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A review: The anaerobic treatment of sewage in UASB and EGSB reactors

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A review: The anaerobic treatment of sewage in UASB and EGSB reactors
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  ELSEVIER PI1:S0960-8524(98)00046-7 Bioresource Technology 65 (1998) 175-190 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0960-8524/98 $19.00 A REVIEW: THE ANAEROBIC TREATMENT OF SEWAGE IN UASB AND EGSB REACTORS Lucas Seghezzo a', Grietje Zeeman b'*, Jules B. van Liel ~, H. V. M. Hamelers h Gatze Lettinga b Universidad Nacional de Salta, Consejo de lnvestigaci6n, Buenos Aires 177, 4400 Salta, Argentina ;'Department of Environmental Technology, Wageningen Agricultural University, P. O. Box 8129, 6700 Ell,, Wageningen, The Netherlands (Received 13 November 1997; revised version received 14 February 1998; accepted 19 February 1998) Abstract The anaerobic treatment process is increasingly recog- nized as the core method of an advanced technology for environmental protection and resource preservation and it represents, combined with other proper methods, a sustainable and appropriate wastewater treatment system for developing countries. Anaerobic treatment of sewage is increasingly attracting the attention of sanitary engineers and dec&ion makers. It is being used successfully in tropical countries, and there are some encouraging results from subtropical and temperate regions. In th& review paper, the main characteristics of anaerobic sewage treatment are summarized, with special emphasis on the upflow anaerobic sludge blanket (UASB) reactor. The application of the UASB process to the direct treatment of sewage is reviewed, with examples from Europe, Asia and the Americas. The UASB reactor appears today as a robust technology and is by far the most widely used high-rate anaerobic process for sewage treatment. © 1998 Elsevier Science Ltd. All rights reserved Key words: anaerobic treatment, sewage, domestic wastewater, UASB reactors, EGSB reactors. INTRODUCTION The term 'sewage' refers to the wastewater produced by a community, which may srcinate from three different sources: (a) domestic wastewater, generated from bathrooms and toilets, and activities such as cooking, washing, etc.; (b) industrial wastewater, from industries using the same sewage system for their effluents (treated or not), and (c) rain-water, particu- larly in the case of sewer systems constructed for both wastewater and storm-water (combined systems) (van Haandel & Lettinga, 1994). The *Author to whom correspondence should be addressed. 175 sewage flow rate and composition vary considerably from place to place, basically depending on economic aspects, social behavior, type and number of industries located in the collection area, climatic conditions, water consumption, type and conditions of the sewer system, and so forth (Haskoning & Wageningen, 1994). Domestic wastewater is usually the main component of sewage, and is often used as a synonym. Table 1 shows the most important constituents of raw sewage in three cities where anaerobic treatment plants are in operation (from van Haandel & Lettinga, 1994). Sand and coarse material (paper, dead animals, bottles) which are retained in the first steps of the treatment process (sand traps, screens) are not considered. In this paper the term 'raw sewage' will be used inter- changeably with 'sewage'. When the sewage is allowed to settle in a primary settler or other settling tank, the result is 'settled sewage' or 'pre-settled sewage'. Provisions for the appropriate handling of sewage date as far back in time as the fourth century B.C., judging by the 'Athenian Constitution' written by Aristotle (van de Kraats, 1997). Thousands of years ahead, direct discharge to the environment is still the most common way of dealing with sewage and domestic wastewater, especially in developing countries. Yet several technological options are available today in the field of wastewater treatment, including conventional aerobic treatment in ponds, trickling filters and activated sludge plants (Metcalf & Eddy, 1991), direct anaerobic treatment (Lettinga, 1995, 1996a), and resource-recovery wastewater treatments with biological systems, in which a combination of anaerobic and aerobic processes is applied (Jewell, 1996). Wastewater purification is the most clear paradigm of environmentally friendly technologies. Some negative aspects of development and urbanization can be diminished, or even elimi-  176 L. Seghezzo et al. Table 1. Composition of sewage in different cities Constituent Pedregal (Brasil) Cali (Colombia) Bennekom (The Netherlands) Settleable solids (ml I ~) 8 2 - - Suspended solids Total 429 215 - Fixed 177 106 - Volatile 252 107 - BOD 368 95 231 COD 727 267 520 Nitrogen (as N) 44 24 45 Organic 10 7 - Ammonia 34 17 - Phosphorus Total 11 1.3 18 Orthophosphate 8 - 14 Organic 3 - 4 Escherichia coli (number in 100 ml) 4 × 107 - - Sulphates 18 - 15 Chlorides 110 - - Alkalinity 388 120 350 Calcium 110 - 4 Magnesium 105 - 2 Temperature (°C) Maximum 26 27 20 Minimum 24 24 8 BOD = biological oxygen demand; COD = chemical oxygen demand (from van Haandel & Lettinga, 1994). Data in mg I ~ unless indicated otherwise. nated, through a comprehensive treatment of domestic and industrial wastewater, directly and immediately enhancing the quality of the environ- ment. Adequate wastewater treatment systems have to be simple in design and efficient in removing the pollutants. Energy consumption in these systems should be low, re-use of water and valuable by-products must be maximized, and the use of sophisticated equipment must be kept to a minimum. These features are required not only in the developing world, but also in industrial countries, where investment costs and energy consumption have to be reduced, while the treat- ment efficiency of the system needs to be optimized. ANAEROBIC TREATMENT Anaerobic treatment of wastewater can be traced from the beginnings of wastewater treatment itself. Anaerobic processes have been used for the treat- ment of concentrated domestic and industrial waste- water for well over a century (McCarty, 1981; McCarty & Smith, 1986). The simplest, oldest, and most widely used process is the septic tank (Jewell, 1987). According to Buswell (1958), a tank designed to retain solids by means of sedimentation, similar to the septic tank, was firstly reported in 1857. Around 1860, a French engineer, Louis H. Mouras, built a closed chamber with a water seal in which all 'excrementitious matter' was 'rapidly transformed'. This system, named 'Mouras' Automatic Scavenger', was first described by Abb6 Moigno in a report which appeared in France in 1881. This invention was enthusiastically defined at that time as 'the most simple, the most beautiful, and perhaps, the grandest of modern inventions' (McCarty, 1985). A chronology of the development of anaerobic diges- tion for waste treatment can be found in Sastry & Vickineswary (1995). The steep increase in energy prices in the 1970s reduced the attractiveness of aerobic methods, contributing to redirecting research efforts towards energy-saving alternatives like anaerobic treatment (van Haandel & Lettinga, 1994). Technical and economical comparisons between aerobic and anaerobic systems have been presented by McKinney (1983), Vochten et al. (1988) and Eckenfelder et al. (1988). A comparison among the most frequently used systems for waste- water treatment in developing countries, including stabilization ponds, activated sludge, trickling filters, anaerobic systems and land disposal, was supplied by von Sperling (1996). High bacterial sensitivity to some environmental conditions (mainly pH, temperature, and toxic compounds), long starting processes, and the production of malodorous compounds, have been commonly cited as disadvan- tages of anaerobic treatment (Jewell, 1987). In fact, pH control may be needed for the treatment of some industrial wastewaters, but for other types of wastewater, including domestic wastewater and sewage, the composition is usually such that the pH will be kept in the optimum range without the need for chemical addition (van Haandel & Lettinga, 1994). Anaerobic bacteria can adapt quite easily to low temperatures, and high-rate anaerobic treatment has been achieved at psychrophilic conditions (Kato, 1994, Kato et al., 1994; Rebae et al., 1995), including  Review: anaerobic sewage treatment 177 some experiences with sewage (Lettinga et al., 1983a; Grin et al., 1983, 1985; de Man et al., 1986, 1988; Sanz & Fdz-Polanco, 1990; van der Last & Lettinga, 1992; Wang, 1994). On the other hand, anaerobic bacteria can tolerate a wide variety of toxicants (Speece, 1983). In fact, aerobic hetero- trophs and methanogens showed similar sensitivities to toxicants, with the exception of an enhanced susceptibility of methanogens to chlorinated aliphatic hydrocarbons and chlorinated alcohols (Blum & Speece, 1991). Anaerobic sludge able to degrade pentachlorophenol (PCP), one of the biocides used in the United States to preserve wood products, was reported by Wu et al. (1993). Removals of PCP up to 99% were also reported by Hendriksen et al. (1992) using a glucose-supple- mented continuous UASB reactor. UASB reactors have also been applied to rapidly detoxify and, under certain circumstances degrade, nitroaromatic compounds (Donlon et al., 1996). The degradation of N-substituted aromatics, alkylphenols, and azo dyes under anaerobic conditions has also been demonstrated (Donlon et al., 1997; Razo-Flores et al., 1996, 1997; Razo-Flores, 1997). The start-up of anaerobic reactors can be satisfactorily achieved in very short times if adequate inoculum is available (de Zeeuw, 1984), and this availability will be progressively greater, as anaerobic treatment plants are built and highly active anaerobic granular sludge becomes available for starting up new plants. Nonetheless, inoculation with active biomass was shown not to be a prerequisite to start-up anaerobic reactors for sewage treatment (Louwe Kooijmans & van Velsen, 1986), and many reactors started up without being inoculated at all, either at pilot scale (Barbosa & Sant'Anna, 1989; Schellinkhout et al., 1985), or full scale (Schellinkhout & Collazos, 1992; Draaijer et al., 1992). At low temperatures the start- up may take longer, but it can be successfully accomplished by inoculating the reactor with digested sludge (Singh et al., 1997). Finally, an adequate construction of the reactor and a proper operation can eliminate completely the problem of bad odours in anaerobic reactors (Conil, 1996). As we can see, substantial improvements have been made in tackling most of the alleged disadvantages of anaerobic treatment, with the result that only a few of the previously presumed drawbacks have remained, while all its principle benefits over conventional aerobic methods still apply (Lettinga, 1995, 1996a,b; Lettinga et al., 1987). According to Jewell (1985), 'there is little doubt that development of a cost-effective and efficient anaerobic sewage treatment alternative would be one of the most significant advances in waste treat- ment history'. Lettinga et al. (1987) fully agreed with this statement by saying that '...a satisfactory appli- cation to raw domestic sewage would represent the maximum possible accomplishment for high-rate anaerobic treatment systems'. The term 'high-rate' was once used for the later designs of sewage sludge digesters, but it is now widely used to refer to anaerobic treatment systems meeting at least the following two conditions: (a) high retention of viable sludge under high loading conditions, and (b) proper contact between incoming wastewater and retained sludge (Lettinga et al., 1987). Anaerobic treatment in high-rate reactors is increasingly recognized as the core method of an advanced technology for environ- mental protection and resource preservation, and it represents, combined with other proper methods, a sustainable and appropriate wastewater treatment system for developing countries (Lettinga et al., 1987, 1993, 1997; van Buuren, 1996; Lettinga, 1996a,b). It is often questioned why aerobic treat- ment of sewage is not replaced more rapidly by the economically more attractive and conceptually more holistic direct anaerobic treatment (Mergaert et al., 1992). Anaerobic treatment would provide tremen- dous advantages over conventional aerobic methods. The costs of aeration and sludge handling, the two largest costs associated with aerobic sewage treat- ment, would be reduced dramatically because (a) no oxygen is needed in the process and (b) the produc- tion of sludge is 3-20 times smaller than in aerobic treatment (Rittmann & Baskin, 1985). Moreover, the sludge (biomass) produced in aerobic processes has to be stabilized in classic anaerobic sludge digesters before it can be safely disposed of, but it was shown to be very resistant to anaerobic degrada- tion (Sanders et al., 1996). Some characteristics of sewage, like low chemical oxygen demand (COD) concentration, high fraction of COD as suspended solids (SS), relatively low temperature, and load fluctuations, are particularly relevant to anaerobic treatment and can have a negative impact on the process performance or costs, exaggerating the diffi- culty of treatment by anaerobic processes (Jewell, 1987). The impact of some of these characteristics was studied by Rittmann & Baskin (1985), who proposed modeling approaches for the purpose of making quantitative evaluations. Careful selection of the technology and appropriate reactor design and operation has overcome most of these possible diffi- culties. Advantages and disadvantages of anaerobic sewage treatment, with special emphasis on high- rate reactors, are summarized in Table 2. THE UASB REACTOR In spite of their early introduction, the interest on anaerobic systems as the main biological step (secondary treatment) in wastewater treatment was scarce until the development of the upflow anaerobic sludge blanket (UASB) reactor in the early 70s (Lettinga et al., 1980; Lettinga & Vinken, 1980). Antecedents of the UASB reactor can be found in the so-called anaerobic contact process studied by Coulter et al. (1957), Ettinger et al.  178 L. Seghezzo et al. Table 2. Advantages and disadvantages of anaerobic wastewater treatment Advantages Disadvantages High efficiency. Good removal efficiency can be achieved in the system, even at high loading rates and low temperatures. Simplicity. The construction and operation of these reactors is relatively simple. Flexibility. Anaerobic treatment can easily be applied on either a very large or a very small scale. Low space requirements. When high loading rates are accommodated, the area needed for the reactor is small. Low energy consumption. As far as no heating of the influent is needed to reach the working temperature and all plant operations can be done by gravity, the energy consumption of the reactor is almost negligible. Moreover, energy is produced during the process in the form of methane. Low sludge production. The sludge production is low, when compared to aerobic methods, due to the slow growth rates of anaerobic bacteria. The sludge is well stabilized for final disposal and has good dewatering characteristics. It can be preserved for long periods of time without a significant reduction of activity, allowing its use as inoculum for the start-up of new reactors. Low nutrients and chemicals requirement. Especially in the case of sewage, an adequate and stable pH can be maintained without the addition of chemicals. Macronutrients (nitrogen and phosphorus) and micronutrients are also available in sewage, while toxic compounds are absent. Low pathogen and nutrient removal. Pathogens are only partially removed, except helminth eggs, which are effectively captured in the sludge bed. Nutrients removal is not complete and therefore a post- treatment is required. Long start-up. Due to the low growth rate of methanogenic organisms, the start-up takes longer as compared to aerobic processes, when no good inoculum is available. Possible bad odors. Hydrogen sulphide is produced during the anaerobic process, especially when there are high concentrations of sulphate in the influent. A proper handling of the biogas is required to avoid bad smell. Necessity of post-treatment. Post-treatment of the anaerobic effluent is generally required to reach the discharge standards for organic matter, nutrients and pathogens. (1958), Fall & Krauss (1961), Simpson (1971), and Pretorius (1971). A similar system called the 'biolytic tank' had been previously used in 1910 by Winslow & Phelps (1911). Now, the UASB reactor is exten- sively used for the treatment of several types of wastewater (Hulshoff Pol & Lettinga, 1986; Lettinga & Hulshoff Poi, 1991; Kato et al., 1994; Lettinga, 1995, 1996a,b). The success of the UASB concept relies on the establishment of a dense sludge bed in the bottom of the reactor, in which all biological processes take place. This sludge bed is basically formed by accumulation of incoming suspended solids and bacterial growth. In upflow anaerobic systems, and under certain conditions, it was also observed that bacteria can naturally aggregate in flocs and granules (Hulshoff Pol et al., 1983; Hulshoff Pol, 1989). These dense aggregates have good settling properties and are not susceptible to wash-out from the system under practical reactor conditions. Retention of active sludge, either granular or flocculent, within the UASB reactor enables good treatment performance at high organic loading rates. Natural turbulence caused by the infiuent flow and the biogas production provides good wastewater-biomass contact in UASB systems (Heertjes & van der Meer, 1978). Higher organic loads can be applied in UASB systems than in aerobic processes (Kato, 1994). Therefore, less reactor ~volume and space is required while, at the same time, high grade energy is produced as biogas. Several configurations can be imagined for a waste- water treatment plant including a UASB reactor. In any case, there must be a sand trap, screens for coarse material, and drying beds for the sludge. The UASB reactor may replace the primary settler, the anaerobic sludge digester, the aerobic step (activated sludge, trickling filter, etc.), and the secondary settler of a conventional aerobic treat- ment plant. However, the effluent from UASB reactors usually needs further treatment, in order to remove remnant organic matter, nutrients and pathogens. This post-treatment can be accomplished in conventional aerobic systems like stabilization ponds, activated sludge plants, and others. The economics of anaerobic treatment in UASB reactors were thoroughly discussed by Lettinga et al. (1983b). A scheme of the UASB reactor is shown in Fig. 1. THE EGSB REACTOR Tracer studies demonstrated that internal mixing was not optimal in a pilot-scale UASB reactor treating sewage at temperatures ranging from 4 to 20°C (de Man et al., 1986). This produced dead space in the reactor, leading to a reduction in the treatment efficiency. In order to improve the sludge-wastewater contact and use the entire reactor volume efficiently a better influent distribu- tion was required. Different feed inlet devices, more feed inlet points per square meter or higher super- ficial velocities have been proposed as solutions. The use of effluent recirculation combined with taller reactors (or a high height/diameter ratio), resulted in the expanded granular sludge bed (EGSB) reactor (Fig. 1), where a high superficial velocity is applied (van der Last & Lettinga, 1992). In this reactor concept, the upflow liquid velocity (>4mh -~) causes the granular sludge bed to expand, eliminating dead zones and resulting in  Review: anaerobic sewage treatment 179 better sludge-wastewater contact. However, a direct relationship between upflow velocity and substrate consumption could not be found, and the granule size and inner structure seem to play a more relevant role in fully expanded EGSB reactors (Seghezzo, 1997). Accumulation of flocculent excess sludge between the sludge granules is also prevented (van der Last & Lettinga, 1992). Soluble pollutants are efficiently treated in EGSB reactors but suspended solids are not substantially removed from the wastewater stream due to the high upflow veloci- ties applied. Recirculation of the effluent dilutes the influent concentration, but it was extensively proven that low strength wastewater can efficiently be treated in EGSB reactors (Kato et al., 1994; Kato, 1994). However, recirculation is not needed for sewage treatment. Influent dilution may also allow the treatment of toxic compounds in these reactors. In UASB reactors, the sludge bed behaves more or less as a static bed, but in fully expanded EGSB reactors, it is considered as a completely mixed tank (Rinzema, 1988). Compared to UASB reactors, higher organic loading rates (as kgCOD -3 - 1) reactor d can be accommodated in EGSB systems. Consequently, the gas production is also higher, improving even more the mixing inside the reactor. The exact mixing pattern cannot be generalized, and it must be evaluated in each reactor by assessing the reactor hydrodynamics (van der Meer, 1979). In tall reactors, the gas loading (in m3m-2h -j) and the hydrostatic pressure at the bottom can be higher than in short reactors and the effect of these parameters on the performance of the process also has to be considered. The main characteristics of EGSB reactors are presented in Table 3. EXAMPLES OF ANAEROBIC SEWAGE TREATMENT IN UPFLOW REACTORS Low temperatures The application of UASB reactors to sewage treat- ment under low temperature conditions has been studied in The Netherlands since 1976 (Lettinga et al., 1981; Grin et al., 1983, 1985; de Man et al., 1986; van Velsen & Wildschut, 1988). Lettinga et al. (1983a) reported results obtained in laboratory UASB reactors with raw domestic sewage using digested sewage and sugar beet cultivated sludge as seed material. Some of their results are summarized in Table 4. At the same time, a 6 m 3 UASB reactor seeded with digested sewage sludge was operated at hydraulic retention times (HRT) of 14-17 h. COD reduction reached 85-65% and 70-55% at 20 and UASB Biogas > < EGSB t Biogas Effl Effluent Recircul Influent dge clge bed lket Influent Fig. 1. Schematic diagrams of UASB (left) and EGSB (right) reactors. Modified from van Haandel & Lettinga (1994) and Wang (1994). P = recirculation pump.
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