludge reed bed system has
the necessary area, number of basins and the correct operation adapted to the climatic
conditions and the process phase of the system (commissioning and normal operation).
Reduction of sludge residue throughout the entire 10-year period of operation is highly
dependent on individual basins continually alternating between loading and rest periods.
Systems with treatment capacities of up to 500, 500-1,000 and more than 1,000 tons of
dry matter per year must be established with 8, 10 and 12-14 basins. Reed beds for
surplus activated and digested sludge must be dimensioned to loading rates of maximum
60 and 50 kg of dry matter/m2/year with minimum 8 and 10-12 basins, respectively.
Experience shows that 10-year periods of operation and a final dry matter content of 30-
20 - Steen Nielsen
40% can only be achieved if the system is dimensioned correctly. The concentration of
LAS and NPE (total) in the sludge residue was reduced by a total of approximately 98
% and 93 , respectively. The reduction in infectious matter in the sludge residue samples
taken 6 – 9 months after the last loading indicated that the content of infectious matter
was reduced approximately 6 log units.
The quality of the final product with respect to heavy metals, hazardous organic
compounds and pathogen removal after 10 years of treatment make it possible to recycle
the biosolids to agriculture as an Advanced Treated product.
REFERENCES
Articles in Journals
Kampf, R., Nolthenius, C.T. (1983). Treatment of sludge in an artificial reed bed. H2O
16, nr. 20, 461-464.
Maeseneer, J., Paelinck, H., Verheven, R., Hulle, D. (1982). Use of artificial Reed
Marshes for Treatment of Industrial Wastewaters and Sludge. In: Studies on Aquatic
Vascular Plants, J.J. Symoens (Ed.), pp 363-369.
Sassaman, M. D, Kauffman, T. R. (1993). Sludge dewatering and disposal using the
reed system. Operations forum 10, nr. 6, 18-21.
Articles in Conference Proceedings
Kim, B. J., Cardenas, R. (1990). Use of Reed Beds for Dewatering Sludge in the USA.
In: Advances in Water Pollution Control (IAWPRC). Constructed Wetlands in Water
Pollution Control, P.F. Cooper, B. C. Findlater (ED.), pp 563-566.
Lienard, A., Esser, D., Deguin, A., Virloget, F. (1990). Sludge Dewatering and Drying
in Reed Beds: An Interesting Solution? General Investigation and first Trials in France.
In : Advances in Water Pollution Control (IAWPRC). Constructed Wetlands in Water
Pollution Control, P. F. Cooper, B. C. Findlater (ED.), pp 245-255.
Nielsen, S. 2003. Mineralisation of Hazardous organic Compounds in Sludge reed bed
and Sludge Storage. (In press). Proceedings of the Seminar : The Use of Aquatic
Macrophyter for WasteWater Treatment in Constructed. Lisbon 8-10 May.
Nielsen, S. 2002. Sludge Drying Reed beds. "Constructed Wetlands in Water Pollution
Control". Proceedings of the International Conference on the use of Constructed
Wetlands in Water Pollution Control, held in Arusha, Tanzania, 16-19 September.
Nielsen, S. 1994. Biological Sludge Drying in Reed Bed Systems - Six years of operation
experience. Proceedings of 4th International Conference on Wetlands Systems for Water
Pollution Control. 6-10 November, 1994. Guangzho People's Republic of China.
Nielsen, S.1991. Biological Sludge Drying in Constructed Wetlands. Proceedings of
International Symposium on Constructed Wetlands for Wastewater Quality Improvement.
1991, 21-24 October, University of West Florida.
Nielsen, S. 1990, Sludge dewatering and mineralization in reed bed systems.
"Constructed Wetlands in Water Pollution Control". Proceedings of the International
Conference on the use of Constructed Wetlands in Water Pollution Control, held in
Cambridge, UK, 24-28 September.
Reports
21 - Steen Nielsen
Danish Environmental Protection Agency (DEPA). 2001. Spildevandsslam fra
kommunale renseanlæg i 1999. Orientering fra Miljøstyrelsen (waste-water sludge from
municipal waste-water treatment plants in 1999 and . Orientation from the Danish EPA)
No. 3, (In Danish)
The Danish Environmental Protection Agency (DEPA). 2000 a. Consolidated act on
the use of waste products for agricultural purposes. Danish consolidated act no. 49 of
January 20, 2000 a (In Danish).
Danish Environmental Protection Agency (DEPA). 2000 b. Investigation and
monitoring program for decomposition of organic matters injurious to the environment in
constructed wetlands – Reed bed plant for sludge drying and treatment and in sludge
deposit. Ministry of the Environment and Energy –Working report no. 22, (In Danish,
summary in English).
Danish Environmental Protection Agency (DEPA). 1987. Spildevandsslam fra
kommunale renseanlæg. Orientering fra Miljøstyrelsen (waste-water sludge from
municipal waste-water treatment plants. Orientation from the Danish EPA In Danish)
Nielsen, S., Andersen, K., Christensen, L.B. 1992 Biological sludge treatment.
Investigation of sludge dewatering and mineralization plants planted with reeds. Ministry
of the Environment and Energy – Danish Environmental Protection Agency. Research in
waste water no. 38, 1992. (In Danish, summary in English).