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Anaerobic digestion of biowaste: test in laboratory scale reactors
N. Labartino*, M. Soldano*, C. Fabbri*, S. Piccinini*
* Research Centre on Animal Production CRPA – V.le Timavo, 43/2 – 42121 Reggio Emilia, Italy [email protected]
Keywords: Biowaste;OFMSW; AD reactors; Biomethane.
1. Introduction
The separate collection of recyclable waste in Italian municipalities (including biowaste, packaging waste, WEEEs and others) has reached a quota of 45.2% of all MSW managed in Italy in 2014 (29.66Mtons/yr), with a rather stable total production of waste (CIC, 2016). Anaerobic digestion (AD) is a feasible biological process to transform biowaste into biogas, a valuable biofuel. The biological process allows material to stabilize before composting, producing less gas emissions and odours, allows an energy and therefore economic value, thanks to the biogas produced.
In Italy, the number of AD plants has constantly increased in the last decade. By 2015, 46 AD-plants have been realized with a total authorized capacity of about 2 million tons of biowaste.
Italy being the third world producer of biogas, the economic valuation of the option of purifying biogas and obtaining biomethane is currently a major line of inquiry. In 2013, the Italian Ministry for Economic Development promoted an important decree on incentives for producing Biomethane.
This study was carried out with an anaerobic laboratory scale reactor filled with OFMSW (organic fraction from separate collection of municipal solid waste, biowaste), in order to evaluate its performance in terms of energy recovery. The study was performed under the LIFE + project Biomether (www.biomether.it) on the promotion of biomethane in the Emilia-Romagna region for vehicle use, as a substitute of fossil fuel, and to integrate the natural gas in domestic, commercial and industrial consumption.
2. Materials and methods
It is the aim of the present contribution to discuss the feasibility and efficiency of biogas production from “biowaste juice”. It was sampled in a full scale plant after treatment with a centrifugal separator machine, from which an organic liquid fraction (juice) was obtained, cleaned from inert (such as glass, plastic, wood).. The dry matter content (10.3%) obtained by the treatment makes it easy to use in an anaerobic digestion plant. Table 1 reports the chemical characteristics of the “biowaste juice” analysed before the use in the laboratory test.
Table 1. Main chemical characteristics of the “biowaste juice”, *(TKN, Total Kjeldahl Nitrgen, NDF Neutral Detergent Fiber, ADF Acid Detergent Fiber, ADL Acid Detergent Lignin), average and standard deviation values.
pH Total
solid (TS) Volatile
solid (VS) Volatile
solid (VS) TKN* TKN* NDF* ADF* ADL*
[-] [g·kg-1] [g·kg-1] [% TS] [% TS] [mg·kg-1] [% TS] [% TS] [% TS]
5.6 102.7±2.7 78.9±2.8 76.8±1.6 4.4±0.1 4511±56.6 28.1 23.6 5.7
The Volatile solid represents 77% of the Total solid of biowaste; about 6% of total solid is lignin, not degradable. Due to the variability of the substrate, it was sampled, analysed and stored in a freezer at -22 °C and defrosted before loading in the reactor.
An anaerobic digestion pilot reactor (CSTR, Completely Stirred Tank Reactor) of 23 litres of volume was loaded daily with such biowaste, and approximately three months experiment has yielded significant data of biogas production, at the same time displaying the biological
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stability of the process. The reactor is equipped with sensors for measuring the pressure generated within the head space due to the production of biogas. Through the solenoid valve, pressure is vented periodically. The composition of the biogas is determined by means of a gas analyser in terms of percentage of CH4, CO2, O2 and H2S concentration.
The methodology consists of the daily loading of biomass and unloading of digestate. The possibility to frequently analyse the digestate allows control of various process conditions such as the balance between acidity and alkalinity, the fatty acid composition, the chemical composition and the presence of inhibitory factors (ammonia, hydrogen sulphide, micronutrients, heavy metals, etc.).
The process was carried out in mesophilic conditions (40°C); in order to reduce the start-up period approximately 16 kg of digestate coming from a biogas plant that treats the same biowaste was put into the reactor, used as inoculum to provide the availability of a bacterial flora already partly adapted.
3. Results and discussion
The test had the duration of 78 days. The quantity of the biowaste treated for the whole period of the trial was 40.3 kg, equal to 4.1 kg of total solids and 3.2 kg of volatile solids. The volumetric organic load (OLR), which expresses the amount of volatile solids loaded per day and per cubic meter of anaerobic reactor was on average equal to 2.9 kg m3 d -1 and the average hydraulic retention time (HRT) was 29 days.
The monitoring of the biological process consisted of taking samples of fresh extracted digestate and measuring frequently the acidity and alkalinity, as well as the pH. The ratio volatile acidity/total alkalinity (FOS/TAC) indicates the balance between volatile acid (produced in the early stages of the anaerobic digestion process by hydrolytic and acidogenic bacteria) and total alkalinity, to buffer the total acidity. The methanogenic bacteria are very sensitive to changes in pH. A FOS/TAC ratio within values of 0.2-0.6 indicates that the process is in a state of equilibrium; Figure 1 shows the values measured during the test.
Figure 1. OLR, FOS and TAC trend during the trial
The value of total acidity (FOS), was higher in the start-up phase (6,824 mg·l-1) and then declined after a retention time re-entering the optimal range of 2,500-4,000 mg·l1. The alkalinity values (TAC) are linear in time but the average value, 16,669 mg·l-1 was higher of the ideal range of 10,000 to 13,000 mgCaCO3·l-1. The ratio between the two parameters (FOS/TAC) was on average, in the whole period, equal to 0,25 to indicates the stability of the process. In the first week values approximately of 0.45 were observed. The pH in the entire trial period was on average 7.9. Instability was observed in the start-up phase probably due to an high
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concentration of volatile acidity in the inoculum that could be affected probably the activity of methanogenic bacteria. The analysis of the inoculum is reported in Table 2.
Table 2. Main chemical characteristics of the inoculum.
pH Total solid (TS)
Volatile solid (VS)
Volatile solid (VS)
TKN
TKN Fos Tac Fos/Tac
[-] [g·kg-1] [g·kg-1] [% TS] [% TS] [mg·kg-1] [mgHAceq·l-1] [mgCaCO3·l-1] -
8.1 48.8 32.6 66.9 10.6 5174 10267 11784 0.87
Digestate unloaded daily from the reactor was collected and weekly analysed for total solid, volatile solid and ammonia. Table 3 shows the average and deviation standard values.
Table 3. Chemical characteristics of the digestate collected weekly.
Total solid (TS) Volatile solid (VS) Volatile solid (VS) Ammonia
(NH4+-N)
Ammonia
(NH4+-N)
[g·kg-1] [g·kg-1] [% TS] [mg·kg-1] [%TS]
38.6±5.3 22.9±4.8 58.8±3.9 3229±49.9 8.5±1.1
Figure 2 shows the weekly methane yield measured during the period of monitoring expressed as normal cubic meters of methane per ton of volatile solids. The average value was 379 Nm3 t VS-1. The trendline is descending and corresponds to an increase of the ORL in a second period of the test, passing from 2.6 to 3.1 kg m3 d -1.
Figure 2. Weekly methane yields , average value during the entire test period and the trend line.
The methane yield on raw materials, an important parameter for the sizing of an anaerobic digestion plant that operates on biowaste, was 30 m3 t-1. The methane percentage measured in biogas produced was good, 67%, due to the high level of organic matter present in biowaste and the absence of unwanted fractions.
The production in methane has reached good values, also observed with the result obtained from the BMP (Biochemical Methane Potential) test in batch (Figure 3), equal to 387
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Nm3 t VS-1, in accordance with the standard UNI EN ISO 11734 for the measure of the maximum methane produced from the degradation of an organic substrate.
Figure 3. The cumulative methane production yield of the “biowaste juice”
4. Conclusion
Due to variability of the composition of the biowaste, it is necessary before its use in the AD plant and in co-digestion with other substrates, the knowledge of the behaviour of the same in anaerobic digestion process, the chemical characteristics, the analysis of the biochemical methane potential (BMP) and the study of the biological process by means of laboratory scale tests. This approach allows to determine the real amount necessary to size the AD plant, to properly assess the cost of the supply, the management of waste collected and allows to set the load planning that will guarantee an efficient biological process and correct in time.
The pre-treatment process of the OFMSW, separating the solid phase, allows to send only the liquid phase to the anaerobic digester thereby avoiding that in the digestate are present large quantities of inert solid substance or not convertible into biogas, resulting in lower production digestate material difficult to treat. The results of the laboratory test confirm that the quality of the biogas is satisfactory, and that the production rate may be considered economically interesting.
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