A comparative kinetic evaluation of the anaerobic digestion of untreated molasses and molasses previously fermented with Penicillium decumbens in batch reactors

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Abstract

A comparative kinetic study was carried out on the anaerobic digestion of untreated beet molasses alcoholic fermentation wastewater and beet molasses previously fermented with Penicillium decumbens. Three 1 l volume stirred tank reactors were used for the study, one with freely suspended biomass (Control), and the other two with biomass supported on Saponite (magnesium silicate) and Esmectite (aluminium silicate), respectively. The reactors were batch fed at mesophilic temperature (35 °C) using COD loadings of between 12 and 140 ml of untreated molasses with a COD of 80.5 g/l and of between 43 and 304 ml of molasses previously fermented with a COD of 23.0 g/l. The anaerobic digestion process of both substrates was found to follow a first-order kinetics and the experimental accumulated methane volume (G)–time (t) data to conform to an equation of the form: G=Gm[1−exp(−K0t)], from which the specific rate constants, K0, were calculated. In the case of untreated molasses, the K0 values decreased considerably from 2.87 to 0.10 (Saponite), from 2.78 to 0.10 (Esmectite) and from 2.16 to 0.07 per days (Control) when the loading of molasses applied and initial COD added were increased from 12 to 140 ml and 1 to 10 g/l, respectively; this showed an inhibition phenomenon in the three reactors studied. In contrast to this, the kinetic constants of the anaerobic digestion of pre-treated molasses were virtually constant over the COD range used (1–7 g/l) in the three reactors considered. Finally, the average methane yield coefficient for pre-treated molasses was 305 ml CH4 STP/g CODremoved, viz. 35% higher than that provided by untreated molasses.

Introduction

Beet or sugar cane molasses are by-products of the sugar-extraction process and are often used as raw material in alcohol distilleries. In Europe and the USA a large number of such industries are currently operating, producing considerable quantities of liquid wastes generally called stillages, distillery slops or vinasses. The production of vinasses in a traditional alcohol factory is in the range of 9–14 l of wastewaters per litre of ethanol obtained. These wastes are strongly acidic (pH 4–5), have a high-organic content (COD in the range of 50–100 g/l). Their free disposal presents a serious challenge to the natural ecosystem and can cause considerable environmental problems [1].

Some researchers [2] have reviewed several methods for the treatment, utilization and disposal of wastewaters from ethanol fermentation industries. Among these are both chemical and biological treatments (aerobic or anaerobic classical methods, trickling filters, lagoons, etc., evaporation–condensation with or without combustion, direct dispersion on soil as a fertilizer, etc.) [2]. A common feature of all these methods is their relatively high cost and, for some, the simultaneous creation of other hazardous by-products/pollutants [3].

The aerobic biological treatment of high-organic load wastes, like molasses, is associated with operational difficulties of sludge bulking, inability of the system to treat high BOD or COD loads economically, relatively high biomass production and high cost in terms of energy. On the other hand, with the diminishing supply of natural gas and other fossil fuels, bacterial conversion of liquid (or solid) wastes to methane and stabilized by-products through anaerobic digestion would be beneficial [3], [4], [5]. These by-products could subsequently serve as food or fertilizer and generally be disposed of with fewer problems (easier dewatering, smaller amounts). Also the high temperatures and high-organic load concentrations of the effluents to be treated, as well as the high-energy requirements of the distillery process, are very suitable conditions for the application of anaerobic digestion.

Anaerobic digestion has a number of advantages. For example, it demands less energy input, anaerobic bacteria are capable of transforming most of the organic substances present into biogas, sludge formation is minimal and nutrient demands are very low. The production of biogas enables the process to generate some energy in addition to the reduced consumption; this can reduce operational costs by a large margin compared with high-energy consumptive aerobic processes [6].

For these reasons, anaerobic digestion of molasses alcoholic fermentation wastewaters have been the subject of a number of studies using laboratory or pilot-scale digesters [7], [8], [9], [10], [11], [12], but studies on full-scale mesophilic plants have been reported less often. Several related reviews have been presented and many pilot-scale investigations have been reported, using different anaerobic reactor configurations [13], [14], [15].

Although anaerobic digestion of most types of distillery wastewaters is feasible and quite appealing from an energy point of view, the presence of inhibitory substances such as phenolic compounds severely hinders the anaerobic process. This slows down the kinetics, and reduces mean rates of methane production, methanogenic activities and yield coefficients. These problems were previously observed in anaerobic batch cultures of wine distillery wastewaters and cane molasses stillages [16], [17], [18].

Many phenolic compounds are known to be toxic and interfere with the activity of methanogenic bacteria. There are numerous reports in the literature showing the toxicity and the inhibitory effects of these compounds on anaerobic digestion processes [19], [20], [21], [22], [23]. In addition, the high salinity of this waste (average conductivity of 40 mS/cm) can also cause osmotic pressure problems to the micro-organisms responsible for the anaerobic process [24].

Therefore, although the anaerobic digestion of this wastewater is attractive and energetically promising, the presence of a high phenolic content slows down the process, hinders the removal of part of its organic content, making necessary the utilization of high hydraulic retention times (HRT). Moreover, the anaerobic digestion process does not remove the intense colour of this wastewater neither an important fraction of the initial COD even working at organic loading rates (OLR) as low as 2–4 kg COD/m3 day.

A previous comparative study of the anaerobic digestion of untreated and previously fermented beet molasses alcoholic fermentation wastewater carried out in suspended cell bioreactors operating in continuous mode showed that for untreated molasses, COD removal decreased considerably from 93.7 to 68.6% when the OLR increased from 1.5 to 7.5 g COD/l day and HRT decreased from 53.5 to 10.6 days [25]. In contrast, for molasses previously fermented with Penicillium decumbens the decrease in the percentage of COD removal with increased OLRs was softer than that observed for untreated molasses in the same range of OLR (1.5–7.5 g COD/l day) [25].

The aim of the present work was to carry out a comparative study of anaerobic digestion of untreated and previously fermented (with P. decumbens) beet molasses alcoholic fermentation wastewater in suspended and immobilized batch reactors and to determine the kinetic parameters and the factors that could affect the process, as a first step toward designing a full-scale plant.

Section snippets

Equipment

The experimental set-up used for the anaerobic digestion experiments of untreated beet molasses and molasses pre-treated with P. decumbens consisted of three anaerobic digestion units (ADU), each one composed of 1 l Pyrex flask with two nozzles at the top—an inlet for loading and an outlet for venting the biogas. The flask was modified by including two further nozzles in the neck, one for the incoming inert gas (nitrogen) required for unloading and the other for the outgoing effluent. The ADU

Operational and control parameters

Table 3, Table 4 show the initial COD values at the beginning of each experimental run carried out in the three reactors used processing untreated and previously fermented molasses, respectively.

The characteristics and operational parameters of the effluents obtained at the end of each experiment performed with untreated and previously fermented molasses in the three reactors used are summarized in Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, respectively.

As can be seen in Table 5,

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