BIOMET PROCESS- MAXIMISING THE ORGANIC WASTE TO ENERGY TRANSFER

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1 BIOMET PROCESS- MAXIMISING THE ORGANIC WASTE TO ENERGY TRANSFER Del Piccolo C. 1, Durand B. 2, Bigot B. 3 1 Veolia Water Solutions & Technologies Italia, 2 Veolia Water STI, 3 Veolia Water Solutions & Technologies UK Corresponding Author Bruno Bigot Abstract A waste- to- energy plant is described for the treatment of about 13 types of different organic substrates in co-digestion with a Two-stage anaerobic digestion process aiming to maximise the degradation into biogas and to assure complete stability to the process. The strategy for the doubling of the treatment capacity without any intervention on the lay-out is also described. Keywords Two-stage anaerobic digestion, waste to energy, stability, thermophilic Introduction It is already a well established knowledge that Anaerobic digestion is one of the most appropriate solutions (Malpei et al. 2007) to treat organic residues, since it s able to get a value out of waste which is a biogas being a potential source of energy such as electrical, thermal or fuel, bringing in this way an added value to the waste management. A composting platform in the Region North of Paris decided to extend its possibilities of waste collection, by installing an anaerobic digestion plant. The major requirement for the anaerobic digestion plant was to be the most reliable, flexible and stable process since the ambition was to have the possibility to feed the plant with a huge number of different difficult substrates and moreover since the beginning the project was intended to be developed to allow to double the capacity of the plant without significant modifications of the original lay-out. Biomet solution, a process developed by Veolia Water Solutions & Technologies Italia, has been retained as the most appropriate for the target. Description of the project The first phase of the project is related to the treatment of about ton/year of substrates with the following average composition. Table 1: Inlet mixture of substrates Type Quantity (ton/year) TS - VS/TS (%) Primary sludge Dairy sludge Cannery sludge Grease Type

2 Grease Type Grease Type Grease Type Filtration media Food Waste (after depackaging) Expired food products Sludge from agro-industry Agricultural by-products Cat. 3 waste Due to this significant variety in the inlet, a refined design in the storage of the different substrates and in their preparation needs to be adopted. The substrates have been classified into families according to a similarity in terms of physical and chemical properties, as well as biogas potential and for every type of family a specific storage strategy has been chosen, in terms of volume, treatment to apply which can be just mixing, dilution and mixing, cutting etc. In this way also new substrates can be accepted after an analysis of their characteristics and will be assigned to the family which is found to be more similar. Three pits of 150 m 3 volume each allow to receive respectively: Liquid or soluble hydrophilic waste which needs just a mixing Pasty waste or suspension of tiny solids which need a more vigorous mixing for homogenisation Solid waste which needs also to be crushed before dilution and mixing Purely liquid wastes are collected in four tanks of 50 m 3 volume each. A covered storage of 300 m 2 for temporary storage of solids before the treatment in the pits completes the reception facility. According to the levels of the different pits and tanks, the daily receipt is prepared in a tank of 280 m 3 volume and from there the soup will feed the anaerobic digestion plant which is constituted of: An hydrolysis reactor of 1300 m 3 A digester of 4400 m 3 A post- digester of 1200 m 3 Pasteurisation constituted of 3 tanks of about 20 m 3 each Solid liquid separation with a centrifuge The biogas produced will be sent, after treatment, to a cogeneration plant of about 1 MW. The solid digestate is sent to the existing composting facility while the excess of the liquid digestate is treated with an evaporation plant before discharge into the river. In the first phase, with an annual load of ton of waste, the plant is supposed to run in mesophilic conditions, while in the second phase of the project, when the load will double, the Temperature will be raised up to Thermophilic to take advantage of the higher kinetics typical of this condition, so that this will balance the reduced Hydraulic retention time which has an

3 impact on the biogas yields, since no volume implementation will be realised to the existing tanks to face the increase of feeding rate. The anaerobic digestion: Biomet process Looking at the reactions involved in an anaerobic digestion process it is possible to identify four main steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis. These steps need to happen one subsequent to the other in order to obtain the final result of the process which is the biogas. The first three phases of degradation (from hydrolysis to acetogenesis) are characterised by a certain degree of similarity and normally involve similar environments, being quite different from the one required by the final stage, the methanogenesis. Most of conventional full scale plants are mono-stage Anaerobic Digestion processes meaning that they integrate in the same fermentation environment all the bio-chemical processes involved: hydrolysis, fermentations (acidogenesis, solventogenesis, acetogenesis) and methanogenesis. A double-stage Anaerobic Digestion (AD) system is aimed at favoring hydrolysis and fermentations in the first stage and segregating methanogenesis in the second stage. The anaerobic process proposed in this project, named Biomet, is a Two-stage AD process. This approach has been chosen to help in optimizing hydrolysis in the first stage, thereby maximizing and/or speeding up the methanogenesis in the second stage. There is a lot of literature giving evidence of the advantages of such an approach (Dennis and Burke 2001; ANPA 2002; Demirer and Othman 2008). The main advantage that the feed backs from site shows is the stability brought to the process and to the biology of the system and this point is a crucial one when AD process needs to be applied to variable substrates and quality and quantity variations need to be managed like in this specific project. The engineering details of the Biomet solution have been carefully studied to assure flexibility and reliability. One of the keys to achieve an efficient biogas production is keeping the reactors completely mixed at all times. On the whole, this ensures a stable process and high-performance (high gas production). This is the reason why a specific mixing device, external for easy servicing has been developed, constituted of a chopping pump and nozzles system. For the same reason also the shape of reactors is important: this engineering point has been optimized also to minimize thermal losses and reduce foam and deposit episodes. Always keeping in mind to provide solutions with a low maintenance impact, the heat exchangers necessary to provide the thermal energy required by the biological fermentation are external, with geometries suitable for an easy and quick maintenance, without any interruption of the process.

4 Figure 1: Typical process scheme of a biogas plant as reference for the description. Preparation tank In the preparation tank the substrates from pits and tanks are introduced and are mixed with a fraction of liquid digestate in order to make the mixture pumpable and homogeneous, so, to reach this goal the preparation tank is equipped with a mixing system and a recirculation pump, which also operates as a shredder. The Total solids content is about 12%. After a residence time which can be set and which typically can be of 4-6 hours, the homogenized substrate is pumped to the hydrolysis reactor. The liquid digestate recirculation allows also to add some alkalinity and to avoid every possible problem caused by the very high content of VFA (Volatile Fatty Acids) in the hydrolysis reactor. Hydrolysis reactor In hydrolysis reactor all the anaerobic digestion phases that are favoured by a lower ph and which tend to bring the ph to acidic values occur, in particular: The hydrolytic phase where lipids are converted in glycerol and long chain fatty acids, proteins are converted into aminoacids and carbohydrates are converted in monosaccharides. The acidogenic phase where carbohydrates and aminoacids are converted into small chain fatty acids, hydrogen, carbon dioxide. For exemple for glucose the reactions that can occur in the acidogenic phase are reported in Table 1

5 Table 2: Acidogenic phase reactions for glucose (Malpei and Gardoni, 2010) Chemical reaction Product C 6 H 12 O 6 + 2H 2 O 2CH 3 COOH + 2CO 2 +4H 2 Acetic Acid 3C 6 H 12 O 6 4CH 3 CH 2 COOH + 2CH 3 COOH + 2CO 2 + 2H 2 O Acetic Acid, Propionic Acid C 6 H 12 O 6 CH 3 CH 2 CH 2 COOH + 2CO 2 + 2H + Butirric Acid C 6 H 12 O 6 2CH 3 CHOHCOOH Lactic Acid The acetogenic phase where the volatile fatty acids like propionic, butyric and lactic acid and the ethanol are converted into acetic acid with production of hydrogen and carbon dioxide. The main reactions that can occur in the acidogenic phase are reported in Table 3 Table 3: Acetogenic phase reactions for glucose (Malpei and Gardoni, 2010) Substrate Chemical reaction G (kjmol -1 ) Propionic Acid CH 3 CH 2 COO - + 3H 2 O CH 3 COO - + HC0 3 + H + + 3H 2 +76,1 Butirric Acid CH 3 CH 2 CH 2 COO - + 2H 2 O 2CH 3 COO - + H + + 2H 2 +48,3 Lactic Acid 2CH 3 CHOHCOO - + 2H 2 O CH 3 COO - + HC0 3 + H + + 2H 2-4,2 Ethanol CH 3 CH 2 OH + H 2 O CH 3 COO - + H + + 2H 2 +9,6 To summarize, the goal to be achieved in hydrolysis reactor is the production of acetic acid and the hydrogen that are the compounds which methanogenic bacteria convert into methane, carbon dioxide and, in the case of hydrogenotrophic methanogenesis, water. Hydrolysis is fed in a discontinuous way, only five days a week and for only several hours per day, working also as a buffer tank. Methanogenesis reactor: digester and post-digester In the methanogenesis reactor the reactions that produce methane occur. The methane is produced by two different ways, the hydrogenotrophic methanogenesis and the acetoclastic methanogenesis. In the hydrogenotrophic methanogenesis, carbon dioxide is reduced and hydrogen is oxidized with production of methane and water in accordance with the reaction: 4H 2 + CO 2 CH 4 + 2H 2 O In the acetoclastic methanogenesis, acetic acid is decomposed by Archea bacteria with production of methane and carbon dioxide in accordance with the reaction: CH 3 COOH CH 4 + CO 2 The most important acknowledged advantages of the separation of the hydrolysis and methanogenesis stages are: Globally smaller size of the reactor (Rapport J et. Al. 2008) ; Less foam problems (Massart N. et al. 2006); Possibility to operate at different temperatures in hydrolysis and methanogenesis (Dugba and Zhang 1999; Dennis and Burke 2001);

6 Increased efficiency: the different microorganisms require different environmental conditions, different nutrients and have different capacities of growth and a different resistance to stress, with the two separated stage both colonies live in the appropriate conditions for their characteristics (Wang and Banks 2003; Nielsen et. Al. 2004; Cirne et. Al. 2007); More flexibility: the introduction of biomass with different physical and chemical characteristics involves fewer problems as the variation is absorbed from the stage of hydrolysis which better tolerates shocks while the methanogenesis will have always a similar diet (Parawira et. Al. 2008). Hydrolysis can in this way act as a buffer tank, smoothing peaks and variations and bringing flexibility to the management of the plant: feeding 5 days/week is in fact possible but with constant biogas production 7 days/week. Digester and post-digester are fed continuously 24/24 at a constant flow rate flow rate from the hydrolysis tank. The post-digester is operated like the primary digester: it is in fact mixed and heated. Biogas management The biogas produced by anaerobic digestion needs to be sent to a purification treatment, with the purpose of eliminating the H 2 S, before being sent to the engine (Muche and Zimmermann 1985; Horikawa et. al. 2004) essentially to avoid corrosion. It is constituted of an active carbon filter, followed by refrigeration to eliminate condensate. Pasteurization According to legislation, a pasteurization treatment at 70 C for 1 hour is required considering the inlet substrates. Three tanks of about 20 m3 each will assure the constancy of the digestate extraction and treatment. Solid liquid separation The digestate from anaerobic digestion process is subjected to a solid-liquid separation performed by means of a centrifuge that allows to obtain a sludge with a dry content of around 20% without use of polymer and with the feed at 70 C coming from the pasteurization step up front. The solid sludge is sent to composting and a liquid fraction with a dry matter content of around 2% which is in part reused for the feeding mixture preparation and partly sent to the post-treatment (evaporation). First results and discussion The plant has been started last April and after few months it has been able to treat the total quantity of available wastes. The typical parameters characterising the two stages of the degradation have rapidly been set: hydrolysis ph is in fact acidic now, while digester ph is around 8 and biological conditions confirm to be very stable and are not affected by the sudden change of inlet substrates in both quality and quantity which are typical of this plant. Some foam was detected inside the reactors and it was most probably due to the high level of grease products fed to the system, but it was very well controlled with dosage of some ppm of antifoam. Biogas production is in line with expectations and methane percentage in the biogas is higher than expected and is constantly above 60%, in many cases it reaches 65%or more giving

7 confidence of a very good energetic conversion of the biomass, much higher than expected from classical one stage digestion return of experience. The technological choices on which the described plant is based have the aims to assure enhanced performance but also a significant stability which is mandatory when biological processes are involved and reliability which is fundamental for a continuous process. Hydrolysis as separate stage acts as smoothing agent, allowing to charge the plant discontinuously, but assuring continuous and constant biogas production and avoiding shocks to the bacteria. The double stage process is stable enough to be managed in Thermophilic conditions, taking advantage of the quicker kinetics and higher degradation results and this will be very useful when the doubling of the capacity will be realized, since it will allow to balance the reduction of the retention time, achieving in any case the guaranteed performance without any extension of the volumes. A more detailed return of experience will be available during the next European Biosolids and Organic Resources Conference, after a longer period in operation and when steady-state conditions will be realized. References ANPA and ONR (2002). Il trattamento anaerobico dei rifiuti aspetti progettuali e gestionali. Cirne D.G., Lehtomäki A., Björnsson L. and Blackall L.L. (2007). Hydrolysis and microbial community analyses in two-stage anaerobic digestion of energy crops. Journal of Applied Microbiology, volume 103, issue 3, September, pp Demirer G. N. and Othman M. (2008). Two-Phase Thermophilic Acidification and Mesophilic Methanogenesis Anaerobic Digestion of Waste-Activated Sludge. Enviromental Engineering science, volume 25, number 9. Dennis A. and Burke P.E. (2001). Dairy Waste Anaerobic Digestion Book. Enviromental Energy Company. Dugba P.N. and Zhang R. (1999). Treatment of dairy wastewater with two-stage anaerobic sequencing bach reactor system thermophilic versus mesophilic operation. Bioresource technology, volume 68, issue 3, June, pp Horikawa M.S., Rossi F., Gimenes M.L., Costa C.M.M. and Da Silva M.G.C. (2004). Chemical absorption of H2S for biogas purification. Brazilian Journal of Chemical Engineering, volume 21, issue 3, September, pp Malpei F. and Gardoni G. (2007). La digestiona anaerobica: I principi del processo biologico e I criteri di dimensionamento. 62 Corso di aggiornamento: Biogas da frazioni organiche di rifiuti solidi urbani in miscela con altri substrati, Maggio. Malpei F., Rigamonti L. and Grosso M. (2007). Il bilancio energetico ed ambientale di alcuni scenari di digestione anaerobica della FORSU. Biogas da frazioni organiche di rifiuti solidi urbani in miscela con altri substrati. Fantigrafica-Cremona.

8 Massart N., Bates R., Corning B and Neun G. (2006). Design and operational considerations to avoid excessive anaerobic digester foaming. Water Environment Foundation. Muche H. and Zimmermann H. (1985). The purification of biogas. Nielsen H.B., Mladenovska Z., Westermann P. and Ahring B.K. (2004). Comparison of two-stage thermophilic anaerobic digestion with one-stage digestion of cattle manure. Biotechnology and Bioengineering, volume 86, issue 3, May, pp Perawira W., read J.S., Mattiasson B. and Björnsson L. (2008). Energy production from agricultural residues: high methane yelds in pilot-scale two-stage anaerobic digestion. Biomass and Bioenergy, volume 32, issue 1, January, pp Rapport J., Zhang R., Jenkins B.M. and Williams R.B. (2008). Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste. California Enviromental Protection Agency. Wang Z. and Banks C.J. (2003). Evaluation of a two stage anaerobic digester for the treatment of mixed abattoir wastes. Process Biochemistry, volume 38, issue 9, April, pp