SIZING OF WASTEWATER SLUDGE ANAEROBIC DIGESTERS*

R. V. ARSOV University of Architecture, Civil Engineering and Geodesy 1 Chr.Smirnensky blvd., 1046 Sofia, Bulgaria

1. Introduction

Wastewater sludge treatment and disposal have always created more problems than wastewater treatment itself. This is rooted in the fact that in contrast to wastewater, which continuously passing the wastewater treatment plants (WWTPs) inflows unaffectedly (in quantitative sense) the natural hydrological cycle, sludge entirely accumulates there. Due to its specific properties, the accumulated sludge inclusion in the natural cycles of mass transfer “in an economically and environmentally acceptable manner” [14] is more difficult. This is because sludge management is associated with overcoming of serious technological and economical problems, some of which have not received satisfactory solution yet. Biological stabilisation of municipal wastewater sludge is one of them, irrespectively of the availability of significant experience and the long historical development of this issue.

Classical technologies for municipal wastewater sludge anaerobic stabisisation proved to be reliable, adequate to the contemporary technological and ecological requirements and are still intensively used in the current sanitary engineering practice. They are applied both for separate or for mixed primary and waste activated sludge stabilisation and usually include the following units: high-rate (heated) anaerobic digesters (methane-tanks), low-rate (conventional, open-air, unheated) anaerobic digesters and Imhoff-tanks (Emscher-wells, two-stage settlers). Despite the biological, chemical and physical processes taking place in these units are now well known and are subject of intensive modelling, adequate generic design procedures for more of them are still missing.

Irrespectively of the remarkable scientific and applied research achievements in this field, out-of-date design methods and parameters, some of which have been established empirically more than 60 years [10, 11] are still intensively use in the current practice for technological design of sludge anaerobic digestion units. Lack of knowledge about the kinetics and mutual impact of relevant biological, chemical and physical processes undoubtedly is not among the reasons for this somewhat “strange” situation. The problem is more complex but its discussion is out of the scope of this paper.

It is than not surprising that in response to the questionnaire, prepared and distributed by IAWQ TG on Anaerobic Digestion Modelling and concerning establishing of new generic model for anaerobic processes in sanitary engineering [13],

68 % of respondents require the future model to be applicable mainly for sludge digestion and 80 % of them want it to perform design procedures.

*Submited to “Water Research”, IWA in 2000

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Therefore creation of a generic design model for wastewater sludge biological stabilisation is very actual task. The general requirements for such a model in our view could be summarised as follows: Reasonable simplicity and adequacy, achievable by respecting only the most

important processes taken in lump which would allow limited number of input parameters to be employed. This is important since the vast part of the model potential users are able to define only the basic sludge quality input parameters, such as suspended solids (SS), volatile suspended solids (VSS), activated sludge age and temperature [13];

Possibility for positioning of the designed digestion system at arbitrary (given) design study-states in order sludge stabilisation to be achieved “in an economically and environmentally acceptable manner” [14]. This would allow conformity of the specified system state to the optimal point of the specific relation between efficiency (respectively – necessary degree of stabilisation at definite unit) and costs, associated with local environmental (hygienic), technical and economical conditions. For this the model have to be based on general kinetic parameters of the relevant biological (anaerobic, aerobic) and physical (thickening) processes, taking into account only limited number of sludge parameters as well as the specific differences between PS and WAS in this respect;

Establishment of relatively simple but adequate kinetic relationships between hydraulic retention time (HRT) and specified degree of sludge stabilisation for the relevant anaerobic, aerobic and physical processes, associated with PS, WAS and mixed primary and waste activated sludge (mixed sludge - MS), respectively. This would allow the necessary volume determination of the relevant bioreactor of definite hydrodynamic type.Description of the basic relationships in a design (bioreactors’ volumes sizing)

procedure, developed in a compliance with the above requirements and the relevant parameters for municipal sludge aerobic and anaerobic digesters are subjects of this paper.

2. Methods

Volumes V of the bioreactors are usually calculated on the base of the continuity equation, which in the context of the reported research could be represented by the following general formulae:

. (1)

Different design models distinguish each to the other by the way they define the values of and , with later in the case if sludge thickening and supernatant

withdrawal take place. The supernatant flow rate depends on the BVSS degree of destruction and sludge thickening kinetics (respectively – on HRT at some anaerobic digesters with cyclic feeding/withdrawing). In turn the hydraulic retention time

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depends on hydrodynamic conditions, BVSS destruction kinetics and degree of destruction. In this respect the design model discussed hereinafter considers the following processes and parameters: kinetics of anaerobic processes (taken in lump), associated with PS, WAS and MS; HRT for relevant hydrodynamic type of bioreactors, associated with given degree of sludge stabilisation; kinetics of the processes of digesting sludge thickening, associated with some anaerobic digesters where thickening and supernatant withdrawal take place; volumes of digesters; input SS and VSS concentrations in PS and MS, as well as SS and sludge age of WAS and temperature in the digesters.

3. Results and discussion

3.1. DETERMINATION OF SLUDGE DIGESTERS HRT

Since the HRT depends on the relevant biological process kinetics, degree of sludge stabilisation and on the hydrodynamic type of the bioreactor, these parameters have to be defined accordingly.

By tradition the degree of sludge stabilisation is controlled by the relevant reduction of its VSS content. The later can be easily defined by a routine analytical procedure at the WWTP laboratory, but it is not reliable base for description of sludge stabilisation kinetics, since the VSS besides biodegradable organic substance include organic part, which is practically resistant to biological destruction. It is more correct the biological processes lump kinetics to be based solely on the biodegradable fraction of the sludge volatile suspended solids (BVSS), which is different from the traditional approach but is more adequate one. The problem is that no any direct method for BVSS determination exists, which imposes an indirect approach to be applied.

The design hydraulic retention time can be determined by the well known relationship (2), valid for completely stirred reactor, which here is presented in a compliance with the assumed approach for raw sludge actual BVSS content determination (Arsov, 1999c):

. (2)

Determination of the first order reaction rate coefficients of sludge BVSS destruction is described in details elsewhere [1, 3].

It is based on plant-scale data gathered in a period of 5 years at the Moscow WWTP high-rate digesters and published by Karpinski [12]. The values of the kinetic parameter , associated with the relevant kinds of biological processes and wastewater sludge can be calculated by the expressions (3a), (3b), (3c[1, 3], respectively:

; (3a)

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; (3b)

. (3c)

The value of residual fraction of BVSS reflects the degree of sludge stabilisation and is subject of evaluation by the designer. High value of is associated with high odour emission potential of digested sludge [19], while adoption of low value of leads to high digester volume and therefore – high costs. Therefore the choice of the value of has to be based on carefully assessed balance between local ecological (hygienic) requirements and costs. Contribution of the active biomass, incorporated in the digested sludge has to be considered as well, since it increases the actual value of residual BVSS fraction in digested sludge. The relevant analyses and recommendations for resuming of the value of are given elsewhere [2, 3]. In this respect a reasonable choice of the value of is 0,15 for PS and MS and 0,20 for WAS. Corresponding values of the actual degree of VSS reduction - (with considering the contribution of the active biomass incorporated in digested sludge) could be calculated by formulas (4a), (4b) and (4c) for PS, WAS and MS, respectively (Arsov, 1999b,c):

; (4a)

; (4b)

(4c)

where the constant values of the “ideal” sludge quality parameters can be chosen, based on averages of the huge amount of published data [3], in the ranges of = 0,70 – 0,75,

. The value of the actual

VSS content in the PS - is an input parameter, which usually vary in the range 0,5 – 0,7. The values of the kinetic parameters Y and kb are usually fixed around 0,06 and 0,03 d-1, respectively.

Following relationships are valid in respect of the correction factors for actual raw sludge BVSS content , associated with the relevant kinds of wastewater sludge [1, 3]):

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; (5a)

; (5b)

. (5c)3.2. THICKENING OF ANAEROBICALLY DIGESTING SLUDGE

Sludge thickening in parallel with anaerobic digestion processes takes place in low-rate digesters and Imhoff-tanks. Because of complexity of the overall process and lack of reliable information about the relevant parameters values, the popular Solids Flux Theory [8] is not applicable in this case. The only available data for the subject are those of Blunk [5] and Pruss [18], concerning primary sludge. They found out exponential relationships between sludge water content and time of thickening, associated with digestion at different temperatures, but nobody of them have specified the value of the relevant coefficient of thickening rate. The graphical relationships of Pruss (1928) however, clearly show acceleration of the sludge thickening with increasing of digestion temperature. This demonstrates that the temperature impact on water viscosity dominates over the one, associated with the sludge reology changes during digestion, which in this respect seems to be negligible. Based on these finding the following relationship has been suggested [1, 3] for general description of the digesting sludge thickening kinetics:

. (6)

Relevant values of the thickening rate coefficients could be obtained by

formula (6), provided the values of , , as well as ,associated with definite period t and temperature , are known. Taking into account the data, published by Pruss [18], Fair et al. [10], Dimovski [7], Imhoff and Imhoff [11] and Tourovski [20], the following values could be accepted: at

= 15o C and t = = 60 d; (optimum value for WAS thickened before

digestion, [3]); ; at = 15o C and t = = 60 d. The

values of and for mixed sludge, generated in Imhoff-tanks with simultaneous precipitation of PS and WAS are 95,5% and 79 %, respectively [7, 11, 20]. The values of and for low-rate anaerobic digesters, where PS and WAS inflow as a separately generated suspensions, could be obtained by formulas (7) and (8) respectively, based on the SS-mass balance:

(7)

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.(8)

Taking into account the above quoted parameters, the following values of the sludge thickening rate coefficients have been obtained:

; (9a)

; (9b)

. (9c)

3.3. DETERMINATION OF THE SLUDGE DIGESTERS’ VOLUMES

The volume occupied by digesting sludge depends on availability, duration and intensity of the sludge thickening process and therefore the value of in the general design formula (1) varies for the different kinds of sludge, hydrodynamic type of digesters and mode of their operation. For continuous-flow completely stirred bioreactors, such as single-stage high-rate anaerobic digesters (methane-tanks) and all kinds of aerobic digesters where sludge thickening doesn’t occur, the following assumption is valid:

. (10)

Intermittent withdrawing of the supernatant along with the digested sludge from low-rate anaerobic digesters, where simultaneous sludge thickening takes place gives opportunity for decreasing of the relevant bioreactors’ volumes. This mode of operation allows periodically mixing of digester just after the supernatant and digested sludge have been replaced by raw sludge. This from the other hand gives a reason the bioreactor to be considered as completely stirred one in respect of the HRT and space necessary to be provided for the relatively slow anaerobic processes.

In the period between mixing the sludge thickening process develops in “batch” hydrodynamic conditions. This in turn gives a reason for assumption of conformity between the sludge thickening process development in the time (equation 6) and the one developing in the space between digester surface and bottom (solids SS concentration – depth profile). This conformity becomes obvious when compare for instance the “standard” sludge SS – depth profile, recommended in the German regulation ATV [4] or the ones published by Ekama et al. [8], with the graphical appearance of the exponential relationship (6), taking into account the sludge SS and water content relation. Based on the above assumption, the mean sludge water

content - along SS – depth profile could be defined as follows:

6

, (11)

where tm = for Imhoff-tanks and low-rate anaerobic digesters without intermittent mechanical mixing.

Then based on the sludge SS mass balance and after some simple rearrangements [3], the net volumes of low-rate anaerobic digesters and Imhoff-tanks can be calculated by equations (12), (13) and (14), valid for PS, WAS and MS, respectively:

; (12)

;

(13)

.(14)3.4. THE DESIGN MODEL VALIDATION AND COMPARISON

A package of design software named STAB has been created, based on the design (sizing) model discussed in general above. The STAB package includes the following modules: AERSTAB – for aerobic digesters design; ANASTAB – for anaerobic digesters design; SEPSTAB – for design of bioreactors for separate sludge digestion; STAB – main menu. Besides its practical value for design, this software facilitated checking of the proposed design model adequacy through validation with available empirical data (Fig 1 and Fig. 2), as well as comparison of its performance with ones of other (classical) design methods (Fig. 3 and Fig.4).

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Figure 1. Proposed design model graphical comparison with pilot-plant data of McKiney [17] and design

method, recommended by US EPA [9] for primary sludge anaerobic digestion at = 0,75 and = 0,65

Figure 2. Proposed design model graphical comparison with pilot-scale data of Malina [15] and the design method, recommended by US EPA [9] for waste activated sludge anaerobic digestion. at = 0,9, = 0,8

Despite the available data (Malina [9, 15], McKiney [17] and EPA [9])

represented by dots in Fig. 1 and Fig. 2 don’t fully characterise the sludge quality to allow precise calibration of the proposed model, they could be fitted quite well by the later (with continuous black lines), if it is run with proper combination of available and default data.

Figure 3. Graphical comparison of the proposed design model for primary sludge anaerobic digestion with other design methods and data from Sofia WWTP. At =0,75 and = 0,65

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Figure 4. Graphical comparison of the proposed design model for mixed sludge anaerobic digestionwith other design methods and data from Sofia WWTP. = 0,75; = 0,65; = 0,9; =0,8

Data presented in Fig. 3 and Fig. 4 show that most of the illustrated design methods (Karpinski [12], Dimovski [7], EPA – 625/4-78-012 [9], CER – 2.04.03-85 [6], Imhoff and Imhof [11]) applying classical (or similar) approaches, as well as the field data from the Sofia WWTP [1, 3] reflect only one study-state (one point) of the relevant systems, excluding the method of Karpinski, which cover some range of the

domain, but with fixed default sludge quality. Most of the data points are located on the left side of the lines with = 0,1 which shows that the relevant systems do not perform digestion processes up to the classical “technical degree of digestion” (90% reduction of BVSS) and that this is the more usual practice [2, 3]. Since the proposed design method is based on biological processes lump kinetics and consider the main sludge quality parameters, it allows positioning of the designed system in an arbitrary study-state point of the domain, chosen by proper analysis and considerations. The relevant analysis performed by the STAB package are discussed elsewhere [2, 3].

4. Conclusions

The necessity of a new approach in design (bioreactors’ volumes sizing) of the municipal wastewater sludge digesters motivates the research reported in this paper and the main reasons for this is discussed.

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The proposed model is founded on the kinetics of the relevant physical and biological processes (the later taken in lump) and needs only few basic sludge quality parameters as input. It allows positioning of the designed digesting system in arbitrary study-state, motivated according to the local environment (hygienic) and economical requirements and conditions. Considering of the different rates of aerobic and anaerobic digestion of primary, waste activated and mixed sludge, along with the relevant hydrodynamic conditions and processes of thickening taking place in some anaerobic digesters are among the unique features and main advantages of the model.

The software package STAB based on the proposed model proved to be useful tool not only for design but also for various technical and economical investigations, concerning determination of the optimal technological scheme and degree of municipal wastewater sludge stabilisation, and costs minimisation.

5. Nomenclature

5.1. ABBREVIATIONS

BVSS biodegradable volatile suspended solidsCSTR completely stirred reactorHRT hydraulic residence timeMS mixed (primary and waste activated) sludgePS primary sludgeSS suspended solidsVSS volatile suspended solidsWAS waste activated sludgeWWTP wastewater treatment plantX general case abbreviation (as a superscript) for PS or WAS or MS

5.2. SIMBOLS

and suspended solids concentrations in PS, WAS and MS, respectively

decay coefficient of the active anaerobic biomass, incorporated in the digested sludge, d-1

rate coefficient of sludge BVSS anaerobic destruction (with the superscript meanings

X PS or X WAS or X MS)

digesting sludge thickening rate coefficient, depending on the temperature, d-1 (with the superscript

meanings X PS or X WAS or X MS)supernatant volumetric flow rate, m3/d (at anaerobic digesters where sludge thickening andsupernatant withdrawal take place)

QX volumetric flow rate of the raw sludge, inflowing the digester, m3/d (with the superscript meanings X PS or X WAS or X MS)

actual degree of VSS reduction, % (with the superscript meanings X PS or X WAS or X MS)

general time variable, T

hydraulic residence time (HRT), dtm period between two consequent sludge mixing when “batch” thickening takes place (in low-rate

anaerobic digesters), ddigester volume, relevant to PS, WAS or MS stabilisation, respectively, m3 (with the superscript meanings X PS or X WAS or X MS)

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sludge water content at the moment t of thickening processes, associated with definite temperature,

% (with the superscript meanings X PS or X WAS or X MS)

initial sludge water content at t = 0, % (with the superscript meanings X PS or X WAS

or X MS)

“first critical sludge water content”, defined as a break-point between the content of “free water”

(removable by gravity thickening) and “physically immobilised water” (removable by mechanical dewatering or drying), % (with the superscript meanings X PS or X WAS or X MS)

Y yield coefficient of the active anaerobic biomass, incorporated in the digested sludgeVSS fraction in an “ideal” raw primary sludge (unaffected by digestion processes usually taking

place in the primary settlers and/or in the sludge accumulation chambers); In the model isassumed as a constant parameter with a value in the range 0,7 – 0,75

actual VSS fraction in the primary sludge at the digester inlet; In the model it is involved as an

input parameterBVSS fraction in an “ideal” raw primary sludge (unaffected by digestion processes usually taking

place in the primary settlers and/or in the sludge accumulation chambers); In the model is assumed as a constant parameter with a value in the range 0,65 – 0,75VSS fraction in waste activated sludge with age 1 d ; In the model is assumed as a constant parameter with a value in the range 0,7 – 0,9BVSS in waste activated sludge with age 1 d; In the model is assumed as a constantparameter with a value in the range 0,7 – 0,8ratio between SS mass of PS and WAS in the MS

volumetric density of the sludge suspension, kg/m3

residual fraction of the BVSS, remaining at the end (or at a definite moment) of stabilisation processtemperature in the digester, grad C

correction factor for the actual raw sludge BVSS content (with the superscript meanings X PS or

X WAS or X MS) ; considers BVSS reduction in PS, WAS and MS, respectively, takingplace in the primary settlers and/or in the sludge accumulation chambers; It is defined analytically by equations (5a), (5b) and (5c), respectively age of the waste activated sludge, d

6. References

1. Arsov R. (1999a) On the kinetics of the biological processes in design of municipal wastewater sludge anaerobic digestion units. Annuals of UACEG 40 (6), Sofia.

2. Arsov R. (1999b) On the reasonable degree of sludge stabilisation at the municipal wastewater treatmentplants. Proceedings of the Specialised Conference on Disposal and Utilisation of Sewage Sludge: Treatment Methods and Application Modalities, Oct. 13 – 15, Athens, Greece.

3. Arsov R. (1999c) Investigations on Rational Technological Flow Sheets and Design Methods for Biological Stabilisation of Municipal Sludge. D.Sc. Thesis, University of Architecture, Civil Engineering and Geodesy (UACEG), Sofia (in Bulgarian)

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4. ATV – Abwassertechnische Vereinigung (1991) Dimensioning of single stage activated sludge plants upwards from 5000 total inhabitants and population equivalents - A 131. ATV rules and Standards. Wastewater-waste, UDC 628.356:628.32-001.2(083).

5. Blunk H. (1925) Contribution to the calculation of digesting wastes removal. Gesundheits Ingeieur 4 (in German).

6. CER 2.04.03-85 (1986) - Civil Engineering Regulations. Sewerage and Wastewater Treatment. Russian State Committee on Civil Engineering, Moscow (in Russian).

7. Dimovski C. (1978) Treatment and utilisation of wastewater sludge. Technika, Sofia, 327 pp (in Bulgarian).

8. Ekama G., J. Barnard, F. Gunthert, P. Krebs, J. McCorquodale, D. Parker and E. Wahlberg (1997) Secondary settling tanks: theory modelling design and operation. Scientific and technical report 6. IAWQ, London, 216 pp.

9. EPA – 625/4-78-012 (1978) Sludge treatment and disposal I. US EPA Technology Transfer, Washington DC.

10. Fair G., J. Geyer and D. Ocun (1968) Water and wastewater engineering 2. J. Wiley & Sons Inc., N.Y.11. Imhoff K. and K. Imhoff (1979) Manual of Urban Sewarage 25. R. Oldendurg Verlag, Munich – Vienna

(inGerman). 12. Karpinski A. (1959) New achievements in wastewater sludge digestion. Academy of Public Works

atRussian Federation, Moscow (in Russian).13. Keller J. (1999) Report from Anaerobic Digestion Task Group. Newsletter 5. IAWQ Specialist group on

Anaerobic Digestion, London.14. Lue-Hing C., P. Matthews, J. Namer, N. Okuno and L. Spinosa (1996) Sludge management in highly

urbanised areas. IAWQ Scientific and Technical Report 4. IAWQ, London, 7 – 12.15. Malina J. (1962) The effect of temperature on high rate digestion of activated sludge. Proceedings of 16

thPardue Industrial Wastes Conference, p. 232.16. Malina J. (1964) Thermal effect on completely mixed anaerobic digestion. Water and Sewage Works 1,

p.52.17. McKiney R. (1963) Advances in biological waste treatment. Pergamon Press, N.Y.18. Pruss M. (1928) Progress in the wastewater sludge thickening. R. Oldenbourg Verlag, Munchen (in

German).19. Rudolph K. (1999) A low-cost approach to tackling odours. Water Quality International 1/2. IAWQ,

London, 28 – 31.20. Tourovski I. (1982) Treatment of wastewater sludge. Stroyizdat, Moscow (in Russian).

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