Enhanced anaerobic co-digestion of agricultural biomass and biological sludge

Aylin Alagöz, B., Erdinçler, A., Yenigün, O., Boğaziçi University, Institute of Environmental Sciences, Turkey



Turkey is an energy-import dependent country while has many available renewable energy sources like “biogas”. The biogas generation has traditionally been from feedstock such as livestock-waste. But, recently conducted studies have indicated that biomass especially the agricultural crops/residues and the sludges generated from the wastewater treatment plants also represent a large unexploited biogas energy potential through the anaerobic digestion. Residuals of agricultural products and the sludges are the waste materials to be treated before the final disposal to the environment. This study investigates the biogas production potential of selected agricultural biomass products (cloverleaf, wheat, nutshell, potato, bagasse, maize silage, grass, barley and manure) through the anaerobic co-digestion with the wastewater treatment plant sludges. The results of the study showed that the co-digestion of wastewater sludge with the agricultural products appeared to be more effective method than the single-substrate digestion in terms of biogas production. The cumulative biogas production was almost ten times higher in reactor having the mixture of wastewater sludge and agricultural biomass product than the reactor containing only wastewater sludge.

Key words: 

Anaerobic digestion; biomass; energy crop; methane; renewable energy; wastewater sludge


A rise in the global population and a consequent increase in demand for oil and gas have resulted in the depletion of the World’s supply of natural gas, which is known to be the cleanest of the fossil fuels. Therefore, as the impact of climate change becomes more apparent and the prospect of energy targets looms, the desire to find alternative and cost-effective renewable sources of sustainable energy that won’t pose any hazards in the long run has come to the fore. As a result, the recent investigations have been focused on for the development and the use of renewable energy sources such as biomass, wind, solar, hydropower, and geothermal that can contribute to securing long term sustainable energy supplies and help to reduce local and global atmospheric emissions (Herzog et al., 2001).

In the near future, several subjects related with the new alternative energy sources from the wastes have been studied. Among them, the municipal organic waste (Vieitez and Ghosh, 1999; Rodriguez-Iglesias et al., 1998; Moller et al., 2004), manure from livestock (Moller et al., 2004) and various industrial wastewaters (Kida et al., 1999) for methane fermentation have been extensively studied in the World. On the other hand, the economics of the production of biogas has recently becoming more favorable. The majority of work done on biogas has used animal wastes as the raw material (Pound et al., 1981) but, there exists a dearth of literature on biogas production from the agricultural products and their residuals. Whereas, biomass especially the agricultural products such as sugar cane, olive or winery residues, etc. and wastes having a considerably high organic content such as the sludge generated from the wastewater treatment plants have a large and unexploited biogas energy potential through the anaerobic digestion and should definitely be focused.

Turkey is one of the economically developing countries dependent on the import of energy requirements and almost more than half of its energy requirement has been supplied by imports (Demirbaş and Bakıs, 2004). However, it is also one of the agricultural countries that can obtain its energy requirement from its available agricultural sources. In Turkey, approximately 65-Mton agricultural wastes are produced annually from the vegetable-processing activities. On the other hand, there is also an increasing sludge generation in Turkey. The production of wastewater treatment plant sludge is estimated to increase up to 1-Mton dry solid per year in 2020.

When these abundant sources of potential agricultural crops and residues are evaluated, the most available and potential renewable energy source in Turkey is revealed as the “biogas”. Turkey has a significant biogas producing potential through biomass (e.g. agricultural products such as winery residuals and sugar production residuals) and the wastewater treatment plant sludge, but the biogas technology has not widely studied yet throughout the country.

On the other hand, besides these huge potential of agricultural products and agricultural wastes, there is an equally significant and inevitable fact in Turkey which is the “sludge problem” generated from the wastewater treatment plants. The stabilization and minimization of the wastewater treatment plant sludge is especially important in Turkey as one of the country suffering from the lack of efficient waste management applications. There are many sludge disposal techniques currently applied such as incineration, landfilling, composting, etc.; but the disposal of sludge with those conventional methods is difficult and expensive, often requiring over 50 per cent of the operating budget for the wastewater treatment plant (Vesilind et al., 2001, Liu and Tay, 2001).

Wastewater sludges can be stabilized using several mechanical, chemical, biological, and thermal treatment methods. However, biological stabilization processes -anaerobic and aerobic digestion- are the most commonly applied sludge stabilization methods among the others. Here, special emphasis has initially been given to the anaerobic digestion system that can generate biogas from the animal manure, agricultural crops and wastes while reducing emissions of the greenhouse gases (Lansing et al., 2007).

Anaerobic digestion is a microbially mediated biochemical degradation of complex organic material into simple organics and dissolved nutrients. The anaerobic digestion process has several advantages over other methods of waste treatment. Most significantly, it is able to accommodate relatively high rates of organic loading. It has also the advantages of producing renewable energy, yielding high quality fertilizer and preventing transmission of disease while stabilizing waste organic matter. As a result, latest progress has been made in cultivating energy crops and crops residual for the biogas production in reactor systems through the anaerobic digestion (Choi et al., 1997; Dohányos et al., 1997; Muller et al., 1998; Muller, 2000; Chu et al., 2001; Tiehm et al., 2001; Bougrier et al., 2005; Ojolo et al., 2007).

Therefore, the main objective of this study was to assess the viability of the anaerobic co-digestion of several agricultural plant materials and energy crops (cloverleaf, wheat, nutshell, potato, bagasse, maize silage, grass, barley and manure) with the municipal wastewater treatment plant sludges in order to investigate the maximum biogas production potential and obtain the maximum productivity of methane rich biogas.

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