Iwa international Specialist Conference

high SRTs in the activated sludge process determine low biogas production in the mesophilic anaerobic stabilisation of waste activated sludge. This causes an upset of the energy balance of the process and therefore an increase in biogas production is needed. Since waste activated sludge is a hard substrate to be treated, the increase in biogas production can be obtained by pre-treatments of wasted sludge or through the co-digestion with other biodegradable substrates rich in organic matter;

a detailed model of the process will be presented in the full paper;

the volatile content of the sludge (VSS) seems not to be a good indicator of its biodegradability and other index should be searched (maybe COD).

References

Bolzonella D., Innocenti L., Cecchi F. BNR wastewater treatments and sewage sludge anaerobic mesophilic digestion performances. Water Science and Technology, 46(10),2002.

44.

Development of Biogas Storage Process with Gas Adsorption Technology Using Activated Carbon

Shuichi Ochi *, Katsumi Maruyama **, Masato Fujino *** and Hiromichi Sawahara ****

* Public Works Research Institute, Minamihara 1-6, Tsukuba-shi, Ibaraki, 305-8516 Japan

** Tsuruoka City, Wastewater Treatment Center, Houden 3-21-1, Tsuruoka-shi, Yamagata, 997-0011 Japan

*** Japan Institute of Wastewater Engineering Technology, Nishi-Ikebukuro 1-22-8, Toshima-ku, Tokyo, 171-0021 Japan

**** Tsukishima Kikai Co., Ltd., Tsukuda 2-17-15, Chuo-ku, Tokyo, 104-0051 Japan

Abstract

The production and utilization of biogases would greatly help curb global warming, which is perhaps the most serious environmental problem faced by mankind today. To ensure the effective and stable use of biogas as a resource, a large gas storage capacity is necessary because the production rate of biogas fluctuates. Conventional storage tanks are not practical, because larger tanks are required.

Therefore, we attempted to develop a practical biogas storage process with a much greater capacity than the conventional process using a gas adsorption technology. Pilot plant experiments were carried out to investigate the characteristics of biogas adsorption, which can be used for the design and operation of the adsorption storage process. The pilot plant was equipped with a storage tank of 0.5 m3 filled with activated carbon, and desulfurized digestion gas generated by anaerobic digestion of sewage sludge was supplied. Four important results were obtained from the experiments: 1) The adsorption storage tank could store 3 to 4 times more biogas than the usual pressured storage tank at the same pressure. 2) The adsorption storage tank generated heat when adsorbing the gas and absorbed heat when releasing the gas. 3) The moisture contained in the digestion gas can turn into droplets or even ice because of the fall in temperature during the release of the gas, thus lowering the storage capacity of the tank, so it is necessary to remove such moisture beforehand. 4) Trace gas matter included in the digestion gas should be removed, because these may accelerate the degradation of the adsorption ability of the activated carbon.

Based on these results, an adsorption storage plant with a gas storage capacity of 600 m3 was constructed for practical use. The storage capacity of the plant with the pressure of 0.7 MPa is 20 times greater than that of the conventional process with the atmospheric pressure, though the empty volume of the tank is only 30 m3. The desulfurized digestion gas, after passing through the dehumidifier unit and the pre-processor unit to remove trace gas matter, is supplied and stored in the absorption storage tank. In the practical research, 610 m3 of digestion gas was repeatedly stored and released at the flow rates of 80 m3/hr for storage and 100 m3/hr for release. The temperature fluctuation of the absorption storage tank was 23.5°C on average

45.

EFFECT OF AN OZONATION ON PATHOGENS AND MICROBIAL COMMUNITY STRUCTRE FROM VARIOUS TREATMENT OF SLUDGE

Kyu-Hong Ahn, Sung-Kyu Maeng*, Bo-Sung Kang, Jun-Seok Hong, Byung-Ran Lim

Future Technology Research Division, Korea Institute of Science and Technology P.O. Box 131, Cheongryang , Seoul, 136-791, Korea

* Tel.: [phone], Fax.: [phone], e-mail: [e-mail]

Due to rising concerns with excess municipal sludge production in Korea, new regulations were promulgated in the year 2001 to prohibit the disposal of sludge containing more than 75% moisture content to landfill. In most countries, the disposal of excess activated sludge has been heavily dependent on landfill. Thus, there have been many studies to develop technologies regarding sludge reduction and recycling, especially in countries with limited land area available. Among the developed technologies, an ozonation has turned out to be particularly effective for sludge mass and volume reduction. It has been reported that a conventional activated sludge system coupled with an ozonation process could be successfully operated without producing excessive sludge. Pathogens are major issues for public concern and ozonation can play an important role in solving such nuisances caused by disposing sludge. In 1993, the US EPA promulgated the 503 regulations, which provide guidelines for the disposal of sewage sludge. Based on the remaining pathogens, the rule classified sludge into two categories; Class A and Class B. Class A sludge may be used in any application without any further treatment. In order to investigate the effect of ozonation on the level of pathogens (Salmonella) and fecal-contamination (indicated by fecal coliform and fecal Streptococcus), optimum ozone dosage for Class A sludge was determined. Furthermore, quinone profiles were carried out to investigate the microbial community structure of the wastewater sludge treated with different ozone dosages. The quinone profiles have recently gained recognition as a tool for the analysis of microbial population dynamics in mixed cultures. Microbial respiratory quinones known as part of the bacterial respiratory chain and there are only one dominant type of quinone exists in a species or genus and it represent as mole fraction of each type. Therefore, by obtaining quinone profiles from an ozonated sludge, the effect of ozone dosage on microorganisms in wastewater sludge can be found not only as an inactivation but also discovering microbial community structure changes.

Sludge used in this study was taken from various sludge treatments in Kyonggi province, Korea. Figure 1 shows the lab scale ozonation process for sludge reduction. Different ozone dosages (mass of ozone consumed per unit mass of sludge solids) were applied to excess

activated sludge by changing the contact time of ozone-containing gas with the sludge. Measurement of parameters including pH, total solids, volatile solids, fecal coliform, fecal Streptococcus, and Salmonella were carried out according to the techniques described by Standard Methods.

In order to determine the optimal ozone dosage for pathogen reduction as well as sludge mass reduction, the ozone dosages 0.05, 0.1, 0.2 and 0.4 gO₃/gSS were applied and the extent of sludge reduction was measured in terms of the SS. For example, more than 25 % of suspended solids were degraded at 0.2 gO₃/gSS. Also, mineralization efficiencies of the sludge based on TS increased greatly with increasing ozone dosage up to 0.4 gO₃/gSS. With higher ozone dosage, a slight increase in mineralization was observed. Similar to mineralization, solubilization increased with increasing ozone dosage up to 0.2 gO₃/gSS.

The initial concentration of fecal coliform in raw sludge was 1.435  10⁶ MPN/gTS. Effectiveness of ozone on fecal coliform inactivation was insignificant when ozone dosage was 0.05 gO₃/gSS. The fecal coliform was not detected when the ozone dosage increased higher than 0.2 gO₃/gSS. An ozone impact on fecal Streptococcus inactivation was also observed, and it shows the similar trend with fecal coliform. The salmonella was not detected in raw sludge from various treatments through out the study. The ozone dosage of 0.2 gO₃/gSS fulfilled the criteria for category A sludge.

The microbial community has changed by the ozonation. Both ubiquinone and menaquinone were decreased by the ozonation and it seems that there is more resistance showed from ubiquinone compared to menaquinone. From the quinone profiles, the resistance from different microorganisms to the ozone was discovered in wastewater sludge of microorganisms. The microbial diversities were found in wastewater sludge treated with various ozone dosage based on quinone profile from the remaining microorganisms.

An ozonation of wastewater sludge could be an available technology for sludge reduction satisfying the Class A sludge regulation based on pathogens. In the future, an ozone treatment for sludge disinfection should be compared with other treatment technologies in terms of cost as well as inactivation performance.

Figure 1. Experimental set up for ozone Treatment

46.

Process Performance And Change In Sludge Characteristcis During Anaerobic Digestion Of Sewage Sludge With Pre/Post Ozonation
Kazuya Komatsu¹, Rajeev Goel², Hidenari Yasui¹, Hideki Harada<sup>3

1,2</sup>Kurita Water Industries, Research and Development Center, 7-1, Wakamiya, Morinosato, Atsugi, Kanagawa, Japan

³Deapartment of Civil and Environmental Engineering, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan

²Corresponding Author – Tel: [phone]; Fax: [phone];

e-mail: [e-mail]

Key words
Anaerobic digestion, dewatering, , inorganic, sludge hydrolysis, solubilization, pretreatment, ozone, viscosity

Introduction

Due to stricter standards for the effluent and solid disposal both the quantities and the unit disposal costs for sludge are on an increase and warrants research in new technologies that can address these issues. Reduction in sludge production from treatment plants can be achieved either during wastewater or sludge treatment processes. In sludge treatment, anaerobic digestion is one of the most fundamental process for solid reduction and many studies have been focused around this process for improving solid reduction through sludge pretreatment. Although, references can be made to many pretreatment methods that use thermal, thermo-chemical and mechanical disintegration, nevertheless, from practical and operational point of view there still remains a need for new effective pretreatment methods. With this background, ozonation was considered as a new pretreatment option. Ozonation was considered to result in more effective treatment as it not only hydrolyzes the sludge solids but also has the potential to improve biodegradability due to its oxidative nature.
The results of our previous experiments in which ozone pretreatment with chemostat anaerobic digestion were studied, ozone pretreatment was found to be effective in partial solubilization of sludge solids leading to subsequent improvement in anaerobic degradability. Although, the extents of solubilization and digestion efficiency depended on the applied ozone doses, a reasonable ozone dose of 0.05 gO₃/gTS was found to be effective. At 0.05 gO₃/gTS, the anaerobic digestion efficiencies improved to about 59% as compared to 30-35% for the control run (Goel et. al. 2002). To further improve the digestion efficiciencies a “closed loop operation” having no intentional solid withdrawal was experimentally studied (Goel et. al. 2002). The closed loop operation was found to be feasible with either pre/post ozone treatment, however, post-ozonation was found to be more effective with respect to minimum requirement of ozone. The volatile solid degradation efficiencies of over 80% were estimated for closed loop configuration which was a significant improvement over the 35-40% degradation efficiencies for conventional and 50-60% for ozone pretreatment with chemostat anaerobic digestion. As these laboratory studies were conducted using synthetic sludge, it was essential to verify and extend these results by conducting pilot scale studies with “sewage sludge”.
Pilot scale experiments at a full-scale sewage treatment plant using sewage sludge were conducted to 1) verify the digestion efficiency of sewage sludge in different process configurations having pre/post ozonation and 2) characterize the anaerobically digested sludge with respect to the inorganic composition, viscosity and dewatering characteristics.
Four anaerobic digesters each with a volume of 1m³ were operated at a full-scale treatment wastewater treatment plant. One of the reactors served as control and other three were operated in closed loop operation (without withdrawal of digested sludge). The sewage sludge used in the pilot scale experiments consisted of primary and secondary sludge in the ratio of 1:3.5 (w/w). Based on the solid balance, the organic solid digestion efficiencies after 3 months of operation for control and closed loop operation were observed to be 52% and 81% respectively. The observed organic removal efficiencies were almost similar to that previouly observed during laboratory scale experiments. However in the case of pilot scale experiments the total solids accumulated in the reactor were about 6% (60g/l) as compared to 3% (30g/l) in the case of laboratory scale experiments. This difference in solid accumulation was largely because of the presence of higher fraction of inorganics (especially Fe²⁺) in the raw sewage sludge used in the pilot scale experiments.
Under the sludge characterization studies, inorganics content, viscosity and dewatering properties of sludge were measured. Inorganic components (Fe, Ca, Mg, Al, Mn, S, P, Si etc.) in raw and anaerobically digested sludge were analytically measured and precipitated components were predicted using Visual MINTEQ ver 2.12 (KTH,2002). Based on the inorganic mass balance, it is estimated that about 90% inorganics in the raw sludge are precipitated in the anaerobic digested sludge. This result is significantly different from that of laboratory scale study in which most of the inorganics in the synthetic sludge escaped in the effluent.
For assessing mixing requirements of closed loop reactors at higher MLSS concentrations, viscosity of anaerobic digested sludge was measured. Relationship between MLSS concentration and viscosity of sludge from different reactor configurations were prepared. At the same MLSS, the viscosity of anaerobically digested sludge from closed loop operation was found to be lower than that of digested sludge from control run. Further, based on the results of more than 15 dewatering tests on individual sludges, it was established that water content of the dewatered cake of anaerobically digested sludge from closed loop operation with ozonation was about 10% lower than the control sludge under similar dewatering conditions. Since, a negative correlation was observed between the water content and inorganic content of sludge, the observed reduction in water content of dewatered cake for sludge from closed loop operation is considered to be due to its high inorganic content of 50-56% as compared to 35-40% in control sludge.
In the proposed paper, we intend to 1) present the comprehensive results of pilot scale experiments along with their importance with respect to process feasibility, operation and economics and 2) consolidate the results of laboratory and pilot scale experiments in such a way that comprehensive assessment of the process of anaerobic digestion with ozonation is possible.

47.

Influences of temperature and feeding mode

on aerobic sludge stabilisation processes
B. Wett, M.F. Soliman and W. Rauch
Department of Environmental Engineering, University of Innsbruck

Technikerstr.13, A-6020 Innsbruck, Austria

(E-mail: [e-mail])
Key Words: Modelling, aerobic sludge stabilisation, temperature, batch system, continuous feeding system
METHODS

Experimental studies

Return sludge has been taken from a municipal wastewater treatment plant in order to study the influence of temperature and feeding mode on the performance of aerobic sludge stabilisation. Each experimental run was conducted in a twin-reactor with a volume of 15.5 litres each. Two different operation systems have been compared: A batch system with initial filling and continuous aeration but no additional feeding. A continuous feeding system which has been fed daily by a constant amount of sludge resulting in a sludge retention time of two weeks at steady state conditions (achieved after about 35 days). Constant temperatures of 35, 25, 15 and 8 °C have been considered and the initial sludge concentration was in the range between 20000 and 35000 mg TSS/l. The controlled air-supply kept the DO concentration between 1.5 and 2.5 mg O₂/l.

Model approaches

As commonly known the Activated Sludge Model No.1 (ASM1) represents sludge stabilisation processes by a cycle of microbial decay, partial hydrolysis of decay products X_S and heterotrophic re-growth from resulting substrate S_S (Fig.1a). This mathematical approach lumps both effects actual lysis of biomass (including predation) and endogenous respiration (degradation of storage compounds). The model’s authors (Henze et al., 1987) point out that it lacks reliability at high sludge retention times. The outlined description of sludge stabilisation processes in ASM1 requires about 7 cycle-runs in order to achieve a state of advanced stabilisation as shown in Fig.1. The approach of ASM3 (Gujer et al., 1999) cuts this cycle and assumes a one-way route of endogenous respiration and inactivation of biomass without feeding into hydrolysis. The storage process introduced by ASM3 mainly aims at the description of an instant uptake and delayed metabolism of substrate X_STO at high loading conditions but also considers endogenous respiration.

X

P

X

I


In

=25%, Out=88%

hydrolysis

In=33%,

Out=96%


X

S

In=25.3%, Out=0.08%

S

S

decay

X

S

decay

hydrolysis


In=0.06%, Out=~ 0%

X

In=2.1%, Out=0.04%

ASM1

ASM-Vienna


storage

V

In=29%, Out=5.6%

S

S


X

STO

In=1.3%, Out=3.4%

X

BH

In=0.03%,

Out=~0%


S

O

In=18%, Out=1.5%

X

BH


growth

growth

In=63%,

Out=4.2%


COD total

S

O

S

O

COD total

a)

b)


In=23445, Out=11183 mg/l

In=23445, Out=11208

mg/l


Fig.1: Flow charts of the sludge stabilisation cycles described by ASM1 and ASM-Vienna (supported by ratios of calibrated input and output compounds at 25 °C and within 14 days of stabilisation).

At extremely low loading conditions or during sludge stabilisation respectively it appears necessary to consider separately a fraction of very slowly degradable organic compounds X_V (Nowak et al., 1999). Finally the so-called ASM-Vienna-model (Winkler et al., 2001) suggests the implementation of both mechanisms – storage of easily degradable compounds and inactivation and slow conversion of heterotrophic organisms - in order to cover the maximum range of applicability in terms of SRT (Fig.1b).

RESULTS AND DISCUSSION

In literature (e.g. Grady et al., 1999) the coupled time-temperature-dependency of VSS-degradation is an acknowledged way to characterize the performance of aerobic sludge stabilisation. Fig.2 shows this relationship at operation temperatures of 8 and 35 °C. Additionally the behavior of both applied models is compared and a higher initial degradation simulated by the ASM-Vienna model is indicated. Both model calculations end up at a final degradation value close to 55 %.

Fig.2: Superimposed impacts of temperature and duration on aerobic batch stabilisation.

The model calibration was based on batch data and simulations of the continuous feeding system validated the models. Simulation and measurement results revealed a significantly higher VSS-degradations in case of batch feeding (55 % in comparison to 49 % at 35 °C shown in Fig.3). Although in both reactor types equal mean retention times are achieved, fresh sludge from continuos feeding shows more relevance to the mean degradation efficiency. This effect from the non-linear degradation performance can be demonstrated in this paper.

Fig. 3: Comparison of VSS-degradation rates in a batch system and in a continuous feeding system after 14 days at 4 different temperature levels (measured and simulated values).