- Email: [email protected]
Storage and feedstock preparation
Storage and feedstock preparation
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Gu¨nther Bochmann Department for Agrobiotechnology, IFA-Tulln—University of Natural Resources and Life Sciences, Vienna, Austria
Chapter Outline Introduction 49 Pretreatment 50 Microbiological systems 51 Enzyme addition 51 Microbiological systems 51 Ensiling 53
Mechanical systems
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Mechanical pretreatment 54 Ultrasound system 55 Electrokinetic disintegration 55 Thermal systems 56 Thermal pretreatment 57
Steam explosion 57 Chemical pretreatment 58 Alkali pretreatment 58 Acid pretreatment 59 Thermochemical pretreatment
Conclusion 59 References 60 Further reading
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Introduction Microbiological processes during anaerobic digestion (AD) are divided into four steps called hydrolysis, acidogenesis, acetogenesis, and methananogenesis. In the first step, hydrolysis, polymers such as cellulose, starch, proteins, or fat, will be broken down to monomers by exoenzymes such as cellulases, amylases, and proteinases from facultative and obligate anaerobic bacteria. The availability of carbohydrates varies depending on the different chemical bonding. Owing to the α-1-4-glycosidic bonding of amylose starch, hydrolysis with amylase takes a short amount of time, but hydrolysis of cellulose takes several days due to the
Substitute Natural Gas from Waste. DOI: https://doi.org/10.1016/B978-0-12-815554-7.00004-0 © 2019 Elsevier Inc. All rights reserved.
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β-1-4-glycosidic bonding. In addition, this bonding leads to tighter packing of the cellulose layers and thus to a lower accessibility of the enzymes. Most of the time cellulose is bound in a lignocellulosic complex. This reduces the anaerobic bioavailability of the glucose in the cellulose. Thus, hydrolysis is the rate-limiting step of difficult-to-degrade substances such as lignocellulose, while for easily available substances such as starch or monomeric compounds, methanogenesis is the rate-limiting step. Many organic agricultural or industrial residues or crops have a high lignocellulose content. Low gas yield and long retention time sometimes make AD of these feedstocks inefficient and offers a huge potential for optimization. Pretreatment technologies have been developed to optimize the AD of lignocellulose. These pretreatment technologies can have various goals: G
G
G
G
G
increasing degradation rate, increasing biogas yield, making new feedstock available for AD, preventing process problems such as floating layers, reducing operational costs such as electricity demand for stirring.
The main objectives of pretreatment are the first two or three bullet points. Assessment of the pretreatment technology is typically performed by measuring the biomethane potential (BMP).
Pretreatment In recent years, various pretreatment technologies have been developed to increase the availability of feedstock in order to enhance biogas production. In most cases these technologies were developed for wastewater treatment, sludge digestion, or bioethanol production from lignocellulosic compounds. The pretreatment technologies can be divided into the different principles and techniques shown in Table 4.1. Table 4.1 List of different pretreatment principles Principle
Techniques
Physical
Mechanical Thermal Ultrasound Electrochemical Alkali or acid Oxidative Enzymes Microbiological Steam explosion Extrusion Thermochemical
Chemical Biological Combined processes
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Microbiological systems Microbiological pretreatment systems can be divided into enzymatic and microbiological systems.
Enzyme addition The purpose of enzyme addition is to increase the digestability of the feedstock by breaking down the polymers in the substrate. Typically a mixture of different enzymes is used. This includes cellulase, xylanase, pectinase, and amylase, although studies have also been carried out on the effect of manganese peroxidase to break down lignin in the feedstock (Frigon et al., 2012). Three different ways of applying enzymes during AD exist: G
G
G
addition to the hydrolysis/acidification step (one step) of a two-step system, addition to a dedicated enzymatic pretreatment vessel, addition to the methanation step (in a single-step digestion).
The addition of enzymes has been analyzed in different studies using different feedstock such as agricultural organic raw material and residues, municipal solid waste and sludge. The effect of enzyme addition on the digestion of pasture grass was analyzed by Romano et al. (2009). This study showed a positive influence on the solubilization of the feedstock. Another positive effect was a faster degradation rate during the anaerobic process but no additional gas yield was achieved. In the first step of a two-step system, brewers’ spent grains were treated additionally with an enzyme mixture. Through this addition, higher volatile fatty acids yields were achieved (Bochmann et al., 2007). The effect of single enzymes such as cellulase, pectinase, or amylase on the degradation of maize and grass silage was analyzed by Ellenrieder et al. (2010). In this study, no additional benefit on gas yield was detected. Frigon et al. (2012) analyzed the effect of lignin and manganese peroxidase and showed a positive effect on the fermentation of switchgrass. Low retention time of 36 days shows high additional gas yield when using these enzymes. In several studies, higher gas yields were measured after enzyme addition. However, while comparing data, the average retention time, the bioavailability of the substrate, and the origin of the inoculum must be taken into consideration in the interpretation of the results.
Microbiological systems Microbiological pretreatment, preacidification or multiphase fermentation is a simple kind of pretreatment technology. In these systems, the first two steps of AD, the hydrolysis and acidogenesis steps, are separated from the last two steps, the acetognesis and methanogenesis steps. The typical equipment for such a process is a two-phase digestion system. The separation of the steps in different vessels is similar to the multiple chambers of the ruminant digestive system. The pH value of the preacidification step should lie between 4 and 6, at which the methane
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production will be inhibited (Deublein and Steinhauser, 2008; Thauer, 1998). Through the inhibition of the methane formation in the first vessel, volatile fatty acids accumulate. In addition to the volatile fatty acids accumulated during the preacidification step, high concentrations of CO2 and hydrogen are produced. Some publications call this step “dark fermentation” with the aim of producing volatile fatty acids and hydrogen. The production of H2 and the formation of volatile fatty acids goes hand in hand. Measuring H2 in the gas phase is an important parameter to evaluate the preacidification step. Measuring H2 can also be a processmonitoring tool to evaluate the whole AD process (Drosg, 2013). The amount of H2 produced is strongly influenced by the pH value. At pH values of 6, H2 production is high at the beginning but then stops. At pH 4, H2 production is lower but prolonged and greater in total (Liu et al., 2006). In continuous fermentation trials, Antonopoulou et al. (2008) demonstrated H2 concentration of 35% 40% v/v during the preacidification step. An additional positive effect of microbiological pretreatment is on the gas quality. In addition to H2 and volatile fatty acid production, CO2 and H2S are formed during the microbiological process. Depending on the pH, CO2 can be present in three forms: at higher pH values in the form of the carbonate ion CO22 3 , at neutral pH as HCO32, and in acidic environments as CO2 (see Fig. 4.1). Due to the low pH of 4 6 during the preacidification, most of the carbonate appears in the volatile form of CO2 and is released into the hydrolysis gas produced during the preacidification step. During the two-stage AD, microbiological processes overlap and CO2 formation also takes place in the second digester. More CO2 can be bound as hydrogen carbonate during the methanation step in the fermentation broth. This means that for the methanogenesis step, a higher CH4 concentration is present in the gas phase. Thus, the results of Nizami et al. (2012), biogas with 71% methane content in a two-phase system digesting grass silage, can be justified. This grass silage showed a methane concentration of 52% in a singlestep system.
2 Figure 4.1 Equilibrium of CO22 3 (gray), HCO3 (dotted), and CO2 (black) depending on the pH.
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The H2S responds in a similar way to CO2 during AD in digester broths. At low pH values H2S is volatile and evaporates to the gas phase. At pH-values higher than 7.0 more HS is present. The higher the pH value, the slower the H2S is released to the gas phase. The microbiological process of sulfate reduction by sulfatereducing bacteria during AD depends strongly on the pH value. Below a pH of 5.8 6 and up to pH 9, sulfate-reducing bacteria are inhibited (Widdel, 1988; Reis et al., 1988). Thus, a pH of about 6 is helpful for increasing the H2S evaporation and can lead to a “cleaner” biogas with lower H2S and CO2 concentrations. Microbiological pretreatment has a very positive effect on the degradation rate of substrates in AD. In general, cellulose-, hemicellulose-, and starch-degrading enzymes work best between pH 4 and 6 at temperatures from 30 C to 50 C, so this preacidification step increases the degradation rate by creating an optimal environment for hydrolytic enzymes, particularly for carbohydrate degradation. Liu et al. (2006) achieved an additional biogas yield of 21% at a hydraulic retention time of approximately 30 days. This was caused by higher degradation through increased hydrolytic enzyme activity. Increasing anaerobic degradation by bioaugmentation showed positive effects in several studies. Cellulolytic consortia, Thermoanaerobacterium thermosaccharolyticum, Caldanaerobacter subterraneus, Thermoanaerobacter pseudethanolicus, and Clostridium cellulolyticum, were added to thermophilic digestion to increase the hydrolysis of cellulose and corn stover. The gas yield was increased by 22% 24% in lab-scale batch reactors. The addition of the mixture of these four organisms in a large-scale biogas plant increased the yield by 10% 11% (Strang et al., 2017). In large-scale biogas plants, preacidification systems are offered by several plant constructers, varying from continuous to batch preacidification systems. Continuous preacidification is offered, for example, by the companies AAT and BDI from Austria. Substrates are fed continuously in a two-reactor continuous stirred tank reactor (CSTR) system. The daily removal of material to feed the second reactor is balanced by a feed of fresh material to the first reactor. Plug-flow reactors are also in use. This technology guarantees the treatment of the required retention time, which is not given at the CSTR system.
Ensiling Storage of feedstock is not normally considered as pretreatment. However, ensiling can be considered as a microbial pretreatment, thus it is included here for completeness. Storing of feedstock for AD is an important factor for an economic process for several reasons. Biogas plants have a demand for several tonnes of feedstock a day. Most feedstock originates from agriculture and thus is harvested seasonally. Depending on the region, harvests are one to three times a year. To guarantee the sufficient supply of the biogas plant, storage of the feedstock must be solved. Ensiling is typically used for the preservation of crops, as whole crop cereals, grasses, etc., using lactic acid bacteria (LAB). Thus, biogas plants can be supplied with carbon source during the whole year. Ensiling can also be carried out with other organic residues such as orange peels (Calabro` et al., 2018). There is some
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discussion on whether ensiling can be referred to as a pretreatment step or not. Several studies show inhomogeneous results on this topic. A huge variety of microorganisms are present on the harvested crops. Apart from LAB, harmful microorganisms such as coliforms, clostridia, and fungi also live on the crops and can potentially influence the storage, for example, a strong growth of fungi during storage would cause loss of carbon and lead to a reduced methane yield. In the case of feeding the silage it might lead to diseases in livestock. Some LABs are already on the ensiling crop but they can also can be inoculated with specific LAB. The LABs produce lactic acid, leading to a decrease the pH in the silage. Within days, LABs consume the free oxygen and form an anaerobic environment which prevents the growth of aerobic organisms. Due to the lack of easily available carbon, straw canot be ensiled as a single substrate. Another benefit of LABs is their ability to grow under relatively dry conditions, such as maize. To avoid oxygen entering the silage, silage bales or heaps are compressed. This compression can be done by driving with a tractor on the silage heaps. These conditions guarantee a stable silage for months (Wilkinson, 2005; McDonald et al., 1991). Another preservation is the production of hay by reducing the grass water content. This includes the loss of the cell plasma and many other cell components, meaning that a reduced gas yield occurs during AD.
Mechanical systems Mechanical pretreatment In many studies the effects of mechanical pretreatment of different feedstock on AD have been tested. In general, mechanical pretreatment increases the specific surface and thus the availability of the biomass to the microorganisms and their enzymes. In addition to increasing bioavailability of the feedstock, particle size reduction affects the viscosity in digesters (Kamarad et al., 2010). For mechanical pretreatment, mainly hammer mills (crushing) and knife mills (chopping) have been tested. Generally, knife and hammer mills are used for dry biomass (,15%) (Taherzadeh and Karimi, 2008; Kratky and Jirout, 2011). Hammer mills are relatively cheap and robust concerning impurities such as stones and metal but have a slightly higher electricity demand compared to knife mills. Kratky and Jirout found out in 2011 that a particle size of 1 2 mm is favorable to increase effective hydrolysis. The effect of knife milling on AD was investigated by several studies. Higher gas yields of approximately 10% were achieved after milling hay to 0.5 mm compared to 20 30 mm (Menind and Normak, 2010). By milling sisal fibers from 100 to 2 mm, higher gas yields of 20% 25% were achieved by Mshandete et al. (2006). Kakuk et al. analyzed the difference between mechanically pretreated corn stover of particle sizes of 10 and 2 mm. During their BMP trials they observed an increase in the degradation rate on no difference in the total yield (Kakuk et al., 2017). Mechanical pretreatment of macroalgae (Tedesco et al., 2013) or waste paper (Rodriguez et al., 2017) showed
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approximately 20% higher methane production within 21 days’ retention time of the BMP trials. Waste paper after mechanical pretreatment (Hollander beater for 60 minutes) produced 20% higher gas yields after 21 days of retention time. Lindern et al. (2015) described the influence of mechanical pretreatment of the hydrolysate from a two-stage maize silage and hay/straw digesting plant and posttreatment of digestate from a crop-digesting biogas plant. Treatment of hydrolysate did not show any significant additional gas yield but the posttreatment of the digestate from 21 to 57 L CH4 3 kg (Lindern et al., 2015). Operation of hammer and knife mills requires energy. The reduction of particle size of wheat straw from 12.5 to 1.6 mm needs between 2.8 and 7.55 kWh/t (Kratky and Jirout, 2011). Slurry-digesting CSTRs require approximately 10 kWh/t (Murphy and McCarthy, 2005). The demand increases for digesters with higher dry matter content and lignocellulosic feedstock. There is still a lack of research on whether or not the energy input for milling is justified compared to additional gas yield, the investment costs, and lower energy demand for agitation devices.
Ultrasound system Ultrasound is defined by a frequency of over 20 kHz. These frequencies cause cavitation, which means the formation of liquid-free bubbles up to a certain size until they implode. Cavitation forms forces that lead to the disruption of cell walls in liquids. Thus, ultrasonic treatment is more common for the treatment of liquid feedstock or waste sludge. However, some studies carried out trials with feedstock such as algae (Alzate et al., 2012) or chicken manure (Braeutigam et al., 2014). The effect on lignocellulosic material was measured in different studies. Delignification is described by the extreme conditions of pressure and temperature during the cavitation process. Without any additional chemicals such as NaOH or potassium permanganate, the effect on delignification is very low (Bundhoo and Mohee, 2018). This is due to the fact that ultrasonic treatment only disintegrates microbiological biomass and does not affect the degradation of lignocellulose in the input material (Onyeche et al., 2002). In general, ultrasonic treatment is more suitable as a posttreatment or sludge treatment than for lignocellulosic biomass. For sludge treatment higher gas yields of approximately 20% were measured, but comparing the higher gas yield to the energy input a negative energy balance was described in most of the studies (Bundhoo and Mohee, 2018).
Electrokinetic disintegration Electrokinetic disintegration is a technology used in modern biotechnology. During AD it is mainly used for disintegration of sewage sludge. The main factor of reduced anaerobic degradation of sewage sludge is due to the presence of flocs and aggregates. These accumulations are formed by ionic bonds between negatively charged molecules with cations on the microbial extracellular polymeric substances (Higgins and Novak, 1997; Tyagi and Lo, 2011). The disruption of this ionic bond by application of electrical fields leads to the disintegration of
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flocs (Tyagi and Lo, 2011). The electrical field changes the charge of the cell membranes and thus disintegrates the cell. In case of treating lignocellulosic compounds with electrical fields, it is not clear whether this affects the degradation of the feedstock. Typically, electrokinetic disintegration works around 30 kV, in a range of 10 100 kV. The sludge is pumped through a pipe with an electrode inside. Companies producing electrokinetic units claim that sewage sludge treatment increases the biogas yield by 20%. Lehner et al. (2009) showed no effect on the application of agricultural residues. Electrokinetic disintegration may suit better to treat sewage sludge or other feedstock with a high content on microbes and not for lignocellulosic material.
Thermal systems Thermal processes have different effects on the lignocellulosic feedstock and subsequently on AD. They can be divided in two major technologies, liquid hot water (LHW) pretreatment and steam explosion. While LHW is described as a typical thermal technology, steam explosion combines thermal with mechanical pretreatment. The process temperature of thermal processes lies in the range of 160 C 220 C. Substrates with low water content such as straw need additional water before treatment. This can be added with direct steam injection. The effects of thermal processes are extensive. The presence of heat and water causes swelling of the feedstock by disrupting the hydrogen bonds that hold together the structure of the crystalline cellulose and other structural complexes. Heat also breaks down hemicellulose, which aids swelling of the whole complex (Garrote et al., 1999). During hydrolysis of the polymer to monomers, sugars such as glucose and xylose are released. Through thermal influence, monomeric sugars are transformed in different products depending on the environment. Various processes work in parallel which include the Maillard reaction (in presence of an amine group) and caramelization (without amine groups). During the Maillard reaction intermediate products such as furfural from pentoses and 5-hydroxy-methyl-furfural (HMF) are formed from fructose and glucose, respectively. The Maillard reaction is responsible for coloring food during food processing such as baking bread and the bread crust or the dark color in beer, and affects its taste. Many studies in the field of bioethanol production have shown high furfural and HMF production during thermal pretreatment. The intermediate products are bacteriostatic and can inhibit the process of AD when present at higher concentrations. Benjamin et al. (1984) and Bochmann et al. (2010) demonstrated that these products also have a negative impact on AD due to their bacteriostatic characteristics. The final products of the Maillard reaction are melanoidins. These brownish to black products are also described in several publications as “pseudo-lignin.” The final products of the Maillard reaction, the melanoidins or “pseudo-lignin,” lead to reduced gas yields (Lizasoain et al., 2017; Bochmann et al., 2015).
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Thermal pretreatment During thermal pretreatment, also called LHW pretreatment, the substrate is heated up under pressure up to 220 C and held for a specific retention time. Then, it is cooled down and used for AD. In the case of low water content of the feedstock, additional water needs to be added before thermal treatment. Some technologies use steam for temperature increase and water addition. At a large scale, one thermal pretreatment technology was realized in Germany several years ago. It is called TDH, which comes from the German name “ThermoDruck-Hydrolyse.” In a first step the reactor is set under pressure of 20 30 bar. The input material is heated up using a heat exchanger. At the same time the effluent of the TDH is cooled down in this counter-current heat exchanger. The Δt in the counter-current heat exchanger is about 20K. The residual heat demand is provided by cooling down the exhaust gas from a CHP unit. Oil is used as the fluid for heat exchange. The temperature of the exhaust gas from CHP units lies between 350 C and 420 C (Bochmann et al., 2015). The retention time in the TDH is 20 min. Finally, the pressure is released (Dinglreiter, 2007). Various studies have shown that thermal pretreatment increases biogas yield only up to a certain temperature. Gas production decreases below this temperature. DiStefano and Ambulkar (2006) describe the maximum temperature as 175 C for sewage sludge. Using TDH to pretreat crops, the maximum temperature lies at 220 C (Dinglreiter, 2007). Thermal pretreatment of brewers’ spent grains shows lower gas yield with pretreatment above 160 C as compared with untreated substrate (Bochmann et al., 2010). The maximum temperature depends on the composition of the substrates and also on the retention time of pretreatment. Thermal energy from LHW can be recovered using a heat exchanger. Bochmann et al. (2015) demonstrated that, through this recovery, a process with a high energy surplus can be realized.
Steam explosion The principle of steam explosion is related to thermal pretreatment. The substrate is heated up in a vessel to a temperature of 160 C 220 C, for example by direct steam injection. The temperature and pressure in the vessel increase. After a process specific retention time of 5 60 min, pressure is released abruptly. This sudden pressure release leads intracellular water to evaporate. Cell walls are disrupted, causing substrates to lose their structure. Application of steam explosion units allows new feedstock such as hay, straw or reed to be used for biogas production. Bauer et al. (2009) analyzed steam explosion tests of straw and showed calculations of ethanol and biogas potentials. Lizasoain et al. (2016) presented a higher gas yield by 89% compared to untreated reed. The optimal pretreatment was at 200 C. Higher pretreatment temperature showed declining gas yield. This is due to the formation of non-degradable substances during the thermal treatment. In 2017, Lizasoain et al. presented data about the
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Figure 4.2 Steam explosion unit in Parndorf, Austria. Source: H. Dauser/Biogas systems.
pretreatment of corn stover. The highest gas yield (compared to untreated corn stover) was demonstrated at a low temperature of 160 C. Increasing temperature, especially combined with increasing duration, showed declining gas yield by the formation of non-degradable or recalcitrant substances, within this publication named “pseudo-lignin” (Lizasoain et al., 2017). The amount of installed steam explosion units is increasing in Europe. Fig. 4.2 shows a pretreatment unit in Parndorf, Austria. In this plant, straw is treated in a steam explosion unit for utilization in an AD plant. Through the steam explosion, higher gas yield and lower floating layers occur.
Chemical pretreatment During chemical pretreatment, mainly acids and bases of different strengths and on different substrates have been investigated. Acids and bases have been left on the substrates for a certain time, typically hours to days, and then the BMP measured. As a combined process thermochemical pretreatment has been analyzed in several studies.
Alkali pretreatment For alkali pretreatment typically NaOH or KOH are used. Alkali pretreatment increases the degradation of hemicellulose and lignin. During alkali pretreatment the acetate group from the hemicellulose will be removed, thus hydrolytic enzymes can more easily access the carbohydrates (Kong et al., 1992). Solubilization of the lignin by the alkali addition results in accessibility of the enzymes in the microorganisms to the cellulose and hemicellulose. AD of Pennisetum hydrid requires a long retention time due to high lignocellulosic content. Alkaline pretreatment with
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NaOH shows the best results with 2% of alkali addition. Increasing the concentration of alkali did not show any additional positive effects. Variations of the process temperature (35 C, 55 C, and 121 C) of the pretreatment showed a negative influence on the biomethane yield (Kang et al., 2018). Alkaline pretreatment with wheat straw involved a 1.6% NaOH solution incubated at 30 C for 24 hours. The pretreatment resulted in a faster production when comparing the BMP after 10 and 40 days’ retention time (Mancini et al., 2018).
Acid pretreatment Acid pretreatment is typically carried out using H2SO4 and leads to the disintegration of the cellulose. Continuous digestion application of sulfur-containing additives leads to the formation of H2S and might cause inhibition effects during AD. Zhang et al. (2018) presented a higher gas yield of 20% 30% depending on the H2SO4 concentration used for pretreatment.
Thermochemical pretreatment Thermochemical pretreatment is carried out with H2SO4 more often than acid pretreatment. Several studies worked with temperatures above 100 C. Kang et al. (2018) showed the negative effect of the combination of alkali pretreatment combined with increased process temperature (see above). Thermochemical pretreatment with H2O2 and acid addition and different temperatures was tested by Venturin et al. (2018). The pretreatment temperature was set to 100 C, 108 C, 116 C, 124 C, and 132 C and the H2SO4 to 0%, 0.75%, 1.5%, 2.25%, and 3%. During these tests only H2O2 addition showed a positive effect on the BMP. Even at 11 days most of the biomethane had been produced. The utilization of H2SO4 showed a negative effect on the BMP.
Conclusion In recent years, extensive research has been carried out in the field of pretreatment of feedstock for AD. The results vary from a huge increase of degradation rate and high additional gas yield to low additional gas yield or even negative influence on AD. Additional gas yield of about 40% 50% have been reached by several authors. But is has reported by several authors that the impact of retention time during BMP to the additional gas yield after treatment of feedstock is huge. The longer the retention time of BMP trials was, the lower the additional biogas yield detected. Thus, many results with very high additional gas yields need to be viewed critically. Many pretreatment technologies are very feedstock specific regarding positive effect on the disintegration effect, and finally the energy balance and economic benefit for the plant operator.
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´ cs, N., Wirth, R., Maro´ti, G., Bagi, Z., Ra´khely, G., et al., 2017. Strang, O., A Bioaugmentation of the thermophilic anaerobic biodegradation of cellulose and corn stover. Anaerobe 46, 104 113. Taherzadeh, J.M., Karimi, K., 2008. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: a review. Int. J. Mol. Sci. 9, 1621 1651. Tedesco, S., Benyounis, K., Olabi, A., 2013. Mechanical pretreatment effects on macroalgaederived biogas production in co-digestion with sludge in Ireland. Energy 61, 27 33. Thauer, R., 1998. Biochemistry of methanogenesis: a tribute to Marjory Stephenson. Microbiology 144, 2377 2406. Tyagi, V.K., Lo, S.L., 2011. Application of physico-chemical pretreatment methods to enhance the sludge disintegration and subsequent anaerobic digestion: an up to date review. Rev. Environ. Sci. Biotechnol. 10 (3), 215 242. Venturin, B., Camargo, A., Scapini, T., Mulinari, J., Bonatto, C., Bazoti, S., et al., 2018. Effect of pretreatment on corn stalk chemical properties for biogas production purposes. Bioresour. Technol. 266, 116 124. Widdel, F., 1988. Microbiology and ecology of sulfate- and sulfur-reducing bacteria. In: Zehnder, A.J.B. (Ed.), Biology of Anaerobic Organisms. John Wiley and Sons, pp. 469 585. Wilkinson, J.M., 2005. Silage. Chalcombe Publications, Southampton UK. Zhang, H., Ning, Z., Khalid, H., Zhang, R., Liu, G., Chen, C., 2018. Enhancement of methane production from cotton stalk using different pretreatment techniques. Sci. Rep. 8, 3463.
Further reading Sepp¨al¨a, M., Paavola, T., Lehtom¨aki, A., Pakarinen, O., Rintala, J., 2008. Biogas from energy crops—optimal pre-treatments and storage, co-digestion and energy balance in boreal conditions. Water Sci. Technol. 58 (9), 1857 1863.
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Gu¨nther Bochmann Department for Agrobiotechnology, IFA-Tulln—University of Natural Resources and Life Sciences, Vienna, Austria
Chapter Outline Introduction 49 Pretreatment 50 Microbiological systems 51 Enzyme addition 51 Microbiological systems 51 Ensiling 53
Mechanical systems
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Mechanical pretreatment 54 Ultrasound system 55 Electrokinetic disintegration 55 Thermal systems 56 Thermal pretreatment 57
Steam explosion 57 Chemical pretreatment 58 Alkali pretreatment 58 Acid pretreatment 59 Thermochemical pretreatment
Conclusion 59 References 60 Further reading
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Introduction Microbiological processes during anaerobic digestion (AD) are divided into four steps called hydrolysis, acidogenesis, acetogenesis, and methananogenesis. In the first step, hydrolysis, polymers such as cellulose, starch, proteins, or fat, will be broken down to monomers by exoenzymes such as cellulases, amylases, and proteinases from facultative and obligate anaerobic bacteria. The availability of carbohydrates varies depending on the different chemical bonding. Owing to the α-1-4-glycosidic bonding of amylose starch, hydrolysis with amylase takes a short amount of time, but hydrolysis of cellulose takes several days due to the
Substitute Natural Gas from Waste. DOI: https://doi.org/10.1016/B978-0-12-815554-7.00004-0 © 2019 Elsevier Inc. All rights reserved.
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β-1-4-glycosidic bonding. In addition, this bonding leads to tighter packing of the cellulose layers and thus to a lower accessibility of the enzymes. Most of the time cellulose is bound in a lignocellulosic complex. This reduces the anaerobic bioavailability of the glucose in the cellulose. Thus, hydrolysis is the rate-limiting step of difficult-to-degrade substances such as lignocellulose, while for easily available substances such as starch or monomeric compounds, methanogenesis is the rate-limiting step. Many organic agricultural or industrial residues or crops have a high lignocellulose content. Low gas yield and long retention time sometimes make AD of these feedstocks inefficient and offers a huge potential for optimization. Pretreatment technologies have been developed to optimize the AD of lignocellulose. These pretreatment technologies can have various goals: G
G
G
G
G
increasing degradation rate, increasing biogas yield, making new feedstock available for AD, preventing process problems such as floating layers, reducing operational costs such as electricity demand for stirring.
The main objectives of pretreatment are the first two or three bullet points. Assessment of the pretreatment technology is typically performed by measuring the biomethane potential (BMP).
Pretreatment In recent years, various pretreatment technologies have been developed to increase the availability of feedstock in order to enhance biogas production. In most cases these technologies were developed for wastewater treatment, sludge digestion, or bioethanol production from lignocellulosic compounds. The pretreatment technologies can be divided into the different principles and techniques shown in Table 4.1. Table 4.1 List of different pretreatment principles Principle
Techniques
Physical
Mechanical Thermal Ultrasound Electrochemical Alkali or acid Oxidative Enzymes Microbiological Steam explosion Extrusion Thermochemical
Chemical Biological Combined processes
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Microbiological systems Microbiological pretreatment systems can be divided into enzymatic and microbiological systems.
Enzyme addition The purpose of enzyme addition is to increase the digestability of the feedstock by breaking down the polymers in the substrate. Typically a mixture of different enzymes is used. This includes cellulase, xylanase, pectinase, and amylase, although studies have also been carried out on the effect of manganese peroxidase to break down lignin in the feedstock (Frigon et al., 2012). Three different ways of applying enzymes during AD exist: G
G
G
addition to the hydrolysis/acidification step (one step) of a two-step system, addition to a dedicated enzymatic pretreatment vessel, addition to the methanation step (in a single-step digestion).
The addition of enzymes has been analyzed in different studies using different feedstock such as agricultural organic raw material and residues, municipal solid waste and sludge. The effect of enzyme addition on the digestion of pasture grass was analyzed by Romano et al. (2009). This study showed a positive influence on the solubilization of the feedstock. Another positive effect was a faster degradation rate during the anaerobic process but no additional gas yield was achieved. In the first step of a two-step system, brewers’ spent grains were treated additionally with an enzyme mixture. Through this addition, higher volatile fatty acids yields were achieved (Bochmann et al., 2007). The effect of single enzymes such as cellulase, pectinase, or amylase on the degradation of maize and grass silage was analyzed by Ellenrieder et al. (2010). In this study, no additional benefit on gas yield was detected. Frigon et al. (2012) analyzed the effect of lignin and manganese peroxidase and showed a positive effect on the fermentation of switchgrass. Low retention time of 36 days shows high additional gas yield when using these enzymes. In several studies, higher gas yields were measured after enzyme addition. However, while comparing data, the average retention time, the bioavailability of the substrate, and the origin of the inoculum must be taken into consideration in the interpretation of the results.
Microbiological systems Microbiological pretreatment, preacidification or multiphase fermentation is a simple kind of pretreatment technology. In these systems, the first two steps of AD, the hydrolysis and acidogenesis steps, are separated from the last two steps, the acetognesis and methanogenesis steps. The typical equipment for such a process is a two-phase digestion system. The separation of the steps in different vessels is similar to the multiple chambers of the ruminant digestive system. The pH value of the preacidification step should lie between 4 and 6, at which the methane
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production will be inhibited (Deublein and Steinhauser, 2008; Thauer, 1998). Through the inhibition of the methane formation in the first vessel, volatile fatty acids accumulate. In addition to the volatile fatty acids accumulated during the preacidification step, high concentrations of CO2 and hydrogen are produced. Some publications call this step “dark fermentation” with the aim of producing volatile fatty acids and hydrogen. The production of H2 and the formation of volatile fatty acids goes hand in hand. Measuring H2 in the gas phase is an important parameter to evaluate the preacidification step. Measuring H2 can also be a processmonitoring tool to evaluate the whole AD process (Drosg, 2013). The amount of H2 produced is strongly influenced by the pH value. At pH values of 6, H2 production is high at the beginning but then stops. At pH 4, H2 production is lower but prolonged and greater in total (Liu et al., 2006). In continuous fermentation trials, Antonopoulou et al. (2008) demonstrated H2 concentration of 35% 40% v/v during the preacidification step. An additional positive effect of microbiological pretreatment is on the gas quality. In addition to H2 and volatile fatty acid production, CO2 and H2S are formed during the microbiological process. Depending on the pH, CO2 can be present in three forms: at higher pH values in the form of the carbonate ion CO22 3 , at neutral pH as HCO32, and in acidic environments as CO2 (see Fig. 4.1). Due to the low pH of 4 6 during the preacidification, most of the carbonate appears in the volatile form of CO2 and is released into the hydrolysis gas produced during the preacidification step. During the two-stage AD, microbiological processes overlap and CO2 formation also takes place in the second digester. More CO2 can be bound as hydrogen carbonate during the methanation step in the fermentation broth. This means that for the methanogenesis step, a higher CH4 concentration is present in the gas phase. Thus, the results of Nizami et al. (2012), biogas with 71% methane content in a two-phase system digesting grass silage, can be justified. This grass silage showed a methane concentration of 52% in a singlestep system.
2 Figure 4.1 Equilibrium of CO22 3 (gray), HCO3 (dotted), and CO2 (black) depending on the pH.
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The H2S responds in a similar way to CO2 during AD in digester broths. At low pH values H2S is volatile and evaporates to the gas phase. At pH-values higher than 7.0 more HS is present. The higher the pH value, the slower the H2S is released to the gas phase. The microbiological process of sulfate reduction by sulfatereducing bacteria during AD depends strongly on the pH value. Below a pH of 5.8 6 and up to pH 9, sulfate-reducing bacteria are inhibited (Widdel, 1988; Reis et al., 1988). Thus, a pH of about 6 is helpful for increasing the H2S evaporation and can lead to a “cleaner” biogas with lower H2S and CO2 concentrations. Microbiological pretreatment has a very positive effect on the degradation rate of substrates in AD. In general, cellulose-, hemicellulose-, and starch-degrading enzymes work best between pH 4 and 6 at temperatures from 30 C to 50 C, so this preacidification step increases the degradation rate by creating an optimal environment for hydrolytic enzymes, particularly for carbohydrate degradation. Liu et al. (2006) achieved an additional biogas yield of 21% at a hydraulic retention time of approximately 30 days. This was caused by higher degradation through increased hydrolytic enzyme activity. Increasing anaerobic degradation by bioaugmentation showed positive effects in several studies. Cellulolytic consortia, Thermoanaerobacterium thermosaccharolyticum, Caldanaerobacter subterraneus, Thermoanaerobacter pseudethanolicus, and Clostridium cellulolyticum, were added to thermophilic digestion to increase the hydrolysis of cellulose and corn stover. The gas yield was increased by 22% 24% in lab-scale batch reactors. The addition of the mixture of these four organisms in a large-scale biogas plant increased the yield by 10% 11% (Strang et al., 2017). In large-scale biogas plants, preacidification systems are offered by several plant constructers, varying from continuous to batch preacidification systems. Continuous preacidification is offered, for example, by the companies AAT and BDI from Austria. Substrates are fed continuously in a two-reactor continuous stirred tank reactor (CSTR) system. The daily removal of material to feed the second reactor is balanced by a feed of fresh material to the first reactor. Plug-flow reactors are also in use. This technology guarantees the treatment of the required retention time, which is not given at the CSTR system.
Ensiling Storage of feedstock is not normally considered as pretreatment. However, ensiling can be considered as a microbial pretreatment, thus it is included here for completeness. Storing of feedstock for AD is an important factor for an economic process for several reasons. Biogas plants have a demand for several tonnes of feedstock a day. Most feedstock originates from agriculture and thus is harvested seasonally. Depending on the region, harvests are one to three times a year. To guarantee the sufficient supply of the biogas plant, storage of the feedstock must be solved. Ensiling is typically used for the preservation of crops, as whole crop cereals, grasses, etc., using lactic acid bacteria (LAB). Thus, biogas plants can be supplied with carbon source during the whole year. Ensiling can also be carried out with other organic residues such as orange peels (Calabro` et al., 2018). There is some
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discussion on whether ensiling can be referred to as a pretreatment step or not. Several studies show inhomogeneous results on this topic. A huge variety of microorganisms are present on the harvested crops. Apart from LAB, harmful microorganisms such as coliforms, clostridia, and fungi also live on the crops and can potentially influence the storage, for example, a strong growth of fungi during storage would cause loss of carbon and lead to a reduced methane yield. In the case of feeding the silage it might lead to diseases in livestock. Some LABs are already on the ensiling crop but they can also can be inoculated with specific LAB. The LABs produce lactic acid, leading to a decrease the pH in the silage. Within days, LABs consume the free oxygen and form an anaerobic environment which prevents the growth of aerobic organisms. Due to the lack of easily available carbon, straw canot be ensiled as a single substrate. Another benefit of LABs is their ability to grow under relatively dry conditions, such as maize. To avoid oxygen entering the silage, silage bales or heaps are compressed. This compression can be done by driving with a tractor on the silage heaps. These conditions guarantee a stable silage for months (Wilkinson, 2005; McDonald et al., 1991). Another preservation is the production of hay by reducing the grass water content. This includes the loss of the cell plasma and many other cell components, meaning that a reduced gas yield occurs during AD.
Mechanical systems Mechanical pretreatment In many studies the effects of mechanical pretreatment of different feedstock on AD have been tested. In general, mechanical pretreatment increases the specific surface and thus the availability of the biomass to the microorganisms and their enzymes. In addition to increasing bioavailability of the feedstock, particle size reduction affects the viscosity in digesters (Kamarad et al., 2010). For mechanical pretreatment, mainly hammer mills (crushing) and knife mills (chopping) have been tested. Generally, knife and hammer mills are used for dry biomass (,15%) (Taherzadeh and Karimi, 2008; Kratky and Jirout, 2011). Hammer mills are relatively cheap and robust concerning impurities such as stones and metal but have a slightly higher electricity demand compared to knife mills. Kratky and Jirout found out in 2011 that a particle size of 1 2 mm is favorable to increase effective hydrolysis. The effect of knife milling on AD was investigated by several studies. Higher gas yields of approximately 10% were achieved after milling hay to 0.5 mm compared to 20 30 mm (Menind and Normak, 2010). By milling sisal fibers from 100 to 2 mm, higher gas yields of 20% 25% were achieved by Mshandete et al. (2006). Kakuk et al. analyzed the difference between mechanically pretreated corn stover of particle sizes of 10 and 2 mm. During their BMP trials they observed an increase in the degradation rate on no difference in the total yield (Kakuk et al., 2017). Mechanical pretreatment of macroalgae (Tedesco et al., 2013) or waste paper (Rodriguez et al., 2017) showed
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approximately 20% higher methane production within 21 days’ retention time of the BMP trials. Waste paper after mechanical pretreatment (Hollander beater for 60 minutes) produced 20% higher gas yields after 21 days of retention time. Lindern et al. (2015) described the influence of mechanical pretreatment of the hydrolysate from a two-stage maize silage and hay/straw digesting plant and posttreatment of digestate from a crop-digesting biogas plant. Treatment of hydrolysate did not show any significant additional gas yield but the posttreatment of the digestate from 21 to 57 L CH4 3 kg (Lindern et al., 2015). Operation of hammer and knife mills requires energy. The reduction of particle size of wheat straw from 12.5 to 1.6 mm needs between 2.8 and 7.55 kWh/t (Kratky and Jirout, 2011). Slurry-digesting CSTRs require approximately 10 kWh/t (Murphy and McCarthy, 2005). The demand increases for digesters with higher dry matter content and lignocellulosic feedstock. There is still a lack of research on whether or not the energy input for milling is justified compared to additional gas yield, the investment costs, and lower energy demand for agitation devices.
Ultrasound system Ultrasound is defined by a frequency of over 20 kHz. These frequencies cause cavitation, which means the formation of liquid-free bubbles up to a certain size until they implode. Cavitation forms forces that lead to the disruption of cell walls in liquids. Thus, ultrasonic treatment is more common for the treatment of liquid feedstock or waste sludge. However, some studies carried out trials with feedstock such as algae (Alzate et al., 2012) or chicken manure (Braeutigam et al., 2014). The effect on lignocellulosic material was measured in different studies. Delignification is described by the extreme conditions of pressure and temperature during the cavitation process. Without any additional chemicals such as NaOH or potassium permanganate, the effect on delignification is very low (Bundhoo and Mohee, 2018). This is due to the fact that ultrasonic treatment only disintegrates microbiological biomass and does not affect the degradation of lignocellulose in the input material (Onyeche et al., 2002). In general, ultrasonic treatment is more suitable as a posttreatment or sludge treatment than for lignocellulosic biomass. For sludge treatment higher gas yields of approximately 20% were measured, but comparing the higher gas yield to the energy input a negative energy balance was described in most of the studies (Bundhoo and Mohee, 2018).
Electrokinetic disintegration Electrokinetic disintegration is a technology used in modern biotechnology. During AD it is mainly used for disintegration of sewage sludge. The main factor of reduced anaerobic degradation of sewage sludge is due to the presence of flocs and aggregates. These accumulations are formed by ionic bonds between negatively charged molecules with cations on the microbial extracellular polymeric substances (Higgins and Novak, 1997; Tyagi and Lo, 2011). The disruption of this ionic bond by application of electrical fields leads to the disintegration of
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flocs (Tyagi and Lo, 2011). The electrical field changes the charge of the cell membranes and thus disintegrates the cell. In case of treating lignocellulosic compounds with electrical fields, it is not clear whether this affects the degradation of the feedstock. Typically, electrokinetic disintegration works around 30 kV, in a range of 10 100 kV. The sludge is pumped through a pipe with an electrode inside. Companies producing electrokinetic units claim that sewage sludge treatment increases the biogas yield by 20%. Lehner et al. (2009) showed no effect on the application of agricultural residues. Electrokinetic disintegration may suit better to treat sewage sludge or other feedstock with a high content on microbes and not for lignocellulosic material.
Thermal systems Thermal processes have different effects on the lignocellulosic feedstock and subsequently on AD. They can be divided in two major technologies, liquid hot water (LHW) pretreatment and steam explosion. While LHW is described as a typical thermal technology, steam explosion combines thermal with mechanical pretreatment. The process temperature of thermal processes lies in the range of 160 C 220 C. Substrates with low water content such as straw need additional water before treatment. This can be added with direct steam injection. The effects of thermal processes are extensive. The presence of heat and water causes swelling of the feedstock by disrupting the hydrogen bonds that hold together the structure of the crystalline cellulose and other structural complexes. Heat also breaks down hemicellulose, which aids swelling of the whole complex (Garrote et al., 1999). During hydrolysis of the polymer to monomers, sugars such as glucose and xylose are released. Through thermal influence, monomeric sugars are transformed in different products depending on the environment. Various processes work in parallel which include the Maillard reaction (in presence of an amine group) and caramelization (without amine groups). During the Maillard reaction intermediate products such as furfural from pentoses and 5-hydroxy-methyl-furfural (HMF) are formed from fructose and glucose, respectively. The Maillard reaction is responsible for coloring food during food processing such as baking bread and the bread crust or the dark color in beer, and affects its taste. Many studies in the field of bioethanol production have shown high furfural and HMF production during thermal pretreatment. The intermediate products are bacteriostatic and can inhibit the process of AD when present at higher concentrations. Benjamin et al. (1984) and Bochmann et al. (2010) demonstrated that these products also have a negative impact on AD due to their bacteriostatic characteristics. The final products of the Maillard reaction are melanoidins. These brownish to black products are also described in several publications as “pseudo-lignin.” The final products of the Maillard reaction, the melanoidins or “pseudo-lignin,” lead to reduced gas yields (Lizasoain et al., 2017; Bochmann et al., 2015).
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Thermal pretreatment During thermal pretreatment, also called LHW pretreatment, the substrate is heated up under pressure up to 220 C and held for a specific retention time. Then, it is cooled down and used for AD. In the case of low water content of the feedstock, additional water needs to be added before thermal treatment. Some technologies use steam for temperature increase and water addition. At a large scale, one thermal pretreatment technology was realized in Germany several years ago. It is called TDH, which comes from the German name “ThermoDruck-Hydrolyse.” In a first step the reactor is set under pressure of 20 30 bar. The input material is heated up using a heat exchanger. At the same time the effluent of the TDH is cooled down in this counter-current heat exchanger. The Δt in the counter-current heat exchanger is about 20K. The residual heat demand is provided by cooling down the exhaust gas from a CHP unit. Oil is used as the fluid for heat exchange. The temperature of the exhaust gas from CHP units lies between 350 C and 420 C (Bochmann et al., 2015). The retention time in the TDH is 20 min. Finally, the pressure is released (Dinglreiter, 2007). Various studies have shown that thermal pretreatment increases biogas yield only up to a certain temperature. Gas production decreases below this temperature. DiStefano and Ambulkar (2006) describe the maximum temperature as 175 C for sewage sludge. Using TDH to pretreat crops, the maximum temperature lies at 220 C (Dinglreiter, 2007). Thermal pretreatment of brewers’ spent grains shows lower gas yield with pretreatment above 160 C as compared with untreated substrate (Bochmann et al., 2010). The maximum temperature depends on the composition of the substrates and also on the retention time of pretreatment. Thermal energy from LHW can be recovered using a heat exchanger. Bochmann et al. (2015) demonstrated that, through this recovery, a process with a high energy surplus can be realized.
Steam explosion The principle of steam explosion is related to thermal pretreatment. The substrate is heated up in a vessel to a temperature of 160 C 220 C, for example by direct steam injection. The temperature and pressure in the vessel increase. After a process specific retention time of 5 60 min, pressure is released abruptly. This sudden pressure release leads intracellular water to evaporate. Cell walls are disrupted, causing substrates to lose their structure. Application of steam explosion units allows new feedstock such as hay, straw or reed to be used for biogas production. Bauer et al. (2009) analyzed steam explosion tests of straw and showed calculations of ethanol and biogas potentials. Lizasoain et al. (2016) presented a higher gas yield by 89% compared to untreated reed. The optimal pretreatment was at 200 C. Higher pretreatment temperature showed declining gas yield. This is due to the formation of non-degradable substances during the thermal treatment. In 2017, Lizasoain et al. presented data about the
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Figure 4.2 Steam explosion unit in Parndorf, Austria. Source: H. Dauser/Biogas systems.
pretreatment of corn stover. The highest gas yield (compared to untreated corn stover) was demonstrated at a low temperature of 160 C. Increasing temperature, especially combined with increasing duration, showed declining gas yield by the formation of non-degradable or recalcitrant substances, within this publication named “pseudo-lignin” (Lizasoain et al., 2017). The amount of installed steam explosion units is increasing in Europe. Fig. 4.2 shows a pretreatment unit in Parndorf, Austria. In this plant, straw is treated in a steam explosion unit for utilization in an AD plant. Through the steam explosion, higher gas yield and lower floating layers occur.
Chemical pretreatment During chemical pretreatment, mainly acids and bases of different strengths and on different substrates have been investigated. Acids and bases have been left on the substrates for a certain time, typically hours to days, and then the BMP measured. As a combined process thermochemical pretreatment has been analyzed in several studies.
Alkali pretreatment For alkali pretreatment typically NaOH or KOH are used. Alkali pretreatment increases the degradation of hemicellulose and lignin. During alkali pretreatment the acetate group from the hemicellulose will be removed, thus hydrolytic enzymes can more easily access the carbohydrates (Kong et al., 1992). Solubilization of the lignin by the alkali addition results in accessibility of the enzymes in the microorganisms to the cellulose and hemicellulose. AD of Pennisetum hydrid requires a long retention time due to high lignocellulosic content. Alkaline pretreatment with
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NaOH shows the best results with 2% of alkali addition. Increasing the concentration of alkali did not show any additional positive effects. Variations of the process temperature (35 C, 55 C, and 121 C) of the pretreatment showed a negative influence on the biomethane yield (Kang et al., 2018). Alkaline pretreatment with wheat straw involved a 1.6% NaOH solution incubated at 30 C for 24 hours. The pretreatment resulted in a faster production when comparing the BMP after 10 and 40 days’ retention time (Mancini et al., 2018).
Acid pretreatment Acid pretreatment is typically carried out using H2SO4 and leads to the disintegration of the cellulose. Continuous digestion application of sulfur-containing additives leads to the formation of H2S and might cause inhibition effects during AD. Zhang et al. (2018) presented a higher gas yield of 20% 30% depending on the H2SO4 concentration used for pretreatment.
Thermochemical pretreatment Thermochemical pretreatment is carried out with H2SO4 more often than acid pretreatment. Several studies worked with temperatures above 100 C. Kang et al. (2018) showed the negative effect of the combination of alkali pretreatment combined with increased process temperature (see above). Thermochemical pretreatment with H2O2 and acid addition and different temperatures was tested by Venturin et al. (2018). The pretreatment temperature was set to 100 C, 108 C, 116 C, 124 C, and 132 C and the H2SO4 to 0%, 0.75%, 1.5%, 2.25%, and 3%. During these tests only H2O2 addition showed a positive effect on the BMP. Even at 11 days most of the biomethane had been produced. The utilization of H2SO4 showed a negative effect on the BMP.
Conclusion In recent years, extensive research has been carried out in the field of pretreatment of feedstock for AD. The results vary from a huge increase of degradation rate and high additional gas yield to low additional gas yield or even negative influence on AD. Additional gas yield of about 40% 50% have been reached by several authors. But is has reported by several authors that the impact of retention time during BMP to the additional gas yield after treatment of feedstock is huge. The longer the retention time of BMP trials was, the lower the additional biogas yield detected. Thus, many results with very high additional gas yields need to be viewed critically. Many pretreatment technologies are very feedstock specific regarding positive effect on the disintegration effect, and finally the energy balance and economic benefit for the plant operator.
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Further reading Sepp¨al¨a, M., Paavola, T., Lehtom¨aki, A., Pakarinen, O., Rintala, J., 2008. Biogas from energy crops—optimal pre-treatments and storage, co-digestion and energy balance in boreal conditions. Water Sci. Technol. 58 (9), 1857 1863.