Valorisation of used cooking oil sludge by codigestion with swine manure

doi:10.1016/j.wasman.2014.02.006

Waste Management

Volume 34, Issue 8, August 2014, Pages 1537-1545

https://doi.org/10.1016/j.wasman.2014.02.006 Get rights and content

Highlights

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Anaerobic codigestion of UCO sludge and swine manure was successful at 50 d HRT.
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VFA build-up was present during the reactor start-up but were reduced after 50 d.
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CH₄ yield was 326 l/kg VS_feed, decreasing HRT to 30 d resulted in poor performance.
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Digestate at 50 d HRT was unstable although the load applied to the reactor was low.

Abstract

The addition of lipid wastes to the digestion of swine manure was studied as a means of increasing biogas production. Lipid waste was obtained from a biodiesel plant where used cooking oil is the feedstock. Digestion of this co-substrate was proposed as a way of valorising residual streams from the process of biodiesel production and to integrate the digestion process into the biorefinery concept.

Batch digestion tests were performed at different co-digesting proportions obtaining as a result an increase in biogas production with the increase in the amount of co-substrate added to the mixture. Semi-continuous digestion was studied at a 7% (w/w) mass fraction of total solids. Co-digestion was successful at a hydraulic retention time (HRT) of 50 d but a decrease to 30 d resulted in a decrease in specific gas production and accumulation of volatile and long chain fatty acids. The CH₄ yield obtained was 326 ± 46 l/kg VS_feed at an HRT of 50 d, while this value was reduced to 274 ± 43 l/kg VS_feed when evaluated at an HRT of 30 d. However these values were higher than the one obtained under batch conditions (266 ± 40 l/kg VS_feed), thus indicating the need of acclimation to the co-substrate. Despite of operating at low organic loading rate (OLR), measurements from respirometry assays of digestate samples (at an HRT of 50 d) suggested that the effluent could not be directly applied to the soil as fertiliser and might have a negative effect over soil or crops.

Introduction

Animal manures and slurries are the largest source of organic waste produced in the UE-27 with more than 1.5 billion tons produced per annum (Holm-Nielsen et al., 2009). In the case of Spain, 46 Mt of swine manure (SM) are produced every year in intensive pig farming facilities. The conventional storage and application of this manure as fertiliser generates a significant environmental concern, due to the large greenhouse gas (GHG) emissions; about 10 Mt of CO_2eq per year; and the pollution of water and groundwater due to nutrient leaching (Fierro et al., 2014).

Anaerobic digestion (AD) is a well-known process widely used to transform organic matter into biogas. This process allows the stabilisation of residues and reduces the amount of solids requiring final disposal (Álvarez et al., 2010). The valorisation of SM through AD would lead to obtaining a great source of renewable energy and recycling of nutrients, reducing the environmental impact of manure management. However, SM digestion is still problematic. The low solid content, and the low biogas yield (10–20 m³ CH₄ per ton) makes its digestion economically unfeasible (Angelidaki et al., 2011, Hartmann and Ahring, 2005). Furthermore, the low carbon to nitrogen (C:N) ratio leads to toxicity problems in digesters. Codigestion has been suggested by different authors as a means of avoiding toxicity associated with high levels of ammonium (Cuetos et al., 2011, Murto et al., 2004, Panichnumsin et al., 2010). It is widely known that codigestion allows the dilution of potential toxic compounds, increases methane production, improves balance of nutrients, and attains synergistic effects between microorganisms (Sosnowski et al., 2003). In this same line, lipid rich wastes are suitable co-substrates which are able to increase biogas yields thanks to their higher biogas potential (1.4 m³ biogas per ton of waste), therefore becoming an important factor for improving the economy of plants (Palatsi et al., 2009).

Lipid rich wastes from different sources have been successfully used as co-substrates for improving biogas yields in digesters of waste water treatment plants (WWTP) (Girault et al., 2012, Martínez et al., 2011) and also in the codigestion of municipal solid wastes and manures (Cuetos et al., 2008, Martín-González et al., 2010, Ferreira et al., 2012, Regueiro et al., 2012), obtaining significant increment in biogas yields. However, the codigestion with this type of co-substrate is not free of operating problems which are usually associated with foaming, clogging, and biomass flotation inside the reactor. In addition, inhibition due to the accumulation of long chain fatty acids (LCFAs) is also a common problem. LCFAs have been reported as inhibitory at low concentrations (Alves et al., 2001, Pereira et al., 2005) particularly affecting methanogenic populations. The adsorption of LCFA on microbial surface has been suggested as the mechanism of inhibition affecting the transport of nutrients to the cell. Nevertheless, this inhibition has proven to be reversible and microorganisms can be adapted by gradual exposure which in turn increases tolerance levels towards LCFA (Alves et al., 2001, Kim et al., 2004, Pereira et al., 2005).

A lipid rich waste which is increasing its production is the sludge obtained from the up-grading process of used cooking oil (UCO). This oil is widely used as a cheap feedstock for biodiesel production in an attempt to reduce production costs. UCO is far less expensive than refined vegetable oils and therefore has become a promising alternative. In addition, the valorisation of this oil prevents environmental contamination if no proper disposal method is implemented (Lam et al., 2010). UCO consists of a mixture of different vegetable oils with a variable composition depending upon the source. Oleic, linoleic and stearic are usually the main fatty acids present (Bautista et al., 2009, Thompson and He, 2006). UCO was traditionally used as a supplement for cattle feeding, but this usage was banned in the EU due to harmful components produced during the repeatedly heating of vegetable oils, mostly oxidation derivatives from polyunsaturated acids (Bautista et al., 2009, Kulkarni and Dalai, 2006). The use of UCO in biodiesel plants involves an initial pre-treatment where solids are eliminated. The pre-treatment generates a solid fraction composed mainly by cooking dregs with high oil content. This waste-oily sludge (WOS) must be properly disposed or used in a way that is not harmful to the environment. WOS obtained from biodiesel plants presents high content in biodegradable organics which makes it a suitable co-substrate for AD.

The aim of this work was the assessment of anaerobic codigestion of swine manure with WOS. Codigestion was studied under batch and semi-continuous operation. Conditions for achieving a successful digestion process were determined. This research opens new opportunities for valorising WOS and incorporates the digestion process into the biorefinery concept. Digestates were evaluated in order to determine their stability by using respirometry assays and phytotoxicity tests with the aim of exploring their use as soil amendments.

Section snippets

Inoculum and substrate

The inoculum was obtained from the anaerobic digester of the WWTP of León and it was composed by a mixed anaerobic microflora. The concentration of total and volatile solids (TS and VS) was 19.5 ± 0.2 and 12.7 ± 0.2 g/l respectively. WOS was obtained from Biocyl biodiesel plant. This plant is located at San Cristobal de Entreviñas (Zamora, Spain). Swine manure (SM) was obtained from a farm located nearby the plant. The manure was screened using a 2 mm sieve to eliminate particles that could obstruct

Batch tests

The characteristics of the two substrates used are given in Table 1. As expected, SM is characterised by a low C:N ratio, consequence of a high nitrogen and low OM content. In contrast, WOS showed a high OM content and much higher TS and VS. The high C:N ratio of this waste makes it a suitable substrate for codigestion with SM.

Fig. 1A shows the evolution of cumulative biogas production data obtained from batch tests for SM and WOS samples. Higher biogas production was observed for WOS sample,

Conclusions

Co-digestion batch tests revealed an increase in biogas production for any of SM–WOS mixtures tested when compared with the single digestion test of SM. Co-substrate addition resulted in an increase in biogas production about 1.5 to 2 fold thanks to the increase in specific biogas production associated to the co-substrate. In addition, a synergistic effect was observed under batch conditions for the different ratios tested. Results obtained from codigestion batch tests presented higher SGP

Acknowledgement

This work was financially supported by project LE091A11-2 of the Junta de Castilla y León

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Valorisation of used cooking oil sludge by codigestion with swine manure

Highlights

Abstract

Introduction

Section snippets

Inoculum and substrate

Batch tests

Conclusions

Acknowledgement

Bioresour. Technol.

Water Res.

Methods Enzymol.

Biomass Bioenergy

Process Biochem.

Bioresour. Technol.

Biochem. Eng. J.

Bioresour. Technol.

Biochem. Eng. J.

Bioresour. Technol.

Bioresour. Technol.

Appl. Energy

Bioresour. Technol.

Water Res.

Water Res.

Bioresour. Technol.

Bioresour. Technol.

Water Res.

Biotechnol. Adv.

Bioresour. Technol.

Waste Manage.