Organic solid waste

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The major options for the stabilization and re-use or disposal of organic solid waste from meat processing and rendering operations are composting (aerobic digestion) and vermicomposting, anaerobic digestion, disposal to landfill and incineration (Salminen and Rintala, 2002).

Incineration has recently been adopted by the meat industry as a viable option as it is considered among the most effective methods for destroying potentially infectious agents (Ritter and Chinside, 1995).

Composting is an aerobic biological process whereby organic materials are decomposed to form a product that has value as a soil conditioner or fertilizer. Composting is generally conducted in windrows (elongated mounds) or reactors, in order to achieve near-optimal conditions. Composting requires particular conditions with respect to temperature, moisture, aeration, pH and carbon/nitrogen (C/N) ratio to optimize biological activity at the various stages of the process (Sharma et al., 1997). If good quality compost is the desired result, then the moisture content (<65%) and the C/N ratio (30-35%) are of particular importance. From the perspective of abattoir operations, the amount of manure, paunch content or sludge intended for composting may be limited by these requirements. Alternatively, if all available material is to be recycled via composting, additional composting raw materials may be obtained from neighbouring industries to balance the moisture content and C/N ratio.

A useful fact sheet (Natural Rendering: Composting Livestock Mortality and Butcher Waste) offering guidelines for on-farm disposal of fallen or dead stock via composting, is available online from the Cornell Waste Management Institute (2002). However, this reference adds a note of caution, notably that any animals exhibiting signs of neurological disease must be reported to the authorities and disposed of in the recommended manner.

Vermicomposting refers to the use of earthworms to augment or replace conventional composting. This process involves the physical breakdown (comminution) of organic matter by the worms and the action of intestinal microorganisms and the microorganisms that inhabit vermiculture substrates (Pizl and Novakova, 2003). The major attraction of vermicomposting is the improved market value of stabilized organic fertilizer produced, as compared with the soil conditioner/fertilizer generated by standard composting. Several commercial vermicomposting facilities have been reported in North America and Asia (Sharma et al., 1997) along with a cautionary mention of issues of increasing production costs and low wholesale price margins while retail profits are high.

Some attention has been focused on the importance of microscopic fungi to the process of vermicomposting cattle manure (Pizl and Novakova, 2003). Earthworm (Eisenia andrei) feeding experiments showed that generally the addition of microfungi to vermiculture substrates resulted in an increase in growth rates; however, the only statistically significant increase was related to the addition of Aspergillusflavus. This result raises the prospect of seeding commercial vermiculture substrates with particular fungi to enhance the vermicomposting process.

Anaerobic digestion is a biological process where organic material is broken down under anaerobic conditions to yield methane, stabilized sludge and partially treated wastewater. Many variations of anaerobic digestion have been, and continue to be, evaluated in order to improve digester performance and maximize the systems' ability to cope with variation in abattoir solid waste output. Some advantages of this system are the production of methane as a by-product, and the ability to handle the high organic loads associated with abattoir waste. Salminen and Rintala (2002) have published an excellent review of the degradation pathways involved in the conversion of carbohydrates, proteins and lipids (typical of solid slaughterhouse waste) through to methane. In addition, these authors have addressed the two major process-limiting by-products (long-chain fatty acids and ammonia) of anaerobic digestion.

The process-limiting effects of ammonia have again been evaluated and these effects have been overcome in trials using a two-stage anaerobic digester (Wang and Banks, 2003). In this trial, the authors de-coupled the solids and liquids retention time in a hydrolysis/acidification reactor. This promoted the effective stripping of ammonia from the process liquors in the first stage, while allowing the necessary solids retention time. This system, described as a hydraulic flush reactor (HFR), showed significant improvements in process efficiency (compared with a conventional singlepass reactor) as measured by total solids (TS) reduction, chemical oxygen demand (COD) removal and biogas conversion.

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