consumers need gas with stable calorific value and/or Wobbe-Index. The allowed tolerance in Germany/Europe is ± 2 %, only.

By means of propane gas which has a higher calorific value (28.1 kWh/ m3, approx.) the biogas will be mixed (conditioned) by special equipment to an appropriate final calorific value. The final value is selected and continuously controlled by the network operator depending on gas type (H or L), network structure, flow situation and consumer requirements.

The conditioning of gas is a high extra cost for the network operator (operational cost and propane, especially). An optimization of gas conditioning means that propane gas usage and cost should be minimized. This can be achieved in the conditioning equipment by selecting and setting the set point of the final calorific value to an acceptable low value. But this value must still be compatible with the calorific values in the surrounding network which — of course — must be known. Appropriate measurements, the mixing equations and simulation help to solve the optimization and set point problem, even on-line.

In gas network the gas flow may have a number feeder points — including different gas qualities — and in addition the intermeshed network structure contains potential mixing points at every pipe crossing or branch. Normally, insight of the gas flow and calorific value at certain interesting points in the intermeshed network is only possible by many measurements (e. g. gas chromatography) and/or network computation and simulation.

Sustainable Process Index (SPI) was developed by Krotscheck and Narodoslawsky in the year 1995 and is part of the ecological footprint family. The SPI represents as a result the area which is required to embed all human activities needed to supply products or services into the ecosphere, following strict sustainability criteria. Based on life cycle input (LCI) data from a life cycle assessment (LCA) study, SPI can be used to cover the life cycle impact assessment (LCIA) part. LCA studies are standardized and described by the ISO norm 14040 (ISO, 2006). Within the methodology there are seven impact categories defined which are indicated by different colors:

A high footprint is equal to a high environmental impact!

The freeware tool SPIonExcel (Sandholzer et. al., 2005) was used to calculate the ecological footprint (Graz University of Technology, n. d.) This offers the possibility to measure not only the economical performance of the PNS scenarios.

To assess the sustainability of biogas production from intercrops it is necessary to consider the whole crop rotation and the effects of intercrop on main crops. A direct comparison of biogas feedstock from main crops (e. g. corn) and intercrops is not possible, because inter crops grow with lower temperatures and less hours of sunshine. Therefore one of the systems compared, was corn as main crop, commonly cultivated with plow, and an intercrop cultivated with conservation tillage and harvested with a chopper for biogas production. It was assumed, that biogas was processed to natural gas quality. In the second system with intercrops corn was cultivated with conservation tillage whereas the intercrop was grown with direct drilling and harvested with a self-loading trailer instead of a chopper. Since a late harvest of a winter intercrop with high yields would reduce corn yields, an early harvest with an average intercrop yield of only 4 tons dry matter was assumed. In the reference system corn was grown without intercrop and the biogas produced in the intercrop systems was substituted by natural gas. The yield of the main crop corn was equal in all systems (15 tons dry matter of the whole plants per hectare for silage).

In these series of experiments we tested several concentrations of inoculum introduced to the medium: 5-40 % v/v (0.086 g dry wt/l — 0.48 g dry wt/l) for standard medium and 10% and 30% (0.086 g dry wt/l and 0.36 dry wt/l) in case of medium containing wastes (Fig. 5). The optimum inoculum concentration in all cases turned out to be 30% v/v. Data in Table 3 indicate that the second higher concentration produces more hydrogen, shorter lag phase

and bigger COD loss. Increase to 40% lead to smaller amount of produced H2. This effect seems to be caused by the fact that with the inoculum, apart from biomass, we also introduced metabolites which in high concentrations negatively influence the efficiency of photofermentation (Waligorska, 2006, Koku, 2002).

Plants, animals and humans require trace amounts of some heavy metals like copper (Cu), zinc (Zn), while others like cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb) are toxic for them. Heavy metal content of the feedstock usually originates from anthropogenic source and is not degraded during AD. The main origins of the heavy metals are animal feed additives, food processing industry, flotation sludge, fat residues and domestic sewage.

With a N load of 150 kg ha-1, the heavy metals load into the soil (Cd, Cr, Cu, Ni, Pb, Zn) were lower in the case of digestate addition comparing to the compost and sewage sludge treatments while were higher in some heavy metals (Cu, Ni, Pb, Zn) comparing to the mineral fertilizer (Pfundtner, 2002).

1.2 Organic matter content of digestate

and 38.4% in the case of different ingestates, highest loading rates and hydraulic retention times while Marcato et al. (2008) found this value of 53%. If the organic loading rate of biogas plant is high and the hydraulic retention time is short, the digestate will contain a considerable amount of undigested OM, which is not economic and not results a good amendment material. However, the OM content of digestate is more recalcitrant and therefore the microbial degradation and soil oxygen consumption can be decreased by its application (Kirchmann & Bernal, 1997).

The adequacy of digestate as soil amendment is based on its modified OM content. Most OM is converted into biogas, while the biological stability of remaining OM was increased during AD with the increase of more recalcitrant molecules like lignin, cutin, humic acids, steroids, complex proteins. These aliphatic and aromatic molecules are possible humus precursors with high biological stability (Tambone et al., 2009). Pognani et al. (2009) found the increase of these macromolecules’ quantities in the course of AD as it can be seen in Table 3.

Similarly, the rate of lignin-C, cellulose-C and hemicellulose-C are increased in the organic matter content after AD of cattle and pig dung (Kirchmann & Bernal, 1997). The increase of these macromelecules-C were 2.4-26.8 %, 14.2-13.9 % and 7.3 % in the manures, respectively. The hemicellulose-C content in the anaerobically treated pig dung was decreased by 23.8 %. However, the increase of non-decomposable carbon content of digestate is always smaller than that of composts (Gomez et al., 2007). On the other hand, improving the fertilizer effect of a digestate with its higher decomposable carbon content results in an increase in roots and crop residues which may have an important effect on the soil organic matter content.

During the last century a number of different types of flows in simple digester have been developed and they can be of the following kinds: (1) batch flow, (2) continuous flow, (3) continuously expanding, (4) plug flow, and (5) contact flow. The conventional digesters are those utilized to process liquid raw materials with a high content in solids, also called rural digesters, the fermentation chamber having a volume below 100 m3. Conventional digesters are installed without any type of mechanism to reduce the retention time during which the biomass remains inside are predominant; these systems are fed discontinuously and known as discontinuous-flow i. e. batch digesters, or fed periodically and known as continuous-flow digesters.

Batch digesters are loaded at once, maintained closed for a convenient period, and the organic matter is fermented and then unloaded at a later time. It is quite a simple system with small operational requirements. Installation can be made in an anaerobic tank or in a series of tanks, depending on the biogas demand, availability and amount of raw materials to be utilized. Batch flow is most suitable for dry organic matters (solid materials), e. g. solid vegetable waste. This type of biowastes is fed into the digester as a single batch. The digester is opened, digestate is removed to be used as biofertilizer and the new batch replaces the digestate. The tank is then resealed and ready for operation. Depending on the waste material and the operating temperature, a batch digester will slowly start producing biogas and increase the production with time and then drop-off after 4 to 8 weeks. Batch digesters are therefore best operated in groups, so that at least one digester is always producing biogas.

Continuous digesters are usually requiring daily loading and residue management. The process is referred to as continuous since to every daily load corresponds a similar volume load of fermented material. The biomass inside the digester moves through by the difference in hydraulic heat, between the substrate entering the digester and the digestate coming out when unloading. Each load requires a retention time, usually between 14 to 40 days. Continuous digesters can have their retention period reduced by the introduction of agitation and heating. The disadvantage of these models is that the raw material needs to be diluted. The great advantage of these digesters over the batch type is that a single unit allows a continuous supply of biogas and biofertilizer and the continuous treatment of small amounts of waste (Florentino, 2003). Biogas production can be accelerated by continuously feeding the digester with small amounts of waste daily. If such a continuous feeding system is used, then it is essential to ensure that the digester is large enough to hold all the material that will be fed into the digester in the whole digestion cycle. One key issue is to implement two digesters, i. e. accomplishing the biodegradation of the organic waste through two stages, with the main part of the biogas is being produced in the first stage and the second stage serves as finishing stage of the digestion at a slower rate.

Regarding the continuously expanding flow, the digester starts one third full and then filled in stages and later emptied. Concerning the plug flow, the wastes are added regularly at one end and over-flows the other. In the contact flow, a support medium is provided.

Two simple biogas digester designs have been developed, the Chinese fixed dome digester and the Indian floating cover biogas digester. The digestion process is the same in both digesters but the gas collection method is different in each. In the Indian-type digester, the water sealed cover of the digester rises as gas is produced and acts as a storage chamber, whereas the Chinese-type digester has a lower gas storage capacity and requires efficient sealing in order to prevent gas leakage. Both have been designed for use with animal waste or dung. Additionally, there are also Philippine and Sri Lankan digesters.

Annika Bjorn, Paula Segura de La Monja, Anna Karlsson, Jorgen Ejlertsson and Bo H. Svensson

Department of Thematic Studies, Water and Environmental Studies,

Linkoping University, Sweden

1. Introduction

The biogas process has long been a part of our biotechnical solutions for the handling of sewage sludge and waste. However, in many cases the existing process applications need to be optimized to improve the extent of biogas production as a part of the energy supply in a sustainable and viable society. Although the principles are well known, process disturbances and poor substrate utilization in existing biogas plants are common and are in many cases likely linked to changes in the substrate composition.

Changes in substrate composition can be done as a means to obtain a more efficient utilization of existing biogas facilities, which today treating mainly manure or sewage sludge. By bring in more energy rich residues and wastes a co-digestion process with higher biogas potential per m3 volatile solids (VS) can often be obtained. However, new and changing feedstocks may result in shift in viscosity of the process liquid and, hence, problems with inadequate mixing, break down of stirrers and foaming. These disturbances may seriously affect the degradation efficiency and, hence, also the gas-production per unit organic material digested. In turn, operational malfunctions will cause significant logistic problems and increased operational costs. Changes of the substrate profile for a biogas plant may also infer modifications of the downstream treatment of the digestate.

Together with high digestion efficiency, i. e. maximum methane formation per reactor volume and time, the economy of a biogas plant operation depends on the energy invested to run the process. A main part of the energy consumed during operation of continuous stirred tank reactors (CSTRs) is due to the mixing of the reactor material (Nordberg and Edstrom, 2005). The shear force needed is dependent on the viscosity of the reactor liquid, where increasing viscosity demands a higher energy input. Active stirring must be implemented in order to bring the microorganisms in contact with the new feedstock, to facilitate the upflow of gas bubbles and to maintain an even temperature distribution in the digester. Up to 90% of biogas CSTR plants use mechanical stirring equipment (Weiland et al, 2010).

solid concentration and on the characteristics of the organic material as well as on the interactions between particles and molecules in the solution (Foster, 2002). Therefore, this type of characterisation can be important in process monitoring and control.

The aim of this chapter is to briefly introduce the area of rheology and to present important parameters for rheological characterization of biogas reactor fluids. Examples are given from investigations on such parameters for lab-scale reactors digesting different substrates.

Many reactor configurations are used for the anaerobic treatment of agro-industrial wastewaters. Among them, the most common types are discussed and illustrated in Fig. 2.

(А) (В) (C) (D)

2.1 Completely stirred anaerobic digester

The completely stirred anaerobic digester (CSTR) is the basic anaerobic treatment system with an equal hydraulic retention time (HRT) and solids retention time (SRT) in the range of 15-40 days in order to provide sufficient retention time for both operation and process stability. Completely mixed anaerobic digesters without recycle are more suitable for wastes with high solids concentrations (Tchobanoglous et al., 2003). A disadvantage of this system

is that a high volumetric loading rate is only obtained with quite concentrated waste streams with a biodegradable COD content between 8,000 and 50,000 mg/L. However, many waste streams are much dilute (Rittmann and McCarty, 2001). Thus, COD loading per unit volume may be very low with the detention times of this system which eliminates the cost advantage of anaerobic treatment technology. Typical the OLR for this digester is between 1-5 kg COD/m3-d (Tchobanoglous et al., 2003).

The small consumer’s individual daily behavior is not known exactly; only an aggregation of all consumers can be calculated. This is due to the fact that consumption values are normally read and collected once a year. For computing purposes a yearly value has to be deduced to a daily or even hourly value by standard load profiles which have implicit uncertainties, of course. One day this shortcoming will be overcome stepwise when Smart Metering will be widely introduced and used.

2.11 Accuracy, cost-efficiency

Many measurements which are useful from the technical or simulation point of view may not be useful for economic reasons. Additional measurements cost extra money: equipment, erection, survey and maintenance, etc. So, in fact any computation will have to cope with less data points and less accuracy than desired or theoretically possible.

2.12 Technical board acceptance procedure

Introduction and use of quality parameter tracking systems (for calorific value) require an individual technical acceptance procedure from the Technical Board and a special operating permit. The required accuracy of the computed result must be better than ± 2% in the network area or ± 0.8% from total measuring range. In addition, a number of permanently recorded control measurements are requested to prove the correctness and integrity of the used data for the Technical Board authority.

3. Conclusion, benefits

Calorific value tracking is a most useful simulation tool in order to derive detailed information all over the network for every node and not only at distinct points. This method replaces many costly measurements in the network. Applying historical data to this tool helps to reconstruct all gas parameters and physical states of the past. Using the forward­looking method it enables even optimization of the gas conditioning for all biogas injections into the network. Aside from existing limitations the precision of the computed calorific value is high. Based on these data billing can be made fair as each consumer could get to know his energy consumption the best way possible.

4. References

[1] Cerbe, G.; Grundlagen der Gastechnik, Hanser 2004

[2] Hass, P.; Rottger, S.; Gasqualitatsberechnung in vermaschten Endverteilnetzen, gwf

Gas/Ergas 9-2009, p. 512 , Oldenbourg Verlag

[3] PTB, Technische Richtlinie G14, Einspeisungen von Biogas in das Erdgasnetz, 11/2007.

[4] Wernekinck, U. et al., Ermittlung des Brennwertes von Erdgasen — Gasmessung und

Abrechnung, 4. Aufl. Vulkan Verlag Essen 2009.

[5] Schley, P.; Schenk, J.; Hielscher, A.; Brennwertverfolgung in Verteilnetzen, gwf

Gas/Erdgas, 9/2011, p. 553-556, Oldenbourg Verlag.

[6] STANET User Manual, Fischer-Uhrig Engineering, Berlin 2011.

The COD removal efficiency, TP removal efficiency, biogas production and composition were markedly influenced by using steel elements as an additional medium in the UASB reaction chamber.

During Stage 1, both UASB reactors reached the steady-state after 25 days of operation. No statistically significant differences (p>0.05) were observed between UASB reactor with steel elements (RFe) and UASB reactor without steel elements (R0) in term of the average COD removal efficiency and biogas production rate (Fig. 2; 3). Nevertheless, RFe indicated higher (p<0.05) removal efficiency in phosphate (86.2%) and TP (81.2%) than R0 in which the analyzed values were 1.8% and 22.8%, respectively (Fig. 3). CH4 content in biogas produced in RFe was as high as 67.1% which was higher by 11.9% than in R0 (p<0.05). In Stage 2 and 3 both UASB reactors demonstrated a stable work, but statistically significant differences in the values of all the monitoring parameters between R0 and RFe were noticed (p<0.05). The duration of each stage were 36 and 48 days, respectively. The average TP removal efficiency and phosphate removal efficiency in RFe were higher by 77.7% and 83.7%, respectively than in R0 during Stage 2, and 68.1% and 73.9%, respectively during Stage 3 (Fig. 3). During Stage 2 and 3 high COD removal efficiencies (95.6%, 94.8%, respectively) were remained in RFe, in contrast to that of 84.2% in Stage 2 and 80.1% in Stage 3 in Ro (Fig. 3). The average CH4 content in biogas of 78.0% and biogas production of 2.59 m3m-3d-1 in RFe, in contrast to that of 60.8% and 0.92 m3m-3d-1, respectively in R0 (p<0.05), were observed during Stage 2. In Stage 3, biogas production increased by 1.12 m3m-3d-1 in R0 and 1.2 m3m-3d-1 in RFe, but it was still significantly higher in RFe (3.79 m3m-3d-1) than in R0 (2.04 m3m-3d-1), p<0.05 (Fig. 2). Moreover in that stage, the highest methane content in biogas of 79.8% in RFe and 68.1% in R0 were achieved (Fig. 2). During the last stage it was found the highest biogas production rate in RFe of 4.01 m3m-3d-1, while 1.86 m3m-3d-1 in R0 was observed (p<0.05). The average

90

80

70

60

50

40

30

20

10

0

CH4 content in biogas decreased to 72.3% in RFe, in contrast to that of 64% in Ro, and the differences between R0 and RFe were statistically significant (p<0.05) (Fig. 2). It was found decrease in TP removal efficiency in RFe and R0 to 72.2 and 10.1%(p<0.05), respectively. According to this, the phosphate removal efficiencies decreased, too (Fig. 3). COD removal efficiency was lower than in Stage 3 and achieved 88.8% in RFe and 71.8% in R0, p<0.05 (Fig. 3).

100

95

90

85

80

75

70

The study demonstrated that the COD removal efficiency was markedly influenced by using steel elements as an additional medium of the UASB reactor. Iron ions generated from the steel elements must have acted as coagulants and were involved in the removal of suspended organic matter. After the operation time of 219 days, sludge samples from both UASB reactors were collected for the determination of TSS, which was higher by 52.1% in RFe than in R0. Moreover, ferrous ions in wastewater could react to form hydroxides which were the sorption areas for suspended organic matter. Additional sorption areas were made by steel elements surface. Enhancement of COD removal efficiency by zero-valent iron processes were reported by Jeon et al. (2003) and Lai et al. (2007). Vlyssides et al. (2009) showed that the addition of ferrous ions in the form of ferrous chloride solution (2% w/v) induced a stable and excellent COD removal efficiency from synthetic milk wastewater, regardless of the increasing in OLR. When the OLR was as high as 10 g COD L-1 d-1, the COD removal efficiency of 98% was achieved.

reacted with ferrous iron to probably form insoluble vivianite precipitated in the reaction chamber. It can be confirmed by significant increasing of TSS (by 52.1% in RFe) and the accumulative iron ions and phosphorus content detected in the anaerobic granular sludge in RFe at the end of the experimental period. The TP and total iron percentage in the dry matter was 0.314 and 0.0981, respectively, in RFe and 0.019 and 0.0129, respectively, in R0. This results confirmed the anaerobic microbial corrosion occurred in RFe. Choung & Jeon (2000) and Jeon et al. (2003) obtained similar trends for domestic wastewater treatment under anaerobic conditions. Moreover, the colour of anaerobic sludge granules from RFe was black, while from R0 was grey with white conglomerates (Fig. 4). It indicates that the presence of iron determine the colour of granules. The black colour of granules is due to the formation of large amounts of iron sulphide precipitate (Vlyssides et al., 2009). It was seen that the granule diameter in the sludge bed in RFe was smaller than in R0. It was different from the data reported by Vlyssides et al. (2009), who observed a considerable increase of 40% in the mean granule diameter resulted in iron accumulation in granules.

During the experimental period high iron concentrations in the RFe effluent were observed. During Stage 1, the highest content of iron was noticed (20.1 mg L-1) and it was consequently decreased to 19.2, 15.8, 14.2 mg L-1 in Stage 2, 3, 4, respectively. The decrease of the total iron in the effluent from the UASB reactor packed with steel elements can indicate the formation of a protective layer on the steel surface. According to Volkland et al. (2001) under certain conditions the vivianite could act as a corrosion-inhibiting layer. Moreover, biofilm-forming bacteria can protect steel from corrosion. With a dense suspension of microorganisms (> 109 cells mL-1) they can protect the steel surface by forming a corrosion-inhibiting layer in consequence of bacterial adsorption and adhesion (Volkland et al., 2001; Yu et al., 2000). Microbial corrosion and the formation of iron precipitates deteriorate the reactive media of steel elements (Karri et al., 2005). It could explain the gradual decrease in phosphorus and TP removal with the duration of the experiment.

Biogas production rate and CH4 content in biogas were higher in RFe than in R0 in all stages (except Stage 1 where the differences in biogas production between RFe and R0 were not statistically significant). According to Karri et al. (2005) zero valent iron was an electron donor for methanogenesis. It suggested that microbial corrosion of steel elements supported methanogenesis which contributed to the more CH4 and biogas production in RFe. Iron may play an important role in granulation phenomena and was found to be a component of essential enzymes that carry out numerous anaerobic reactions (Vlyssides et al., 2009; Yu et al., 2000). The conversion of COD to biogas components and bacterial growth may be limited at iron deficient concentrations. However, the accumulation of iron ions may decrease the specific activity of the bacterial groups, including methanogens (Yu et al., 2000). It was reported that high Fe2+ concentration in the anaerobic sludge granules led to decrease of the specific activity of biomass due to the presence of a large amount of minerals deposited within the granules, a significant decrease in the water content in granules, and the possible toxicity of high-concentration Fe2+ accumulated inside the granules (Yu et al., 2000). During the experiment, biogas production rate was not decreased from Stage 1 to 4, which could indicate that the activity of methanogenic bacteria was not inhibited by anaerobic steel corrosion process. The maximum value for biogas rate was 8.22 L d-1 in RFe and 4.2 L d-1 in R0. Najafpour et al. (2008) achieved the biogas production of 3.6 L d-1 for HRT of 48 h with the methane content of 76% from UF whey permeate. Venetsaneas et al. (2009) achieved about 1 L CH4 d-1 and 68% v/v methane content in biogas in the two-stage process for cheese whey fermentation.

Fig.2-2 showed the quality percentage of cellulose, hemi-cellulose, and lignin in biogas residue.

According to research results (Gao Zhenghua, 2008; Chen Hongzhang, 2008), the cellulose, hemi-cellulose, lignin of straw, wheat and corn stalks compared with these of biogas residue fibre were shown in table 2-4.

The comparative analysis results showed that, cellulose quality percentage of biogas residue fibre after anaerobic fermentation was 5% higher than straw, wheat stalk, corn stalk; while hemi-cellulose quality percentage was 5%lower than straw, wheat stalk, corn stalk; lignin quality percentage did not change. The result showed that anaerobic fermentation to lignin content was not influence, hemi-cellulose relatively reduced, cellulose relatively increased, it is positive to the resources utilization of biogas residue.