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14th European Biosolids and Organic Resources Conference and Exhibition 1
Organised by Aqua Enviro Technology Transfer
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DEWATERING DRINKING WATER SLUDGE’S IN REED BED SYSTEMS
Nielsen, S1. and Cooper D.J.,2
1Orbicon A/S, Denmark, 2ARM Ltd, UK
Author Emails smn@orbicon.dk; info@armreedbeds.co.uk
Abstract
Sludge drying reed beds have been used for dewatering and mineralisation of sewage sludge’s since
the late 1980’s, however there has been no experience of treating drinking water sludge’s in reed
bed systems.
Water Treatment Works (WTW) treat water to a potable standard, part of this treatment includes
dosing with Iron Sulphate to help dirt particles to coagulate, producing a ferric sludge. Historically
the sludge wastes have been pumped to the sludge lagoons at Hanningfield Reservoir. However,
these are now nearing the end of their serviceable life and a new sludge handling process is
required.
As part of a trial with Essex and Suffolk Water, ARM Ltd and Orbicon A/S set up a series of six trial
beds, each 20m2 at Hanningfield Reservoir in Essex to examine the dewatering processes of the
sludge produced from the water treatment process. These were monitored from 2008 – 2009. The
purpose of the test is to clarify whether the sludge is suitable for treatment in a sludge reed bed
system, the dimensions criteria and the quality of reject water from the sludge reed bed system.
The operation of the test system has been very positive especially because of the very intensive
loading program for some of the basins on such a young system. It is possible to drain and treat
ferric sludge (approximately 300,000 mg Fe/kg dry solid) in a reed bed system.
The system has a good draining efficiency and the filtrate water from the sludge loading is out of the
system the next day. The sludge residue surface is drying as indicated by evidence of desiccation
cracks shortly after the water drains out of the system. In spite of the different loading programs,
volume reduction is very high at over 99%. The ferric sludge is dewatered to approximately 30-40 %
dry solid and desiccates in the trial beds. It is possible to get the vegetation to grow in ferric sludge,
where the pH was measured to 7, 7. It has not been necessary to use fertilizer.
The proposed use of reed bed systems not only reduces the capital and operating cost, but also
provides the site with an environmental friendly operation area. The overall reduction of the sludge
volume occurs without the use of chemicals. The process involves only a very low level of energy
consumption for pumping the sludge and reject water.
Key words
Sludge treatment, reed bed System, drinking water sludge, load, dewatering, ferric sludge
Introduction
It has been identified by the asset management strategy for Essex and Suffolk Water that there is a
need to provide a new process for sludge handling at the Hanningfield Water Treatment Works.
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Hanningfield WTW (Figure 1) is supplied with raw water from Hanningfield Reservoir. The existing
treatment works comprises pre-ozonation, clarification, filtration, main ozonation, pesticide
removal, chlorine disinfection prior to the pumping of treated water. Sludge waste is generated
primarily from the de-sludging of the Pulsator clarifiers producing a mineral sludge with seasonally
fluctuating levels of algae and suspended solids.
Figure 1 Hanningfield Water Works and the water works sludge lagoon
The final part of the solids removal process is the rapid gravity filters and the granular activated
carbon contactors used during backwashing. Historically these sludge wastes have been pumped to
the sludge lagoons at Hanningfield Reservoir. However, these are now nearing the end of their
serviceable life and a new sludge handling process is required. The two options being considered are
a mechanical centrifugal solution and sludge treatment reed beds. The proposed use of reed bed
systems not only reduces the capital and operating cost, but also provides the site with an
environmentally friendly operation area.
The purpose of the trial is to clarify:
Whether the sludge from Hanningfield Water Treatment Works is suitable for further
treatment in a sludge reed bed system.
The dimensions required (capacity, operations, loads, area, number of basins, etc.) for a full
scale plant at Hanningfield Water Treatment Works.
The quality of reject water from a sludge reed bed system treating sludge from Hanningfield
Water Treatment Works.
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Historically, sludge reed beds (Figure 2) have been used for the dewatering and mineralization of
sewage sludge in Europe since 1986. The European Union Water Framework Directive calls for
cleaner discharges from our waste water treatment facilities. Improved treatments to achieve higher
quality effluents results in the production of more sludges. In countries like Denmark, France,
Germany and Sweden, sludge treatment in Reed Bed Systems are a common and a well-proven
method. Long-term sludge reduction takes place in reed-planted basins, partly due to dewatering
(draining, evapotranspiration) and partly due to mineralisation of the organic solids in the sludge.
Sludge from wastewater treatment plants is pumped onto the basin surface. The dewatering phase
results in the dry solids 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.
The water content is further reduced through evapotranspiration.
Figure 2 Sludge reed bed system
A sludge treatment reed bed system utilizes the forces of nature to reduce and treat sludge. The
only appreciable power consumption is by the pumps used to transport sludge and reject water. This
means that the reed bed system uses much less power than other systems. Transport costs will be
reduced substantially, while the volume of sludge can be reduced to approximately 1.5-2.5 % of its
original volume.
The sizing and design of reed bed systems depends on the sludge production (TDS per annum),
sludge type, quality and regional climate. The treatment period is approximately 8 - 12 years and the
operation of the system may be divided into a number of phases related to different periods in the
lifetime of a system. Each phase consists of commissioning, full operation, emptying and re-
establishment of the system. When the system is setup, it requires only a weekly control-visit to the
site of about one to two hours, and there is no contact with the sludge, which gives a better working
environment.
The plan is to empty the Sludge Reed Bed System over a period, with 2-3 basins selected for
emptying per year. Capacity during the emptying period is maintained despite the reduction in
number of the basins the emptying phase. After a period of years, the basins are emptied, and its
contents can be recycled and spread on farm land.
After a basin has been emptied you can start the load for another operations period. Maintaining full
capacity during emptying is possible provided that the basins are re-established after emptying with
14th European Biosolids and Organic Resources Conference and Exhibition 4
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sufficient regeneration of vegetation, and provided that the loading rate is adapted to vegetation
growth. So re-planting basins has not been necessary.
Sludge treatment in reed bed systems is a thoroughly tested method with a number of proven
advantages. The overall reduction of the sludge volume occurs without the use of chemicals. The
process involves only a very low level of energy consumption for pumping the sludge and reject
water resulting in a minimum of CO2-emissions.
The control system (SCADA) can be setup at the main working site and the daily surveillance can be
done from there. There is no noise from the system as there is from many other types of treatment
systems. The system works effectively to reduce pathogenic bacteria like Salmonella, Enterococci
and E. Coli, thus making it a lot safer to be on site.
Sludge treatment in Reed Bed Systems uses no chemicals in the dewatering process. This means a
considerable improvement in the working environment along with a reduction of the chemical
residue in the treated waste water passing into the environment. The content of substances in
sludge that are foreign to the environment can be reduced. After the treatment of sludge, recycling
options are good, particularly in agriculture. The sludge quality is cleaner and more adaptable in the
natural cycle than mechanically dewatered sludge.
Methods and results
The Hanningfield test system (Figure 3) was built at Hanningfield Water Treatment Works with 6
basins each of 20m2 with a design comparable to a full-scale system with reeds, ventilation, sludge
input; reject water systems as well as filters and drains. The reeds were planted in February 2008.
The development during 2008 was good. The reeds grew well and began to cover the whole surface
and achieved a height of approximately 1 meter. It hasn’t been necessary to replant as the reeds
have thrived in the test system. In 2009 the reed growth continued and the surface of the basins was
approximately 100 % covered with reeds. In July the reeds had achieved a height over 2 meters
(Figure 4). It has not been necessary to add fertilizer.
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Figure 3 Basin no. 1 – 29.07.2009 (Hanningfield Sludge Treatment System)
Sludge quality
The data (Figure 4) shows that there is a large variation in the total solids being loaded into the
basins. Therefore it is very important to know the % of dry solid in each load in order to calculate the
load (kg ds/m2/year) as precisely as possible. The contents of solids are lower in 2009 and it has been
necessary to adjust the loading program to achieve the demanded area loads. The sludge quality
seems to have good sedimentation ability (Figure 5) and settles well within a few minutes.
0
0,1
0,2
0,3
0,4
0,5
0,6
01.03.08 01.06.08 01.09.08 02.12.08 04.03.09 04.06.09
%
Total SolidsAverage2008: 0,1862009: 0,163
Figure 4 Total solid (%) – Sludge
2008-2009
average
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Figure 5 Sedimentation of the sludge. A: Sludge sample (28.07.2009). B: Sedimentation period.
Load
The basins have been loaded for approximately 7 and 4 months in 2008 and 2009, respectively.
During the test period the load has been intensified. Each load representing a volume of 3m3, has
been loaded over a period of approximately 1 hour. The accumulated volume has resulted in a total
loaded volume of 69 - 180 m3 and 45 – 150 m3 in 2008 and 2009 (as of 21.07.2009), respectively
(Figure 6).
The dry solid content in the sludge has varied during the test. The average values were 0.18 % and
0.16 % in 2008 and 2009, respectively (Figure 5). The accumulated load has resulted in a total load of
110 - 300 kg and 70 - 240 kg in 2008 and 2009 (21.07.2009), respectively. During the test-loading
period in 2008, each of the basins have been loaded for 1-3 days (loading days) with 1-4 batches
daily, followed by a rest period of approx. 10-30 days (resting days). In 2009 the basins have and will
be loaded for 2-5 days with 2-4 batches daily, followed by a rest period of approx. 30-55 days.
0
50
100
150
200
250
300
01.03.08 01.06.08 01.09.08 02.12.08 04.03.09 04.06.09 04.09.09
m3
Loading basin 1
Figure 6 Load of basin no 1 (m3)
A
B A B
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In 2008 the area load had been applied for a whole year the area load would have varied between
10 to 26 Kg ds/m2/year. In 2009 the area load was varied between an area load 25 – 50 kg
ds/m2/year for a whole year.
-0,005
0
0,005
0,01
0,015
0,02
0,025
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
06-07-09 00:00 06-07-09 12:00 07-07-09 00:00
l/se
c/m
2
m3
/hBasin no 1
Inlet
Outlet
Figure 7 Four loads and the resulting dewatering profile.
Dewatering (l/sec/m2)
The majority of the water in a sludge load continues to flow vertically through the sludge residue
and filter. The dewatering (liters/second/m2) is good (Figure 7 and 8). The dewatering efficiency has
a positive development in 2009.
The basins are now more mature with a good crop of reeds. The decrease in the dewatering profile
has not been so pronounced and the variation in the dewatering efficiency from the basins has been
reduced. The load has increased in 2009 for all the basins. The maximum loads are 2-4 loads per day
per basins (approx. 6-12 m3/day).
0
0,005
0,01
0,015
0,02
0,025
0,03
0,035
0,04
0,045
0,05
3000 3100 3200 3300 3400 3500 3600 3700 3800
Reject (l/s/m2)BASIN 6
Figure 8 Reject water from the basin no. 6 (Liters/second/m2)
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Generally the dewatering profile is a peak with a maximum over 0.015 l/sec/m2. In some basins the
maximum dewatering speed is over 0.020 – 0.025 l/sec/m2 (Figure 8).
Dewatering (m3)
The times for dewatering of 6-12 m3 are approximately 15 hours and over 90 % of the load is
dewatered in that period (Figure 9). Other positive results are that the dewatering profiles generally
are very narrow, short, steep (up and down) and reach the bottom line with a short period of time,
even for the basins which had been loaded with 12 m3 each day for 4 days.
0
20
40
60
80
100
120
140
1 2 3 4 5 6
%
Basin no.
Reject - % of load Average system
Figure 9 Volume of reject water - percentage of load (basin no 1-6)
Dewatering – Volume reduction
The dewatering phase results in the dry solids content of the sludge remaining on the basin surface
as sludge residue. The water content is further reduced through evapotranspiration. The sludge
residue shows evidence of desiccation very quickly (Figure 10). In spite of the different loading
programs, volume reduction is very high over 99 %.
In 2009 the reduction is slightly higher than in 2008 with an increase in the loads (Table 1), and with
more mature basins. The dry solid (0.16 -0.20) in the sludge has been concentrated approximately
200 times to a dry solid estimate in the sludge residue of approximately 28 - 42 % (Table 1).
Table 1 Reduction in sludge volume – 01.03.2008-31.01.2009
Bassin
no.
Sludgevol.
m3
Sludge dry solids (%) Reduction
%
Sludge
residue
vol.
m3
Sludge residue dry
solids (%)
Min. Max. Avg. Min. Max. Avg.
1 180 0.16 0.2 0.186 99.33 1.2 23.9 29.9 27.8
2 159 0.16 0.2 0.186 99.55 0.7 35.6 44.4 41.3
3 120 0.16 0.2 0.186 99.48 0.6 30.8 38.5 35.8
4 81 0.16 0.2 0.186 99.56 0.4 36.4 45.5 42.3
5 105 0.16 0.2 0.186 99.43 0.6 28.1 35.1 32.6
6 87 0.16 0.2 0.186 99.43 0.5 28.1 35.1 32.6
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Figure 10 Dewatering efficiency and change in the sludge surface in 14 days (September/October
2008)
Reject Water - Quality
The dewatering phase results in the dry solids content of the sludge remaining on the basin surface
as sludge residue. Samples of Water Works sludge and samples of reject water treated in the reed
bed system show the efficiency of the filtration through the reed beds (Figure 11).
Samples of sludge and reject water taken in the hours after the sludge load. Sludge treatment in the
reed bed system showing the high efficiency of the filtration. The quality of the reject water during
the hours after the loading has a very low content of suspended solid after one load. More than one
load on the same basin on the same day results in turbidity variations in the reject water (Figure 11).
Figure 11 Sludge samples (A) and samples of reject water (1, 2, 3, 4,) during a period with first 3 of 4
loads
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0
50
100
150
200
250
300
350
400
19-05-09 03-06-09 18-06-09 03-07-09 18-07-09
NTU
Turbidity - Total
Figure 12 Reject water – Turbidity (NTU)
In May 2009 a turbidity meter was installed on the outlet pipe. The data recording started in May
(Figure 12) before it was calibrated in May and early June. In the initial period before the meter was
calibrated it shows incorrect values.
Generally since June the turbidity is lower than 5 NTU (Figure 13). During the loading (eg. 4 loads in
one day) there is an increase in the turbidity, up to approximately 20 NTU. A few results reach 50
NTU. The trend shows that the main volume of water has a turbidity level below 5 NTU even in the
loading periods (Figure 12).
Sludge residue
Sludge residue levels are measured approximately every 14 days. There has been a reduction in
depth from 12 to 6 cm over a period of approximately 15 weeks which indicates the effect
dewatering in the resting period (Figure 13).
The results have shown that the system has good dewatering properties resulting in good dry solid
%. The amount of nitrogen and phosphorus in the sludge residue appears at present, to be sufficient
to support reed growth (Table 2).
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-2
0
2
4
6
8
10
12
14
01-06-2008 01-10-2008 01-02-2009 01-06-2009 01-10-2009
cm
Basin 1Average depth gauge 1-3
Scale pole
Figure 13 Basin no 1 - Sludge residue height
Table 2 Sludge residue quality in the lagoon and in the basins (29.07.2009). The numbers in the
bracket are days since last load
Conclusions
In the first phases (March 2008 – January 2009) of the test the results have shown that:
1. The operation of the test system has been very positive especially because of the very
intensive loading program for some of the basins on such a young system.
2. The reeds are approximately 2.0 meters high and they are beginning to cover the whole
surface.
Parameters Unit Basin 1 Basin 2 Basin 3 Basin 4 Basin 6 Lagoon Site 1 Lagoon Site 2
Dry solid % 39 (21) 40 (18) 44 (28) 42 (4) 31 (6) 28 28
Loss on
ignition
% of DS 24 25 26 25 28 22 14
Total
nitrogen
mg/kg DS 4,800 7,200 7,900 8,600 7,600 5,300 3,900
Total
phosphor
mg/kg DS 11,000 10,000 9,700 9,600 8,100 9,100 58,000
Aluminium mg/kg DS 1,700 1,400 1,400 1,300 2,200 4,100 2,700
Calcium mg/kg DS 47,000 47,000 46,000 43,000 50,000 160,000 200,000
Iron mg/kg DS 370,000 390,000 390,000 370,000 370,000 230,000 130,000
FeS2 (pyrite) mg/kg DS 610 830 920 180 140 1,900 250
Grease + Oil mg/kg DS <60 81 140 110 160 190 110
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3. The filters have a good draining efficiency and the water from the loads is out of the system
the next day. The sludge residue is dewatered to approximately 30-40 % dry solid.
4. Shortly after the water is out of the system the sludge residue surface is cracking up very
well.
5. The dewatering is good.
6. Samples of the reject water have shown that the filter has a good filtration capability.
7. A full scale system will need 16 basins and the loading can be 30 kg dry solid/m2/year.
References
Nielsen, S. (2003) Sludge Drying Reed beds. Water Sci. and Technology.48, No 5 pp. 101 – 109.
Nielsen, S. (2003) Paper: 16 years of experience with sludge treatment in Reed beds system
Conference: Achievements and Prospects of Phythoremediation in Europe. COST Action 837. 15 - 18
October 2003 Vienna.
Nielsen, S.(2005) Sludge Reed beds facilities: Operation and problems. Water Sci. and Technology, 51
No 9 pp. 99-107.
Christensen, L.B. (1991) Acidification and Ochre formation in Fenlands. BSSS and IPS Symposium,
Cambridge, England April 1991.
Acknowledgements
The authors do wish to thank the different partners involved in this study: Essex & Suffolk Water,
Hanningfield Treatment Works, and Northumbrian Water Limited.
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