Sounding Out: Treatment for Slaughterhouses

Combining Anaerobic Digestion with Ultrasound Despite its potential, Anaerobic Digestion for slaughterhouse waste currently offers lower conversion rates of only 37%. A European Commission funded ADAW (Anaerobic Digestion for Animal Waste) project aims to produce biogas with high yield levels, with material post-treated by ultrasound. By Neil Marshall Livestock production makes up approximately one quarter of the total value of agricultural production within the EU 27 member states. The industry is characterised by a complex network of farmers, farmer cooperatives, slaughterhouses, processing, rendering plants (converters of waste animal tissue into stable, value-added materials), and retailers involved in distribution and marketing. Every year nearly 360 million pigs, sheep, goats and cattle as well as several billion poultry are killed in EU slaughterhouses. The European fur industry adds another 25 million animals to the figure. Hatcheries kill around 330 million day-old-chicks. As a result, the slaughtering industry in Europe produces large amounts of different wastes and wastewaters, with about 25 million tons each year. This industry is currently suffering problems due to due two major factors. First up is legislation. Since 2002, and due to the EU Animal By-products Regulation (Regulation (EC) No 1774/2002 repealed by 1069/2009) land disposal of all animal wastes is prohibited, with the exception of manure and digestive tract contents. By banning the land disposal of slaughterhouse waste, the European Regulation has led to a drastic reduction in management options, and an increase in total slaughtering industry costs. Secondly are the difficulties in treating animal wastes. European slaughterhouse operators currently have only three wastewater treatment options: incineration, composting or anaerobic digestion. However, they have to invest in costly waste management facilities that cannot treat high lipid and protein waste such as animal wastewater coming from the slaughterhouses. If, however, they choose to use external treatment options, the gate fee for the slaughterhouse waste to be treated by commercial biogas plants varies between €11/ton to €35/ton. If we extrapolate this data, European slaughterhouse waste would cost between €275 million to €875 million per year. Environmental and Health Concerns Like other food processing industry sectors, the slaughtering industry faces environmental issues. For example - water and soil pollution. Lipid-rich-wastes are characterised by their hydrophobicity, containing functional groups that are not degradable for microorganism degradation pathways. This results in a decreased concentration of the contaminants in the water-phase, which is the most accessible form for organisms to degrade the material. This reduces the degradation rate and increases the resident time and therefore contaminant concentrations in the soil. Furthermore, slaughterhouse wastewater has a complex composition and contains a large amount of organic compounds. After the initial screening of coarse solids, slaughterhouse wastewater is mainly composed of diluted blood, fat and suspended solids. Wastewater from slaughter houses is heavy in pollution and, therefore, it should not be allowed to mix with the municipal drain system without pre-treatment. If this waste reaches water sources it could create eutrophication problems. Finally, there is a high energy consumption associated with slaughterhouse SMEs. The sector needs strategies to reduce the energy consumption. Some operators have introduced new equipment and operation processes but energy is still a challenge for slaughterhouses. Current technologies to treat slaughtering animal by-products present various environmental issues that must be considered. While incinerating animal wastes may be an efficient means for generating energy and getting rid of excess nutrients, it releases CO2 to the atmosphere and cannot be considered a sustainable practice in the context of large scale nutrient recycling, nor for maintaining long-term soil quality. Incineration and anaerobic solutions can offer a solution to both problems, the treatment of the organic compounds and an alternative to recover energy and to reuse it in the own slaughterhouse facilities. Composting allows for a treatment of the organic waste but the process energy balance is negative. Legislation The slaughtering industry has limitations on the options of waste disposal and use depending on their nature. Slaughterhouse wastes are considered as Animal by-products (ABP) which are regulated by the EU Directive 1069/2009, categorising slaughterhouse waste into three categories depending on the level of risk to public and animal health. Incineration is the unique option that is common to the three categories of animal by-products. Moreover, Category I and II can be composted or transformed into biogas. Slaughtering industry SMEs need to find a definitive solution for the residues in those categories where valuable transformation is possible (Categories II and III). Among all potential alternatives, they will choose the most cost-efficient management solution for their activity waste. None of current solutions can offer a suitable return of investment for Small and Medium sized Enterprises. Anaerobic Digestion Anaerobic Digestion of animal by-products constitutes a possible method of treating the by-products and at the same time produces energy in the form of methane (biogas), and utilisation of the digestion effluents as fertiliser for application on agricultural fields. However, slaughterhouse wastes are generally regarded as difficult substrates for Anaerobic Digestion, mainly because of their typically high protein and lipid content. Lipid-rich wastes have a higher biogas potential than other type of wastes. Lipids as a substrate for AD processes show a higher theoretical methane yield (0.99 LCH4/g) compared to carbohydrates (0.42 LCH4/g) (10). Table 1 shows the biogas generation potential of diverse substrates, compiled from different sources: Anaerobic Digestion uses commonly available agricultural wastes as substrate, which can offer a conversion rate between 40-45%. The vegetable substrate with the higher bio-gasification capacity is corn/grass silage. However, energy crops consumed an area that exceeded two million hectares in 2007. This high demand for agricultural areas generates new competition within the food industries. Technological Obstacles for Animal Waste Treatment by AD There are four technological obstacles that hinder the use of AD in animal wastes from slaughterhouses: LCFAs accumulation: Lipids create problems in Anaerobic Digestion because of the possible accumulation of inhibiting degradation intermediates such as long-chain fatty acids (LCFAs). Even at very low concentrations such as 0.5 kgm-3, LCFAs, especially unsaturated LCFAs, are suggested to be inhibitory to acetogenic and methanogenic bacteria, responsible for fat degradation and biogas formation respectively. Consequently, avoidance of LCFA formation is the main challenge for animal waste anaerobic digestion. Current solutions consist of physical separation of fat or a physic-chemical process in which the LCFAs are transformed in less harmful compounds. None of these methods can lower LCFA concentration to the minimum required. Particle agglomeration: Proteins and lipids are trapped into agglomerated particles and they become insoluble substrates, and therefore proteins cannot be adsorbed by the microbial organisms for their degradation. Lipids also cause operational problems in anaerobic digesters due to clogging (obstruction). The flotation of biomass due to adhesion of fat also causes loss of active biomass because of washout. Therefore, treating waste and wastewater rich in lipids in a high rate anaerobic digester requires applying a pre-treatment process in order to eliminate the potential problems associated to the agglomeration. Instability of the digestion process: detection of imbalance in the anaerobic digestion is critical for an improved performance. Existing waste and wastewater treatment plants were not designed to use real-time control systems. Treatment plants lack flexibility and controllable actuators which represents a fundamental barrier for the widespread acceptance of new parameter control sensors. Foam formation: In agricultural biogas plants ammonia concentrations are of concern when protein rich co-substrate is digested such as slaughterhouse waste (15). Foaming results in inefficient gas recovery from the digesters creating additional costs for electricity production. Other problems caused by foaming can include blockages of gas mixing devices, foam binding of sludge recirculation pumps, fouling of gas collection pipes due to entrapped foam solids, foam penetration between floating covers and digester walls and tipping of floating covers during foam expansion and collapse. Due to the above obstacles, and despite its high potential conversion rate, Anaerobic Digestion for slaughterhouse waste currently offers lower conversion rates of only 37%. New Project Concept ADAW (Anaerobic Digestion for Animal Waste) is a project funded by the European Commission 7th Framework Program. The project started on 1st March 2013 and lasted two years. Developed by a consortium set up by partners from different European Countries, the project was formed by four Industrial SMEs, three research centres and one end-user in the sector of slaughter houses. It aims to realise a new process for the treatment of wastes with high-lipid and protein content (mainly coming from slaughterhouses) that valorises their carbon content (energy potential) in the form of biogas with very high yield levels. The novelty of this approach relies on the combination of three different technologies; Anaerobic digestion, thermal pre-treatment and ultrasound to Category II and III residues, thus complying with current Animal Waste Regulations. The ADAW Project concept is for the incremental improvement in the degradability of wastes with high-lipid and protein content in anaerobic digestion and its enhanced conversion to biogas by: selection of the most suitable micro-organism culture, feed pre-treatment and digestate post-treatment. Selection of a more suitable inoculum makes the Anaerobic Digestion process more robust and resistance to inhibition. Thermo-chemical pre-treatment of the feed material allows modification to its chemical structure and makes it more compatible with the aqueous medium and provides a more accessible feedstock to bacterial species. The use of ultrasonic energy after this pre-treatment favours this phenomenon (by particle de-agglomeration) with the associated benefit in organic matter degradation. Material in the digester can also be intermittently post-treated by ultrasound. This activity produces additional benefits; decreasing foam formation and activating the biomass. The key benefit of the technology is the proper use of these techniques combined with the use of a designed liquid-phase on-line monitoring system that allows operating at Organic Loading Rate (OLR) as high as 4 kg COD/m3.d with 80% yields. In conclusion, global meat production has tripled in the past three decades and could double its present level by 2050. Consequently, the rate of animal waste produced will increase in similar proportions. Therefore it's important that new technologies and techniques are investigated so that this growing waste stream and be efficiently treated while meeting regulations. Neil Marshall is director of EST (Electrochemical Sensor Technology), one of the partners in the ADAW project. For more information on the ADAW project, please visit: www.adawprojet.eu. ADAW Technical Features Coupling of saponification and the compulsory pressure sterilization processes in a single step with energy recovery, meeting safety regulation for these wastes. An ultrasonic dispersing and de-agglomeration system customized for pre and post treatment. A liquid-phase on-line sensing and measuring system for Volatile Fatty Acids (VFA) and Alkalinity in the digester. Improving biogas yields by an advanced and customised control system for real time monitoring to facilitate process control and optimised energy management. Selection of a well-adapted and specific Bacterial population to work on high organic loads of wastes with high-lipid and protein content. More Waste Management World Articles Waste Management World Issue Archives