Anaerobic treatment of biodiesel production wastes

Transcription

1 Anaerobic treatment of biodiesel production wastes I. Bodík, M. Hutňan, T. Petheöová and A. Kalina Dept. Environmental Engineering, Faculty of Chemical and Food Technology, Slovak University of Technology Bratislava, Radlinského 9, Bratislava, Slovak Republic ( Abstract Production of biodiesel rapidly increases mainly in the EU countries. Relatively high portion (15-18 %) of glycerine waste stream (g-phase) is one of the negative aspects of real biodiesel production. It is expected the thousands tons of g-phase are and will rise in the all developed countries annually. The main goal of presented paper is to find a way for energy use of g-phase waste stream. The anaerobic fermentation and biogas production from g-phase were tested under laboratory anaerobic conditions during a few months. The obtained results confirm the high energy content of g-phase and possibility of its utilization in full-scale biogas applications. The specific biogas production of 98 liters per liter of tested g-phase was measured. Keywords anaerobic treatment, biodiesel, glycerine, biogas, waste oils INTRODUCTION The biodiesel is an ecological alternative of the fuel based on methyl esters of long chain fatty acids (LFCAs) designated for diesel vehicles. The biodiesel, which is produced by the process of transesterification of vegetable oils, animal fats or wastes fats, is an attractive alternative to diesel fuel because it is produced from renewable resources (Ma and Hanna, 1999). Europe leads worldwide production with more than 1.5 Mil tons of biodiesel in EU-25 in 23 (Bosbas, 28). Actual production of biodiesel in 25 in EU-25 is 3.2 Mil tons and the forecast for 26 is more than 6.1 Mil tons, which represent about 9% of total world biodiesel production. The biggest worldwide biodiesel producer is Germany (1.7 Mil tons in 25). In the Slovak Republic the biodiesel is mostly produced from the rape seed oil with a base catalyst and methanol (Fukuda et al., 21). Production of the biodiesel rapidly increases also in the Slovak Republic and in the 25 about 78 ton of biodiesel was produced totally in the Slovak Republic. The EU has defined the program of replacement of 5.75 % of the consumption of the liquid fuels with biofuels to 21, with will accelerate biodiesel production in the next years in European countries. During the process of the biodiesel production a heavier separate liquid phase called glycerol phase (g-phase) is formed. The portion of the g-phase represents approximately % of the weight of the oil/fat input and its composition is not constant and is influenced by several factors, especially by the acidity number of the input oil. It contains 5 6 % of the glycerol, % of the alkalis especially in the form of alkali soaps and hydroxides, 15-18% of methyl esters, 8 12 % of methanol, 2 3% of water and other components. After treatment of the g-phase with strong mineral acids the crude glycerol with the glycerol content of % is obtained (Zhang et al., 23).

2 Under the Slovak conditions the annual production of the diesel fuel in the Slovak refinery Slovnaft is approximately 2.4 Mil tons and replacement of 5.75 % by biodiesel fuel represents demand of about 14 tons of the biodiesel. This amount of the biodiesel represents a production of 2 tons of the g-phase which has no wide use on Slovak and foreign market. The crude glycerol is used in a pharmaceutics, cosmetic and in food industry. Demand of glycerol in these industrial branches represents only a small fragment of the future g-phase production. One of the possibilities of its use is the anaerobic treatment. During an anaerobic digestion biogas is produced, which can be used for a production of energy. There is an absence of relevant information about the anaerobic treatment of the g-phase in the literature. Only the information about the anaerobic degradation and transformation of pure glycerol into some chemical substances are referred in literature. Anaerobic transformation of glycerol to 1, 3-propanediol as an intermediate product of anaerobic degradation of fats is the most used process (Kocsisová and Cvengroš, 26; Cvengroš and Považanec, 1996; Barbirato et al., 1997). The suitable use of glycerol as a source for anaerobic treatment confirms a work (Perle et al., 1995), where the concentration of ATP was double in compare to other substrates (oil acid). A positive effect of glycerol as a co-fermentation substrate is supported by Amon (26). A 6% supplementation of glycerol to pig manure and maize silage resulted in a significant increase in CH 4 production from 569 to 679 litre CH 4. kg VSS -1. The interesting energy content of 16.3 MJ/kg (Thompsom and He, 26) in the g-phase gives an assumption of high biogas production. The main topic of this paper is to test g-phase under anaerobic conditions with the aim of optimal biogas production. MATERIAL AND METHODS G-phase as an anaerobic substrate Tests of anaerobic long-term degradation in the laboratory scale model were realised with the use of raw g-phase from the Slovak biodiesel producer. The average values of the primary characteristics of this material are shown in the Table 1. As it is evident from the Table 1 the very high organic content (COD, BOD 5 ) represents very good conditions for efficient biogas production. The ratio rate of BOD 5 : COD has a value of.57 which is very favourable for anaerobic conditions. From this value the very good anaerobic degradation of g-phase can be expected. On the basis of the rate it can be expected that concentrations of nutrients N and P are not sufficient in the tested g-phase and it will be probably necessary to add them to anaerobic process. On the other hand the very high content of DIS and relatively high ph-value are parameters that can affect anaerobic process. Table 1 Primary characteristics of raw g-phase Parameter COD BOD 5 Total N Total P DIS density ph Measured values kg/m 3 1,4 - Reactor configuration and operation Laboratory anaerobic scale model with effective volume of 4 litres was situated in the room with the stable temperature of 37 o C; therefore it worked under mesophilic conditions. G-phase was once a day dosed into the anaerobic reactor through the filler hole. The content of reactor was sporadically (more times per day) stirred. The produced biogas was lead through the bubbler filled with water that served as indicator of biogas production. The amount of produced biogas was measured by wet laboratory gas meter. Long-term investigation of the g-phase treatment was realised in the laboratory scale model with the scheme shown in Figure 1. The laboratory-scale experiments with anaerobic biodegradable of g-phase described in this study were running from January till September 26. During the experiment, basic technological parameters in reactor temperature, ph, COD, sludge concentration (SS), organic portion of sludge (VSS), volatile fatty acids (VFA), NH4-N, PO4-P and dissolved inorganic salts (DIS) were regularly monitored.

3 RESULTS AND DISCUSSION Start-up of the anaerobic laboratory reactor The anaerobic reactor was filled with two litres of anaerobic digested sludge from municipal WWTP Bratislava. The sludge concentration was 2.6 g.l -1 ; organic portion in sludge was 49.6 %. Tap water was added to the reactor to reach 4-litre volume. The sludge concentration during the start-time of the reactor was 1.3 g.l -1. The long term operation of the laboratory model consisted of daily dosing of g-phase into the reactor and monitoring the biogas production and sludge and sludge water quality in the reactor. Long term operation of the anaerobic laboratory reactor The real long-term operation began on 12 th January 26. The initial dose of g-phase was 4 ml per day which represents hydraulic load of reactor 1. ml.l -1.d -1. The range of doses during the longterm operation varied between 1 ml.l -1 and 5 ml.l -1 and was controlled depending on technological reactor conditions (concentration on VFA, COD, actually biogas production, etc.). The course of g-phase doses during the reactor operation is described in Figure 1. Volumetric loads correspond to the g-phase dosing and represent values from 1.6 kg.m -3.d -1 up to maximal value of 8. kg.m -3.d g-phase doses per reactor volume (ml.l -1.d -1 ) Date Figure 1 G-phase dosing course during long-term anaerobic operation of reactor The biogas production corresponds to individual doses of g-phase, or with individual volume loadings respectively, and is illustrated in Figure 2. As it is evident from Figure 2, the biogas production increased with increasing g-phase dose into anaerobic reactor. The average daily biogas production during the start-up of process was 2.75 L.d -1 for g-phase dose of 1 ml.l -1.d -1 and after 69 days of operation for dose of 3.5.mL.L -1.d -1 was the average production about 12.3 L.d -1. The specific biogas production per 1 ml of dose increased from 688 ml.ml -1 during the start-up period up to 88 ml.ml -1 for dose of 3.5.mL.L -1.d -1. Increased amounts of specific biogas production can be explained by gradual adaptation of the anaerobic biomass to the given substrate and improved homogenization of the reactor content caused by increased biogas production. The biogas production was equivalent to the amount of 3.1 m 3 per 1 m 3 of the reactor at 5.6 kg/m 3.d of the volumetric load. Till the 23 rd of March 26 (7 th

4 day of operation) the anaerobic reactor had very good parameters from the point of view of biogas production as well as other monitored parameters. The development of COD concentrations (average 56 mg.l -1 ), VFA (average 156 mg.l -1 ) and ph (average 7.1) ranged in standard values and didn t show any problem. On the other hand, the increase of g-phase dose during the first three months of operation was relatively high (see Figure 1). First of all the adaptation time between dose changes was short and increase of dose between 2. and ml.l -1 was very rapid. That was probably the main reason for the total failure of anaerobic reactor after the 7th operation day. 3 1, 1,5 2, 2,5 3,5 5 1, 1,5 2, 2,5 3,5 2,5 dose ml/l 1,5 Biogas production per g-phase dose (Litre) ,5 Specific biogas production per doses (ml/l) Date Figure 2 Biogas and specific biogas production during long-term operation of reactor The increase of dose to the value of 5. ml.l -1 resulted into sharp decrease of biogas production (see Figure 2) simultaneously followed by sudden increase of COD and VFA concentration (see Figure 3). As shown in Figures 3 the rapid increase of COD (as for 55 ), VFA concentration (as for 172 ) lead to inhibition of methanogenic processes in reactor. The values of ph decreased to the value of 6.. Neither ph regulated by NaHCO 3 to the value of 7 improved the situation. The anaerobic reactor negotiated the shocked change nearly three weeks. During the first four weeks after failure the lowest substrate doses (1 ml.l -1 ) were added into the reactor. The COD concentration gradually decreased below 1, as VFA concentration (Figure 3). At the dose of g-phase 3.5 ml per litre of the reactor the COD concentration increased above 2 mg/l, the VFA concentration also increased and biogas production decreased. After 6-day interruption of g-phase dosing the volumetric load reached the value of 4 kg.m -3.d -1 and this value of the volumetric load was remained. The reactor showed stable operation with specific biogas production of 979 ml per 1 ml of g-phase during 65 days at this volumetric load value. The average specific biogas production during the whole anaerobic reactor operation was 96 ml per 1 ml of g-phase. After calculation to standard conditions (temperature o C, atmospheric pressure Pa) the specific biogas production was 798 Nm 3 per 1 m 3 of g-phase. These are values that can be used in the technology proposal of the anaerobic reactor for g-phase treatment. The average values of the biogas composition during the long-term operation of reactor ranged in the standard values: methane (61.1%), carbon dioxide (38.6%), hydrogen and hydrosulphide were not present.

5 6 1, 1,5 2, 2,5 3,5 5 1, 1,5 2, 2,5 3,5 2,5 ml/l concentration (mg.l -1 ) VFA COD Date Figure 3 COD and VFA concentration during long-term operation of reactor The sludge concentration in the anaerobic reactor increased from initial 1.3 g.l -1 up to 3 g.l -1 at the end of experiments. It is expected that the sludge concentration will continuously increase. Real concentration that can be reached in the stirred reactor using still effective stirring is app. 7 g.l -1. During the whole operation time no excess sludge was removed. The only sludge that was removed from the reactor was the sludge for laboratory analysis in the volume of maximum 5 ml per week. Despite of the ph high value of g-phase (ph = 1.4 see Table 1) it was not necessary to regulate the ph value in the anaerobic reactor with the exception of the shocked change, when ph was regulated by NaHCO 3 to the value of ph = 7.. According to g-phase composition the dosing of nutrients during the reactor operation was expected. The initial nitrogen concentration (NH 4 -N in filtered sample) in reactor was 2 mg.l -1 and it was represented by nitrogen from seeded digested sludge. The anaerobic reactor was maintained without nitrogen addition during the first 7 days of operation. The slowly consumption of nitrogen was measured and after 7 days the concentration of nitrogen decreased to 5 mg.l -1. Concentrations of phosphorus (PO 4 -P in filtered sample) were relatively stable (about 1 mg.l -1) during this starting period. After the failure of anaerobic process both concentrations rapidly increased (NH 4 -N up to 3 mg.l -1 and PO 4 -P up to 4 mg.l -1 ) and after the re-activation of process they slowly decreased to the formerly values. The dosing of external nitrogen was realized after the failure of reactor in week intervals, the dosing of phosphorus was realized only twice during the whole laboratory operation. G-phase contains considerable amount of DIS g.l -1, which can influence the anaerobic degradation. The initial DIS concentration was about 1.3 g.l -1 and slowly increased during the whole experiment up to 14 g.l -1 and no negative affect on g-phase degradation was observed. Next increasing of DIS and long-term operation of reactor under very high concentration of DIS (about 2 g.l -1 ) could cause operation problems. We recommend performing the DIS concentration in the values that were achieved in the laboratory scale model; 15 g.l -1, optimum of 1 g.l -1. These values can be achieved by double dilution with water.

6 CONCLUSIONS On the basis of long-term operation of the laboratory scale model of anaerobic treatment of g-phase the following conclusions can be stated: The operation of mesophillic anaerobic degradation of g-phase as the only organic substrate is feasible; the process operation is very sensible to organic over-loading of reactor (extremely high COD); and process operation requires addition of small amounts of nutrients; G-phase represents the material with high content of organic substances that can be easily decomposed with the very high specific biogas production. The anaerobic reactor achieved stable operation at the volume loading of 4 kg.m -3.d -1 with ca 98 ml of biogas production per ml of dosed g-phase. In the laboratory scale model of anaerobic treatment of g-phase the maximal reached volume loading was 8-1 kg.m -3.d -1, but this loading was very sensitive and un-stable; During the monitored period maximum sludge concentration in the reactor was 3 g.l -1. According to the high concentration of the organic substances in the substrate and small volume of the removed excess sludge it is supposed that in the reactor the sludge concentration 7 g.l -1 can be kept, that represents the value of efficient stirring of the reactor. Real volume loading at 7 g.l -1 sludge concentration is considered to be 6 kg.m -3.d -1 (COD); The concentration of DIS increased very slowly but continuously from 1.3 g.l -1 up to 15 g.l -1. Under condition with concentration of DIS > 15 g.l -1 the process operation could become very sensitive and biogas production decreases, but is feasible; Very effective transformation of g-phase into biogas was measured more then 95% which gives very good assumptions for post-treatment of sludge water. Due to this fact the extremely HRT of g-phase was measured in the reactor, and extremely low portions of excess sludge were produced, respectively; On the basis of these long-term operation experiences the first full-scale Slovak (European) biogas station with g-phase as main substrate is being prepared with the total electric capacity 1 MW. REFERENCES Amon Th., Amon B., Kryvoruchko V., Bodiroza V., Pötsch E. and Zollitsch W. (26). Optimising methane yield from anaerobic digestion of manure: Effects of diary system and glycerine supplementation. International Congress Series 1293, Barbirato F., Astruc S., Soucaille P., Camarasa C., Salmon J.M., Bories A. (1997). Anaerobic pathways of glycerol dissimilation by Enterobacter agglomerans CNCM 121: limitation and regulation. Microbiology 143, Bozbas K. (28). Biodiesel as an alternative motor fuel: Production and policies in the European Union. Renewable and Sustainable Energy Reviews, 12 (2), Cvengroš J., Považanec F. (1996). Production and treatment of rapseed oil methyl ester as alternative fuels for diesel engines. Biores. Technol. 55, Fukuda H., Kondo A., Noda H. (21). Biodiesel fuel production by transesterification of oils. J. Biosci. Bioeng. 92, Kocsisová T., Cvengros J. (26). G-phase from methyl ester production splitting and refining. Petroleum & Coal 48(2), 1-5. Ma F., Hanna M. A.(1999). Biodiesel production: a review. Biores. Technol. 7, Thompson J.C., He B.(26). Characterization of crude glycerol from biodiesel production from multiple feedstocks. Applied Eng. Agri. 22, Zhang Y., Dubé M.A., McLean D.D., Kates M. (23). Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Biores. Technol. 89, 1-16.