Topic of the Month: May 2015 (Background Information)

Processing of crab shells to generate biogas

The main components of crab shells – apart from chitin – are mineral materials, particularly potassium salts, and proteins [1]. The first operation in the processing of chitin is extraction of it. This can be done chemically (acid-alkaline extraction) or enzymatically. A protein-rich alkaline solution and a calcium-rich acid solution are produced in the chemical process. The protein-rich waste stream can be used to generate energy in a biorefinery concept that is as waste-free as possible. The advantages of anaerobic fermentation of organic substances was identified by McCarty [2] as long ago as 1964: a high level of stabilisation of the organic material, a low nutrient requirement, no need for oxygen and the obtainment of biogas that can be used as a renewable energy source.

In principle, biogas can be generated from practically any kind of organic substance. In Europe, biogas production and use is common in sewage treatment and agriculture in the meantime. The anaerobic technology is even used in industry in more than 65 different countries and a total of over 1,400 plants have been installed so far by 16 equipment manufacturers [3]. One of the challenges that is being tackled in the ChiBio project is technical implementation of the most sustainable processing as possible of the by-products that are created. In this context, the biogas generation potential of the by-products that are created by the chemical extraction of chitin from shrimp and crab shells was determined.

Compared with other substrates, these by-products have a low organic dry matter content, extreme pH levels and high salt concentrations. While typical substrates like sludge from waste water purification or liquid manure have a ratio of organic dry matter to total dry matter (oDM/DM ratio) of 60 to 80 per cent, the oDM/DM ratio of the by-products created in the chitin extraction of crab and shrimp shells is about 15 / 30 per cent.

To determine the biogas generation potential, 1-litre reactors were operated on a laboratory scale with the side stream of chitin-extracted shrimp shells containing protein and two different batches of crab shells (May and June) as the substrate. The experiments were carried out as fed batches with two feeding cycles. The specific biogas production of the shrimp shell side stream amounted to 1,125 mlN/goDM in the first feeding cycle and 830 mlN/goDM in the second feeding cycle. This was higher than the specific biogas yield with the crab shell side streams, for which 305 / 319 mlN/goDM were measured in the first feeding cycle and 283 / 391 mlN/goDM were determined in the second feeding cycle.

This means that the specific biogas yield of the side stream in the processing of shrimp shells was even higher than the typical biogas yield with renewable raw materials (about 500 – 700 mlN/goDM). The explanation for this is that the main component of renewable raw materials is sugar, whereas the main components of the shrimp stream are proteins, which produce a higher specific biogas yield. The specific biogas yield of the side streams in the processing of crab shells is more comparable to the biogas yield obtained with organic household waste (about 350 mlN/goDM).

The biogas production rates were primarily linear, which indicates a growth limitation of the anaerobic micro-organisms. Further investigations of the reasons for this growth limitation provide an opportunity to improve anaerobic degradation and thus the specific biogas yield. (Source: Fraunhofer IGB)

[1] Boßelmann, F.; Romano, P.; Fabritius, H.; Raabe, D.; Epple, M. (2007) The composition of the exoskeleton of two crustacea: The American lobster Homarus americanus and the edible crab Cancer pagurus, Thermochimica Acta, Vol. 463: 65-68
[2] McCarty, P.L. (1964) Anaerobic waste treatment fundamentals, Public Works for September-December, Princeton University
[3] Show, K. Y.; Tay, J. H.; Hung, Y.T. (2010) Global perspective of anaerobic treatment of industrial wastewater, Handbook of Environmental Engineering 11: Environmental Bioengineering Show, K. Y.; Tay, J. H.; Hung, Y.T. (2010) Global perspective of anaerobic treatment of industrial wastewater, Handbook of Environmental Engineering 11: Environmental BioengineeringElastomere (Sing. das Elastomer, auch Elaste) sind formfeste, aber elastisch verformbare Kunststoffe, deren Glasübergangspunkt sich unterhalb der Einsatztemperatur befindet. Die Kunststoffe können sich bei Zug- und Druckbelastung elastisch verformen, finden aber danach wieder in ihre ursprüngliche, unverformte Gestalt zurück. Elastomere finden Verwendung als Material für Reifen, Gummibänder, Dichtungsringe usw. Die bekanntesten Elastomere sind die Vulkanisate von Naturkautschuk und Silikonkautschuk.
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