Evaluation of a two stage anaerobic digester for the treatment of mixed abattoir wastes
Introduction
The potential for rapid anaerobic stabilisation of organic solid wastes has received considerable attention during recent years, with a variety of substrates proving the general success of in-vessel digester systems. Stable anaerobic digestion is highly dependent upon maintenance of adequate buffering capacity in the digester, and in this respect the carbon to nitrogen ratio of the substrate is influential as ammonia plays a key role [1]. Ammonia itself, however, is toxic to methanogenic bacteria at relatively low concentrations [2]; and in the case of anaerobic digestion of organic nitrogen-rich solid waste materials from abattoirs, its accumulation as a result of protein degradation can lead to process failure [3]. The technique employed in this study to overcome the problem was first used as a means of stabilising the pH during the digestion of a poorly buffered cellulosic waste in which even low concentrations of volatile fatty acid (VFA) led to process instability through a lowering of pH to sub-optimal levels for the methanogenic population [1]. The principle employed is to decouple the solids and liquids retention time in a hydrolysis/acidification reactor, effectively stripping the fermentation intermediates and hydrolysis products from the reactor as they are formed. In this work the principle is applied to the stripping of ammonia from the process liquors in the first stage, using the excess process washwaters generated in the abattoir, which are then treated in a second phase methanogenic reactor. This system thus allows for the necessary solids retention in the first stage for cellulose hydrolysis but overcomes any accumulation of intermediates by using a high-rate anaerobic system with a short retention time as the second phase.
Anaerobic treatment of abattoir wastes is not new and the use of systems for research, demonstration and full-scale application has been reported since the 1950s [4]. However, many of the early treatment systems were simple anaerobic lagoons or ponds [5], [6], [7], [8], [9], [10], [11], [12], [13] although several mechanical plants were built in the USA and New Zealand, at both a pilot and full scale, to establish the treatment of slaughterhouse wastewaters [14], [15], [16], [17], [18], [19], [20]. In the UK the first laboratory research on anaerobic digestion of slaughterhouse wastes was reported by Lloyd and Ware [21] and the first full-scale plant was installed in 1962 [22]. Since the 1970s, with the development in applications for anaerobic technology, more and more research work on the anaerobic treatment of abattoir wastes has been undertaken using high-rate anaerobic systems [23], [24], [25], [26], [27], [28], [29], [30]. Despite the research effort, the anaerobic digestion of mixed abattoir wastes is still not popular, possibly due to the inherent instability of the process [3].
Conventional single-stage digestion of mixed waste (cattle blood and intestinal contents) has proved difficult to control resulting in high levels of VFAs and ammonia in the digester mixed liquor. Small-scale experiments [31] using a hydraulic flush regime as a first stage digester appeared to overcome the problem of accumulation of fermentation intermediates. The current work scaled up the first stage hydraulic flush reactor (HFR) to 30 l and looked at the efficiency of a high rate anaerobic filter (AF) as a second stage. The HFRs were operated with solids loadings ranging from 3 up to 8 kg total solid (TS) m−3 per day and their performance was compared with a conventional completely mixed single-pass reactor (SPR).
Section snippets
Feedstock and sludge inoculum
The substrate for the digestion trials was collected from Manchester Abattoir, Manchester, UK and was a mixture of cattle blood and cattle paunch (ruminant stomach) contents. The substrate for the experiment was prepared by homogenising a mixture comprising one part blood and three parts paunch contents (w/w) and then diluting the mix with water to give a TS content about 3, 4, 5, 6, 7 and 8% for feeding the digesters. Analytical determinants of the feedstock prepared in this way are shown in
Single-pass reactor
This was operated with a 10-day HRT and was used as the control. Its performance was assayed at three levels of organic loading using substrate made up with a solids content of 3, 4 and 5%. The digester effluent was sampled daily and analysed for pH, TS content (TS%), TVS content (TVS%), COD, VFAs and ammonia concentration. Gas production was recorded daily and the methane composition determined once a week. After the digester had reached a steady state it was run for 30 days at each organic
Conclusions
Use of a conventional single-pass digester system for the treatment of mixed abattoir wastes showed a low process efficiency. This is believed to be due to the build-up of fermentation intermediates in the form of VFAs, and to the high levels of ammonia. These arise from the hydrolysis of blood and cellulosic materials with a low carbon to nitrogen ratio [31] and may have led to process instability. Accumulation of ammonia-nitrogen concentrations in the digester can significantly inhibit the
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