Stoichiometry of activated sludge process for the treatment of synthetic organic wastewater

Jotntnh~L or FERMENTATIONAND BIOENGINEERING Vo1. 78, No. 2, 175-178. 1994

Stoichiometry of Activated Sludge Process for the Treatment of Synthetic Organic Wastewater KENJI FURUKAWA, 1. NORIKO TANAKA, 2 AND MASANORI FUJITA 1 Department of Environmental Engineering, Faculty of Engineering, Osaka University, Yamadaoka, Suita 5651 and Chemical Division Organic Chemical Sales Department Tokyo Sales Section, Shikoku Chemicals Corporation, 3-13 Nihonbashi, Chuo-ku, Tokyo, 2 Japan Received 18 March 1994/Accepted 6 June 1994 For the establishment of a more general and comprehensive activated sludge stoichiometry, which can be applied to the treatment of complex organic wastewater, experimental studies of activated sludge process were carried out. Changes in the elemental composition of activated sludge with the change of sludge retention time (SRT) were investigated. Elemental composition of activated sludge treating synthetic organic wastewater composed of peptone and yeast extract was constant at CsH9NO3 over a wide range of process operating conditions. A definite elemental composition of activated sludge including phosphorus cannot be successfully obtained because of the variations of phosphorus content. Average chemical compositions of activated sludge which was cultured below SRT of 3 d and above SRT of 5 d were C60HluN13040Pz and C60HmN1303~P, respectively. Yield coefficient (Y) and decay constant (b) were experimentally determined to be 1.22 g-MLSS/ g-TOC and 0.126 l/d, respectively. Using the experimentally determined kinetic constants, stoichiometric equation for the treatment of complex organic synthetic wastewater can be successfully developed.

The stoichiometry of activated sludge process may be useful not only for the estimation of the amounts of excess sludge production and oxygen consumption but also for the understanding of its treatment characteristics. For a wastewater containing casein, CsH,2N203, the following stoichiometric equation was obtained (1).

activated sludge process treating synthetic organic wastewater: (i) to investigate the changes in the elemental composition of activated sludge with the change of 0c and (ii) to construct a more realistic activated sludge stoichiometry for the treatment of complex organic wastewater.

CsHI2N203 -t- 302 CsH7NO2 + NH3 + 3CO2 + H20

MATERIALS AND METHODS

(1)

The experimental setup used for this investigation is shown in Fig. 1. The concentrated synthetic organic wastewater used in this study had the following composition (g/l tap water): peptone 60, meat extract 40, NaC1 3.0, MgSO4-7H20 2.5, KC1 4, CaC12 1.85, and NaHCO3 42. Peptone and meat extract were used as the carbon, nitrogen and phosphorus sources. This concentrated synthetic organic wastewater was kept in the refrigerator and was diluted to the proper concentration with tap water prior to use. As the seed sludge, an activated sludge which had already been acclimated to this synthetic organic wastewater by the fill and draw cultivation method was used. The mixed liquor of the aeration tank was filtered through a sludge filter separator (filter area was 50 cm 2) attached to the aeration tank. The filter attached to the top of the acrylic cylinder with an inner diameter of 8 cm was made of an industrial pad (Scotch Brite brand type A, very fine). The effluent passed from this filter separator was clear. Oc of this treatment process can be calculated by the following equation:

This stoichiometry, however, does not take into consideration an important controlling parameter of sludge retention time (SRT: Oc). Sherrad and Schroeder integrated the mathematical models and chemical stoichiometric equations for the description of biological wastewater treatment systems and proposed the following stoichiometry for synthetic wastewater containing glucose as the sole carbon source (2). 3C6Hi206 + x I N H + +x2H2PO~- -~X302 --,yIC60Ha7023NI2P +y2H20 +y3CO2 +y4 H +

(2)

Coefficients of this stoichiometry except y~ at each SRT can be determined from balancing of C, H, N and O after kinetic determination of coefficient y~ (2). Both of these equations describe only known carbon sources, and therefore, cannot be applied to actual wastewater which contains various kinds of organic compounds. Moreover, a definite cell composition is assumed in the two stoichiometric equations regardless of the process operating conditions. Hence, the development of a more general and comprehensive stoichiometry, which can be applied to the treatment of complex organic wastewaters, is currently required. The objectives of this paper are to undertake the following two points through experimental studies of the

VX (3) ((2- Qw)Xe+ QwX where V, X, Xe, Q and Qw denote aeration tank volume (/), mixed liquor suspended solids; (MLSS; rag//), effluent suspended solids (rag//), influent flow rate (Ud) and sludge withdrawal rate (//d) respectively. The effluent is clear when the filter separator is funOc=

* Corresponding author. 175

176

FURUKAWAET AL.

a

si

J. FERMI~NT.BIO~NO., 0

)a.

TABLE 1. Kineticequations used for the calculation

xs Xe

S

1

_

1

S=X{----y--~--}

(5)

Activated sludge cone. (rag//)

Y(Si-S) 0c X= (1 + bOc) 0

(6)a

Sludge production rate (nag/d)

Ps= (1 +bOc)O

(7)a

Observed yield (mg/mg)

P, Yob Q(Si-S)

(8)a

S

FIG. 1. Schematic diagram of experimental apparatus. 1, Influent; 2, influent pump; 3, aeration tank; 4, filter separator; 5, air; 6, sludge withdrawal; 7, effluent. ctioning properly, i.e., Xe=Omg/l. Therefore, Oc of this treatment system is reduced to the following simple equation. V

(4)

Qw

Therefore, the Oc value for this completely mixed experimental activated sludge process can be easily controlled merely by changing the sludge withdrawal rate. In order to obtain the steady-state condition at each Oc, the activated sludge process was operated for at least 3 times of operating Oc. Steady state was judged both by the stable effluent quality and MLSS concentration. All experiments were conducted at 20°C. Activated sludge under steady-state condition was washed twice with deionized water and lyophilized. The contents of C, H and N of activated sludge were determined by CHN corder MT-5 (Yanako Analytical Instruments Corp., Kyoto). The phosphorus content of activated sludge was determined using the methods outlined in Sewage Analytical Methods (3). Unless otherwise specified, water analyses were carried out according to the Japanese Industrial Standard (JIS)K0102 (4). MATHEMATICAL

MODEL

A mathematical model descriptive of the completely mixed biological wastewater treatment system adopted for the construction of activated sludge stoichiometry of synthetic organic wastewater treatment is shown in Table 1. The effluent substrate concentration was derived using the modified Michaelis-Menten equation (5) for the analysis of substrate utilization of activated sludge instead of the commonly used Michaelis-Menten equation:

1 dS X dt

k(S/X)" Ks+ (S/X)"

(9)

Under proper operation of the activated sludge process, effluent substrate concentration is generally low, and therefore Eq. 9 reduces to the following equation. X

dt =

-X-

VY(Si- S)

Y

7

Oc-~--

(Oc-l+b) 1/,

Effluent substrate conc. (mg//)

2

(10)

in which K=k/Ks. The empirical relationship between the growth rate and the substrate removal rate for activated sludge is as follows:

S~: Influent substrate cone. (mg-TOC//); S: effluent substrate cone. (mg-TOC//); 0: hydraulic retention time (d); Y: yield coeffÉcient (mg/mg); b: decay constant (l/d); K: coefficient (mg//.d); n: coefficient (--). a From Ref. 2. 1 dX 1 dS ---Y-----b X dt X dt

(11)

Assuming that steady-state conditions prevail, specific growth rate of activated sludge equals the reciprocal of Oc. Substituting Eq. 10 into Eq. 11 yields

s=~Oc-l+b) YK

l/n (5)

RESULTS AND DISCUSSION Table 2 illustrates the activated sludge treatment results of synthetic organic wastewater under steady-state conditions at each 0c. Table 3 shows the results of elemental analyses for activated sludge at each 0c and for influent synthetic organic wastewater. A slight difference in the values of C, H, N and O was recognized, but these values can be regarded to be constant over a wide range of operational process conditions. Although phosphorus content of activated sludge was in the range of 1.7 to 3.4%, a clear inverse relationship between the phosphorus content of activated sludge and 0c was observed in this study; i.e., phosphorus content of activated sludge is high under low Oc and vice versa. Phosphorus in the activated sludge cells is generally found in the form of ATP, nucleic acid, phospholipid, or polyphosphate. The high phosphorus content at low 0c may indicate the dominance of activated sludge microorganisms with higher metabolic activities. At higher 0c, however, slow-growing microorganisms with low phosphorus content may survive in the activated sludge. The elemental composition of activated sludge is shown in Table 4. By excluding the phosphorus content, the elemental composition of activated sludge is found to be CsHgNO3 irrespective of Oc. However, a definite TABLE 2. Treatment result of activated sludge for synthetic organic wastewater SRT (d) 0.5 1.0 2.0 3.0 5.0 10

Influent(rag//) TOC T-N T-P 106.4 37.2 4.40 114.6 35.3 7.02 114.0 29.1 8.32 104.5 29.2 11.1 106.0 31.7 6.50 107.5 30.2 7.69

Effluent (mg//) TOC T-N T-P 14.4 18.1 2.00 12.6 20.3 1.73 12.3 8.77 2.43 10.6 15.5 4.65 8.0 20.4 3.70 7.0 19.4 4.48

X (mg//) 157 307 705 682 1040 1136

VoL 78, 1994

STOICHIOMETRY OF ACTIVATED SLUDGE PROCESS

TABLE 3. Results of elemental analyses for the activated sludge and synthetic organic wastewater

TABLE 5. Chemical composition of activated sludge and synthetic wastewater (carbon as standard)

s•S

S

C

SRT= 0.5 d 40.59 1.0 40.54 2.0 37.34 3.0 40.13 5.0 40.70 10.0 40.92 Synthetic WW• 38.57 Wastewater. unit:

H 6.37 6.39 5.96 6.30 6.42 6.34 7.03

N

O

10.07 9.99 9.62 9.78 9.96 9.93 12.08

P

34.44 38.73 33.09 32.23 33.89 32.56 30.89

Ash

3.36 5.1 2.68 1.68 2 . 4 7 11.53 2.90 8.68 2.17 6.86 1.65 8.61 0.89 10.55

elemental composition o f activated sludge including phosphorus cannot be successfully obtained from this study because o f the variations in phosphorus content. Table 5 shows the chemical composition of activated sludge and influent synthetic organic wastewater including phosphorus using C as standard. As mentioned before, it is again impossible to define a definite chemical composition including phosphorus. Average chemical compositions o f activated sludges which were cultured below Oc o f 3 d and above Oc o f 5 d were C6oHl14N13040P2 and C6oHH3N13037P, respectively. Comparing these experimentally determined elemental compositions with the well-accepted elemental activated sludge compositions like CsH7NO 2 and C60Hs7023NI2P, hydrogen and oxygen contents were high in the activated sludge cells cultured on synthetic organic wastewater. These differences may be due to the influent synthetic organic wastewater used in this study. The organic wastewater composition o f synthetic wastewater used in this study was determined to be CgHaNO2 from the result of elemental analysis for lyophilized synthetic organic wastewater. In this study, influent T O C concentration was fixed at 112 m g / l . Since 102 m g / l C4HaNO2 is equivalent to 48 mg-TOC/l, influent wastewater concentration in this study was 2.33 mmol-C4HsNO2/L Neglecting the small amounts o f unmetabolized organics in the effluent, the following general stoichiometry for the activated sludge treatment o f synthetic organic wastewater can be deduced. 2.33C4HsNO2+xO2 -* y l C s H g N O 3 + Y 2 C O 2 + y 3 H 2 0 + y 4 ( T - N )

(12)

where T-N denotes effluent total-N conc. (mg//). If the coefficient Yt is determined, the rest of the coefficients can be easily determined by balancing o f C, H, N and O. The relationship between Yl and Yob can be shown as follows:

•

C

0.5 60 1.0 60 2.0 60 3.0 60 5.0 60 10 60 Synthetic OWa 60 a Organic wastewater.

H

N

O

P

113 113 115 113 114 112 131

12.8 12.7 13.2 12.5 12,6 12.5 16.1

38.2 43.0 39.9 36.1 37.5 35.8 36.0

1.89 1.51 1.51 1.65 1.22 0.92 0.53

Composition C6oHn3Nl303sP2 C60HII~NI3043P2 C~0HH~NI304oP2 C60HmNj3037P2 C6oHH4N1303sP C~0HH2NI3036P C~HL31NI6036P

Yob--(weight o f activated sludge produced)/ (weight o f T O C utilized) [ 131 (13)

-----Yl ~ Si__S ]

Coefficient Yl can be determined if Yob is known. Yob at each 0c can be kinetically obtained using Eqs. 5 to 8. For this determination of Yob, kinetic coefficients for activated sludge treatment of synthetic organic wastewater must be known. Figure 2 shows the relationship between 1 / 0 c and specific substrate utilization rate (see Eq. 11). F r o m this figure, Y and b were graphically determined to be 1.22g-MLSS/g-TOC and 0.126 l / d , respectively. Figure 3 shows the determination o f K and n from Eq. 10. From this plot, K and n were graphically determined to be 7.55 m g / / / d and 0.75, respectively. Substituting Eq. 6 into Eq. 5 and rearranging gives: y1 - llnK -

1/n S i 0 -

1

S---- (Oc- l + b)i- l/n + y l - linK - 1in0-1

(14)

Substituting the above-determined kinetic coefficients (Y, b, K and n) and the experimental conditions for test activated sludge process ( V = 5 . 3 / , Q = 1 2 / / d and Si = l l 2 m g / / ) yields the following kinetic equation for effluent substrate concentration. S=

15.7 (0c- 1+ 0.126)-°'33 + 0.14

(15)

Table 6 shows the process variables for the activated sludge treatment of synthetic organic wastewater, which were calculated using the experimentally determined kinetic coefficients. 1.5

A

TABLE 4. Chemical composition of activated sludge and synthetic wastewater (nitrogen as standard) ~--.....~ment C Sampk SRT(d)-"'--..~ 0.5 4.70 1.0 4.74 2.0 4.53 3.0 4.79 5.0 4.77 10 4.81 Synthetic OWa 3.72 Organic wastewater.

H

N

O

P

8.85 8.95 8.67 9.02 9.02 8.95 8.15

1 1 1 1 1 1 1

2.99 3.39 3.01 2.88 2.98 2.87 2.24

0.15 0.12 0.12 0.13 0.10 0.07 0.03

Composition

o

0.5

CsH9NO3 0 0

C4HsNO2

177

0.5

1

Sp. substrate uUllzaUon rate(g/g/d)

FIG. 2.

Y and b determination.

1.5

178

FURUKAWA ET AL.

J. FERMENT.BIOENG.,

10

TABLE 6. Process variables for the activated sludge treatment of synthetic organic wastewater SRT (d) 0.5 1.0 2.0 3.0 5.0 10

0.1

I

I

0.01

0.1

SRT (d)

As an example, the stoichiometric equation at Oc o f 0.5 d was determined. It is very difficult to distinguish a m o n g unmetabolized substrate, intermediate m e t a b o lites and cellular excreta in the effluent. Hence small a m o u n t s o f unmetabolized organics appearing in the effluent were neglected in order to simplify the following calculation. Yob for 0C o f 0.5 d is 1.21; therefore the coefficient Y~ was determined as follows, neglecting the unmetabolized organics in the effluent: Y~=l.03

By substituting the Y1 value o f 1.03 into Eq. 12 and balancing the expression, the following stoichiometry is obtained. 2.33C4HsNO2+5.7302 --, 1.03CsH9NO3 + 4 . 1 7 C O 2 + 4 . 6 9 H 2 0 + 1.3(T-N)

X (rag//) 130.5 254 467 643 915 1333

Ps (mg/d) 1383 1344 1344 1135 970 707

Yob (g/g) 1.21 1.15 1.03 0.936 0.791 0.571

TABLE 7. Stoichiometry for activated sludge treatment of synthetic organic wastewater

(s/x) FIG. 3. K and n determination.

1.21= Y,(131)/(112),

S (mg//) 17.1 14.2 12.0 10.9 9.88 8.84

(16)

Similar stoichiometric equations at various values o f 0c can be obtained by the same p r o c e d u r e and are shown in Table 7. W i t h a knowledge o f these stoichiometries, i m p o r t a n t process design i n f o r m a t i o n on the oxygen requirement, the a m o u n t o f excess sludge p r o d u c t i o n and the effluent total-N concentration for the treatment o f synthetic organic wastewater can be easily obtained. Also these stoichiometric equations m a y be o f much help for the selection o f suitable o p e r a t i o n a l 0c. The results o f this study on the establishment o f activated sludge stoichiometry treating synthetic organic wastewater are summarized as follows. (i) The elemental c o m p o s i t i o n o f activated sludge treating synthetic organic wastewater c o m p o s e d o f p e p t o n e and meat extract was

0.5 1.0 2.0 3.0 5.0 10.0

Stoichiometric equation 2.33C4HsNO2+ 5.7302 1.03CsHgNO3 + 4.17CO2 + 4.69H20 + 1.30(T-N) 2.33C4HsNO2+ 6.0002 ---~0.983CsH9NO3+ 4.41CO2 + 4.90H20 + 1.35(T-N) 2.33C4HsNO2+ 6.5902 --~0.881C5HoNO3+ 4.92CO2 + 5.36H20 + 1.45(T-N) 2.33C4HsNO2+ 7.0302 -+0.804CsHgNO3 + 5.30CO2 + 5.70H20 + 1.53(T-N) 2.33C4HsNO2+ 7.7702 -+0.675CsH9NO3 + 5.95CO2 + 6.28H20 + 1.66(T-N) 2.33C4HsNO2+ 8.8602 --'0.487CsH9NO3+ 6.89CO2 + 7.13H20 + 1.84(T-N)

constant at CsH9NO3 over a wide range o f process operating conditions. However, definite elemental composition including p h o s p h o r u s could not be obtained owing to the variations in p h o s p h o r u s content. (ii) Stoichiometric equations for the treatment o f complex organic synthetic wastewater can be successfully developed using a similar m e t h o d developed by Sherrard and Schroeder (2). REFERENCES 1. Sherrard, J. H.: Kinetics and stoichiometry of completely mixed activated sludge. J. War. PoUut. Control Fed., 49, 1968-1975 (1977). 2. Sherrard, J . H . and Schroeder, E.D.: Stoichiometry of industrial biological wastewater treatment. J. Wat. Pollut. Control Fed., 48, 742-747 (1976). 3. Japan Sewage Works Association: Sewage Analytical Methods (1984). 4. Japanese Industrial Standard: K0102, 134-145 (1986). 5. Hashimoto, S. and Furukawa, K.: Growth kinetic studies on organic oxidation and nitrification by activated sludge. J. Ferment. Technol., 60, 525-536 (1982).