anaerobic waste-activated sludge digestion–a bioconversion mechanism and kinetic model

Anaerobic Waste-Activated Sludge Digestion-A Bioconversion Mechanism and Kinetic Model

Tatsuo Shimizu," Kenzo Kudo, and Yoshikazu Nasu Department of Sanitary and Environmental Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Japan

Received December 21, 1992/Accepted July 9, 1992

The anaerobic bioconversion of raw and mechanically lysed waste-activated sludge was kinetically investi- gated. The hydrolysis of the biopolymers, such as protein, which leaked out from the biological sludge with ultrasonic lysis, was a first-order reaction in anaerobic digestion and the rate constant was much higher than the decay rate constant of the raw waste activated sludge. An anaerobic digestion model that is capable of evaluating the effect of the mechanical sludge lysis on digestive performance was developed. The present model includes four major biological processes-the release of intracellular matter with sludge lysis; hydrolysis of biopolymers to volatile acids; the degradation of various volatile acids to acetate; and the conversion of acetate and hydrogen to methane. Each process was assumed to follow first- order kinetics. The model approximately simulated the overall process performance of the anaerobic digestion of waste-activated sludge. The model suggested that when the lysed waste-activated sludge was fed, the overall digestive performance remarkably increased in the two-phase system consisting of an acid forming process and a methanogenic process, which ensured the symbiotic growth of acetogenic and methanogenic bacteria. 0 1993 John Wiley & Sons, Inc. Key words: waste-activated sludge two-phase digestion - sludge solubilization * biopolymer hydrolysis - kinetic model

I NTROD U CTlON

The anaerobic digestion conversion process of a waste- activated sludge biomass to methane gas includes several biological reaction steps. It is thought that when waste-activated sludge is transferred from aerobic to anaerobic surroundings, part of the biopolymer is hydrolyt- ically converted to lower-molecular-weight components and released immediately outside cells with other com- paratively low-molecular-weight intracellular substances. Furthermore, upon hydrolysis of the cell walls and membrane rupture (i.e., lyse), high biopolymer substances are released outside cell walls. These released biopolymeric proteins, lipids, nucleic acids, and carbohydrates are

* To whom all correspondence should be addressed.

hydrolyzed by the extracellular enzymes produced by anaerobic microflora. The biopolymers are then converted to methane gas after having been converted to various volatile organic acids. The complete digestion time requirements are extensive because the sludge solubilization and conversion to the lower-molecular-weight compounds through hydrolysis of intracellular biopolymers is the rate- limiting step in the anaerobic digestion p r o c e s ~ . ~ ~ ~ ~ ' ~ There are technical difficulties in rapidly treating the sludge by adding the waste-activated sludge directly to the recently developed anaerobic bioreactorZ6 that maintains a high microorganism concentration.

To improve the digestive efficiency, as well as dehy- dration, by solubilizing the decomposable organic substances in the sludge, thermal pretreatment of sludge was

production rate and methane conversion efficiency can be increased. Lee et al." demonstrated that thermal pretreatment improves the hydrolysis of a particulate organic matter and promotes effective solubilization of organic substances. This produces an organic substance that is easily converted to methane gas via volatile organic acids. Researchers also reported that the retention time can be reduced to less than 10 days in anaerobic digestion of thermally pretreated, waste-activated sludge. Improvement of sludge digestion efficiency using a pretreatment method that combines heat treatment with acid or alkali treatment" and ultrasonic pretreatment23-25*27 has also been researched. However, in anaerobic digestion, the conversion mechanism and the kinetics of the pretreated, solubilized sludge to methane gas cannot be fully explained.

In this study, to clarify the mechanism of sludge component decomposition, the biopolymer ingredient extracted after treating ultrasonically the waste-activated sludge was added to an anaerobic digestion process. The decomposition and convertibility of the intracellular biopolymers to methane gas was studied. A kinetic analysis of the biopolymer component decomposition to volatile organic acids and of the conversion of the various organic acids to methane gas was also done. Finally, a kinetic model based on experimental results was proposed, and simulation studies were conducted on the validity of the model and the

re~earched.2,3,6,7,9,lO~ll,l~l~~l8~l9,~7,~~ It showed that the gas

Biotechnology and Bioengineering, Vol. 41, Pp. 1082-1091 (1993) 0 1993 John Wiley & Sons, Inc. CCC 0006-3592/93/01101082-10

improvement of treatment efficiency with waste-activated sludge solubilization.

MATERIALS AND METHODS

Sludge Solubilization

One to 2-L of waste-activated sludge with an 18,000 mg/L MLSS concentration were solubilized by a treatment lysing cell walls with a 200-W, oscillation frequency of 20 KHz, ultrasonic generator for 1 hour at 25" to 30°C. The sludge solubilization ratio was approximately 60%. The ratio was defined as a supernatant organic substance concentration after centrifuging at 10,000 rpm divided by the organic substance concentration of the waste-activated sludge and multiplied by 100. TOC concentration was used as the organic substance concentration. The raw waste-activated sludge with the dry basis composition, shown in Table I, and 60% of the solubilized sludge was used in the continuous treatment experiment.

Waste-Activated Sludge and lntracellular Biopolymer Decomposition Rates

A complete-mixing anaerobic digestion tank with a 2.5-L capacity was used. The digestion tank was maintained at pH 7 and at a temperature of 37°C. The raw waste- activated sludge and the supernatant of solubilized sludge were treated continuously with various retention times. Each decomposition rate was examined.

Continuous Treatment Apparatus

The one-phase anaerobic treatment process consisted of a complete-mixing tank with a 2.5-L operation capacity and a 3-L sedimentation tank. Figure 1 shows a schematic outline of the two-phase process in which acidogenesis and methanogenesis is achieved in two serial reactors. The acid producing process used a 0.5-L operation capacity complete-mixing tank to which the anaerobic digestion sludge was returned from a sedimentation tank. The methane-producing tank was a 2-L fluidized-bed reactor with agitation. In this reactor, a pipe 5 cm in diameter and 50 cm in length was installed for the solid and liquid separation.

The anaerobic sludge floc that was flowed out from the pipe was separated in the 3-L sedimentation tank and

Table I. Composition of waste-activated sludge.

Figure 1. -0-phase digestion process apparatus: 1, waste-activated sludge stock tank; 2, acidogenesis digester; 3, methanogenesis digester; 4, sedimentation tank; 5, final effluent; 6, anaerobic digestion sludge withdrawal.

returned to the methane-producing tank. Because pumice has been proven to have an agglomerating effect on sludge, and works as a carrier of microorganism^,^^,^^*^^ between 20 g/L and 30 g/L of pumice with a particle diameter of 200 p m and an apparent density of 1.1 g/cm3 were added to each reactor to increase the MLVSS concentration. In each reactor, VSS concentration was 15,000 mg/L, and the temperature was kept at 37°C.

Analytical Method

The supernatant obtained after centrifuging the mixed liquor at 0°C and 15,000 rpm for 5 minutes was used to determine a concentration of various volatile organic acids using gas chromatography. The total organic carbon (TOC) and total nitrogen (TN) concentrations were determined by Sumitomo Kagaku gas chromatography (Model GCT-12N). The protein was measured by Lowry's method and the lipids by Kate's method. The carbohydrates were measured by the phenol sulfuric acid method. Nucleic acids were extracted by the STS method and the ultraviolet absorption spectrum was measured. The nucleic acid concentration was determined from absorbance at 260 nm.

RESULTS AND DISCUSSION

Sludge Solubilization and lntracellular Biopolymer Decomposition

According to both Pfefer's'l and our data, when waste- activated sludge is digested anaerobically without pretreatment, at least 15 to 20 days of solid retention time,

~~~~~~~~~~~~~~~~~~~~~

Total solid Volatile solid Fixed solid TOC TN Protein Nucleic acid Lipid Carbohydrate

Concentration 19,700-17,600 17,000- 14,900 2840-2700 8300-7760 1990-1270 10,700-9310 1910-1540 2320-935 1700-1470 ( W L ) (18,300) ( W 0 0 ) (2740) (7980) (1650) (10,000) (1780) (1450) (1 680)

Content 100 86.3-84.4 15.6-13.7 45.7-39.3 11.3-8.7 60.7-50.7 10.7-8.5 10.7-6.1 10.5-8.1 ("/.I (85.0) (15.0) (43.8) (9.0) (54.2) (9.6) (7.9) (9.2)

Numbers in parentheses indicate average values.

SHIMIZU, KUDO, AND NASU: ANAEROBIC WASTE-ACTIVATED SLUDGE DIGESTION KINETIC MODEL 1083

sometimes more, are required to obtain greater than 60% digestion efficiency of mesophilic digestion, as shown in Fig- ure 2. The first-order digestion rate constant was approximately 0.15 day -l . Therefore, pretreatment of sludge is necessary to promote solubilization and hydrolysis in order to achieve rapid sludge digestion in an anaerobic process.

Figure 3 shows the relationship between the ultrasonic pretreatment time and the percentage of solubilization. Fifty percent solubilization requires 30 to 40 minutes of treatment. Ninety minutes were required to achieve the maximum solubilization rate of 75% to 80%. Surfactants are known to enhance the lysing and the destruction of microorganisms.' 'heen-80 (polyoxyethylene sorbitan monooleate) was used in this study to determine the effect of the surfactants on waste-activated sludge solubilization. Adding about 50 mg/L of Tween-80 doubled the solubilization rate of the waste sludge in the ultrasonic treatment, but had no inhibiting effect on the sludge digestibility in the anaerobic digestion process.23

To clarify the effect of sludge solubilization on the anaerobic digestion efficiency, supernatants obtained from the solubilized sludge and the raw waste-activated sludge were added to an anaerobic digester continuously. The decomposition rate of the lysed intracellular organic substance such as proteins and the digestion rate of the raw waste- activated sludge were examined and compared. From the mass balance expression in a continuous treatment, the following was obtained:

V * dL/dt = F(L, - L) - V(-dL/dt)deg (1) In a steady state,

(-d~/dt),, = (L, - L)/e (2)

Lo: VSS concentration of influent waste-activated sludge or TOC concentration of released organic substance (mg/L). VSS concentration in digester or soluble TOC (except volatile organic acid TOC) concentration in digester (mg/L).

8 : mean retention time (day). Q: sludge input volume per unit time (L/d).

L:

Solids retention time (day)

Figure 2. Degradation of volatile solid in the raw waste-activated sludge as a function of solids retention time: (0), Pfefer's data with hydraulic retention time, 10 days; 0, Pfefer's data with hydraulic retention time, 5 days; (U), data obtained from this study.

' 0 20 40 60 8 0 T i m e ( m i n )

Figure 3. addition concentration (mg/L): (0), 0; @), 50; (A), 100; (D), 500.

Ultrasonic pretreatment of waste-activated sludge. Tween-80

When the waste-activated sludge digestion or the released organic substance decomposition rates are assumed to be the first-order reaction, the following is derived:

Therefore,

L' : nonbiodegradable organic substance concentration or nonbiodegradable waste-activated sludge

digestion or hydrolysis rate constant (day-'). (mg/L).

L: Figure 4 indicates the waste-activated sludge and its released organic substance decomposition characteristics. The results show that, in an anaerobic digestion process, the digestion rate of the waste-activated sludge and the hydrolysis rate of the released components follow the first-order reaction. The rate constant calculated from the straight line slope is 0.16 day-' for waste-activated sludge digestion and 1.2 day-' for released organic substance decomposition. This indicates that release of cell materials by cell wall lysing can increase the decomposition rate of the waste- activated sludge component. The maximum anaerobic decomposition efficiency of the waste-activated sludge was about 65%, and of the released cell components was about 90%. In evaluating the effect of the sludge pretreatment, both the decomposition rate and maximum decomposition efficiency are important factors. Sludge solubilizing by destroying cell walls improves both.

L I Lo

Figure 4. L/Lo vs. (1 - L/Lo)/O plot. (0), Raw waste-activated sludge, X = 0.16 day-'; (O), solubilized organic substance, L = 1.2 day-'.

1084 BIOTECHNOLOGY AND BIOENGINEERING, VOL. 41, NO. 11, MAY 1993

Next, the biodegradability of an aerobic digestion of the main waste-activated sludge organic components, protein, lipid, carbohydrate, and nucleic acid, were examined. Fig- ure 5 shows the result of a batch digestion study of the raw waste-activated sludge and the biopolymers released from sludge. In this experiment, raw waste-activated sludge and solubilized sludge supernatants were added in the pulse form to an anaerobic digestion tank where they had been continuously treating the raw waste sludge and solubilized sludge, respectively, with a retention time of 10 days.

The sludge digestion rate and the hydrolysis rate of intracellular biopolymers were obtained from the decreasing curve of MLVSS concentration and TOC concentration, respectively. The hydrolysis rates of the proteins, carbohydrates, and nucleic acids were much higher than the raw waste-activated sludge digestion rate. The nucleic acids decomposed particularly rapidly in solubilized sludge.

Figure 6 indicates sludge component decomposition characteristics in a continuous treatment system. The intracellular biopolymer substance decomposition rate followed the first-order reaction, and was considerably higher than the sludge digestion rate of 0.16 day-'. For proteins, the rate was 1.3 day-'. The nucleic acids rate was 1.8 day-'. For carbohydrates, it was 1.2 day-', and for lipids it was 0.76 day-'.

Table I1 indicates the results of the anaerobic hydrolysis of the main sludge components, i.e., proteins, lipids, nucleic acids, and carbohydrates. The hydrolysis rate of the intracellular biopolymer was 10 times higher than the digestion rate of the waste-activated sludge. The maximum decomposition efficiency was about 90%. These results indicate that it is possible to increase the operating performance of the anaerobic digestion process and greatly improve the conversion rate of sludge to methane gas using solubilized sludge with destroyed cell walls.

Continuous Treatment of Solubilized Waste-Activated Sludge

Sludge solubilization increases the treatment efficiency and performance of the waste-activated sludge digestion.

I I I

100 200 300 Digestion time (hr)

Figure 5. Degradation of waste-activated sludge components: (O), MLVSS; @), sludge TOC; (O), protein; (A), carbohydrate; (A), nucleic acid.

a31 s'

; I ,<- 1 ; ,

O ! l 1 ' a2 0.4 0.8 LI Lo

Figure 6. L/Lo vs. (1 - L/Lo) /B plot for intracellular biopolymer. (O), Protein, L = 1.3 day-'; (O), carbohydrate, L = 1.2 day-'; (A), lipid L = 0.76 day-'.

Figure 7 shows the relationship between the overall gas production rate and the retention times in anaerobic digestion. Sludge with the composition shown in Table I was put into the digestion tank and treated continuously with various hydraulic retention times.

In the one-phase treatment process of the raw waste- activated sludge, when the retention time was reduced (i.e., when the input loading rate was increased) there was no gas production rate increase. Alternatively, in the two-phase treatment with ultrasonically lysed, solubilized sludge, the gas production rate increased remarkably as the retention time was reduced. With a 2.5-day retention time and a VSS input loading rate of about 6.0 kg/m3 day, the overall gas production rate of the two-phase process was about four times greater than that of the one-phase treatment process.

Figure 8 shows the relationship between gas conversion efficiency and retention times. Gas conversion efficiency, based on the TOC balance, was calculated with Eq. (5). Gas conversion

efficiency - -

overall gas production rate (kg-C/m3 - day)

TOC loading rate of input organic substance (5)

(kg-C/m3 day)

Table 11. biopolymers.

Hydrolysis of waste-activated sludge and intracellulai

Hydrolysis rate Maximum percent constant of hydrolysis

(day-') ("/.I Raw sludge 0.16

Protein 1.3 Nucleic acid 1.8a Lipid 0.76

Soluhilized sludge 1.2

Carbohydrate 1.2

Cellulose 0.95b 0.52'

65 90 95 95 88 90 90b 9 o c

a Batch data. Continuous treatment with sludge recycle. Chemostat data.


10 2 0 30 Hydravlic retention time(&yJ

Figure 7. Relationship between overall gas production rate and hydraulic retention time. Solid lines indicate theoretical values. Experimental data were obtained from two-phase digestion process where solubilized sludge was added 0, and single-phase digestion process where solubilized sludge (A) and raw waste-activated sludge (M) were added.

In a two-phase treatment system with solubilized sludge, the gas conversion efficiency was greater than 40% and the digestion efficiency was greater than 60% with a retention time of over 2.5 days. On the other hand, in the one-phase treatment of the raw sludge, no gas production increase was achieved, even when the retention time was reduced and the sludge input loading rate was increased. Consequently, the gas conversion efficiency decreased to less than 10% with 2.5 days of retention time.

Kinetic Model and Simulation Results

Experimental results indicated efficiency improvements with solubilized sludge in the anaerobic digestion treatment process. A kinetic model that might be used to estab- lish the operating conditions for the solubilized and raw waste-activated sludge digestion processes was proposed. Solubilized sludge with ultrasonic lysis include intracellular biopolymers and nonsolubilized parts, which were solubilized by releasing intracellular biopolymer in the digestion tank. The intracellular biopolymers in the digestion tank were assumed to be hydrolyzed by anaerobic bacteria. This produced various volatile organic acids,

Hydraulic retention tirne(day1

Figure 8. Relationship between gas conversion rate and hydraulic retention time. Solid lines indicate theoretical values. Experimental data were obtained from a two-phase digestion process where solubilized sludge was added 0, and single-phase digestion process where solubilized sludge (A) and raw waste-activated sludge (W) were added.

which were decomposed to acetic acid and H2 and then converted to methane gas.

Furthermore, it was assumed that the waste-activated sludge solubilization, the hydrolysis of the intracellular biopolymer substances, the decomposition of the volatile organic acids to acetic acid and H2, and the conversion to methane gas followed a first-order reaction. The microorganism population in the anaerobic digestion process was assumed to consist of acid-forming bacteria, Hz-producing acetogenic bacteria, and methanogenic bacteria. The growth yield and the decay rate constant of the acid-forming bacteria and the Hz-producing acetogenic bacteria were assumed to be the same.” The following mass balance expression would be obtained in a one-phase treatment process. Waste-activated sludge:

- = dLo D(L,,I - (1 + CY - ac)LO) - R L ~ (6) dt R L ~ = KLo(Lo - Lo,*) = K L ~ F ~ 0 . m Lo (7)

Biopolymer substance:

Volatile organic acid:

Acid-forming bacteria and acetogenic bacteria:

Methanogenic bacteria:

Hydrogen and carbon dioxide:

Methane gas:

where:

Li organic substance concentration (mg/L) [raw waste activated sludge (i = 0), protein


(i = l), nucleic acid (i = 2), lipid (i = 3), carbohydrate ( i = 4)]. volatile organic acid concentration (mg/L) [acetic acid ( j = l), propionic acid (j = 2), butyric acid ( j = 3), valeric acid ( j = 4), caproic acid ( j = 5 ) ] . gas concentration (mg/L) [H2 ( k = l), C02

organic substance decomposition rate constant (day-'). volatile organic acid decomposition rate constant, (day-'). maximum decomposition rate (-). biopolymer production ratio with solubilizing waste-activated sludge (-). organic acid production ratio with decomposing organic substance (-). other volatile organic acid production ratio with decomposing organic volatile acid (-). decomposition rate of organic substance and volatile organic acid (mg/L * day).

(k = 2), CH4 (k = 3)].

RA, RL, RL1 decomposition rate of organic substance,

X A

X M

X T

Y A

Y M

L A

AM

RGK R&1

FGKILi

FGKISj

UGK D a C

Subscripts

I *

volatile organic acids, except acetic acid and acetic acid based on COD (mg COD/L - day). acid-forming and acetogenic bacteria concentration (mg/L). methanogenic bacteria concentration (mg/ L). total bacteria concentration (mg/L). acid-forming and acetogenic bacteria growth yield based on COD (-). methanogenic bacteria growth yield based on COD (-). acid-forming and acetogenic bacteria decay rate constant (day-'). methanogenic bacteria decay rate constant (day-'). gas conversion rate (L/L day). gas conversion rate of H2 to CH4 based on COD (mg COD/L - day). gas volume conversion factor for organic substance decomposition (L/mg). gas volume conversion factor for volatile organic acid decomposition (L/mg). gas evolution rate (L/L - day). dilution rate (day-'). recycle ratio (-). concentration factor (-).

input. nonbiodegradable organic substance.

Eq. (6) is the mass balance expression for the waste- activated sludge, and Eq. (8) is for the intracellular biopolymers. It was assumed that the biopolymers released from

the waste-activated sludge consisted of proteins, nucleic acids, lipids, and carbohydrates. It was further assumed that the volatile organic acids produced from the biopolymer decomposition were acetic, propionic, butyric, valeric, and caproic acids. Given this, the mass balance of the volatile organic acids can be expressed in Eq. (10). In the case of acetic acid, the first term in Eq. (10) represents the outflow changes, and the second term represents the acetic acid production from the biopolymer component decomposition. The third term represents the acetic acid production from the decomposition of the other organic acids, e.g., propionic and butyric acids. The fourth term is the conversion of acetic acid to CH4.

H2 and C02 are formed by biopolymer and volatile organic acid decomposition. CHq is formed by acetic acid decomposition as well as H2 and C02, as expressed in Eq. (16). The mathematical model is applicable to a two-phase

treatment process as well. To simulate the experimental results with the above

mathematical model, it is necessary to determine the stoichiometric coefficient and kinetic parameters. For the waste-activated sludge solubilization and biopolymer substance decomposition rate constants, the experimental values shown in Table I1 are used.

To obtain the organic acid yield coefficient from the biopolymer substance decomposition, a bovine serum as the protein, RNA from yeast as the nucleic acid, and a palmitic acid as the lipid were added into the acid-producing tank. Yield coefficients were obtained from the ratio of the amount of each volatile organic acid produced to the amount of the biopolymers consumed in the batch culture.

At least 50% of the total CHq evolution occurred via the volatile organic acids, e.g., propionic, butyric, valeric, and caproic acids in the anaerobic digestion of the waste- activated sludge. This result indicates that the acetogenic bacteria in a symbiotic relation with methanogen takes an important role in the anaerobic digestion of waste-activated sludge, as reported by Bryant4 and Kaspar et a1.16 It is thought that the higher fatty acids are decomposed by

oxidation, ultimately producing acetic acid. However, various volatile organic acids were produced in this experiment, indicating that bacterial organic acid metabolism is influenced by the partial pressure of hydrogen in the reaction system and that acetic acid may be converted to other organic acids. The organic acid decomposition kinetic parameter was obtained by analyses of the time courses of the various volatile acids in the batch e~pe r imen t .~~

In this experiment, various volatile organic acids were put into each anaerobic digestion in pulse form. The decomposition of the volatile organic acids is approximated by first-order reaction kinetics. The first-order rate constant indicates the organic acid decomposition ability, which is an important parameter when evaluating the process performance. The decomposition ability of the volatile organic acid in the methane-producing tank for the two- phase digestion process was much higher than in the one-phase process. In a two-phase system, various volatile


organic acids, i.e., acetic, propionic, butyric acids, etc., are constantly supplied from the acid-producing tank to the methane-producing tank. This increases the activity of the acetogenic and the acetic acid-utilizing methanogenic bacteria. When raw waste-activated sludge was added to the one-phase system, an extremely low amount of volatile organic acid was produced because the sludge solubilization was the rate-limiting step.

The yield coefficients of H2 and C02 from the biopolymer decomposition were calculated from the stoichiometric relationship, e.g., for protein, C39H63013N10.

C39H63013N10 + 24H20 - 5.1CH3COOH + lSCH3CH2COOH + 0.6CH3(CH2)2COOH

+ 0.5CHg(CH2)3COOH + 0.08CH3(CH2)4COOH

+ 1.8CsH903N + 8.2NH3 + 15H2 + 9.7C02 (17)

In stoichiometric Eq. (18), CsH903N represents the molecular formula of bacteria. The H2, C02, and CH4 yield coefficients in the volatile organic acid decomposition were also calculated from the stoichiometric relationship. The values reported by Henze et a1.I2 were used for the growth yield of the acidogenic, the acetogenic, and the methanogenic bacteria. For the decay rate constant, the values reported by Pavlostathis et a1.20 were used. Table I11 indicates the stoichiometric coefficient and the kinetic parameters obtained in the experiment. Table IV shows the raw waste- activated sludge and the biopolymer concentrations input to the anaerobic digestion tank. Using the recycle ratio of cr = 0.25 and a sludge concentration factor of c = 1.5, a simulation study was conducted assuming a steady state.

Figure 9 compares the simulation and experimental results of the volatile organic acid concentration in the effluent. The model could simulate approximately the in- creasing tendency of the volatile organic acid concentration with a decreasing retention time in the two-phase digestion process. The observed values, however, were lower than the calculated values. The difference between the observed and the calculated curve might be caused by underestimation of the decomposition rate constant of the volatile organic acids. The rate constant was obtained from the batch culture's data after pulse input of the volatile acids mentioned above. In this experiment, the rapid increase of the volatile acid concentration might have inhibited the degradation reaction.

Figure 10 shows the simulation results of the protein concentration in the digester. In the solubilized sludge digest system, as retention time increased, the protein concentration decreased because the decomposition efficiency increased. However, in the raw waste sludge digester system, the protein concentration was quite low because solubilization was the rate-limiting step. Quantitatively, the proposed model tends to follow the above, as well as with other biopolymer components.

Figure 7 shows the simulation results of the overall gas production rate. The mathematical model demonstrated that when the raw waste-activated sludge was added to

a digester, even if retention time was reduced, the gas production did not increase. When the solubilized sludge was added to the digester, the gas production rate greatly increased with a decreased retention time.

Figure 8 shows that the retention time and the gas conversion rate relationship can be expressed quantitatively with the model. In the two-phase digestion systems, where solubilized sludge was added, gas conversion efficiency increased to over 60%. However, in the one-phase systems where the raw waste sludge was added, the efficiency did not exceed 25%, demonstrating that the solubilized sludge improves treatment performance. The model simulated the above experimental findings.

When the solubilized sludge was treated in an anaerobic digestion process, the various volatile organic acids produced as intermediates were converted effectively to CHq in the methane-producing tank through a syntrophic associa- tion between the acetogenic and the methanogenic bacteria. The experimental results and the simulation demonstrate that, when solubilized waste-activated sludge is digested, a two-phase anaerobic digestion process greatly enhances the treatment's efficiency.

CONCLUSION

The study of anaerobic digestion of ultrasonically solubilized waste-activated sludge leads to the following con- clusions:

The digestion rate of the waste-activated sludge and the hydrolysis rate of the biopolymers released from the waste-activated sludge follow first-order reaction kinetics. The respective rate constants were 0.16 day-' and 1.2 day-'. The digestibility of the waste-activated sludge can be greatly improved by lysing cell walls and releasing the sludge components from the cells. When the raw waste-activated sludge is treated in a continuous digestion process, no increase in gas production can be expected, even if the retention times are reduced. However, when solubilized sludge was used, the gas production rate increased until the retention time was under 2.5 days. With a 2.5-day retention time, digestibility improved to 60% and the gas conversion efficiency improved to 40%. The kinetic model, which assumes that sludge solubilization, biopolymer substance hydrolysis, volatile organic acid decomposition to acetic acid, and conversion of acetic acid to CH4 follow the first-order reaction, demonstrates quantitatively that sludge solubilization improves treatment efficiency. Compared with a one-phase digestion system, the conversion efficiency of waste-activated sludge to CHq is greatly improved in a two-phase anaerobic digestion process with the biological symbiosis of the acetogenic and methanogenic bacteria.


Tabl

e 11

1. K

inet

ic p

aram

eter

s us

ed f

or s

imul

atio

n st

udy.

Was

te-a

ctiv

ated

slud

ge (i

= 0

)

Prot

ein

(i =

1)

Nuc

leic

aci

d

(i =

2)

Lipi

d C

arbo

hydr

ate

(i =

3)

(i =

4)

(a)D

ecom

posi

tion of

was

te-a

ctiv

ated

slu

dge

and

biop

olym

er

Dec

ompo

sitio

n ra

te c

onst

ant

KL

~

(day

-’)

Max

imum

dec

ompo

sitio

n ra

te F

L~

,~

~

(-)

Con

vers

ion

ratio

to v

olat

ile o

rgan

ic a

cid

Fs,

l~i (-

) A

cetic

aci

d (j = 1

) Pr

opio

nic

acid

(j

= 2

) B

utyr

ic a

cid

(j

= 3

) V

aler

ic a

cid

(j

= 4

) C

apro

ic a

cid

(j

= 5

)

0.16

0.

65

1.30

0.

95

1.80

0.

95

0.76

0.

88

1.20

0.

90

L a

0.36

0.

29

0.05

0 0

0.40

0.

40

0.16

0.

12

0.05

0.31

0.

13

0.13

0.

05

0.05

0.25

0.

13

0.06

0.

06

0.01

G

as c

onve

rsio

n ra

tio w

ith o

rgan

ic s

ubst

ance

de

com

posi

tion

FG

K/L

~ (mL/

mg)

H

ydro

gen (k

= 1

) -

0.40

0

1.80

0.

18

Con

tent

of

biop

olym

er F

L~

/L~

(-)

1.

00

0.64

0.

12

0.09

4 0.

11

Car

bon

diox

ide (k

= 2

) -

0.27

0.

10

0.40

0.

16

Ace

tic a

cid

(j

= 1

) Pr

opio

nic

acid

B

utyr

ic a

cid

Val

eric

aci

d C

apro

ic a

cid

(j

= 2

) (

j =

3)

(j

= 4

) (

j = 5

)

(b)

Dec

ompo

sitio

n of

vol

atile

org

anic

aci

d D

ecom

posi

tion

rate

con

stan

t K

sj (

day-

’)

Sing

le p

hase

Tw

o ph

ase

Ace

tic a

cid

(j’ =

1)

Prop

ioni

c ac

id (j’

= 2

) B

utyr

ic a

cid

(j’ =

3)

Val

eric

aci

d (j

’ = 4

) C

apro

ic a

cid

(j’ =

5)

Vol

atile

org

anic

aci

d pr

oduc

tion

ratio

Fs,l

s,i (-)

Gas

con

vers

ion

ratio

with

org

anic

sub

stan

ce

deco

mpo

sitio

n F

GU

S~ (m

L/m

g)

Hyd

roge

n (k

= 1

) C

arbo

n di

oxid

e (k

= 2

) M

etha

ne (k =

3)

4.62

8.

20

1.11

3.

00

4.78

16

.3

1.85

6.

84

0.77

1.

02

-

0.81

1.

36

0.59

0.

52

-

-

0.76

0.47

-

-

0.41

-

-

0.40

0.

32

-

0.55

-

-

-

0.40

0.

40

Aci

doge

n A

ceto

gen

Met

hano

gen

(c)

Bio

mas

s gr

owth

G

row

th y

ield

(V

SS k

dkg

CO

Da

0.15

0.

15

0.03

D

ecay

con

stan

t (da

y-’)

b 0.

10

0.10

0.

015

a H

eme

et a

1.l’;

Pavl

osta

this

et

al?’

Table IV. Input concentration of waste-activated sludge and intracellular biopolymer.

Waste activated Protein Nucleic acid Lipid Carbohydrate sludge (i = 0) ( i = 1) ( i = 2) ( i = 3) ( i = 4)

(1) Raw waste-activted sludge

(2) Solubilized waste-activated sludge Input concentratioin Li, (mg/L) 15,000 0 0 0 0

Input concentration Li,o (mg/L) 6000 5500 1500 1000 1000

Hydraulic retention time(day) (a) Single phase digestion process where raw sludge was added.

-._ -- " -"- -. " .::- ~ ~ : ~ - . . " ~ . ; ~ . * . - . ; - ~ . ; ~ ~ ............................. -

'0 10 20 30 Hydraulic retention time(day)

(bl Two phase digestion process where soclbilized sludge w added.

Figure 9. Relationship between effluent volatile organic acid concentration and hydraulic retention time. Theoretical values: (-), acetic acid; ( 0 - . * - . . -), propionic acid; ( * - . - * - a ) , butyric acid; (--------), valeric acid; (. . . - .), caproic acid. Experimental data: 0, acetic acid; (W), propionic acid.

Physical treatments (e.g., thermal pretreatment and mechanical sludge lysis, chemical pretreatment with acids, alkalis and surfactants, or a combination of both the physical and chemical treatments) could enhance the overall

Hydraulic retention time (day) 10 2 0 3 0

Hydraulic retention time (day)

Figure 10. Relationship between protein concentration in the digesters and hydraulic retention time. Solid lines indicate theoretical values. Q (a), Single-phase digestion process where raw waste-activated sludge was added; 9 (W), single-phase digestion process where solubilized sludge was added; 9 (O), methane-producing tank of two-phase digestion process where solubilized sludge was added; Q m, acid-producing tank of two-phase digestion process where solubilized sludge was added.

digestion performance. An anaerobic, thermophilic bacteria which produces a lysozyme enzyme lysing cell wall has been found. There is a great possibility of lysozyme- producing bacteria existing in anaerobic digestion tanks. Therefore, in the future, it is also necessary to find highly active bacteria and study their method of separation, incubation, and their application to a sludge-treatment process.

References

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13.

14.

15.

16.

17.

18.

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anaerobic waste-activated sludge digestion–a bioconversion mechanism and kinetic model

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