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Operation strategy of grease and municipal sewage sludge co-digestion: Results of continuous trials
To this day, biogas production on wastewater treatment plant (WWTP) and use has not been optimised, as in many cases it was operated as a means of stabilising sludge, and not recovering energy. Furthermore, many sectors of activity lead to the production of different types of organic wastes. For these reasons, co-digestion strategies are considered a good way to enhance anaerobic digestion performances and the production of biogas (Mata-Álvarez et al., 2000). Grease is a waste produced on WWTP and usually disposed of.
Further Authors:
R. Cano - University of Valladolid
A. Nielfa - University of Valladolid
Furthermore, this waste is also produced by many other industries (biofuel, restaurants...) and therefore it is an interesting market opportunity. The availability on site, its high production, its biodegradability potential and previous works that have shown interesting results using this waste for sludge co-digestion (Bouchy et al., 2012) are the main reasons to select grease as a promising co-substrate. Several assays in batch mode have been carried out with sewage sludge and grease as co-substrate, showing that grease waste is an adequate co-substrate for anaerobic digestion of sewage sludge. However trials in continuous operation are necessary in order to verify biogas yields, the impact on the WWTP and dewatering performance. The aim of this study is to evaluate the co-digestion of sewage sludge and grease from WWTP in a continuous reactor operation. The continuous experiments were carried out in two reactors of 20L of useful capacity and 10L of gas chamber. Both reactors were operated at mesophilic temperature range (35ºC), with a hydraulic retention time (HRT) of 20 days, and was conducted during 470 days. Biogas production was measured by a pulse electrical system and analyzed by gas chromatography. The stability of the operation was controlled carrying out periodical analysis of several parameters of the digestates such as solids content, total and soluble chemical oxygen demand (COD), volatile fatty acids, alkalinity, pH and ammonium. Initially the reactor start-up was carried out using WWTP digested sludge as inoculum. Then, the reactors were fed with primary and secondary thickened sludge, mixed in the same proportion in weight. Once the desired feed of sludge was reached, grease from the dissolved air flotation (DAF) unit of a WWTP was mixed with the mixed sludge to feed one of the reactors in a weight ratio of 15/85% respectively (33/66% COD ratio). This ratio was selected based on previous results carried out in batch mode (Bouchy et al., 2012). The operation of two reactors made possible the comparison between the effects of the co-digestion versus a digestion of mixed sludge alone. The start-up of both reactors was carried out during 4 months to achieve the desired HRT (20 days) and was finished when all the parameters were stabilized and the desired organic loading rate, 2kgVS/m3d, was reached. The average value for the methane potential of the mixed sludge was 220 NmLCH4/kgVSadded. In the behaviour of the methane productivity curves once the digester is fed with the mixture grease/sludge it can be observed that the methane production increases with the addition of the co-substrate (grease), obtaining an increase in biogas production of almost 100%, reaching a final methane yield of 400 NmLCH4/gVSadded. The better the acclimation of the biomass, the higher is the biogas production. Additional experiments were carried out in a second phase in order to study the limits of the process. With this objective, the Organic Loading Rate (OLR) was increased until reaching the operation limit of the digester. The HRT was highly decreased during these assays, reaching values lower than 10 days without significant operational problems. The process has shown a great robustness since it accepts high OLR (7 kgVS/m3d) without significant perturbation, even at HRT lower than 9 days.
| Copyright: | © European Compost Network ECN e.V. | |
| Quelle: | Orbit 2012 (Juni 2012) | |
| Seiten: | 8 | |
| Preis inkl. MwSt.: | € 8,00 | |
| Autor: | Ana Pérez  Lynne Bouchy Maria Fdz-Polanco Patricia Camacho | |
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