SLUDGE TREATMENT AND DRYING REED BED SYSTEMS IN DENMARK

SLUDGE TREATMENT AND DRYING REED BED SYSTEMS IN DENMARK

Seen Nielsen MSc*and Neil Willoughby* *

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 (2,50(F125,000 pe) is treated in sludge reed bed systems with 1-18 basins with loading rates of 252,200 tonnes dry sofidslyear for fen years. in 2002, approximate& 95 systems were in operation. Dimensloning and design of reed bed systems depends on the sludge production rate, sludge type, quality and reglonal cllmate.

The maximum sludge loading rate is approximately 5oM) kg DS/nY/year. Loading cycles are related to the sludge type and the age of the sludge reed systems. The sludge residue will, after approximately ten years of operation, reach an approximate height of 1.2-1.5 metres with dry solids content of 3@40%. Experience has shown that the quality of the final product with respect to heavy metals. hazardous organic compounds and pathogen removal after ten years of treatment make it possible to recycle the biosolids to agriculture as an enhanced treated product.

Key words: Advanced treated; biosolids; enhanced heated,,; sludge dewatering; sludge drying; reed beds, loadlng rate, paihogen removal.

'Hedeselskabet, Environment and Energy A/S, Ringstedvej 20, DKdOOO Roskilde, Denmark

Telephone: 0045-46 30 03 10

Email: smnrShedeselskabet. dk

"ARM Ltd, Rydal House, Colton Road, Rugeley. Staffordshire, WS15 3HE UK.

Telephone: 0044-(0) 188W838 I I , Fax: O W ( 0 ) 1889684998

Email: neil. [email protected]. uk

INTRODUCTION Reed beds have been used for sludge reduction in Denmark since 1988 when the first sludge processing system was introduced (1015), Long-term sludge reduction takes place in reed-pianted basins. partly due to dewatering (draining, evapotranspiration) and partty due to mineralisation of the organic matter in the siudge. From wastewater treatment

plants the sludge is pumped onto the basin surface. The dewatering phase thus results in the dry solids content of the sludge remaining on the basin surface as siudge residue, whilst 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.

Oxygen diffusion via filter aeration and through the cracked sludge surface, and oxygen diffusion from the roots into the sludge residue enable aerobic micro-organisms to exist close to the roots and in the sludge residue. The overall siudge 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 flable 1) is that this type of system is capable of treating a number of sludge types with o dry matter content of between 0.5 and 5%. Furthermore, the overall performance of the the systems is not compromised by heavy rainfall or freezing condictions.

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Table 1. Reference Plonts (March 2003) in Denmark and Sweden (S: Swedish plant)

Municipality Cityflown

Skovde (S) Skovde

Nordborg Himrnark

Sludge I Established Year Number of Baslns

2003 10

2003 10

Rmnede

Hadsten

Greve

Simrishamn (S)

Kongsted 2000 8

Hadsten 1999 8

Mosede 1999 10

Sirnrisharnn 1998 8

1000

450

Y

Activated sludge 60,000

Activated sludae 25,000

Tidoholm (S)

Sams0

RudkBbing

Tidoholm 1 1995 8

I 1995 8 Rudkdbing I 1992 8

Ballen

Galten

Nokskov

Galten 1990 6

Skovby 1995 8

Nakskov 1990 10

Prmt0

Jernlise

Allerslev 1988 2

Regstrup 1988 4

Regstrup 1992 2

Undlise 1992 3

40

30

-u ~



Tannesdslyr I Type I PE (Apprax.) 1200 1 Activoted sludge 60,000

and digested

and digested sludge

350

Skagen Test plant (digestion tank)

fish industry wastewater

I I 10 Sor0 Tuelsm 2003

Tinglev Gdrdeby I 2001 8

300 I Activated sludge 1 1,000

250 I Activated sludge 1 15,000

230 I Activated sludae I 15,000 I

300 I Activated siudae I 11.000

1375 I Activated sludoe I 123,000 Skive Skive

Vall0 Str0by Ladeplads

Kolding Kolding

2;o i Activated s l u ~ / 9,000 ~

Activated sludge 125,000 and digested

sludae

Helsinae I Helsinae I 1996 I 10 630 I Activated sludae I 40.000 I I I

H0je-Taostrup Kallerup I 2003 I 2 I I

60 I Activated sludge I 9,000

H~ie-Toastru~ I Kallerup I 1996 I 8 240 I Activoted sludge I 360

1 30

232


Svinninae I Gislinae I 1991 I 3 42 Activated sludae I 3,000 152 Activated sludge 10,000

Activated sludge


185

870

14 I Activated sludge I 1 ,000 25 I Activated sludoe I 2.500

iast two years, more systems have been established with a capacity of between 300 and 1 . 0 0 tonnes of dry solids. in 1998. the two largest systems in Kolding (Figure 1) and Skive

were brought into operation and now have a processing capacity of 2,168 and 2,000 tonnes of dry solids, respectively.

According to a statement by the Danish EPA (I2), sludge production (excluding industrial sludge) from municipal wastewater treatment plants amounts to approximately 155,000 tonnes of dry solids. By about 2003, approximutely 30,000 tonnes of dry solids, or 19% of the total sludge quantity, will be processed in sludge reed bed systems (Figure 2). The majority of the sludge may be recycled on agricultural land according to requirements applicable as at 2000 ('I).

SYSTEMS IN DENMARK Since the sludge reed bed treatment method was introduced in Denmark its use 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, approximately 95 systems were in operation. Interest in the

systems remains high and in 2003. approximately 105 systems are expected to be In operation.

The majority of the systems (approximutely 55%) have a capacity of up to 200 tonnes of dry solids per year. Within the

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Fig 'ure 1. Kolding Sludge Reed Bed System (September 2ow

IZ0 1

1988 1W 1992 1994 16% IS99 2wO 2W2

Figure 2. Increase in processing capaclty (TDS/annum). Total processing capacity (April 2002) - (Nielsen, 2000)

Kolding Wastewater Treatment Plant The Kolding Sludge Reed Bed System noble 1) was built to treat the sludge from the Kolding Wastewater Treatment Plant (125,ooO pe). The sludge production of 2.100 tonnes of dry matter per year comprises surplus activated sludge with biological removal of phosphorous (65%) and sludge from the sludge digester (35%).

The sludge system was brought into operation in 1998 and has 13 planted basins and operation and control systems for sludge processing. The reed bed system covers an area of approximately 6.2 hectares (62.000 m2). Since the system has been established the decanter centrifuges have been removed and their building redeployed as laboratory. storage and messing facilities.

Skive Wastewater Treatment Plant Skive Sludge Reed Bed System was brought into operation in 1998, and is sltuated 6km from the wastewater treatment plant (123,000 pe). The sludge Is pumped to a buffer tank before it is processed in the siudge reed bed system consisting of 18 planted basins.

The sludge reed bed system is divided into four stages. Stages one and two are designed and dimensioned to be

iii) Climate

Periods of operation A sludge reed bed system operation cycle lasts for an average of ten years. The first part of the cycle Includes a commissioning period of two years After commissioning, the system runs at full capacity for subsequent ten year cycles of operation, including periods of emptying, Normally, emptying is planned to start in year eight and is completed In year 12 of

each operation cycle. In accordance with the operation plan, the order of succession in which basins are emptied during the period between years eight and 12 must be established in years SIX and seven. In order to meet the requirements of capacity for a ten-year treatment period of operation, as well as dewatering of the sludge residue to a dry

solids content of approximately 30-40%, the sludge reed bed system must be correctly dimensioned.

Sludge quality The physical quality of the sludge changes at different stages of the dewatering process m. The content of fat and starch etc,. in the sludge, as well as the form of production (e.g. low sludge age, concentratlon, pre-dewatering using polymer, mesophlllic or thermophiiiic digestion) are of Importance to the sludge dewatering capacity and consequently to the final size and number of basins.

Area loading rate The area loading rate is determined in relation to the sludge lype and climate and in connection with emptying. Over a commissioning period of approximately two years, the loading rate is increased to full capacity, With regard to loading of surplus activated sludge. the area loading rate is set to maximum 60 kg DS/m'/year after commissioning, With regard to sludge from digesters, sludge with a high fat content, or sludge with a low sludge age (<20 days), an area loading rate of maximum 50 kg DSlrnYyear is recommended.

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SYSTEM DESCRIPTION AND DESIGN Sludge from wastewater treatment plants and buffer tanks Sludge from the wastewater treatment piant may be taken from the activated sludge plant, final settling tanks, concentration tanks or digesters.Sludge is pumped 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 solids content of the sludge pumped into the sludge reed beds may be regulated

Filter design Each basin forms o unit consisting of a waterproof membrane, filter, sludge loading system and reject water and aeration system. The filter consists of several different layers of gravel, filter sand and the growth layer at the top. The total filter height is 0.550.60m before sludge loading Sludge residue heights in each basin are monitored by means of indicator poles.

The reeds (Phragmites australis) The reeds are critical to the efiiciency of the system and to the extent of reduction in the sludge volume reduction (I5). The reeds contribute to dewatering the sludge via increased evapotranspiration from the sludge residue and by enhancing the flltratlon properties of the dewatered sludge. The movement of the stems in the wind and the mechanical effect of 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 roots. The subsequent decay of the roots creates a flne and tlght pore system which increases the run-off capaclty of the water. Finally, the presence of reeds contributes to the mineralisation (6.13) of the organic solids in the sludge. In the vast majority of installations, reeds were planted in the growth layer with four pots of reeds (0.5 I) per m2 or one pot (1.5 i) per m?

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.The sludge loading system is designed to ensure a uniform hydraulic and dry solids load across the entire basin surface, regardless of basin size and distance to the sludge pump. ('I.

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 and

the sludge residue. 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 (10,'5).

LOADING - OPERATIONAL STRATEGY Operation of reed bed systems may be divided into a number of perlods 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. Each phase consists of commissloning, normal loading, emptying and reestablishment of the system @),

Commissioning Commissioning of each basin commences immediately after planting. The loading is managed such that reed development keeps pace with the increasing sludge residue. The comrnissionlng phase comprises the planting season, the first growing season and the second growing season. During the commissioning phase the aims are:

i) To create the best growing conditions for the reeds and to avoid weeds in the basins

ii)To adapt the loading to the development of the vegetation, so that replanting is avoided

ili) To establish the variation of the sludge production on an annual basis and to work out operational plans for the final joint operation of the wastewater treatment plant and sludge reed bed system.

loading and dewatering Following the commfssioning phase, In the thlrd and fourth year of the operation the loading rate is increased to match the sludge production from the wastewater treatment plant. If the reeds are planted in May. the plant wiil have the capacity for full operation already in the third year of operation.

The loading strategy involves assigning an individual quota to each individual basin. The length of the loading periods and rest periods between loadings dependsQn the age of the system or bash, the dry solids content and thickness of the sludge residue and the Intensity of partial loadings during the period of loading v).

Emptying prognosis An emptying prognosis is a management tool to estimate when it would be appropriate to empty a basin. Such a prognosis ensures that emptying is commenced in due time so that the sludge treatment facility us a whole operates optimally during the emptying phase and overloading of the last basins to be emptied is prevented.An emptying prognosis is based on the records of the sludge residue height and accumulated area-specific loading rates after

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commissioning. Two basins selected for emptying are excluded from the loading plan approximately 112-1 year before emptying.

Lead

Lead Cadmium

Cadmium Mercury

Mercuw Nickel

Nickel

Reestablishment The re-establishment phase begins after emptying, at the latest in September. The operation of the sludge reed bed 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 ten (the first year in a new operating phase), year 1 1 (Pa.) and year 12 (3 " . ) will probably be heavier than in the corresponding years during the commissioning

1,300 2,900 10,000 Mg/kg p

15 34 100 msncs p

0.8 mdks ds

26 95 200 m g m p

0 8 mdks ds 400 690 2,500 mg/kg p

30 m&a ds

120 mgikg ds

Final Disposal The finished biosolids created by the sludge reed bed system have a consistency which makes it easy to spread onto agricultural or forest land or to use as landfill cover. The final disposal route for the sludge residue from the basins will depend on whether the quality of the biosolids meets with the Danish standards (Table 2) in particular lead, cadmium, mercury. nickel and hazardous organic compounds

concentrations ( 1 2 ) ,

Chromium

Copper Zinc

PAH

Table 2. Analysis results. Content of heavy metals (mg/kg dry solids) and phosphorus (mg/kg total phosphorus) af Rudk~bing 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 whlch are finally disposed of on agricultural land (DEPA. 2000 a)

Parameter I Basin 3 I Bash 7 I Limit value I Units

39 99 100 mgkg ds

260 470 1,000 mgkg ds

410 1,100 4,000 mglkg ds

2 9 2 0 3 mdko ds

OEHP

NPE

IAS

3.4 3.7 50 mg/kg ds

3.1 1.5 10 mdkg ds 4 0 <50 1,300 mg/kg ds

OPERATION AND CONTROL The sludge reed bed system is monitored from the wastewater

treatment plant computer and operated automatically. In the past systems have been manually controlled, but since 1995. they are increasingly automated gable 1). The automatic control system, including software, consists of two main modules, an operation module and a data collection module. The operation module contains graphlcs illustrating what actually happens In the system. All sludge processing systems gable 1) are equipped with automatic control and operation systems as standard. The sludge pumping is activated automatlcaily in the event of excess sludge production in the wastewater treatment plant, and pumping

sludge into the sludge reed bed system. Operation is optimlsed through monltorlng and automation, thus improving dewatering and minerailsation.

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. Sludge flow and dry solids 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 optimlsed, thereby securing a high dry solids percentage and also prolonging the life of the sludge reed bed system.

The loading rate at system and basin level Is computed on an ongoing basis. Increase in sludge residue height In relation to loading rate and dewatering efficiency in the form of drained volume per 24 hours and area-speclflc run-off (I/sec/m*) are also recorded

The data are not just 'nice to have' but are Important control parameters used to monitor the system's function and dewatering capacity and whlch provide a basis for future loading plans. Prognosis are prepared concerning the contribution of individual basins to the operation based on whether dewatering is stable or if there is a tendency for reduced dewaterlng efficiency.

RESULTS This paper presents experience, guidelines for dimensioning, operations and descriptions and know-how from a 15 year period (1988-2002). primarily with references from Denmark

and Sweden.

LOADING Helsinge Sludge Reed Bed System The Helsinge Sludge Reed Bed System was established and planted with reeds in October 1996 (Table 1). The sludge system has a capacity of 630 tonnes dry solids per year. ten basins, each of an area of 1.050 m2 at the filter surface and a maximum area loadlng rate of 60 kg dry soiids/m2/ year.

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Sludge production from the Helsinge Wastewater Treatment

Plant consists of activated sludge and sludge from final settling tanks and constitutes approximately 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 approximately 540 tonnes dry solids.

Since 1998, Individual basins, here represented by basin number 1, were subjected to an average loading rate of approximately 55 tonnes dry solids per year after commissioning (Figure 3A). resulting in an average area- specific loading rate of 52.4 kg dry solids/ m2/year (Figure. 38). 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 4).

I -6%

I 1898

I ,949

I 1989

I xo

I 2oM

I zmr

w

I ?M1

Figure 3. Helsinge Sludge Reed Bed System - Basin number 1. A. Annual loading rate (tonnes dry solids). B. Areaspecific loading rate (kg dry solids/m2/year) - (Nielsen, 2002).

Rudksbing Sludge Reed Bed System The system was established in 1992 with eight basins (Figure 5) and a capacity of 240 tonnes of dry sdlids per year. Each basin has a filter area of approximately 500 m2.

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

’- I

I*.

Figure 4. Helsinge Sludge Reed Bed System - Basin number 1. Increase in the sludge residue height (Nielsen, 2002).

subjected to a loading rate of approximately 233 tonnes of dry solids per year, resulting In an average area loading rate

of 59 kg DS/m2/year (Figure 6).

Figure 5. Rudkpbing Sludge Reed Bed System.( 1998) showing music festival unaffected by odour

EMPTYING PHASE Helsinge Sludge Reed Bed System The plan is to empty the Helsinge Sludge Reed Bed System over a five-year period with two of the ten basins selected for emptying per year. According to the plan, emptying will commence in 2004. Thus, ail ten basins in the system will be emptied after five years. Capacity during the emptying period (five years) is maintained at 630 tonnes of dry solids per year during the emptying phase.

Rudksbing Sludge Reed Bed System The Rudkplblng Sludge Reed Bed System has lived up to expectations regarding its operation and efficiency. The eight basins in the Rudkplblng Sludge Reed Bed System are emptied

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F 1.40 I

3 120-

: i m . I

Figure 6. Rudke~bing Sludge Reed Bed System - Basin number 2. A. Development of sludge residue heighf Annual loading. B. Annual loading (Nielsen, 2002)

over a four-year period where two basins per year are taken out of operation and then emptied (Figure 7).

With regard to the operation of the remaining basins (minimum six), experience has shown that it is best to empty only two basins per year. It was originally planned that the emptying phase for the first two basins would commence in 2ooo. however, this was postponed and took place in August "l2. Thus, the period of operation was increased from 8-12

years to 1&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 2M32 was approximately 80 kg DS/m2/year. The loading rate was not changed during the emptying period. The loading cycle continues to consist of a loading period of three weeks followed by a 15week rest Deriod.

QUANTITY AND QUALITY OF SLUDGE RESIDUE RudkBbing Sludge Reed Bed System The quality of the sludge residue in The Rudkrabing 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 agricuilural land (la.

basins 3 and 7, September 2002.

During the entire ten-year period of operation, the sludge

from the Rudk~bing 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 me reed bed system was brought into operation in 1992, the average height increase was 0.1 m per year. After a ten-year period of operation, the sludge residue in basins number three and seven was a total of approximately 940 m3.

During the entire period, the quality of the sludge from the Rudkabing Wastewater Treatment Plant which was loaded in the sludge reed bed system met the criieria of the

Statutory Order (I2) regarding content of heavy metals. Analysis of the sludge residue for heavy metals and hazardous organlc compounds (m) were performed prior to emptying the basins. The dry solids content in the sludge residue was up to 40%.The nitrogen and phosphorus content were in the order of 25,000 and 40,000 mg/kg dry solids, respectively. The quality of the sludge residue after ten years of biological treatment In the sludge reed bed system met the valid Statutory Order criieria

for use on agricultural land gable 2). The sludge residue from basins three and seven was spread

on agricultural land with a muck spreader and then ploughed under.The area required for spreading the residual sludge was determined based on the Statutory Order criteria(I2) of

maximum 90kg phosphorus per hectare resulting in approximately 9 tonnesJha and 47-56 kg N/ha on a total of approximately 100 ha. According to the Statutory Order, the

land may 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 vegetution is again reestablished. the basins will gradually regain their treatment capacity, which will steadily decrease the total area loading. The regrowth was satisfactory and it is

not expected that replanting will be necessary.

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Pathogen Reduction A large reduction in pathogens in the sludge means that the treated sludge complies with the quality guidelines in the

Statutory Order regarding controlled sanitation(I". Wastewater sludge contains a large number of bacteria. Salmonella, Coli

bacteria and faecal Streptococci are found in wastewater sludge (raw and mesophillic-digested) in the following quantities per mi: 10-100, 10,000-1,000,000 and 10,000- 1 ,ooO,oOO, respectively.

As a general rule, pathogenic bacteria which are excreted and end in an alien environment, only live for a short period of time depending upon various environmental factors and the bacteria's own characteristics.

According to the Statutory Order from the Ministry of the Environment regarding sewage sludgec? sludge which is to be spread on agricultural land must meet the following quality guidelines for controlled sanitised products:

(i) Saimonella must not be detected (ii)The number of faecal Streptococci must be less than

1 m/g

Salmonella

Faecal streptococci

E. Coli

Analysis of the reduction in pathogens in the sludge residue from Gaiten Sludge Reed Bed Plant in sludge residue samples

taken 6 - 9 months after the last loading indicated that the pathogen content was reduced by approximately 106 units based on a dry solids basis to a level corresponding to the requirements for controlled sanitation (Table 3). 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).

Not detected

Less thon 1 OO/g Less thon 2O/a

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 'limit vaiues'were brought into effect to regulate the content of hazardous organic compounds such as linear alkyl benzene suifonates (US), poly aromatic hydrocarbons (PAH), diethyihexyl phthalate (DEHP) and

nonylphenoi ethoxylates (NPE) in sewage sludge being spread on agricultural land. Consequently there is increasing interest in finding methods which can reduce the content of hazordous 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 siudgecI3). The treatment of digested sludge Involved three separate three sludge treatment methods for a period of about nine months:

(i) Sludge mineralisation in a full-scale reed bed system (ii) Storing in storage containers (iii) 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 solids (Figure 8). During the period from day 35 (30 March 1999) to day 150 (23 July 1999). the concentration of LAS was reduced by 90%. At the end of the experiment, the sludge had a LAS content of approximately 1M) mg/kg dry matter, which represents approximately 2% of the initial concentration (Figure 8). The total mass of LAS

added at the start of the experiment was approximately 24.5 kg. After nine months there was 0.4 kg remaining.

A

-9

" 1 B

D

Figure 8. Mineralisation of LAS in the digested sewage sludge. Concentration (mg/kg dry matter) as a function of time. A: sludge reed bed. 8: contulner and sludge pile (Danish €PA, 2000 b).

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Mineralisation of NPE in the sludge reed beds The concentration of NPE (total) at the start of the experiment (23.02.1999) was approximately 54mg/kg dry matter (Figure 9).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(W

Long-termsloroge (0-20 crn) Long-term storage (20-1 20 cm)

Mechanical turning

a -

m -

20 -

61% 7590

-0% -0% 43% 90%

A

. -4. . '. h

Figure 9. Mineralisation of NPE in digested sewage sludge. Concentration (mg/kg dry mafter) a s a function of time with treatment in A: sludge reed bed. 6: container and sludge pile (Danish EPA. 2OOOb).

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 4mglkg DS at the end of the experiment. Mineralisation of NPE in the sludge pile resulted in a 43% reduction (Figure 9 and Table 4).

Table A. Reduction of U S and NPE (%) achieved using vorlous treatment methods (Danish EPA, 2000 b).

Method I NPE I LAS

Lona-term storaae (0 -1 20 cml I - 10% I -41 %

Sludae reed bed I 93% I 98%

Composting I 78-95% I 100%

Mechanical oerotion 75.95% I 95%

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 posltive 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 (Jorgensen, 1999) and from mechanical aeration (J~rgensen et 01.. 1999) in a wastewater treatment plant flabie 4). 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 nine months investigations period,

CONCLUSIONS 1 , The sludge drying reed bed systems in Denmark are

built to treat Sludge for an average period of ten years until emptied. The total lifetime of a piant is at least 30 years.

2. 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.

3. Reduction of sludge residue throughout the entire ten-

year period of operation is highly dependent on individual basins continually alternating between loading and rest periods.

4. Experience shows that ten-year periods of operation and a final dry solids content of 30-40% can be reliably achieved if the system is sized correctly.

5. The concentration of LAS and NPE (total) in the sludge residue was reduced by a total of approximately 98 %

and 93%. respectively. Based on the results of the three monitoring experiments, in respect of the mineralisation of hazardous organic compounds, the following main conclusions may be made:

i) Oxygen is a crucial factor for mineralisation of LAS

ii) Mineralisation under anaerobic conditions was

iii)Temperature affects the rate at which

6.The reduction in pathogens in the sludge residue samples taken 6 - 9 months after the last loading indicated that the pathogen content was reduced by aDDroxlmatelv 100.

and NPE

limited

mineralisation occurs

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7,No fly or odour problems have been observed or reported even though many of the systems are In close proximity to houses.

8. The quality of the final product with respect to heavy metals, hazardous organic compounds and pathogen removal after ten years of treatment make it possible to recycle the biosoiids to agriculture as an Enhanced Treated product.

REFERENCES 1. KAMPF. R., NOLTHENIUS, C.T. (1983).Treatment of sludge in an

artificial reed bed. H20 16. no 20.461 -464. 2. MAESENEER, J., PAELINCK. H.. VERHEVEN. R., HULLE. D. (1982).

Use of artificial Reed Marshes for Treatment of Industrial Wastewaters and Sludge. Studies on Aquatic Vascular Plants, J .J . Symoens (Ed.). pp 363-369.

3. SASSAMAN, M. D, KAUFFMAN, T. R. (1993). Sludge dewatering and disposal using the reed system. Operations forum 10. no6.18-21.

4.KIM, B. J., CARDENAS, R. (1990). Use of Reed Beds for Dewatering Sludge in the USA. Advances in Water Pollution Control (IAWPRC). Constructed Wetlands in Water Pollution Control, PF. Cooper, B.C. Findlater (Ed.). pp 563-566.

5. 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.

Advances in Water Pollution Control (IAWPRC). Constructed Wetlands in Water Pollution Control. PF. Cooper, B.C. Findlater (Ed.). pp 245255.

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SLUDGE TREATMENT AND DRYING REED BED SYSTEMS IN DENMARK

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