Biogas from cattle slaughterhouse waste: Energy recovery towards an energy self-sufficient industry in Ireland

Highlights

Substantial methane yields achieved from individual slaughterhouse waste streams.

Digestion of combined waste streams viable with no decrease in methane yield.

Digestion of combined waste available annually allows for ample energy recovery.

Potential to subsidise 100% of the sampled slaughtering facility energy demands.

On a national scale, potentially increase RES by 0.33% in Ireland.

Abstract

This study was carried out to assess the energy recovery potential from organic industrial by-products of a cattle slaughtering facility. There are several processes to convert organic material to energy; the technology of interest in this study was anaerobic digestion, the biological conversion of degradable organic material into methane. The scenario was initially confined to a full scale cattle slaughtering facility processing 3.28% of heads slaughtered in Ireland. The methane potential of dissolved air flotation sludge, paunch, soft offal as well as a mixed waste stream (combination of individual waste streams) was determined through a series of biochemical methane potential assays under mesophilic conditions. The methane potential of the characterised waste streams ranged from 49.5 to 650.9 mLCH4 gVS−1. The potential energy recovery from the mixed waste stream resulted in the prospective subsidy of 100% of the energy demands of the slaughtering facility as well as the energy demands for the production of the biogas. When investigating the impact of energy recovery from the entire sector the potential energy recovery equated to 1.63% of the final energy demands of the Irish industrial sector. This could potentially increase the RES in Ireland from 7.8% to 8.13% contributing to both RES-E and RES-H.

Keywords

  • Agroindustrial organic waste;
  • Anaerobic digestion;
  • Biogas;
  • Combined heat and power;
  • Power generation

Nomenclature

ABPR

animal by-products regulation

AD

anaerobic digestion

BMP

biochemical methane potential

C

carbohydrates

CHP

combined heat and power

CSO

central statistics office

DAF

dissolved air flotation

GFC

gross final energy consumption

GVA

gross value added

H

hydrogen

HRT

hydraulic retention time

LCFA

long chain fatty acids

N

nitrogen

NaOH

sodium hydroxide

NREAP

national renewable energy action plan

PA

paunch

RES

renewable energy share

RES-E

renewable energy share-electricity

RES-H

renewable energy share-heat/thermal

SHWM

slaughterhouse waste mixed at annual production ratios

SMY

specific methane yield

SO

soft offal

SRM

specified risk material

TS

total solids

UNFCCC

United Nations framework convention on climate change

VS

volatile solids

1. Introduction

1.1. Beef industry in Ireland

Despite unprecedented growth of the Irish economy since the early 1990s, the agri-food sector remains one of Ireland’s largest industries as measured by wealth generation (7.7% of GVA), exports (11.5% of total merchandise exports) and employment (9.2% of total employment) [1] and [2]. A major facet of this sector is Irelands beef industry producing 516,900 tonnes of meat from the slaughtering of over 1.59 million heads of cattle in 2013 [3]. Ireland is the biggest net exporter of beef in the EU and the 5th largest in the world [4]. Export volumes stand at approximately 90% of annual production and contribute 22.3% (2.57% of total merchandise exports) of exports in the agri-food sector in Ireland. High amounts, as much as 45–53% of the live weight of the animal, of organic by-products which are considered to be industrial organic wastes are generated from this industry [5]. As regards the main organic wastes streams, there is blood of the bleeding process, paunch from the removal of the rumen and intestinal content, the intestinal residues from the evisceration processes, fat from the meat trim step as well as the head and the limbs (mostly bone). Moreover, sludge from the wastewater treatment plant of the slaughterhouse is generated. These wastes are characterised by high organic content mainly composed of animal proteins and fats [6], [7] and [8]. They are strictly managed by legislation, Animal By-Products Regulation (ABPR 1069/2009/EC), in order to prevent the outbreak and spread of diseases such as Bovine Spongiform Encephalopathy and the dangerous human disease Creutzfeld-Jacob [9].

1.2. Treatment of organic waste streams

There are a number of permissible disposal routes under the ABPR with the most common being; material sent for rendering (bones, inedible offal, blood, trimmings etc.) or land spreading (sludge’s, paunch, lairage washings etc.). The high organic content of the waste streams generated from the slaughterhouse make them an attractive feedstock for anaerobic digestion (AD) which is considered a suitable treatment method provided approved pre-treatments are applied if required under the ABPR, excluding SRM; material with the highest risk or carrying disease (heads, spinal cord, condemned meat etc.) which is only suitable for incineration or landfilling [7], [10], [11] and [12]. AD has long been considered to be one of the best alternatives for nutrient and energy recovery from organic solid wastes with high protein and fat content [13]. In AD the organic waste is converted to biogas, primarily methane, and a nutrient rich digestate through a series of biochemical processes. The methane produced can be utilised for energy production while the nutrient rich digestate can be employed as a soil conditioner [6]. This alternative treatment method is an effective option, combining material and energy recovery allowing the possibility of an energy self-sufficient industry while incorporating a holistic waste treatment system [6], [7], [14] and [15].

1.3. Focus of paper

The focus of this paper is to determine the methane potential of the available organic waste streams, in order to identify the potential energy that could be recovered through the exploitation of AD as an alternative waste treatment within the confines of a full-scale cattle slaughterhouse. The potential energy recovery is assessed in terms of subsidising the process energy of the slaughtering facility and evaluating the degree of energy self-sufficiency that could be achieved. Advancing from the boundaries of a single slaughtering facility, the cattle slaughtering sector in Ireland is also appraised. The contribution of the potential renewable energy generation from the entire sector is assessed in terms of progress towards meeting Irelands 2020 renewable energy targets, RES of 16%, mandated under the Renewable Energy Directive (2009/28/EC) [16].

2. Materials and methods

2.1. Determination of potential methane yield

2.1.1. Slaughterhouse wastes

The sampled slaughterhouse was located in Cork, Ireland and processed approximately 52,000 heads of cattle annually (3.28% of total annual slaughterings in 2013). The slaughtering waste streams considered for this study were paunch (PA), soft offal (SO) (intestinal residues, fat and meat trimmings and some blood) as well as dissolved air flotation sludge (DAF) from the wastewater treatment facility onsite. SRM was not included as per the ABPR regulations while the limbs were not included due to their low biodegradability (primarily made up of bone). As well as treating the three selected waste streams on an individual basis they were mixed together according to their annual production ratios (1:2.55:3.22-PA:DAF:SO), referred to as SHWM from this point on, in order to investigate the implications of treating the three waste streams collectively. Pasteurisation (70 °C for min 1 h) was applied to the SO prior to testing in all cases as per the ABPR for the treatment of category 3 material. The consistency of the wastes in their sampled state did not permit their direct use in accurate BMP assays or composition analysis and thus all samples were mixed and blended thoroughly in order to reduce particle size (<8 mm) and create representative specimens with a uniform particle size. It is important to note that even after the preparation process the offals are still characterised as heterogeneous this reality enforces the need for triplicate testing [6].

2.1.2. Analytical systems

The composition analysis was carried out in terms of basic, organic and elemental characterisation. The basic parameters used for substrate and inoculum description were the Total Solids (TS) and Volatile Solids (VS) content determined in accordance to Method 1684 of the U.S. EPA for Total, Fixed and Volatile Solids in Water, Solids and Biosolids [17]. The organics (VS) within the substrates were further broken down into primary constituents of fats, proteins and carbohydrates. Fats and proteins were determined by an approved laboratory for the microbiological testing of animal by-products in accordance with Commission Regulation 142/2011/EU implementing the ABPR [9] and [18]. The difference between VS, fats and protein content was designated as carbohydrates. The elemental composition (C, H, N) was determined following the standard operating procedure of a CE440 Elemental Analyser, with O being designated as the difference between VS and the C, H and N content.

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