Biogas from Slaughterhouse Wastes

slaughterhouse-wasteSlaughterhouse waste (or abattoir waste) disposal has been a major environmental challenge in all parts of the world. The chemical properties of slaughterhouse wastes are similar to that of municipal sewage, however the former is highly concentrated wastewater with 45% soluble and 55% suspended organic composition. Blood has a very high COD of around 375,000 mg/L and is one of the major dissolved pollutants in slaughterhouse wastewater.

In most of the developing countries, there is no organized strategy for disposal of solid as well as liquid wastes generated in abattoirs. The solid slaughterhouse waste is collected and dumped in landfills or open areas while the liquid waste is sent to municipal sewerage system or water bodies, thus endangering public health as well as terrestrial and aquatic life. Wastewater from slaughterhouses is known to cause an increase in the BOD, COD, total solids, pH, temperature and turbidity, and may even cause deoxygenation of water bodies.

Anaerobic Digestion of Slaughterhouse Wastes

There are several methods for beneficial use of slaughterhouse wastes including biogas generation, fertilizer production and utilization as animal feed. Anaerobic digestion is one of the best options for slaughterhouse waste management which will lead to production of energy-rich biogas, reduction in GHGs emissions and effective pollution control in abattoirs. Anaerobic digestion can achieve a high degree of COD and BOD removal from slaughterhouse effluent at a significantly lower cost than comparable aerobic systems. The biogas potential of slaughterhouse waste is higher than animal manure, and reported to be in the range of 120-160 m3 biogas per ton of wastes. However the C:N ratio of slaughterhouse waste is quite low (4:1) which demands its co-digestion with high C:N substrates like animal manure, food waste, crop residues, poultry litter etc.

Slaughterhouse effluent has high COD, high BOD, and high moisture content which make it well-suited to anaerobic digestion process. Slaughterhouse wastewater also contains high concentrations of suspended organic solids including pieces of fat, grease, hair, feathers, manure, grit, and undigested feed which will contribute the slowly biodegradable of organic matter. Amongst anaerobic treatment processes, the up-flow anaerobic sludge blanket (UASB) process is widely used in developing countries for biogas production from abattoir wastes.

Slaughterhouse waste is a protein-rich substrate and may result in sulfide formation during anaerobic degradation. The increased concentration of sulfides in the digester can lead to higher concentrations of hydrogen sulfide in the biogas which may inhibit methanogens. In addition to sulfides, ammonia is also formed during the anaerobic digestion process which may increase the pH in the digester (>8.0) which can be growth limiting for some VFA-consuming methanogens.

Waste Management in Olive Oil Industry

olive-oil-wastesThe olive oil industry offers valuable opportunities to farmers in terms of seasonal employment as well as significant employment to the off-farm milling and processing industry.  While this industry has significant economic benefits in regards to profit and jobs; the downside is it leads to severe environmental harm and degradation. In 2012, an estimated 2,903,676 tons of olive oil was produced worldwide, the largest olive oil producers being Spain, Italy, and Greece followed by Turkey and Tunisia and to a lesser extent Portugal, Morocco and Algeria. Within the European Union’s olive sector alone, there are roughly 2.5 million producers, who make up roughly one-third of all EU farmers.

Types of Wastes

Currently, there are two processes that are used for the extraction of olive oil, the three-phase and the two-phase. Both systems generate large amounts of byproducts.  The two byproducts  produced by the three-phase system are a solid residue known as olive press cake (OPC) and large amounts of aqueous liquid known as olive-mill wastewater (OMW).  The three-phase process usually yields 20% olive oil, 30% OPC waste, and 50% OMW.  This equates to 80% more waste being produced than actual product.

Regardless of system used, the effluents produced from olive oil production exhibit highly phytotoxic and antimicrobial properties, mainly due to phenols.  Phenols are a poisonous caustic crystalline compound.  These effluents unless disposed of properly can result in serious environmental damage.  There is no general policy for waste management in the olive oil producing nations around the world.  This results in inconsistent monitoring and non-uniform application of guidelines across these regions.

State of Affairs

Around 30 million m3 of olive mill wastewater is produced annually in the Mediterranean area.  This wastewater cannot be sent to ordinary wastewater treatment systems, thus, safe disposal of this waste is of serious environmental concern.  Moreover, due to its complex compounds, olive processing waste (OPW) is not easily biodegradable and needs to be detoxified before it can properly be used in agricultural and other industrial processes.

This poses a serious problem when the sophisticated treatment and detoxification solutions needed are too expensive for developing countries in North Africa, such as Morocco, Algeria and Tunisia, where it is common for OMW to be dumped into rivers and lakes or used for farming irrigation.  This results in the contamination of ground water and eutrophication of lakes, rivers and canals.  Eutrophication results in reductions in aquatic plants, fish and other animal populations as it promotes excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of oxygen, causing aquatic populations to plummet.

Another common tactic for disposal of olive mill wastewater is to collect and retain it in large evaporation basins or ponds.  It is then dried to a semi-solid fraction. In less developed countries where olive processing wastes is disposed of, this waste, as well as olive processing cake and SOR waste is commonly unloaded and spread across the surrounding lands where it sits building up throughout the olive oil production season.  Over time these toxic compounds accumulate in the soil, saturating it, and are often transported by rain water to other nearby areas, causing serious hazardous runoff. Because these effluents are generally untreated it leads to land degradation, soil contamination as well as contamination of groundwater and of the water table itself.

Even a small quantity of olive wastewater in contact with groundwater has the potential to cause significant pollution to drinking water sources. The problem is more serious where chlorine is used to disinfect drinking water. Chlorine in contact with phenol reacts to form chlorophenol which is even more dangerous to human health than phenol alone.

Remedial Measures

The problems associated with olive processing wastes have been extensively studied for the past 50 years.  Unfortunately, research has continued to fall short on discovering a technologically feasible, economically viable, and socially acceptable solution to OPW.  The most common solutions to date have been strategies of detoxification, production system modification, and recycling and recovery of valuable components.  Because the latter results in reductions in the pollution and transformation of OPW into valuable products, it has gained popularity over the past decade. Weed control is a common example of reusing OPW; due to its plant inhibiting characteristics OPW once properly treated can be used as an alternative to chemical weed control.

Research has also been done on using the semisolid waste generated from olive oil production to absorb oil from hazardous oil spills.  Finally, in terms of health, studies are suggesting that due to OPW containing high amounts of phenolic compounds, which have high in antioxidant rates, OPW may be an affordable source of natural antioxidants. Still, none of these techniques on an individual basis solve the problem of disposal of OMW to a complete and exhaustive extent.

At the present state of olive mill wastewater treatment technology, industry has shown little interest in supporting any traditional process (physical, chemical, thermal or biological) on a wide scale.This is because of the high investment and operational costs, the short duration of the production period (3-5 months) and the small size of the olive mills.

Conclusion

Overall, the problems associated with olive processing wastes are further exemplified by lack of common policy among the olive oil producing regions, funding and infrastructure for proper treatment and disposal, and a general lack of education on the environmental and health effects caused by olive processing wastes.   While some progress has been made with regards to methods of treatment and detoxification of OPW there is still significant scope for further research.  Given the severity of environmental impact of olive processing wastes, it is imperative on policy-makers and industry leaders to undertake more concrete initiatives to develop a sustainable framework to tackle the problem of olive oil waste disposal.