The aim of this work was to assess the fur farming by-products as substrates for anaerobic digestion plants with special attention given on their bio-methane potential and biodegradability.
The production of fur based commodities in Greece is driven by a dynamic and well established industry with long tradition in the production of quality products, which moreover presenting significant exporting experience. The Western Macedonia region is the area where nearly all of the fur production in Greece is taking place including the feed manufacturing, the farming and the transformation of the pelts into final commodities. The specie that is most commonly farmed for fur production is the American Mink (Mustela vison) which is a carnivore, semi-aquatic mammal, with the full grown males reaching the 45cm in length, 20cm in height and the 2 kg in weight. The diet of the farmed animals is composed of low cost protein and fat rich byproducts, including slaughterhouse and aquaculture wastes. During the mink feed preparation the high protein feed is mixed with cereal based products in order to create a total mixed daily ratio in the form of a thick paste that allows the feed to stay firm on the feeders minimizing loses.
The wastes that are being generated from the mink breading facilities are manures and waste feed both of which are collected underneath the cages in small piles of up to 50cm in height. The waste management options for this waste stream is restricted by the high solids, organics and nitrogen content that are significantly hampering the abilities of aerobic biological processes to effectively treat these waste streams. Even so, composting is widely employed, due to the low cost applied, with however mixed results. Mink farmers in order to stimulate application of the generated compost are offering it free of charge, unfortunately the acceptance and application rates to the fields by local farmers being disappointing.
A waste management option which can be employed for the management of the fur farming wastes is the anaerobic digestion (AD). AD is a biomass bio-conversion process disengaged from the weather conditions which offers the advantages of self-sustentation, income generation, and waste valorization with limited material requirements. Anaerobic digestion of manures and plant waste materials it is offering both the opportunity for the recovery of biogas (a methane rich gas) but also of the hygienized and stabilized digestate which can be applied onto agricultural land as an organic fertilizer. In addition, odorous emissions, a feature of open manure storage or treatment systems, are avoided.
The substrates assessed in this work where the fresh and weathered mink manure (FMM, WMM respectively) waste feed (WF) and the bone and meat meal (BMM) that is being generated by the dead mink bodies after pelting. The two main characteristics of all the wastes assessed were the high total solid levels and the high protein concentrations reaching as high as 97% and 58% for the BMM respectively.
The substrates assessed in 500 mL glass vials and under mesophilic conditions mainly due to the known problems related to the inhibition of the process by the elevated concentrations of unionized ammonia driven by the organic protein rich substrates used and the high temperatures experienced in thermophilic systems. The inoculum utilized where collected from two 6 L mesophilic CSTR reactors operating within the facilities of the University of Thessaloniki treating a mixture of cattle manure and food wastes with an OLR of 3.1 kgVSm3R-d and a HRT of 28 days. Prior to the preparation of the batches the inoculum degased for 1 week. During the preparation of the batched the vials where filled with the inoculum and the substrate in a VSinoc to VSsubs ratio of approximately 2, followed by oxygen removal from the vials through nitrogen bubbling and placed within a temperature controlled cabinet at 36 ±1 oC for up to 30 days.
During the batch digestion of the different substrates no problems related to inhibitory phenomena were observed and the biogas production initiated rapidly, with most of the biogas being recovered within the first two weeks.
The specific bio-methane potential of the assessed substrates found as 367 and 428 mLCH4/gVSadded for the fresh and weathered mink manures respectively, 548 mLCH4/gVSadded for the waste mink feed and 591 mLCH4/gVSadded for the mink bone and meat meal, corresponding in all cases to a VS reductions higher than 65%. While the bio-methane production of weathered manure it appears abnormally high, compared to the fresh manure, a primary explanation of this phenomenon can be due to an initial hydrolysis stage that is taking place within the manure piles as they are formed and which is stimulated by the anaerobic conditions and the flow of biological fluids within the piles.
When the specific methane production of these wastes are compared with other common substrates used in anaerobic digestion plants it can be seen that the production is higher by at least 100% when compared to cattle manures, approximately 80% when compared to pig manures, with the generated volumes being in par with data reported for the anaerobic digestion of source separated high quality food wastes.
This work investigated, for the first time, the potential of the mink farming wastes as substrates to AD systems. According to the results, the mink farming wastes can be considered as excellent substrates for AD systems where large volumes of methane gas can be recovered. At the same time the generated digestate, which after the anaerobic treatment is presenting improved characteristics compared to the initial substrate, can be utilized as an organic soil amendment, minimizing in this way the land application of expensive chemical products.
The main limitation identified at the present stage is the seasonal availability of the substrates with most of the manures and waste feed to becoming available between July and October, while between October and November the mink bone and meat meal is becoming available, rendering the mono-digestion of mink farming wastes as at least challenging. Better utilization of these waste streams can be achieved in centralized digestion plants where their high bio-methane potential could significant improve cash flows during the period where they are becoming available.