Evaluation of a two stage anaerobic digester for the treatment of mixed abattoir wastes

Abstract

A two-stage anaerobic digestion system for treating mixed abattoir wastes was compared with a conventional single-pass reactor (SPR). The aim of the work was to investigate a means of overcoming the problems caused by accumulation of volatile fatty acids (VFA) and ammonia in conventional completely mixed SPRs, which are thought to arise as a result of the low carbon: nitrogen ratio of the feedstock. The two-stage digester consisted of a first stage 30 l working volume completely mixed reactor in which the hydraulic retention time (HRT) was significantly shorter than the solid retention time (SRT). This mode of operation was referred to as a hydraulic flush, and was designed to retain the fibrous components of the feedstock within the reactor whilst rapidly washing out hydrolysis and fermentation intermediates. The hydraulic flush reactor (HFR) operated at a 10-day SRT and 2-day HRT. The performance of the HFR was compared with that of an SPR at total solids (TS) loading rates of 3, 4, 5, 6, 7 and 8 kg TS m⁻³ per day. The SPR reactor failed at a solids loading rate of 5 kg TS m⁻³ per day and only showed a maximum of 41% solids reduction whereas the HFR showed up to 66% solids reductions and performed satisfactorily up to 7 kg TS m⁻³ per day. An anaerobic filter (AF) was employed as second stage to the HFR to provide a means of treating the liquid effluents derived from first stage. The AF was run at a HRT of 1 day with loading rates in the range 4.0–13.1 kg COD m⁻³ per day. A COD removal of around 95% and a methane production rate of up to 0.34 m³ CH₄ per kg COD removed was achieved. The best performance of the two-stage system was 66% solids reduction, 81.7% COD removal and an overall methane yield of 0.21 m³ CH₄ per kg TS added, at a solids loading rate of 7.02 kg TS m⁻³ per day.

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).