Modelling anaerobic digestion of microalgae grown in wastewater treatment systems using ADM1 Fabiana Passos*, Doris Brockmann**, Jean-Phillipe Steyer**, Ivet Ferrer* *GEMMA - Group of Environmental Engineering and Microbiology, Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de Catalunya·BarcelonaTech, c/ Jordi Girona 1-3, Building D1, E-08034, Barcelona, Spain. (E-mail:
[email protected],
[email protected]) **INRA, UR0050, Laboratoire de Biotecnologie de l’Environnement, Avenue des Etangs, Narbonne F11100, France (E-mail:
[email protected],
[email protected])
Abstract: This study aimed at evaluating the anaerobic digestion performance and biogas production from microalgal biomass grown in high rate algal ponds (HRAP) for wastewater treatment. Experimental results obtained in lab-scale reactors indicated that the methane yield increased with HRT, reaching 0.21 L CH4/g COD when anaerobic reactors were operated at 20 days HRT. Variations in anaerobic digestion performance were detected in accordance with harvested biomass characteristics, which showed a strong seasonality. The Anaerobic Digestion Model No. 1 (ADM1) was used to model anaerobic digestion of microalgae grown in HRAPs. After adjusting the hydrolysis rates and the fraction of particulate inerts in the influent, the model described well the performance of the lab-scale reactors. This was the first time that experimental data from wastewater treatment HRAP was used for model calibration and, therefore, this study may contribute to the understanding and optimization of anaerobic digestion performance. Keywords: ADM1; Algae; Biogas; High rate algal pond; Methane; Wastewater
INTRODUCTION In the last decades, microalgae have been in the focus of research regarding new environmentally-friendly energy sources. Although these photoautotrophic organisms have many advantages compared to oil crops for biofuel production, microalgae biomass production and optimization at full-scale is not yet feasible due to high costs of harvesting and downstream processing. On the contrary, wastewater treatment by symbiosis between microalgae and bacteria grown in high rate algal ponds (HRAP) has already been demonstrated [1]. In fact, this treatment technology could be preferred to activated sludge systems for small communities, since no aeration is needed in the biological step. As for activated sludge, microalgal biomass produced in HRAPs during the wastewater treatment process could also undergo anaerobic digestion to produce methane and electricity through cogeneration. So far, only a few studies on microalgae anaerobic digestion in continuous reactors have been conducted, most of them with pure cultures and/or with residual biomass after lipid extraction for biodiesel production. Nevertheless, experimental results indicate that the methane yield from microalgae is similar values to that of secondary sludge (around 0.25 L CH4/g VS at 28-30 days HRT) [1, 2]. Therefore, microalgae anaerobic digestion could be coupled to HRAP treating wastewater. Mathematical models are commonly used for predicting anaerobic digestion performance and methane production, reducing time and cost associated with experimental procedures. Modelling anaerobic digestion of organic substrates has been intensively developed, in particular for sewage sludge. The Anaerobic Digestion Model no.1 (ADM1) is a well-accepted biokinetic model used to describe the main processes taking place during anaerobic digestion [3]. A study on modelling microalgae anaerobic digestion indicated that ADM1 showed good agreement with
experimental data; however modelling of hydrolysis with Contois kinetics was crucial [4]. The aim of the current study was, firstly to study anaerobic digestion of microalgal biomass harvested from HRAP for wastewater treatment over a period of one year; and secondly to calibrate the ADM1 for microalgae anaerobic digestion using obtained experimental data. MATERIAL AND METHODS Microalgal biomass was grown in a pilot HRAP treating real wastewater from the municipal sewer of Barcelona (Spain). The experimental set-up consisted of a gravity settler (0.007 m3, 0.025 m2), a HRAP (0.5 m3, 1.5 m2) and a clarifier (0.01 m3, 0.025 m2). Harvested biomass was thickened in laboratory Imhoff cones before undergoing anaerobic digestion in a bench-scale reactor (1.5 L) under mesophilic conditions (35 ± 2 ºC). The reactor was operated on a continuous feeding basis and two hydraulic retention times (HRT) were studied: 15 days (July - October 2012) and 20 days (November 2012 – July 2013). Biogas production was measured by water displacement, and the methane content was analysed by gas chromatography twice a week. The ADM1 was used to model anaerobic digestion of microalgal biomass from the HRAP. The model was calibrated using experimental data measured during the described period. Simulations were carried out in MATLAB® using the ADM1 implementation of Rosen and Jeppsson [5]. In this study, we used the Contois model to describe microalgae hydrolysis as proposed by Mairet et al. [4]. To describe the variability in microalgal biomass characteristics over the year, two different approaches were used: i) variation of the fraction of particulate inerts (Xi); and ii) variation of the maximum specific hydrolysis rate of carbohydrates, proteins and lipids (khyd). The fraction of carbohydrates (20%), proteins (58%) and lipids (22%) was kept constant during the whole period, since experimental data showed no significant seasonal variation. The Contois half saturation constants of hydrolysis of carbohydrates, proteins and lipids were taken from a previous study on ADM1 modelling of Chlorella vulgaris, and are 0.50, 0.26 and 0.49 kg COD/m3, respectively [4]. All other parameters were maintained as in the original ADM1. RESULTS AND DISCUSSION Anaerobic digestion of microalgal biomass from HRAP showed a high variability during the studied period (0.08-0.21 L CH4/g COD). The methane yield more than doubled (from 0.08 to 0.15-0.21 L CH4/g COD) when the HRT was raised from 15 to 20 days. This was attributed to the slow hydrolysis of the microalgae complex cell wall structure. In a previous study, the methane yield of Chlorella vulgaris was improved from 0.11 to 0.18 L CH4/g COD by increasing the HRT from 16 to 28 days [2]. On the whole, anaerobic digestion of biomass produced in wastewater treatment HRAP seems to depend greatly on the microalgae species growing in these systems, i.e. on their cell wall structure and macromolecular composition. Therefore, the methane yield will be higher if the cell wall structure is less complex and contains less recalcitrant substances. Moreover, the potential methane yield is different for each macromolecular compound in microalgae, which again depends on the microalgae species, but also on the growing conditions. In fact, experimental results showed how the relative proportion of microalgae species and other microorganisms varied
weekly, which possibly influenced the anaerobic digestion performance and methane yield. The high variability of biomass composition in these treatment systems makes modelling anaerobic digestion more difficult. Calibration of the ADM1 was carried out with experimental data obtained over one year. In general, ADM1 demonstrated a good ability to describe microalgal biomass anaerobic digestion during the whole period (Figure 1). However, the hydrolysis rates of different macromolecular compounds, as well as the fraction of particular inerts were modified to better fit experimental data. These adjustments were based on observations of the microalgae community. For instance, the variation of hydrolysis rates was defined according to the abundance of diatoms, which are slowly degradable due to the silicate cell wall; and to the increase of predators, which could have promoted the formation of exopolymers as mechanism of defense.
Figure 1 Effluent COD and methane yield after mesophilic anaerobic digestion of microalgal biomass from HRAP: experimental data (black spots) and model output (blue line).
CONCLUSIONS Anaerobic digestion of microalgal biomass from wastewater treatment HRAP resulted in a methane yield around 0.08 L CH4/g COD for an HRT of 15 days and in 0.15-0.20 L CH4/g COD when the HRT was increased to 20 days. Variations in anaerobic digestion performance over the year were attributed to changes in the microalgae community. Therefore, adjustments on the hydrolysis rates and the fraction of particulate inerts in ADM1 were needed to improve the fit with experimental data. The calibrated model satisfactory simulated the anaerobic digestion performance, meaning that the model could be used for microalgae anaerobic digestion simulation and prediction. REFERENCES [1] C. G. Golueke, W. J. Oswald, H. B. Gotaas, "Anaerobic Digestion of Algae," Applied Microbiology, vol. 5, no. 1 pp. 47-55, 1957. [2] M. Ras, L. Lardon, B. Sialve, N. Bernet, J. P. Steyer, "Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris," Bioresource Technology, vol. 102, no. 1, pp. 200-206, 2011 [3] D. Batstone, J. Keller, R. I. Angelidaki, S. V. Kalyuzhnyi, S. G. Pavlostathis, A. Rossi, W. T. M. Sanders, H. Siegrist, V. A. Vavilin, “Anaerobic Digestion Model No. 1 (ADM1)," IWA Publishing, London, 2002. [4] F. Mairet, O. Bernard, M. Ras, L. Lardon, J. P. Steyer, “Modeling anaerobic digestion of microalgae using ADM1,” Bioresource Technology, vol. 102, pp. 6823–6829, 2011. [5] C. Rosén, U. Jeppsson, "Aspects on ADM1 Implementation within the BSM2 Framework," Department of Industrial Electrical Engineering and Automation, Lund University, Lund, Sweden, 2006.
