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An examination of the treatment ofiron‐dosed waste activated sludge byanaerobic digestionD. K. Johnson a , C. M. Carliell‐Marquet a & C. F. Forster a

a School of Engineering , University of Birmingham , Edgbaston,Birmingham, B15 2TT, UKPublished online: 17 Dec 2008.

To cite this article: D. K. Johnson , C. M. Carliell‐Marquet & C. F. Forster (2003) An examination of thetreatment of iron‐dosed waste activated sludge by anaerobic digestion, Environmental Technology, 24:8,937-945, DOI: 10.1080/09593330309385632

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Environmental Technology, Vol. 24. pp 937-945© Selper Ltd, 2003

AN EXAMINATION OF THE TREATMENT OF IRON-DOSED WASTE ACTIVATED SLUDGE BY ANAEROBIC

DIGESTION

D. K. JOHNSON, C. M. CARLIELL-MARQUET AND C. F. FORSTER*

School of Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

(Received 22 August 2002; Accepted 20 February 2003)

ABSTRACT

Anaerobic digestion is an important sludge treatment process enabling stabilisation of the organic fraction of sewage sludgeprior to land application. Any practice which might retard the anaerobic digestion process will jeopardize the stabiKty of theresulting digested sludge. This paper reports on an investigation into the relative digestibility of iron-dosed waste activatedsludge (WAS) from a sewage treatment works (STW) with chemical phosphorus removal (CPR), in comparison to WAS froma works without phosphorus removal. Two laboratory scale anaerobic digesters (51) were fed initially with non iron-dosedWAS (Works M) at a solids retention time of 19 days. After 2 months the iron-dosed CPR sludge (Works R) was introducedinto the second digester, resulting in a 32 % decrease in biogas production and an increase in the methane content of thebiogas from an average of 74 % to 81 %. Pre-treatment of the CPR sludge with sodium sulphide and shear, both alone and incombination, caused the gas production to deteriorate further. Pre-acidification and pre-treatment with EDTA did result inan enhanced gas production but it was still not comparable with that of the digester being fed with non-iron-dosed sludge.The daily gas production was found to be linearly related to the amount of bound iron in the sludge.

Keywords: Activated sludge, iron dosing, anaerobic digestion, gas production.

INTRODUCTION

The phasing out of the marine sites for the disposal ofsludge in 1998 meant that there was a need to find additionaldisposal routes for some 200,000 tonnes of sludge each year inthe UK. Traditionally, a significant proportion of the sludgeproduced during the treatment of municipal sewage has beenapplied to agricultural land and this is still considered to bethe Best Practical Environmental Option [1]. However, thecombination of increased environmental awareness, publicconcern and European legislation has resulted in attentionbeing re-focussed on many aspects of sludge processing.Arguably, the most significant aspects which must beconsidered are the requirements of the Safe Sludge Matrix,together with the proposed EU Directive, the MunicipalWastewater Treatment Directive and the Codes of Practice forAgricultural Use of Sewage Sludge [2,3]. In other words, thereis a need to consider the stability of the sludge and the fate ofinorganic nutrients, particularly phosphate.

The removal of phosphate from municipal sewage canbe achieved biologically [4,5] or by the use of inorganic salts,such as ferrous sulphate, to form insoluble phosphates whichcan then be removed by settlement [5,6]. In the activatedsludge process, these inorganic phosphates are incorporatedinto the sludge floes and change their characteristics

appreciably. The precise speciation of the iron which isformed in the sludge floes when phosphate is removed in thisway is not clear but it is likely to be a complex hydroxy ironphosphate [7].

Mesophilic anaerobic digestion has always been one ofthe options for treating sewage sludge prior to landapplication in that it stabilises the organic fraction of thesludge and, whilst there will be a need to consider additionalpasteurising processes for some sludges to conform to thespecifications for extended treatment [2], it is likely to have anappreciable role for some time.

Waste activated sludges which have been generated ina chemical phosphate removal (CPR) process will also have tobe treated by anaerobic digestion to acquire the status oftreated sludge as defined by the Safe Sludge Matrix. Althoughsome earlier work has shown that this type of sludge willhave no effect on digestion [8,9], there is a strong body ofopinion which suggests that the use of CPR sludges mayresult in an adverse effect on the digestion process [10,11].The pre-treatment of sludges to enhance the anaerobicdigestion process has been the subject of an appreciableamount of research in recent years. This has, essentially, beenaimed at disintegrating the sludge particles so that thebacteria can degrade them more quickly. Techniques whichhave been examined include the use of ultrasound [12], the

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use of chemicals combined with shear [13] and the use of

gamma radiation [14]. One approach which was specific for

iron-dosed sludge was the use of sulphide [15]. This paper,

therefore, describes an examination of a series of pre-

trearments of an iron-dosed activated sludge and their effects

on the anaerobic digestion process. It also compares the

results with a sludge which had not been iron-dosed.

MATERIALS AND METHODS

The Digesters

The digestion systems were constructed from two

identical continuously stirred tank reactors each of which

consisted of a purpose-built flanged glass tank with a side

port such that the working volume was 5 1 (Figure 1). The

feed of thickened waste activated sludge was pumped to the

base of the reactor by peristaltic pumps (Watson Marlow,

Model 302S). The concentrations of the feed were 25.82 ± 3.62

and 28.92 ± 5.27 gl1 for the sludges from Works M and R

respectively. Feeding was done five times a week but the

solids' retention time was calculated on the basis of a seven

day week. Both digesters were operated at a solids' retention

time of 19 days. The stirrer speed was controlled at 100 rpm

(Electrolab, Bredon, Glos.) and the temperature was

controlled at 35°C by a heating pad/thermister system

(Electrolab, Bredon, Glos.). Both digesters contained an active

biomass which had been digesting waste activated sludge for

several years. The biogas was collected by the downward

displacement of acidified water (0.05M H2SO4) and measured

at standard temperature and pressure (760 mm Hg; 273 °K).

The two waste activated sludges which were used as

feed for the digestion systems were obtained from full-scale

activated sludge plants operated by Severn Trent Water Ltd.

After sampling, they were stored in refrigerated (5°C) tanks

(3001) until use. Works R was an activated sludge plant where

iron dosing was being employed for the removal of

phosphate. Works M was not being treated with iron.

Pre-treatments

The pre-treatments which were used to modify the

sludge from Works R are summarized in Table 1. The

sulphide treatment involved the addition of 200mM sodium

FromC

Feed

ToB

Gas

D

Waste

Figure 1. Schematic diagram of the digester (A. Stirrer control, B Heater control, C Heater pad).

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Table 1. Sequence of treatments and pre-treatments applied to the sludge from Works R.

Start

1-11-009-1-01

22-5-0128-8-0125-10-0110-1-028^-02

Finish

21-12-0031-3-01

28-8-0119-10-0121-12-0118-3-0219-5-02

Trial Number--

12345

Pre-treatment

Digester compatibility trialD-l/Non-iron dosed sludge; D-2/Iron dosedsludge with no pre-treatmentSulphide treatmentSulphide treatment plus rapid mixingRapid mixing (shear)Pre-acidificationEDTA

sulphide (NaS.9H2O) (32 ml) to unthickened sludge (968 ml)with a contact time of 10 minutes. This was based on theconcentration of iron in the sludge and a preliminary studywhich showed that this concentration would leave a lowconcentration of sulphide (< 40 mgl"1) in the liquors [16]. Shear(425.5 s"1) was imparted using a six-bladed paddle in a fullybaffled tank with a shearing time of 15 minutes [16]. Pre-acidification was achieved by mixing the sludge underanaerobic conditions for 18 hours at 55°C [17]. The treatmentwith EDTA involved the addition of di-sodium ethylenediamine terra-acetic acid to the unthickened sludge to give afinal concentration of 0.1M. This resulted in a soluble ironconcentration of 688.7 mgl'1 in the feed sludge. AH the treatedsludges were thickened by centrifugation (1000 x g; 1 min).

Analytical Methods

Total and volatile solids were measured by the standardgravimetric methods [18]. Alkalinity was measured bytitration with 0.05M H2SO4 and the pH by using a standardelectrode/meter (Mettler Toledo, Model 320). Volatile fattyacids (VFAs) were measured with a gas Chromatograph (GC)(Cambridge Ai GC94) which had been calibrated withdilutions of a standard mixture of acids (acetic = 500 mgl'1,propionic to caproic = 250 mgl"1). A megabore column (D-BFFAP, 30m x 0.536 mm ID) was used and the carrier gas washelium (3.2 ml min'1 ). The initial column temperature of105°C was increased at the rate of 30°C per minute until atemperature of 145°C had been reached and then at a rate of15°C per minute until a temperature of 190°C was achieved.Samples were centrifuged for 30 minutes at 1250 x g (MSEScientific Instruments Model GF-8) and then filtered througha 0.2 urn nitro-cellulose membrane. Before analysis, thesample (1.0 ml) was acidified with formic acid (4 ul) and asample volume of 0.7 \il was injected into the GC. Thecomposition of the gas was also measured by gaschromatography (Pye, Model 104) using a glass column(16.3m x 3 mm ID) with a Porapack Q (mesh size 80-100)support. The column temperature was 50°C, helium was usedas the carrier gas (40 ml min'1) and the sample size was 1 ml.

Phosphate, both soluble and acid-digestible, was

measured by the standard vanadomolybdophosphoric acidmethod [18]. The digestion was done with nitric and sulphuricacid. This digest was also used for the measurement of boundiron using atomic adsorption spectrometry (Unicam 939).

Statistical analyses were done with the AnalysisToolpak in Microsoft Excel.

RESULTS AND DISCUSSION

Initially, both digesters were fed with sludge fromWorks M to determine whether there were any digester-specific differences in their performance. The results, over atwo month period (Figure 2), showed that this was not thecase. The results in Figure 2 also show that there was a cyclicsequence in gas production throughout the week. This wasdue to the digesters not being fed or attended over theweekend period. The peaks were, therefore, the cumulativegas production over the weekends. The average methanecontent of the gas from D-l was 74.6 ± 3.4% and that of thegas from D-2 was 74.7 ± 3.9%.

The feed to the digester designated as D-2 was thenchanged to waste activated sludge from Works R. Works Msludge continued to be fed to the other digester; D-l. Figure 3shows that this change caused the long-term mean daily gasproduction to drop from 1890 mid"1 to 1280 mid"1,demonstrating that the use of an iron-dosed sludge did affectthis parameter in an adverse way. Figure 3 also shows that,although there were occasions when the gas production wassimilar to that of the untreated sludge, sulphide dosingcaused an appreciable deterioration in the gas production.The reason for this deterioration in performance is unclear atthe moment, unless trace metals such as cobalt, which arenecessary for the digestion process, were also beingimmobilised as insoluble sulphides.

The original study by Nielsen and Keiding [15] showedthat the sulphide, in reacting with the iron to form ferroussulphide, weakened the sludge floes with the result that up to10% of the total organic matter in the floe was solubilised.This pattern of behaviour would be expected to result in anenhancement of the digestion process. The introduction ofshear would be expected to solublise organic matter further

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4000

3500

3000

"•o

g 2500

o1 20003"O

a 1500M(S

O1000

500

<v*N

Figure 2. Gas production by D-1 (A) and D-2 (•) fed with sludge from Works M.

2500

12-Apr-01 04-Jul-01 15-Aug-01 21-Sep-01 16-Nov-01

Figure 3. Mean daily gas production by D-2 with pre-treated sludge (•) showing the long-term mean values for D-1 (A) and D-2 with untreated sludge (•).

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[19]. However, an analysis of the data showed that thecombination of shear and sulphide (Trial 2) decreased thegas production further from the Trial 1 mean value of 940mid"1 to one of 602 mid"1 (Figure 3). This difference wasstatistically significant (ANOVA; p < 0.05). An examination ofthe physico-chemical characteristics of the various digestedsludges (Table 2) shows that there were no abnormalparameters. However, if the two criteria which are frequentlyused to judge the stability of an anaerobic digestion; Ripley'sratio, which is the ratio of intermediate alkalinity, the titrefrom pH = 5.7 to pH 4.3, to total alkalinity [20], and the ratioof total VF A: alkalinity [20]; were examined, the former wasabove the recommended ideal of 0.1 to 0.35 although the latter

was well below the value deemed to indicate digesterinstability. The use of shear without the addition of sulphide,Trial 3, resulted in a significant increase in the gas production(ANOVA; p < 0.05) to a mean value over the two months ofthe trial of 840 mid"1. However, this value was significantlylower than that obtained from both the untreated iron-dosedsludge and the non-iron-dosed sludge.

Figure 4 presents the daily gas production by D-2during the periods when the sludge from Works R wasreceiving pre-acidification and when it was being pre-treatedwith EDTA and shows that both of these pre-treatmentprocedures gave better results than when there was nopre-treatment. However, a statistical analysis showed that the

Table 2. Physico-chemical characteristics of the digested sludges from D2.

Trial

Alkalinity(mgl1)pH

Ripley's ratio

VFA:alk

Untreated4933±4447.38±0.040.43±0.090.004±0.003

1

5068±4367.55± 0.1190.5

±0.090.010±0.004

2

5519±9247.41±0.080.69±0.140.009±0.003

3

4986±5027.36±0.100.73±0.100.033± 0.031

4

6310±5497.26±0.080.75±0.120.038± 0.013

5

5511±788

7.1

±0.100.66±0.170.007± 0.005

2500

Figure 4. Mean daily gas production in Trials 6 and 7 (•) showing the long-term mean values for D-1 ( • ) and D-2 usinguntreated iron-dosed sludge (•).

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pre-acidification data were not significantly differentfrom those obtained when there was no pre-treatment(ANOVA; p > 0.05). The same analysis showed thatthe EDTA treatment did give a significant enhancement ingas production. Indeed, it did appear to give a gasproduction, which for a limited period, was comparablewith that from D-1. However, a statistical comparison of thedata with the mean value for the gas produced by D-1 showedthat neither of these pre-treahnents enabled the iron-dosedsludge to be digested as effectively as the non-iron-dosedsludge.

The gas composition is summarised in Table 3 andshows that the introduction of the iron-dosed sludge broughtabout a significant increase in the methane content of the gas.However, this was not a sufficiently large increase to alter theoverall effect. The non-iron-dosed sludge produced thegreater quantity of methane, 1597 mid'1, on average,compared to 1036 mid"1 from the iron-dosed sludge. The otherpre-treatments produced the same effect.

The reduction in the volatile solids' concentration is ascritical in assessing a digestion process as is the production ofthe gas. The summary results in Table 4 show that the use ofthe iron-dosed sludge resulted in a lower reduction in volatilesolids and that, in general, this difference was statisticallysignificant.

A VFA:alkalinity ratio in a digester of greater than 0.4implies that there is a degree of instability [21] and, althoughthe results in Table 2 show that the ratios in D-1 were well

below this critical value, Figure 5 shows that there werefluctuations in the total VFA concentrations. Generally, theconcentrations were lower than those in the digester treatingthe non-iron-dosed sludge, but, during the periods whenshear and pre-acidification were being used, there weresignificantly higher concentrations of VFA in the digester.Their presence during the pre-acidification trial isunderstandable, but the fact that shear induced higherconcentrations of VF As in 20% of the samples at the end of thetrial cannot be explained at the present time. However, it ispossible that the shear could have affected the particle size bybreaking up the sludge floes, increasing the surface areaavailable for enzymic attack and, thus, increasing the rate ofVFA production.

In Trials 1 and 2, the iron-dosed sludge fed to digesterD-2 and the resultant digested sludges contained lowconcentrations of soluble phosphate, the averageconcentrations were 7.8 and 15.4 mg I"1 respectively. The pre-treatment by shear alone resulted in a significant increase inthe soluble phosphate being released during digestion,generating a mean concentration of 33.1 mg I"1 (ANOVA: p <0.05). Pre-acidification had the same effect, with digestionresulting in an increase in the soluble phosphateconcentration from 6.5 mg I"1 in the feed to one of 23.0 mg I"1.The treatment with EDTA, however, resulted in phosphatebeing released so that the sludge fed to D-2 had a meansoluble phosphate concentration of 197 mg I"1. A significantproportion of this was taken up again by the sludge during

Table 3. Comparison of the methane content of the gas.

Treatment for D-2 Feed

No treatmentSulphideSulphide + stirringStirringPre-acidification

EDTA

Methane content (%)

D-1

74.4 ± 1.9

72.4 ± 2.3

73.3 ± 2.3

75.3 ± 2.5

73.8 ± 2.3*

73.8 ± 2.3*

D-2

80.7 ± 1.9

82.3 ± 1.5

80.3 ± 2.0

83.2 ± 2.5

80.7 ± 1.2

74.3 ± 6.7

ANOVA; p

7x10*1.6 x lCT*

1.5X10"5

5x10"*

1.27x 1 0 "

0.63

* Mean value for the preceding year

Table 4. Reduction in volatile solids.

Treatment for D-2 Feed

No treatment

Sulphide

Sulphide + stirring

StirringPre-acidification

EDTA

VS reduction (%)

D-1

43.7 ±4.5

37.7 ±8.5

35.1 ± 8.5

41.5 ±5.1

40.0 ±7.2*

40.0 ± 7.2*

D-2

36.7 ± 7.2

26.5 ± 12.6

26.9 ± 11.3

11.9 ± 13.5

27.7 ±8.5

18.9 ± 16.6

ANOVA; p

0.06

0.01

0.06

0.005-

-

* Mean value for the preceding year

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06- 04- 13- 22- 02- 05- 23- 14- 17-Feb- Apr- Jun- Aug- Oct- Dec- Jan- Mar- May-01 01 01 01 01 01 02 02 02

Figure 5. Variation in the total VFA concentrations in D-2.

the digestion process so that the digested sludge had a meansoluble phosphate concentration of 83 mg I"1.

The untreated iron-dosed sludge, as would be expected,had a very low concentration of soluble iron. Typically, it was

< 1 mg I'1. Figure 6 shows how the various pre-treatmentsaffected the sludge and what impact the digestion processhad. Neither of the sulphide treatments had any effect on thesoluble iron concentration. The use of sulphide on its own

45

40

en 35Eco 30

t 25ooo 20! 15oS= 10oW

i 3000

3 4 5 6 7

Trial

Figure 6. Mean soluble iron concentrations in the sludges ( • D-2 E3 Works R).

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gave a digested sludge with a mean concentration of 3.4 mg I'1

and when sulphide and shear were combined, the digestedsludge had a mean soluble iron concentration of 13.3 mg I*1.When only shear was used the soluble iron concentration was< 1 mg I"1 but after digestion, this had increased to 27.9 mg I'1.These results are to be expected as the sulphide ions wouldbind any iron which was in solution and, whilst shear wouldbreak up the sludge floes, there is no reason to expect it toaffect any pre-precipitated iron. The fact that some of the ironin the feed sludges was released into solution during theanaerobic digestion process has been noted previously [15],

Pre-acidification released a small amount (2.1 mg I"1) ofiron from the raw sludge and after digestion the soluble ironconcentration had risen to an average value of 39.7 mg I"1. Thepre-acidification stage involves the hydrolysis of largemolecules present in the sludge and the fact that some ironwas released into solution suggests that some of the iron wasbound to these molecules, possibly to the extracellularsubstances which are present in activated sludge and areknown have metal binding properties [22]. The use of EDTAcaused an appreciable amount of iron to be released intosolution, both at the pre-treatment stage and after digestion,689 mg I"1 and 2723 mg I'1 respectively. The various pre-treatments affected the bound iron in the sludges fed to D-2

and an examination showed that these concentrations werelinearly related to the daily gas production (Figure 7). Therelation described by the regression equation is significant atthe 99% level. There was also a trend, rather than amathematical relationship, which indicated that, as theamount of bound iron in the sludge increased, the reductionin the volatile solids decreased.

CONCLUSIONS

1. The anaerobic digestion of an iron-dosed sludgeproduced less gas than that of a non-iron-dosed sludge.

2. The use of sulphide and shear, both alone and incombination, brought about a further reduction in thegas production.

3. EDTA addition and pre-acidification improved gasproduction in comparison to iron-dosed sludge with nopre-treatment. However, the performance was not asgood as with non-iron-dosed sludge.

4 . The gas production was linearly related to theconcentration of bound iron in the sludge.

5. The anaerobic digestion of an iron-dosed sludge gavea lower reduction of volatile solids than that of a non-iron-dosed sludge.

2500

~ 2000

|

o

(0mw

1500

1000

s 500

y = -6.3909X + 2044.1R2 = 0.9274

50 100 150 200

Bound iron (g Fe kg'1 TS)

250

Figure 7. Relationship between the bound iron concentration and the daily gas production.

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REFERENCES

1. Hickman G. A. W., Sustainable beneficial biosolids recycling to land - Future issues, Sludge 9 - Seminar on Developments in

Sludge Treatment and Disposal, University of Surrey, September (1999).

2. Anon., Safe Sludge Matrix: Guidelines for the Application of Sewage Sludge to Agricultural Land, ADAS (AMPU 1234/C)

(1999).

3. Evans, T., A comparison of international standards for the beneficial use of biomaterials: can we see where we might be

going, Water Environ. Manage., 15, 64-71 (2001).

4. Upton J., Hayes E. and Churchley J., Biological phosphorus removal at Stratford upon Avon; the effect of influent

wastewater characteristics on effluent phosphate, Water Sci. Technol., 33, 73-80 (1996).

5. Morse G. K., Brett S. W., Guy J. A. and Lester J. N., Review: phosphorus removal and recovery technologies, Sci. Total

Environ., 212, 69-81 (1998).

6. Strickland J., The development and application of phosphorus removal from wastewater using biological and precipitation

techniques, Water Environ. Manage., 12, 30-37 (1998).

7. De Haas D. W., Wetzel M. C. and Ekama G. A., The use of simultaneous chemical precipitation in modified activated

sludge systems exhibiting biological excess phosphate removal. Part 5: Experimental periods using a ferrous-ferric

chloride blend, Water SA, 27, 117-134 (2001).

8. O'Shaughnessy J. C., Nesbitt J. B., Long D. A. and Kuntz R. R., Digestion and dewatering of phosphorus enriched

sludges. J. Water Pollut. Control Fed., 46, 1914-1925 (1974).

9. Malhotra S. K., Parrillo, T. P. and Hartenstein, A. G., Anaerobic digestion of sludges containing iron phosphate. J. Sanitary

Eng. Div. Proc. Am. Soc. Civil Eng., 97, 629-645 (1971).

10. Dentel K. S. and Gossett J. M., Effects of chemical coagulation on anaerobic digestibility of organic materials. Water Res.,

16, 707-718 (1982).

11. Gossett, J. M. and McCarty P. L., Anaerobic digestion of sludge from chemical treatment. J. Water Pollut. Control Fed., 9,

533-542 (1978).

12. Clark P. B. and Nujjoo I., Ultrasonic sludge pretreatment for enhanced sludge digestion., Water Environ. Manage., 14,66-71

(2000).

13. Müller J., Lehne G., Schwedes J., Battenberg S., Naveke R., Kopp J., Dichtl N., Scheminski A., Krull R. and Hempel D. C.,

Disintegration of sewage sludges and influence on anaerobic digestion, Water Res., 38,425-433 (1998).

14. Lafitte-Trouqué S. and Forster C. F., The use of ultrasound and γ-irradiation as pretreatment for the anaerobic digestion of

waste activated sludge at mesophilic and thermophilic temperatures. Bioresource Technol., 84, 113-118 (2002).

15. Nielsen P. H. and Keiding, K., Disintegration of activated sludge flocs in the presence of sulfide, Water Res., 32, 313-320

(1998).

16. Johnson D. K., Effect of Iron Dosing on the Anaerobic Digestion of Sludge, PhD Thesis, University of Birmingham, Birmingham,

UK. (2003).

17. Roberts R., An Evaluation of Anaerobic Thermophilic/Mesophilic Dual Digestion of Sludge, PhD Thesis, University of

Birmingham, Birmingham, UK. (1999).

18. Clesceri L. S., Greenberg A. E. and Eaton A. D., (Eds.), Standard Methods for the Examination of Water and Wastewater, 20th

Edn., APHA, AWWA, WEF. Washington, DC, USA. (1998).

19. Forster C. F., The effect of centrifugal pumping on the physical characteristics of activated sludge, Environ. Technol. Lett, 9,

245-250 (1988).

20. Ripley L. E., Boyle W. C. and Converse J. C, Improved alkalimetric monitoring for anaerobic digestion of high-strength

wastes, J. Water Pollut. Control Fed., 58, 406-411 (1986).

21. Zickefoose C. and Hayes R. B., Anaerobic Sludge Digestion: Operations Manual, EPA 430/9-76-001 (1976).

22. Forster C. F., Factors involved in the settlement of activated sludge - II The binding of polyvalent metals. Water Res., 19,

1265-1272 (1985).

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