Intelligent Carbon and Energy Utilization

Transcription

1 Intelligent Carbon and Energy Utilization Johansen, R *. EnviDan A/S, Vejlsøvej 23, 8600 Silkeborg, Denmark, raj@envidan.dk Abstract This research and development project is about intelligent use of the carbon and energy resources in raw municipal wastewater. The project evaluates the energy potential in wastewater and opportunities to optimize wastewater treatment plants to become energy neutral or even produce energy in excess. Optimization of certain unit operations at wastewater treatment plants to achieve a neutral or positive total energy balance is possible in several cases. Hence the aim of this project is to obtain knowledge for developing a best practice for utilizing carbon resources in the wastewater in the most sustainable way regarding environmental impact and energy consumption. The study has two different main themes: 1) Interaction and dependence of pre-settlement and biological removal of nutrients in the main plant will be investigated and by mapping the total plant energy and carbon balance the most advantageous operation is found. This study combines and utilizes existing technologies in a comprehensive and documented Intelligent Carbon and Energy Utilization concept. This research is specifically looking into enhanced pre-settlement with a COD controlling loop between primary sludge fed to digester and primary sludge recycled to the main WWTP depending on the nitrogen removal. Test results are not shown but tests at Thisted WWTP indicate that it is possible to enhance the amount of primary sludge fed to the digester and by doing so improve the gas production and minimize the carbon-foot print. 2) Another part of the study is looking at the specific parameters for enhanced anaerobic digestion concerning dry matter content and temperature especially for digestion of fat. Furthermore the optimum mesophilic temperature is evaluated concerning heat input versus biogas production. Also the methane emission from the digested sludge and the gas potential in already digested sludge is investigated. A survey of the Danish CSTR digesters show in general that the temperature and dry matter content in the digesters probably could be optimized and thereby enhance the gas production. At 43 C the mesophilic digestion seems unstable probably due to inhibitory concentrations of accumulated LCFA. This was not detected at 35 C and 39 C. The test results from digestion of animal fat indicates that the optimum temperature range for mesophilic anaerobic digestion is between 39 C to 40 C when it comes to stable gas production. In a temperate climate the total methane emission from pumping and storage of digested sludge was found to be 0.1 % % of the total methane production and therefore could be ignored. Keywords Biogas production, Pre-settlement, Carbon utilization, Anaerobic digestion, Municipal wastewater treatment, Optimization, Carbon footprint. Introduction In the past focus has been on decreasing energy consumption related to biological removal of nitrogen and organic matter at wastewater treatment plants amongst others by introducing advanced on-line control systems. Moving the Page 1

2 WWTP from being energy consuming to becoming energy neutral or even energy producing involves optimizing every stage of the treatment process. This project turn the way of looking at wastewater upside down as it is seen as a valuable resource and not as a waste product. By looking at wastewater as a resource it also supports the global effort to reduce emissions of green house gases by decreasing the carbon footprint from wastewater treatment plants in general. EnviDan A/S, a Danish engineering consulting company, has received grants from the Danish Environmental Protection Agency in order to clarify and document optimized use of the carbon and energy content in municipal wastewater. Other partners are a number of Danish municipalities and master students from Aarhus University School of Engineering. Materials and methods The research and development project also includes literature studies, laboratory and full scale tests. Carbon and energy mass balances are established using data provided by the wastewater companies involved. These balances will be used as baseline for optimization of carbon and energy utilization on wastewater treatment plants. At most wastewater treatment plants primary settlement is carried out to remove suspended solids and a fraction of the organic matter to decrease the nutrient load and energy consumption in the aeration tanks. The organic matter or primary sludge is then led to anaerobic digesters where the carbon source is utilized for biogas production. On the basis of the theoretically estimated carbon mass balance the potential for minimizing the carbon foot print from biological consumption is studied. The carbon mass balance is completed with data from full scale wastewater treatment plants. These plants are Thisted Spildevand A/S, Spildevandscenter Avedøre I/S and NK Forsyning, all located in Denmark. The purpose of this focus area is to verify the basic relation between the amount of primary sludge precipitated, the biological reaction rate and the gas yield. Through theoretical calculations the consequences of enhancing the primary settlement and removal of carbon and primary sludge is evaluated. Thisted WWTP has a biosorption process combined with two primary settling tanks as a pre-treatment. At Thisted WWTP the effect of a differentiated chemical dose of Fe 3+ is investigated to optimize the amount of primary sludge led to the digester. To carry out the full scale experiment a setup with a suspended solid online sensor is placed at the outlet of the primary clarifier combined with a dry solid online sensor measuring the dry matter content in the primary sludge taken out. 2-3 times a week the content of COD, Total-N and Total-P are analyzed as 24 hour flow proportional samples from the outlet of the primary settling tanks. The level of the sludge blanket in the tanks is measured manually. The nitrate concentration is followed to evaluate the online denitrification and the overall nitrogen removal. Primary settled sludge is fed to the biological stage in case of unsatisfied nitrate removal to improve the COD/Total-N ratio. The full scale experiment started in late May 2013 and proceeds until November The study regarding optimization of digesters includes experience obtained from full scale operation, laboratory tests and scientific articles documenting tests in laboratory and pilot scale. Data from 23 plants were collected to map the operation of anaerobic digesters in Denmark. Recent research [1] showed that the gas potential increased with increasing grease content in mesophilic anaerobic digestion of sewage sludge and lipids. Co-digestion of a pork by-product was investigated. According to [2] a substrate mixture with 5 % pork by-product and pig manure digested at 37 C showed an increased methane production. Co-digestion of municipal sludge with fat-oil-grease (FOG) increased the destruction of sludge COD and VS as compared to digestion of municipal sludge alone. Degradation was enhanced by approximately 11 % based on COD and VS data. The enhanced methane production decreased when the waste blend FOG:sludge-mix COD ratio was above 20:80 %, which corresponded to a total waste loading of 5.8 g COD/l [3]. Page 2

3 Following previous research the effect on biodegradation of lipids (animal fat) in co-digestion with sewage sludge with increasing temperature in the mesophilic range is investigated. Potential biogas yield for co-digestion of municipal waste sludge and lipids is documented by laboratory scale tests at three different temperatures in the mesophilic area (35 C, 39 C and 43 C) to investigate the availability of the lipids as substrate. The laboratory scale tests have been carried out by Bioprocess Control in Sweden in December 2012 and January Tests were run during 40 days under the same conditions, with the same inoculum substrate ratio based on VS. The optimum temperature range for mesophilic anaerobic digestion is between 39 C to 40 C when taking gas production and heat balances into account according to [4]. According to [5] the methane production rate increased between 28 C and 49 C following Arrhenius law. The level of VFA accumulation was also found to increase with temperature and propionate was the most abundant acid present. In the aim to reduce the overall greenhouse gas emission from wastewater treatment it is necessary to investigate the amount of methane degassed from digested sludge. The undegraded organic matter will during the following sludge treatment be converted to biogas with a methane content of approximately % [6]. Through laboratory scale tests the potential of degassing will be investigated using three different types of anaerobic digestion- mesophilic digested sludge, termophilic digested sludge and sludge from a combined treatment with termophilic and mesophilic digestion as an inoculum. The tests were carried out by master students at Aarhus University at 10 C and 20 C. Figure 1: 1: sample, 2: sample outlet, 3: valve between sample and GC-glass, 4: GC-glass, 5: valve between GC-glass and column, 6: inlet column, 7: outlet column, 8: column with acetic water (ph 2), 9: valve between column and pump, 10: pump, 11: outlet, 12: container with acetic water (ph 2). Results and discussion Carbon and energy mass balances are established using data from the municipalities involved in the development project creating a baseline and starting point for optimization. The knowledge and outcome from the different activities will result in recommendations for a general and more intelligent management strategy at WWTP. Literature studies, laboratory tests e.g. after gas potential, trials by adjusting and optimizing full scale pre-treatment processes as well as operation of digesters all contribute to a better understanding of the energy potential of wastewater as a resource. Page 3

4 Optimization of digesters Optimized operation of anaerobic digesters gives a better utilization of the carbon source. Furthermore, by reducing the greenhouse gas emissions the environmental impact is decreased. There is a large potential for improvement of anaerobic digestion of primary and secondary sludge from wastewater treatment plants. Status Danish digesters The operation parameters of 23 Danish CSTR digesters is shown on Figure 2 regarding temperature, retention time (HRT) and solid content in the anaerobic digester. Figure 2: Temperature, HRT and dry matter content shown for 23 different Danish digesters. In general the temperature is about two degrees below optimum temperature C according to [4] for mesophilic digestion and the dry matter content in the digesters could most likely without any problems be about 4-5 % DS to minimize the energy input for heating and to extend the HRT and thereby enhance the gas production. To reduce the energy input for heating a sludge/sludge heat exchanger has been placed at a few plants to increase the temperature of the digester. Digestion of animal fat The methane yield (Figure 3a) is directly related to digestion of fat compensating for the methane yield from the inoculum. The gas production each day during the test period is shown in Figure 3b. Page 4

5 Figure 3a: Methane yield. Figure 3b: Flow rate. Long Chain Fatty Acids (LCFA) has often caused problems in anaerobic digestions receiving large amounts of grease. The decomposition of LCFA could be the rate limited step of complex substrates which will demand a careful feed of grease to the digester to avoid LCFA accumulation and inhibition. LCFA can inhibit from 500 mg/l according to [2]. At 43 C the mesophilic digestion seems unstable, hypothetically due to inhibitory concentrations of accumulated LCFA caused by a too short adaption time at 43 C. This was not detected at 35 C and 39 C. The results support the statement with the optimum temperature range for mesophilic anaerobic digestion between 39 C to 40 C regarding gas production. However a longer adaption time could support the statement proposed in [5], that the methane production rate is following Arrhenius law between 28 C and 49 C. Results showed a large gas potential in co-digesting lipids in a mesophilic driven digester, but further investigations of how to manage the feed pattern and characteristics of the digested sludge should be made. However there is an opportunity to maximize reactor utilization and profits by a mixture of substrates and increase the temperature of the digester. The emission of methane (degassing) from the digested sludge has to be included in the carbon footprint calculations to achieve an accurate assumption of the environmental impact. Henrys Law describing gas liquid equilibrium is used to calculate the methane emission from pumping digested sludge to a storage tank or homogenization tank and later dewatering. KH (Henrys constant) is substance related and temperature dependent. Dissolved in clean water at 25 C K H (CH 4 ) is 714 [L atm/mol], [7], [8] and [9]. At any other temperatures K H is calculated as: K H (T) = K H (T θ -C (1/T 1/Tθ) ) e C Temperature dependent constant ( K), [8], [9] and [10] The calculated methane emission for the three WWTP were between 0.1 % % of the total methane production and therefore could be ignored. The gas potential in already digested sludge was investigated at Stegholt WWTP (termophilic digestion), see Figure 4 and Herning WWTP (Temperature phased anaerobic digestion, TPAD with a termophilic and mesophilic step), see Figure 5. Page 5

6 Figure 4a: Development in methane production. Figure 4b: Development in methane content.. Figure 5a: Development in methane production. Figure 5b: Development in methane content. Results from Thisted WWTP (mesophilic digestion) are not shown because the tests where carried out with primary and secondary sludge with a very short retention time in the digester due to a hydraulic shortcut. The methane production at 10 C gives the most realistic view of the methane emission over the year, because this temperature represents the average temperature in Denmark (temperate climate). At Stegholt WWTP the digested sludge is stored in a tank for 3 days before final dewatering and disposal. The calculation of the methane emission at 10 C is between m 3 CH 4 /year which is less than 0.5 of the total methane production at the plant. The sludge retention time in the digester at Stegholt WWTP is between days. At Herning WWTP there is no methane production at 10 C and therefore no methane emission from storage tanks before sludge dewatering and disposal. The total sludge retention time in the two digesters at Herning WWTP is approximately 30 days. The total methane emission from pumping and storage of digested sludge is between 0.1 % % of the total methane production and therefore could be ignored. In both cases the methane content is only rising in the test bottles incubated at 20 C. This is probably due to a new methanogen population of microorganisms adapted the new temperature area. The late methane production from Herning WWTP could be explained by a better digestion due to a longer total SRT which results in a higher slowly degradable COD fraction as an output. The TPAD solution also has the hydraulic advantages of a seriel CSTR compared to a single CSTR. Page 6

7 Acknowledgements The author would like to thank the participating wastewater treatment plans for providing knowledge and data, and a special thanks to; Inger Christensen at Thisted Spildevand A/S, Lilian Vangsgaard at Stegholt wastewater treatment plant, Jan Ravn at Herning wastewater treatment plant. Thanks to Peder Maribo and master students from Aarhus University School of Engineering for planning and performing degassing tests and finally thanks to Niels Østergaard from Westcome Renewable for setting up energy balances and calculations. This study was partially funded by the Danish Environmental Protection Agency. References [1] S. Luostarinen, S. Luste, M. Sillanpää (2009) Increased biogas production at wastewater treatment plants through co-digestion of sewage sludge with grease trap sludge from a meat processing plant. Finland, University of Kuopio. Bioresource Technology 100 p [2] A. Hejnfelt, I. Angelidaki. (2009) Anaerobic digestion of slaughterhouse by-products. Denmark, Department of Environmental Engineering, Technical University of Denmark. Biomass and bioenergy 33 p [3] Tandukar M. et al. (2013) Anaerobic co-digestion of municipal sludge with FOG enhances the destruction of sludge solids. Proceedings of 13 th World Congress on Anaerobic Digestion International Water Association. School of Civil & Environmental Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, USA. [4] Ogbonna E. et al. (2013) Answering the Case for Optimum Mesophilic Reaction Temperature. Poster from proceedings of 13 th World Congress on Anaerobic Digestion International Water Association. University of Hertfordshire, College Lane, Hatfield, Hertfordshire, UK. [5] Buffiére P. et al. (2013) Kinetics of primary sludge digestion at various temperatures. Proceedings of 13 th World Congress on Anaerobic Digestion International Water Association. SUEZ Environment, CIRSEE, France. [6] A. Alonso-Vicario, José R. Ochoa-Gómes et al. (2010) Purification and upgrading of biogas by pressure swing adsorption on synthetic and natural zeolites. Spain, Department of Industrial Technology. Microporous and Mesoporous Materials 134 p [7] SANDER, R., Compilation of Henry s Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry. Version 3 ed. Germany: Air Chemistry Department Max-Planck Institute of Chemistry. [8] Lide and Frederikse, CRC Handbook of Chemistry and Physics, 76th Edition, D. R. Lide and H. P. R. Frederikse, ed(s)., CRC Press, Inc., Boca Raton, FL, [9] Wilhelm, Battino, et al., Wilhelm, E.; Battino, R.; Wilcock, R.J., Low-pressure solubility of gases in liquid water, Chem. Rev., 1977, 77, [10] Dean, Lange's Handbook of Chemistry, McGraw-Hill, Inc., Page 7