Impact of industrial sludge and cattle manure on anaerobic digestion of the OFMSW under mesophilic conditions
Introduction
During the last 20 years, developments in wastewater treatment strategies, such as upgrading of the existing treatment plants and the implementation of new and more effective treatment processes, lead to a continuous increase on sludge production. At the same time, the awareness on the problems associated with sludge handling and the risks to the environment and to human health had been increasing. Besides sludge, human activity produces other organic solid wastes that can be regarded as resources. This comprises manures and slurries from farm animals, agricultural wastes, non-hazardous industrial wastes and the organic fraction of municipal solid wastes (OFMSW). For many years, landfilling was the most common disposal given to the organic solid wastes. However, the high costs associated with waste disposal, the reduction of the lifetime of landfills, the negative impact on the environment and the European legislation made it necessary to put forward new scenarios for the management of these types of wastes.
The use of an anaerobic digestion process for the treatment of organic solid wastes has gained importance and has increased in recent years due to the new European legislation that regulates both the disposal in landfill of this type of wastes [1] and the production of electricity from sources of renewable energy [2]. The benefits of the co-digestion of two or more organic residues are well described in the literature [3], [4], [5], [6]. The quality of organic wastes to be treated by anaerobic digestion is crucial for the technical feasibility of the process and for the use of the treated waste as a fertilizer on agricultural soil. In addition, the biogas yield will also be much dependent on waste composition. Due to the very large biodegradable organic content of OFMSW, the major limitation of the anaerobic digestion of this waste is the rapid acidification, which is responsible for the pH decrease inside the reactor and for large volatile fatty acids (VFAs) production, which stress and inhibit the activity of methanogenic bacteria [7]. One way to control this problem is anaerobic co-digestion with other organic solid wastes with a higher buffer capacity. In Denmark, the anaerobic digestion of OFMSW is generally applied as co-digestion with other types of organic wastes such as manures, sewage sludge and industrial organic waste [8].
One of the most important parameters used to choose the technology and equipment to be used, especially in what concerns mixing and pumping needs, is the percentage of total solids (TS) inside the reactor. Dry processes (above 15% TS) have less sensitivity to the input of untreatable material to the reactor, because there is no segregation of particles inside the reactor as in wet systems. On the other hand, dry processes are not able to achieve removals as high as the ones for wet processes, something that will be reflected on the end product quality [9], [10], [11], [12], [13], [14].
Anaerobic digestion is a proven and established technology for the treatment of organic solid wastes. However, studies are still needed to improve knowledge concerning the effects of changing the input to a digester and how the waste composition influences the overall stability of the process. In addition, although many studies have been performed on anaerobic co-digestion using different organic wastes [5], [6], [7], the application of industrial sludge (IS) as a co-substrate is limited. Therefore, the purpose of this research was to evaluate the effects of co-digestion municipal solid waste with both IS and cattle manure (CM) on TVS reduction, VFA accumulation, pH variation, methane production and process stability at mesophilic conditions and 5% TS reactor content. Furthermore, another aim of this work was to obtain response surfaces that could predict the behaviour of anaerobic co-digestion of these residues in order to allow the establishment of management strategies for handling these organic wastes in future.
Section snippets
Operating procedure
Batch biodegradability assays were carried out with 14 different proportions of the three solid wastes (Table 1) at mesophilic conditions (35 °C) and for a period of 65 days. A description of the experimental set-up is found in Capela [15]. The TS concentration in each reactor was 5% (wet digestion). Nutrient solutions consisting of macro- and micronutrients according to Flor [16] were added to all assays to supplement the lack of any chemical constituent that was not incorporated in the
Results and discussion
The co-digestion of organic solid wastes involves the mixing of various substrates in varying proportions. If all other factors are held constant, the TVS removal and the specific methane yield (SMY) are only functions of the different proportions of wastes used. Hence, the mixture design technique developed by Cornell [19] can be applied in this case to obtain response surfaces that predict the behaviour of anaerobic co-digestion of different mixtures of three wastes under certain conditions
Conclusions
This study demonstrates the effects of the addition of a co-substrate, cattle manure and/or industrial sludge (IS) on the stability and performance of the anaerobic digestion of OFMSW, mainly during the start-up period.
The OFMSW is the waste with higher biodegradability, so the increase in the amount of OFMSW in the mixture results both in higher SMY and TVS removals. Besides the OFMSW, the IS is the organic waste that has the major contribution to both the SMY and the TVS removal, probably due
Acknowledgements
The authors highly acknowledge the financial support rendered by Portuguese CCRC—Centro, Program PRAI-Centro 2002/2003.
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