Actual sludge production in municipal secondary wastewater treatment plants

Environment International, Vol. 14, pp. 29-35, 1988

0160-4120/88 $3.00 + .00 Copyright © 1988 Pergamon Press plc

Printed in the USA. All rights reserved.

ACTUAL SLUDGE PRODUCTION IN MUNICIPAL SECONDARY WASTEWATER TREATMENT PLANTS M. F. Hamoda Department of Civil Engineering, Kuwait University, P.O. Box 5969, Kuwait

(Received 15 August 1987; Accepted 10 February 1988) This study presents actual sludge production data from 26 activated sludge treatment plants which were evaluated in an attempt to provide a better basis for design of sludge facilities. The plants studied received normal pollutant loadings and showed good performance. Actual sludge production rates obtained, varied according to plant size and were different from those typically used in design. The mass of waste activated sludge solids, produced per mass of biochemical oxygen demand (BOD) removed, increased with organic loading applied and a linear correlation was found. Values obtained were higher than cell yield coefficients based on process kinetics. The results suggest that empirical sludge production coefficients developed in well-performing plants could provide a more realistic design basis.

Introduction

surveyed employ the conventional activated sludge process for biological treatment of wastewaters. This process is widely used in different parts of the world.

Wastewater treatment involves not only the treatment and renovation of the liquid, but also the processing and disposal of the solids removed or generated during treatment of the liquid. Sludge may be defined as a mixture of the solids and associated liquid produced during the treatment of wastewater. Processing and disposal of sludge may account for up to 50% of the total cost of treating wastewater (Vesilind, 1979). Proper design of sludge facilities requires careful evaluation of sludge quantities produced at wastewater treatment plants. The quantity of sludge requiring processing has increased considerably in most countries in the past decade (Bruce et al., 1984). An engineer must be able to estimate the total quantity of sludge produced during treatment before the design of sludge processing and disposal facilities can be completed. Mathematical calculation of sludge quantities, generated by each treatment process, can be made based on process fundamentals (Benefield et al, 1975; Metcalf and Eddy, 1979). However, it will generally be necessary to assume sludge production rates based on "typical" values, taking into account the probable nature of the wastewater, the capacity of the works, the type of wastewater treatment processes to be employed and likely seasonal variations. This study was initiated to provide a better evaluation of sludge quantities based on actual sludge production data obtained through a survey of operating secondary wastewater treatment plants. The plants

Methodology Sludge production data were collected from 25 operating activated sludge plants in the province of Ontario, Canada, during a survey of municipal wastewater treatment facilities. In a similar investigation, the main activated sludge plant in Kuwait was surveyed. The plants surveyed ranged in capacity from about 500 to 200,000 m3/day of wastewater treated. All these plants treated, primarily, domestic wastewaters and encompassed primary and secondary (conventional activated sludge) treatment stages. Data on sludge quantities were obtained from the plants by questionnaires, site visits, and review of sludge inventories and operating records. Population was based on census data for the area served by each plant with allowances made for industrial discharge loads whereever existed. In addition, more extensive data were available from the main plant in Kuwait during a controlled study of plant performance which allowed further evaluation of factors affecting sludge production. It is commonly difficult to obtain a precise estimate of the mass of sludge solids generated at a particular wastewater treatment plant due to lack of representitive samples for solids determination. Changes in the sludge inventory, within the treatment process, also 29

30

M.F. Hamoda Table 1. Raw wastewater characteristics and loading factors. Parameter

Ontario

Kuwait

Suspended Solids Concentration (mg/L)a Biochemical Oxygen Demand Concentration (mg/L)a Flow (L/capita.day) b Suspended Solids Loading (g/capita.day) b Biochemical Oxygen Demand Loading (g/capita.day)b

212 _+ 78

280 _+ 120

190 _+ 65

415 _+ 74

485 105

273 80

92

113

aMean _+ standard deviation. bBased on mean values.

contribute to this difficulty. H o w e v e r , such p r o b l e m s are o v e r c o m e by evaluating sludge production on a monthly or longer basis. With detailed data collected from m a n y t r e a t m e n t plants s u r v e y e d in this study, the results obtained are considered reliable. Sludge production can be given in mass (dry solids) or volume, in total or specific quantity produced (per capita, per volume of w a s t e w a t e r treated, or per mass of pollutants removed). In this study, attempts were made to relate sludge production with c o m m o n operating parameters. Statistical correlations were examined in each case. Data from all plants studied were analysed to formulate general relationships, whereas information from K u w a i t ' s main plant was used to examine specific p a r a m e t e r s .

Results and Discussion The results of this study involve w a s t e w a t e r characteristics, total sludge production, waste activated sludge quantities, and sludge characteristics at the plants surveyed.

Wastewater characteristics Perhaps the m o s t important w a s t e w a t e r characteristics influencing sludge production rates are the suspended solids (SS) and the biochemical oxygen d e m a n d (BOD) concentrations or per capita figures. Average annual concentrations reported for raw w a s t e w a t e r SS and five-day B O D in the studied plants are presented in Table 1 along with the per capita loading values of these principal pollutants. These figures show that Ontario plants s u r v e y e d receive mediumstrength w a s t e w a t e r , whereas K u w a i t ' s plant treats strong w a s t e w a t e r according to the classification of typical domestic w a s t e w a t e r composition (Metcalf and Eddy, 1979). The high strength of K u w a i t ' s wastewater is p r e s u m a b l y due to the relatively low per capita w a t e r used and the established habits of c o m m u n i t y residents. The unit waste SS and BOD loading factors presented in Table 1 lie within the normal range of 90-150 g SS/capita'day and 80-120 g B O D / c a p i t a ' d a y reported

for various municipalities in the U S A (Metcalf and Eddy, 1979) but are considerably higher than typical figures o f 40-60 for SS and 45-55 for BOD, respectively, reported for urban areas in Western E u r o p e (Puolanne, 1984). These differences m a y be due to resident habits and possible industrial discharges.

Total sludge production The quantity and characteristics of sludge produced are related to the w a s t e w a t e r flow and characteristics, type of process, and degree of treatment. The two main sources o f sludge generated at secondary (activated sludge) plants are suspended solids f r o m the influent w a s t e w a t e r (raw primary sludge) and biological solids synthesized from B O D r e m o v e d in the activated sludge process (waste activated sludge). Information from t r e a t m e n t plants concerning the relative proportions of raw primary sludge and waste activated sludge was generally not available. Some plants combine both types o f sludge before processing and disposal while others p u m p the waste activated sludge to the primary sedimentation tanks for cosettling with p r i m a r y sludge. Consequently, total (prim a r y plus waste activated) sludge production figures are considered in these analyses. Table 2 presents average sludge volume generated based on w a s t e w a t e r volume treated. It also shows average sludge quantity in t e r m s of dry matter (total solids) produced, based on w a s t e w a t e r volume treated, population served, and pollutants (SS + BOD) removed. T h e average volume of sludge produced per volume of w a s t e w a t e r treated (about 0.5%) found in this study is similar to that typically reported (BenTable 2. Sludge production unit values. Unit

Value

m3 sludge/ma treated wastewater g dry solids/m3 treated wastewater g dry solids/capita.day g dry solids/g (SS + BOD) removed a

0.005 219 95 0.83

aSS = suspended solids, BOD = biochemical oxygen demand.

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Fig. 1. Relationship between sludge production (megagram dry total solids per year) and actual flow of wastewater treated (thousand cubic meter per day).

efield et al., 1975; Metcaff and Eddy, 1979) although a higher value (1%) is sometimes assumed in design (US EPA, 1979). The per capita sludge dry solids production values obtained are also lower than the design figures in USA (Metcalf and Eddy, 1979) but are higher than most actual European figures (Haugan and Mininni, 1981; Duvoort-Van Engers, 1981; Puolanne, 1984). The COST 68 report (1975) on European communities gives total annual national sludge production figures which differ from one country to the other and reveal great differences in treatment practice. Figures on national statistics reveal per capita sludge production rates in Western Europe ranging between 30 and 124 g total solids/capita.day (Puolanne, 1984). The differences found in the values obtained in this study and those reported on European communities are most likely explained by the differences in treatment processes and their design principles. They also reflect the differences in SS and BOD loading factors as discussed earlier. An attempt was made to relate the yearly sludge quantity produced in terms of dry matter (total solids) obtained from plants of various sizes to the corresponding flow rates, population served, and (SS + BOD) removed at these plants. Figures 1 and 3 display

the relationships obtained on a logarithmic scale, respectively. The mass of dry solids is expressed in megagram (tonne). The line of best fit found in each case shows a good correlation. Table 3 gives the correlation coefficients and regression equations obtained. It can be noted from Fig. 1 that as the plant size increased from 1,000 to 100,000 m3/day the amount of dry solids produced per unit of flow decreased by about 35%. Waste activated sludge Waste (excess) activated sludge solids contribute significantly to the total sludge production in secondary wastewater treatment plants. Data were available from Kuwait's main plant to allow further evaluation of the waste activated sludge production. The performance of the activated sludge process was examined using monthly operational data collected in the years 1975, 1980, 1983, 1986, during which the plant passed through various stages of increasing hydraulic and organic loadings. As the plant capacity exceeded its design figures in the early 1980's, plant extension was undertaken and additional facilities were comissioned in 1985. Activated sludge production and wasting requirements may vary both between plants and also within a

32

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SS+ BOD REMOVED (Mg/yr) Fig. 3. Relationship between sludge production (megagram dry total solids per year) and suspended solids plus biochemical oxygen demand of pollutants removed (megagram dry mass per year).

Sludge production in w a s t e w a t e r t r e a t m e n t plants

33

Table 3. Sludge production relationships a. Relationship Obtained Figure 1 Figure 2 Figure 3

Correlation Coefficient, r

Regression Equation

+ 0.81 + 0.82 + 0.94

log Y = 0.881 log Q + 1.972 log Y = 1.027 log P + 1.518 log Y = 0.970 log X - 0.005

a n u m b e r of plants = 26. Y = sludge p r o d u c e d in m e g a g r a m (Mg) o f dry total solids per year. Q = w a s t e w a t e r treated in t h o u s a n d cubic m e t e r per day. P = population served in t h o u s a n d s . X = s u s p e n d e d solids + biochemical o x y g e n d e m a n d r e m o v e d from wastewater in m e g a g r a m (Mg) of dry m a s s per year.

index (mL/g) on the organic loading factor, respectively. The relationships obtained clearly indicate the importance of organic loading on process performance. Consequently, this factor would most likely influence activated sludge production. In order to compare activated sludge production at different plants, a common basis for documenting sludge generated is used which expresses the mass of volatile suspended solids (VSS) produced per mass of BOD removed as a coefficient (kg VSS/kg BODR). Volatile suspended solids is likely a better measure of sludge biomass than total suspended solids, although the later parameter is perhaps more commonly determined in practice. The mass of BOD removed was calculated using influent and effluent total BOD results

single plant for different periods. However, sludge production rates could be predicted based on the organic loading factor, (i.e., food-to-microorganism (F/M) ratio), or other parameters that influence activated sludge process performance. Efforts were made to examine the effect of F/M ratio, expressed as mass of BOD applied per day per mass of mixed liquor suspended solids (MLSS) in the activated sludge aeration tank (kg BODA/kg MLSS.day), on BOD removal efficiency and on sludge settleability. The sludge volume index (SVI), def'med as volume of sludge settled after 30 min in a one-litre graduated cylinder/suspended solids concentration, was used to measure sludge settleability. Figures 4 and 5 show the dependency of BOD removal efficiency (%) and the sludge volume

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Fig. 4. Effect of organic loading on biochemical o x y g e n d e m a n d removal efficiency in the activated sludge process (BODA = biochemical o x y g e n d e m a n d applied to the process).

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of the activated sludge process. Activated sludge solids intentionally wasted were only used in calculating sludge production. In this study, the solids lost in the activated sludge system effluent were usually insignificant to contribute to total solids production. However, significant quantities of effluent solids, if present, may introduce an unnecessary variable into the evaluation. The relationship between the loading factor (F/M) and the sludge wasting coefficient is presented in Fig. 6 for the loading range of the conventional activated sludge process. The least-squares line of best fit obtained shows a correlation coefficient, r, 0.86 indicating a reasonably good correlation between these two parameters within the studied range of F/M loadings. However, the linear correlation obtained may not be valid for lower or higher F/M loadings indicative of different microbial growth phases (Shultz et al., 1982). The values obtained for the activated sludge wasting coefficient shown in Fig. 6 are somewhat higher than the typical cell yield coefficient of 0.45 kg VSS/kg BOD removed or 0.65 kg TSS/kg BOD removed commonly used in system design based on process kinetics (Benedict et al., 1979; Shultz et al., 1982), knowing that the average volatile content of the sludge suspended solids observed in this study was 70%. Thus,

cell yield coefficients that address only microbial solids growth might not be used as estimates of actual sludge production. Sludge characteristics

In order to design sludge treatment and disposal facilities properly, not only the quantities of sludge solids are required, but also the characteristics of the sludge to be handled must be known. These characteristics vary depending on the origin of the sludge. Some particular characteristics, which are important in considering the fertilizing value of the sludge, were studied using the data obtained from the plants under study. These plants practice the anaerobic digestion of raw sludge and the land disposal of digested sludge. Table 4 summarizes the data obtained on sludge solids, nitrogen and phosphorus contents, which are important characteristics where the sludge is to be used in agriculture. Data on heavy metals present in the sludge were scarce. The data presented in Table 4 are comparable to those reported in the literature (Metcalf and Eddy, 1979). In general, these data indicate a high nutrient content of the sludge. This quality was not greatly influenced by anaerobic digestion.

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Table 4. Average nutrient characteristics of mixed primary and activated sludge produced a

Constituent Total Solids (TS) Volatile Solids (VS) Total Kjeldahl Nitrogen (N) Total Phosphorus

Untreated Sludge b (mg/L)

Anaerobically Digested Sludge b (mg/L)

20.1 ± 8.5

40.1 _+ 18.2

100

13.1 +_ 2.3

20.5 +_ 3.4

51

2.4-+ 0.61

2.1

±

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Anaerobically Digested Sludge c (% of dry wt.)

0.5

5.16

0.61

2.38

(P) aNumber of plants = 26. bMean -+ standard deviation. CBased on mean values.

References Benedict, A. H., Merrill, M. S., and Mauseth, G. S. (1979) Sludge production, waste composition, and BOD loading effects for activated sludge systems, J Water Pollut. Control Fed. 51, 28982915. Benefield, L. D., Randall, C. W., and Sherrard, J. H. (1975) Estimating sludge production aids in facilities design - - Parts I & II, Water & Sewage Works 122, 52-55, 100-102. Bruce, A. M., Campell, H. W., and Balmer, P. (1984) Development and trends in sludge processing techniques, in Processing and Use of Sewage Sludge P. L'Hermite and H. Ott, eds., pp. 19-38. Commission of the European Communities, D. Reidel Publishing Co., Dordrecht, Holland. COST 68- Project (1975) Final report of the management committee, Commission of the European Communities, EUCO/SP/48/75, Brussel. Duvoort-Van Engers, L. E. (1981) Sludge production in the Netherlands in Characterization, Treatment and Use of Sewage Sludge P. L'Hermite and H. Ott, eds., pp. 27-37. Commission of the

European Communities, D. Reidel Publishing Co., Dordrecht, Holland. Haugan, B. E. and Mininni, G. (1981) Characterization of sewage sludges, ibid 13-26. Metcalf and Eddy (1979) Wastewater Engineering: Treatment, Disposal, Reuse. McGraw-Hill, Inc., New York. Puolanne, J. (1984) Sludge production rates in Processing and Use of Sewage Sludge P. L'Hermite and H. Ott, eds., pp. 39-51. Commission of the European Communities, D. Reidel Publishing Co., Dorderecht, Holland. Schultz, J. R., Hegg, B. A., and Rakness, K. L. (1982) Realistic sludge production for activated sludge plants without primary clarifiers, J. Water Pollut. Control Fed. 54, 1355-1360. U.S. Environmental Protection Agency (1979) Process design manual for sludge treatment and disposal, EPA 625/1-79-011, Washington, D.C. Vesilind, P. A. (1979) Treatment and Disposal of Wastewater Sludges. Ann Arbor Science, Ann Arbor, MI.