THE USE OF REED-BEDS FOR SLUDGE DRYING

P. Cooper. BTech. MSc, CEng. MlChernE (Fellow), N. Willoughby, and 0. Cooper. BSc. MSc, MIAgrE*

ABSTRACT This paper reviews the design and performance experience with sludge- dying reed-beds over the past fourteen years. Whilst there are very few of these systems in the UK, there is much experience in Europe and particularly in Denmark. The Danish experience is reviewed in some detail. The design of and experience gained from two UK systems is described. The final dry-solids concentration depends upon the concentration in the initial sludge dose. It is possible, when treating anaerobically digested sludges containing 3-4% DS, to achieve about 90% volume reduction and a final dy-solids content of up to 40%. With thinner activated sludges containing 0 .346% DS, a reduction (in volume) of greater than 97% is possible with a final solids concentration in the range 10-20%.

Key words: Agricultural sludge; constructed wetlands; reed-beds; sludge drying.

This paper was presented at the 7th CIWEMAquaEnviro Conference on Biosolids and Organic Residuals, held in Wakefield on 17-20 November 2002.

* Associate, Director and Managing Director; respectively, ARM Ltd., Rugelel UK.

INTRODUCTION Reed-beds (constructed wetlands) have been used in the UK since the mid-l980s, and there are now more than 600 plants in operation. There are many variants of these systems for different purposes in use throughout the world and most of them have examples in the UK, except for the sludge-drying reed-bed (SDRB). There are only six full-scale systems and a few pilot tests of SORBS in the UK. It is suspected that this is because conventional sludge-drying beds were in common use at sewage-treatment works (STWs) in the UK up to the 1960s, and these were gradually phased out during the 1960s and 1970s. Some of these were large mechanised systems, but most were simple beds at small village works. They were gradually replaced because (a) in the British climate they did not dry the sludge well enough, and (b) better sludge-drying techniques were being introduced in the 1970s.

Sludge removal from rural STWs poses a problem to operators because it is rarely cost-effective or practical to provide mechanical dewatering to reduce the sludge volume. It is expensive to transport this high water-content sludge and, environmentally, it is regarded as undesirable to have sludge tankers travelling along small country roads.

During the last twenty-five years, interest has developed in using reeds (Phragmites Australis) to enhance conventional drying-bed performance. Lienard"' reports that Seidel and Kickuth, tried using reeds in sludge-drying beds for physical-chemical sludges at the nuclear research centre in Karlsruhe in the late 1 9 6 0 ~ [ ~ , ~ ~ . The process was then used at a number of other German STWs in the 1970s and 1980s. The first UK contact with SDRBs was in 1985 at a textile factory in Windelsbleiche, GermanyI4' where a vertical-flow bed was treating chemical sludges.

The use of reed-beds for sludge drying has been taken up enthusiastically and successfully in Denmark'5,6,781, the USA(9101 and France""), but not in the UK. In Denmark, large SDRBs are remotely dosed by computer control via the telephone network and are monitored by video links. They are widely used in the USA for village systems, especially on the East Coast1Iz1. SDRBs are now being introduced at

small village systems in Francell,"' and Belgiurn1l3'. There are only about ten examples in the UK where early attempts at designing and operating SDRBs in the late 1980s failed because of inadequate design or operation. One system was built in an old concrete tank which had leaks, and the Phragmites died; another system was abandoned after the operators overdosed it with sludge, and the reeds became submerged and subsequently died. Early mistakes might have set back the adoption of SDRBs in the UK, but these elementary mistakes could have been easily rectified by good design and operation. Subsequent installations have proved to be far more successful, and the benefits claimed for application at small or remote sites are: (i) Many years of operational security and low maintenance; (ii) In some cases they have no power requirement; (iii) They can be self-built by operators in remote locations; (iv) Tanker access is not required; (v) Dewatering can be carried out on sites where, previously, this was

non-viable; and (vi) The production of high-quality return liquors.

For larger systems the benefits are: (a) Operational security for up to a decade and low power usage; (b) Low maintenance and produce no noise; (c) Unmanned operation if carried out using remote sensors and

control; (d) No chemical additives for dewatering; and (e) The production of high-quality return liquors.

REVIEW OF SLUDGE-DRYING REED-BED DESIGN The principles that allow reed-planted sludge-drying beds to dry the sludge more rapidly than the old unplanted systems are as follows: (i) Stem, rhizomes and roots enhance water drainage by providing

channels in depth (probably the most significant effect); (ii) Wind-rock produces holes in the sludge surface at the base of

stems; (iii) Evapo-transpiration is enhanced by the presence of leaves; and

THE JOURNAL I V18 N2 1 MAY 2004 85

(iv) Mineralisation.

'opulation

?quivalent

:pel 2000 1000 30 000 10000 3 520 15000

Media Beds are normally about 800 mm deep, made up of gravel layers topped off with sand. The WRc/Severn Trent Water recommends 700 mm of 5-10 mm gravel topped with 100 mm of sharp sand. Nielsen@) used 350-450 mm of gravel topped with 150 mm of filter sand,

Sludge Type of load(tTS1 sludge

25 AS, DAS

14 AS, DAS 870 RS 152 CS, AS

42 AS 232 CS, AS

Design Loading Rates Estimating the sludge volumes and mass to be treated is the most important factor in correct design. Once this is determined, the sludge- loading rate is the next most important factor. Various loading rates have been reported.

Sludge loaded (m3) 263" 18 253" 23

Sludge residue (m3)

OPERATIONAL EXPERIENCE Experience in USA Sludge-drying reed-beds have been used since the mid-1980s for villages and small towns in Eastern USA, where a sludge-loading rate 30 kg DS/m2/annum is n 0 r m a 1 ~ ~ ~ ~ ~ ) . In Pacific North Western USA, Burgoon et all5' gradually increased the sludge dose from 10 kg DS/m2/annum to a design loading rate of 65 kg DS/m2/annum. The sludge loading was gradually increased in two-weekly steps.

Reduction (%)

93.2 90.9

Experience in France Lienard'lI] reported that a high sludge-loading rate in the first growing year could lead to the death of the Phragmites, and a similar sensitivity has been noted for agricultural sludges with a loading of 70 g DS/m2/d (equivalent to 25 kg DS/m2/annum). Lienard recommended a dose of only 18 kg DS/m2/annum in the first year; then with the reeds fully established, the loading could be increased to 60 kg DS/m*/annum.

Experience in Belgium De Maeseneeri13' recommended growing the reeds for one year before introducing sludge, then loading at 20-30 kg DS/m2/annum for aerobically-stabilised sludge containing 45-65% organic matter. This is equivalent to 1.0-1.5 m3 of 2% DS sludge/m*/annum or 1.5 population equivalent (pe)/m2 for a sludge.

Experience in Denmark A loading rate of 60 kg DS/m2/annum is used in Denmark'6,81 for surplus activated sludge (SAS) from plants with a high sludge age (>20 days). For activated-sludge plants with a lower sludge age, or for a mixture containing SAS and anaerobically digested sludge, the loading rate is reduced to 50 kg DS/m*/annum. Care has to be taken in allowing for beds being rested and for the planned emptying of beds@). There are two large systems in Denmark: (i) Kolding deals with a 125 000 pe and 2168 tDS/annum in an area of 62 000 m2; and (ii) Skive deals with a 123 000 pe and 2000 tDS/annum (but may be increased to 2850 t/annum) in an area of 99 000 m2.

The loading at Kolding is 35 kg DS/m2/annum and at Skive it is 29 kg DS/m'/annum. This shows the effect of allowance for beds being out of commission, to plan for the resting and emptying upon the spot loading of 60 kg DS/m2/annum. The quality and type of sludge can have a substantial effect. Nielsen@' reports treating mainly SAS or mixtures of SAS and anaerobically-digested sludge in Denmark and recommends using the capillary suction time (CST) test to assess sludge quality and define the loading rate (Table 1).

Overall Volume Reduction Nielsen@' reports the results from six beds in Denmark: with three beds receiving activated sludge (two anaerobically digested), two beds treating chemical sludge mixed with activated sludge, and one receiving primary sludge; ranging in size from 215 m2 to 11 6000 m2 and 1000-30 000 pe. Table 2 gives the basic data on these systems, and Tables 3 and 4 show the percentage reductions which were achieved in two of the beds.

Table 1. Correlation between CS7; area loading rate and number of beds"

CST ( 5 )

30 30-100 100-500 500-1000 1000 2000

Area loading rate (kg DS/m2/annum) 60 60

50 50 40

No. of beds

opening

8 10 10 12-14 12-14

Table 2. Reference plants in Denmark6'

11 600 2 377 700 4,000

Table 3. Sludge loading and reduction at Regstrup, Denmark6)

BedNo.

89.6 2154" 56 97.4

N.B. a sludge DS approx. 0.4% sludge DS approx. 3.0%

Table 4. Sludge loading and reduction at Allerslev, Denmark5'

Bed No. I

THE JOURNAL 1 V l 8 N 2 I MAY 2004 86

be used increasingly in the UK, especially at works which are remote and difficult to service. There is an urgent need for data to be collected from existing UK plants, so that the design of these systems can be refined for future use. Work in Denmark has shown their application at large works, where there are already two sites treating sludges from populations greater than 120 000. Volume reduction wil l depend upon the in i t ia l sludge concentration. For th in sludges such as activated sludge containing about 0.6% DS, the reduction can be greater than 97%. For thickened sludges, such as anaerobically digested sludges containing 3-4% DS, the reduction will be about 90%. The final sludge concentration is usually in the range 20-40% DS. The 40% DS value could be achieved with surplus activated sludge. Sludge-loading rates of 60 kg DS/mz/annum are acceptable; however, this is for a fully- commissioned plant and does not take into account the time required for resting and emptying at the end of the fi l l ing cycle. When these periods are taken into account, the overall loading rate could decrease to 30-35 kg DS/m2/a nn um. Care has to be taken not to overload the beds in the first year and particularly in the spring, when a high loading of raw sludge can be deleterious to the growth of new shoots. Some designers recommend growing the bed for up to a year before adding any sludge.

LIENARD, A., ESSER, D., DEQUIN, A. AND VIRLOGET, F. Sludge dewatering and drying in reed beds: an interesting solution? General investigations and first trials in France. In: Constructed Wetlands in Water Pollution Control. Pergamon Press, Oxford, 1990,257. BITTAMANN, M. AND SEIDEL, K. Entwaesserung und aufbereitung von chemieschlamm wit hilfevon pflanzen. GWF$ 1967, 108, (18), 488. KICKUTH, R. Hoehere wasserpflanzen und gewaesserreinhaltung. Schriftenreihe Vereinigung Deutscher Gewaesserschutz, 1969, EV VDG, 19, 3. BOON, A. G. Report of a Visit by Members and Staff of WRc to Investigate the Root-Zone Method for Treatment of Wastewaters. WRc Report 37641, WRc, Medmenham.1985. NIELSEN, S. M. Sludge dewatering and mineralization in reed bed systems. In: Constructed Wetlands in Water Pollution Control. Pergamon Press, Oxford, 1990, 245. NIELSEN, S. M. Biological sludge drying in constructed wetlands. In: Constructed Wetlands for Water Quality lmprovement. Lewis Publishers, Boca Raton, 1993, 549. NIELSEN, S. M. Biological sludge drying in reed bed systems -six years of operational experience. In Proc. 4th lnf. Conf. on Constructed Wetlands for Water Pollution Control. Guangzhou, China, November, 1994. NIELSEN, S. M. Sludge drying reed beds. In Proc. 8th lnt. Conf. on Wetland Systems for WaterPollution Control. Arusha, Tanzania, 16- 19 September, 2002. (In press.) KIM, B. J. AND CARDENAS, R. Use of reed beds for dewatering sludge in the USA. In: Constructed Wetlands in Water Pollution Control. Pergamon Press, Oxford, 1990, 563.

(10) KIM, B. J. Field evaluations of reed bed sludge dewatering

technology: Summary of benefits and limitations based on 4 years experience at Fort Campbell, Kentucky. In Proc. 67thAnn. Conf. of Wat. and Envir: Fed., Chicago, October, 1994.

(11) LIENARD, A., DUCHENE, P. AND GORINI, D. A study of activated sludge dewatering in experimental reed-planted or unplanted sludge drying beds. Wat. Sci. & Technol., 1995, 32, (31, 252.

(12) BOON, A. G. Report of a Visit to Canada and the USA to Investigate the Use of Wetlands for Treatment of Wastewater. WRc Report425-S, WRc, Medmenham. 1986.

(13) DE MAESENEER, J. L. Constructed wetlands for sludge dewatering. Wat, Sci. & Technol., 1997, 35, (5), 279.

(14) COOPER, P. F., JOB, G. D., GREEN, M. B. AND SHUTES, R. B. E. Reed Beds and Constructed Wetlands for Wastewater Treatment. WRc, Medmenham, 1996,202.

(15) BURGOON, P. S., KIRKBRIDE, K. F., HENDERSON, M. AND LANDON, E. Reed beds for biosolids drying in the arid North Western United States. Waf. Sci. & Technol., 1997, 35, (5), 287.

(16) COOPER, P. F. Design of Sludge Drying Reed Beds for Ballygrant STW and Kiells STW, Isle of Islay, Argyle Division, Strathclyde Regional Council. WRc Report UC 2591, WRc, Swindon, 1995.

(17) EDWARDS, J. K., GRAY, K. R., COOPER, D. J., BIDDLESTONE, A. J. AND WILLOUGHBY, N. Reed bed dewatering of agricultural sludges and slurries. In Proc. 7th lnt. Conf. on Wetland Systems for Water Pollution Control. Orlando, September, 2000.

(18) COOPER, P. F., COOPER, D. J., EDWARDS, J. AND BIDDLESTONE, J. Treatment of sludges from sewage and agricultural wastewaters using sludge drying reed beds. In Proc. 4th Workshop on Transformations of Nutrients in Natural and Constructed Wetlands. Trebon, Czech Republic, 2001.

THE JOURNAL I V18 N 2 I MAY 2001 8 9