Modelling sludge accumulation in anaerobic wastewater stabilization ponds

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Pergamon 0273-1223(95)00505-6

War. Sci. Tech. VoU" No. 12. pp. 185--190, 1995. Copynght @ 1995 IAWQ Printed in areat Bnlam. All rights reserved. 0273-1223/95 $9'50 + 0-00

MODELLING SLUDGE ACCUMULATION IN ANAEROBIC WASTEWATER STABILIZATION PONDS Muwaffaq M. Saqqar* and M. B. Pescod**

* Jordan Environmental Study Group. P.O. Box 540116. Amman. Jordan ** Department of Civil Engineering. University ofNewcastle upon Tyne.

Newcastle NE1 7RU. UK

ABSTRACT Sludge accumulation in the ftrst anaerobiC pond at the A1samra Wastewater Treatment Plant in Jordan bas been monitored over a period of years. Homogeneous distribution of sludge over the pond bottom bas not been achieved. The maximum amount of sludge has not accumulated near the inleL This is due to scouring of the settled materials near the pond inlet and outlet by the high jet velocity of the incoming flow. A model bas been developed to describe the volume of sludge accumulated (V AS) in the primary anaerobic pond. The model bas been derived on the basis of the non- biodegradable materials in the settled sludge. V AS bas been described in terms of the mass rates (F) of suspended solids and total BOD5 in the raw wastewater and an accumulated sludge coefficient (K AS)'

KEYWORDS Wastewater stabilization ponds; biodegradability; modelling.

anaerobic

pond;

settled

solids;

sludge

accumulation;

sludge

INTRODUCTION Sludge accumulation in primary anaerobic ponds is normally expressed irrationally by assuming an amount produced per capita per year. In spite of the fact that it is not always easy to determine accurately the equivalent population served. sludge accumulation per capita can vary considerably from place to place. It is not strange. therefore. to find figures reported in the literature to describe sludge accumulation exhibiting a wide variation. For example. Gloyna (1971) estimated sludge accumulation as 0.03-0.05 m 3/cap. year while Arceivala (1986) reported a value of 0.08 m 3/cap. year for India.

MODEL DEVELOPMENT The volume of sludge accumulating in a primary anaerobic pond is controlled by the non-biodegradable portion of the settled solids which either enter the system or are produced as a result of the microorganisms' biological activities. Hence. model development will be based on this simple principle. Fig. I illustrates the components controlling the volume of settled sludge in a primary anaerobic pond.

18S

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M. M. SAQQAR and M. B. PESCOD

Dealing with mass rates F (kg/d) for the parameters, the mass rate of the sludge settled into the bottom layer (kg/d) can be described as: = (FXSS,O - FXSS,I) + .iFX

(1)

where. FxSS,O = flow rate of the suspended solids at the pond inlet (kg/d) FXSS,I = flow rate of the suspended solids at the pond outlet (kg/d) M x = net solids produced by biological action (kg/d)

= YM CBOD = Y (FCBOD.O - FCBOD.I) where. FCBOD.O = flow rate of the total BODs at the pond inlet (kg/d) FCBOD,I == flow rate ofthe total BOD5 at the pond outlet (kg/d) Y == yield coefficient == .iFx / M CBOD Due to the difference in the biodegradability of volatile suspended solids (X VSS) and fIXed suspended solids

(X FSS )' equation (I) can be written as:

== (FxvsS.O - FXVSS.I)

(2)

+ (FXFSS •O- FXFSS •I) + Y (FCBOD.O - F CBOD,I)

f l , f 2 and f3 are defmed as average fractions which are removed by the pond for the incoming FxvsS () FXFSS •O and FCBOD O. respectively. jl. h and h are defmed as fractions of solids not destroyed by the digestion process for'FxvSs , FXFSS and those produced by biological action, respectively.

xn"'l'~] ..... x ..... X

INLET

UG • 1 . .

J.t. 1

1

OUTLET ~Y~Q

SLUDGE Figure I. Schematic of componeots controlling volume of accumulated sludge in the first anaerobic pond.

The volume rate of the wet accumulated sludge (QAS) (m 3/d) can now be expressed as:

<2.\1 - [(j, f, ) (Fxvss.O)+(j2 f 2) (FXFSS ,o)+(j3 Y f 3) (FCBOD,O)] [SGa

where. SOs == specific gravity of the sludge Pw == water density - 1000 kglm3 WS = water content of the sludge.

Pw (l-Ws )]

(3)

The sludge ,:"ater co.ntent (Ws) and the sludge specific gravity (S0s> were measured for the sludge in the flfst anaerobtc pond to the Alsamra system and found to be 0.88 and 1.03. respectively.

Modellmg sludge accumulation

187

Therefore, the value of [SGs (1- WS)] in the denominator of equation (3) can be considered constant and an average value of 0.1236 can be assumed. Equation (3) can now be written in simplified form as: QAS = [(YI FXyss.o + Y2 Fxss .o + Y3 FCBOD.O> / (1000)] where. 'YI = 0I f 1)/0.1236. 'Y2 = 02 f2)/0. 1236. 'Y3 = (h Y f3)/0.1236.

(4)

To examine what these three values ('YI' Y2' 'Y3) are likely to equal if an anaerobic pond is assumed to behave similarly to an anaerobic digester. well-documented information from the literature on anaerobic digestion was utilized. For YI and Y2' it is well known that long time digestion (say 100 days retention) causes about 70% destruction of X ySS 0 and 20% of X FSS 0 (EPA, 1978; Vesilind, 1979; WPCF, 1985). This implies that jl and jz can be approximated by 0.3 and 0.8, respectively. f l and f2 will be assumed to be equal to the fraction of X SS •o removed in the first pond and equal to 0.74 according to the finding presented by Saqqar and Pescod (1995). This means that 'YI and Y2 can be approximated as 1.8 and 4.8, respectively. For 'Y3' it is well established that bacterial cells are comprised of about 60% volatile suspended solids and the remaining 40% are fixed suspended solids (Benefield and Randall, 1980). This means that h=0.50. An average value for f 3, as found by Saqqar and Pescod (1995), was 0.53 and Y can reasonably be assumed to be 0.5 (Metcalf and Eddy. 1979). This yields an approximation ofY3 at 1.07. All the above estimated values for YI' 'Y2 and Y3 can serve as guides to those values existing in operating anaerobic digesters. Equation (4) can now be written to determine the volume of accumulated sludge VAS (m 3) in terms of the masses (kg) of F XYSS, FXFSS and FCBOD (assuming a pond acts as an anaerobic digester) as: VAS = 1.07 [(1.7 FXYSS.o + 4.5 FXFSS •O+ FCBOD.O) / (1000))

(5)

It should be remembered that decomposition of the settled sludge in primary anaerobic ponds occurs over a very long time (normally over 5 years), compared with retention in an anaerobic digester, and this allows for the solids which have a slow biodegradability rate to be decomposed and hence the sludge volume will be further reduced. Such a variation can be modelled by establishing a general formula for equation (5). so that: VAS = KAS [(1.7 FxySS.o + 4.5 FXFSS •O+ 1 FCBOD.O) / (1000)]

(6)

where. KAS is called the accumulated sludge coeffIcient. Decomposition of the settled sludge will be assumed to be proportional to (1.7 FXYSS.O + 4.5 FXFSS •O + FCBOD 0) in all pond systems. The continuity of the decomposition will be reflected by a decrease in the value ~f KAS till it reaches an ultimate stationary value when decomposition is completed. To reach approximately the ultimate KAS value, at least one year retention for the sludge is needed. The expected limited variation of VAS with the same amount of (1.7 FXYSS.O + 4.5 FXFSS.O + I FCBOD,O> in two different pond systems will also be reflected by a slight variation in the coeffIcient K AS ' Knowledge of the value of KAS for a particular system can lead to an estimate of the volume of the accumulated sludge (V AS), knowing some of the raw wastewater characteristics expressed in masses of FXYSS.O> FXFSS,O and FCBOD () The coeffIcient KAS can also be seen as a comparative measure of the biodegradability of the settled shidge in two pond systems for the same retention time; the lower the value, the higher the biodegradability of the settled sludge. To o~tain the KAS value for the Alsamra system, in order to proceed to model development, the left side of equat1o~ (6) (VAS) should be determined. The volume of the accumulated sludge (V AS) was calculated by measuring sludge depths at 72 grid points (8 x 9) in the first anaerobic pond. Sludge depths were determined to the nc:arc: st 1.0 c~ based on data provided by Miqdadi (1989) and Fig. 2 is a three-dimensional plot of sludge dlstnbutlon 10 the pond. The figure does not support the common belief that the maximum amount of sludge is acc~mulated near the inlet o~ a primary pon~. However, it clearly demonstrates that scouring of the settled material occurs near the pond mlet and outlet 10 the zone along the direction of the incoming flow, at which higher jet velocities normally prevail. This creates a higher shear velocity on the top of the settled

M. M. SAQQAR and M. B. PESCOD

188

solids in that zone and, hence, lower sludge depths were found in that zone. Settled solids in the zone were either dispersed into the other two longitudinal pond strips or carried out with the pond effluent during periods of high jet velocity. In addition. turbulence created by the high jet velocity promotes unfavourable conditions for the settling process itself. This fmding demonstrates the importance of controlling the jet velocity of the incoming wastewater in order to minimize its adverse influence and prevent sludge scouring. This can be achieved by judicious design of the inlet and proper choice of the inlet-outlet configuration. The figure also indicates that a homogeneous distribution of sludge over the pond bottom, which is always assumed by designers. was not achieved. In addition, the symmetrical distribution of the sludge suggests that there wa.~ no influence exerted by the prevailing wind direction. It was found that the average depth of the accumulated sludge was 1.7 m (ranging from 0.2 to 2.7 m) after 44 months of operation (from the start up in July, 1985 to March. 1989) during which V AS was measured. This depth is equivalent to a sludge volume (V AS) of 45660 m3. On the basis of these results, the sludge depth would reach 2 m, at which time the pond was to be desludged, in the summer of 1990, after 5 years of operation. This finding highlights another failure in design expectations since Binnie and Partners (1983) predicted that volume to be reached after 12.5 years of operation. The volume of the accumulated sludge was equivalent to 1.28 m3 per 1000 m3 of flow (QQ>. This figure, although it can be assumed as a constant for the Alsamra system, is not the same for all pond systems. The figure should be seen as a variable depending on the raw wastewater characteristics.

J

1

2.5

£

2

"

1.5

a-

't>

'""

't>

"

0;

100

0.5 0

0

40

80

120

160

pond length (m)

200

120

240

Figure 2. Sludge distribution in the first anaerobic pond.

...... .., E

8o

100 90

~ASrO.6

K AS -0.7 K AS -0.5

80

K AS -1.0

70 60

....

50

40 3D

o §" '0

>

20 10

0

o

10

20

JO 40 50 60 70 80 1.7 f[XVSS]H.5 f[XfSS]+f[CBOD] (t.t kg)

90

100

Figure 3. Variation of VOlume of accumulated sludge (1000 m3) in terms of (I.7 FXVSS + 4.5 FXFSS + FCBOD)(million kg) for different KAS values.

Modelling sludge accumulation

189

Knowing monthly means of FxvSS,Oo FXFSS,O and FCBOD.O (kg), KAS was found to equal 0.59 (=0.6) in the Alsamra system. In order to generalize the use of equation (6), slight variations in other pond systems can be reflected by a slight variation in the coefficient K AS' Graphical solutions for the equation with K AS = 0.5, 0.6 and 0.7 are presented in Fig. 3. For comparison, a graphical presentation for KAS =1. which approximates the case of equation (5) and presents the highest possible level of sludge accumulation in any primary pond, is also illustrated in Fig. 3. A value of 0.6 for K AS can be taken as an average for pond systems and. hence. the equation determining the volume of the accumulated sludge can be finalized as: VAS = 0.6 [(1.7 Fxvss.o + 4.5 FXFSS •O+ FCBOD.O) I (1000)]

(7)

Equation (7) can be used to estimate the volume of accumulated sludge with a knowledge of raw wastewater characteris~cs. in terms of X vss •Oo XFSS •O and C BOD.O (mg/l) and the flow rate entering the pond system (Qo)(m 3/uOlt tune). It can be noted from equation (2) that all the parameters controlling the settled sludge are correlated to the XSS removed. Hence. a more simplified equation. but less accurate. can be established to estimate V AS by the knowledge of only the FxsS (kg) removed. Data analysis for the Alsamra system shows that: V AS = 2.8 [(Fxss,o - FXSS,I) / (1000)]

(8a)

This means that each 1000 kg of XSS removed was equivalent to 2.8 m3 of the accumulated sludge. Utilizing the fmding presented in Saqqar and Pescod (1995), from which the average X SS•I = 0.26 X SS,<> then equation (8a) can be written for the Alsamra pond system as: VAS = 2.1 [(FxSS.O> / (looO)J

(8b)

Equation (8) can be used as a rule of thumb to estimate approximately the expected volume of accumulated sludge when only limited information is available.

CONCLUSIONS Following 44 months of operation of the Alsamra Wastewater Stabilization Ponds in Jordan, the distribution of sludge depth across the fIrst anaerobic pond was determined and gave an average depth of l.7m. The volume of the accumulated sludge was equivalent to 1.28m 3 per IOOOm 3 of influent wastewater, which had a high suspended solids concentration. The water content of the sludge accumulated in the first anaerobic pond at Alsamra was found to be 0.88 and the sludge specific gravity to be 1.03. A model was derived to predict the volume of accumulated sludge in a primary anaerobic pond in terms of the masses (in kg/d) of influent volatile suspended solids, fixed (inert) suspended solids and BOD concentration. A correction factor. termed the accumulated sludge coefficient, was introduced to allow for anaerobic decomposition of the settled sludge. A simplified form of the model was suggested as a rule of thumb to predict accumulated sludge volume in any primary anaerobic pond, based on the influent fixed solids concentration.

REFERENCES An:eivaJa, S. J. (1986). Wast<'Water Treatmelll for Pollution COlllrol. Tala McGraw-HiU Publishing Company Limiled, Delhi. India. Benefield, L. D. and Randal\, C. W. (1980). Biological Process Design/or Wastewater Treatmelll. Prentice-Hall. Englewood Oilfs.

Binnie and Panners (1983). Design Documentation 0/!./samra Waste Stabilization Ponds. London, UK. EPA (1978). Sludge Treatment and DisposaL EPAl62SI4-781012. Environmental Researcb Center. Cincinnati. OH. USA. GIoyna, E. F. (1971). Waste Stabililalion Ponds. WHO, MonoJtl'8Ph. Series 60. Geneva.

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M. M. SAQQAR and M. B. PESCOD

Metealf and Eddy, Inc. (1979). Wastewater Engineering: Treatment, Disposal, Reuse. 2nd edn. McGraw-HilI, NY, USA. Miqdadi. I. M. (1989). Personal communication. Saqqar. M. M. and Pescod, M. B. (1995). Modelling the performance of anaerobic wastewater stabilization ponds. Wal. Sci. Tech. 31(12). (this issue). Vesilind, P. A. (1979). Treatment and Disposal afWastewater Sludge. Rev. edn., Ann Arbor Science Publishers. WPCF (1985). Sludge Stabilization. Manual of Practice FD-9, Water Pollution Control Federation, WashingtOn, DC, USA.