How the concept of co-digestion can be applied at a university campus canteen

Carvalho, L. and Duarte, E., Technical University of Lisbon, Portugal



The present work pretends to improve the waste management policy at the ISA campus canteen. The main idea was to generate biogas by co-digesting both the produced canteen waste streams (solid and liquid) with WWTP sludge, and use it to feed the kitchen’s stove. An experimental assay was carried out to evaluate the process of co-digestion and biogas yield using at first step only the generated liquid stream with WWTP sludge, in a CSTR working in mesophilic conditions (35±2°C). Liquid stream is mainly composed by soup surplus, which has to be disclaimed directly into the sewer due to food safety compliances. The experimental results obtained showed that between digesting the soup as a single substrate and when WWTP sludge was added as co-substrate, an increment of the methane yield from 275 to 288 L CH4/kg CODFEED was achieved. Moreover, co-digestion showed to promote a much more stable process.

Key words: 

Anaerobic co-digestion, biogas, vegetable soup waste, WWTP sludge.


At the University campus canteen the food waste can be separated in two different streams, one composed by the solid fraction and other by the liquid fraction, which is mainly represented by the soup left-over. The solid fraction is being daily collected by an external company that send it to a composting facility, on the other hand the liquid stream is dumped into the sewer, since it cannot be taken by the canteen employees or others due to food safety compliances. The company collects only the solid fraction because their composting facility is not prepared to receive liquid substrates.

So, to avoid the environmental constrains related to the soup disposal and to promote a better waste management policy at the university campus canteen, the idea was to convert this wasted liquid stream into a sub-product, to be used as substrate for anaerobic digestion and consequently production of biogas ─ a source of renewable energy. This technical solution will solve the environmental problems associated with the soup disposal and produce a renewable fuel that can be used to replace the utilisation of fossil gas on the canteen stoves, allowing not only to reduce the canteen’s carbon footprint but also the costs associated with gas purchasing.

The soup is always made from vegetables, which are boiled in water with salt and olive oil. Its confection can be considered as a thermal pre-treatment, since the boiled vegetables will became easily degradable for the microorganisms and part of its organic matter is solubilised during that process, making it a potential interesting substrate to digest anaerobically.

Some substrates used alone in anaerobic digestion (AD) do not allow reaching valuable biogas production yields, because they do not provide all the required nutrients for a vigorous development of the methanogenic microbial consortium. To fulfil nutritional deficits other substrates (co-substrates) can be added to the medium, so that the required microorganisms can grow and enhance the AD process. Co-digestion has been defined as the anaerobic treatment of a mixture of at least two different substrates with the aim of improving the efficiency of the anaerobic digestion process (Álvarez et al., 2010). The benefits of co-digesting distinct substrates also allows balancing the C:N ratio, pH, buffer capacity, nutrients, moisture content, dilution of inhibitory compounds, better biogas yield and increased digestion rate. (Agdag and Sponza, 2007; Angelidaki & Ellegaard, 2003; Bouallagui et al., 2009; Callaghan et al., 1999; Mata-Alvarez et al., 2000). However, it is not clear whether some by-products might have adverse effects when added to a stable digester or used in conjunction with other types of residues (Ferreira et al., 2007; Fountoulakis et al., 2008). Co-digestion also presents economic advantages, such as minimizing equipment needs by sharing the same equipment for different residues and easier handling of mixed waste (Mata-Alvarez et al., 2000).

The aim of this study was to evaluate the potential use of vegetable soup (VGS) surplus as a substrate for biogas production and to examine various conditions in order to reach the optimal performance of the anaerobic system. Co-digestion of the VGS with wastewater treatment sludge (WWS) was evaluated in a semi-continuously fed stirred tank reactor (CSTR) at 35±2°C. Experimental results from mesophilic co-digestion with different operational variables were compared in regard to organic matter removal efficiency, biogas quality and specific yield.

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