02 - Impact of Food Industrial Waste on Anaerobic Co-digestion of Sewage Sludge and Pig Manure

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  • 7/27/2019 02 - Impact of Food Industrial Waste on Anaerobic Co-digestion of Sewage Sludge and Pig Manure

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    Impact of food industrial waste on anaerobic co-digestion of sewagesludge and pig manure

    M. Murto*, L. Bjornsson, B. Mattiasson

    Department of Biotechnology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

    Received 11 April 2003; revised 19 October 2003; accepted 12 November 2003

    Abstract

    The performance of an anaerobic digestion process is much dependent on the type and the composition of the material to be digested. The

    effects on the degradation process of co-digesting different types of waste were examined in two laboratory-scale studies. In the first

    investigation, sewage sludge was co-digested with industrial waste from potato processing. The co-digestion resulted in a low buffered

    system and when the fraction of starch-rich waste was increased, the result was a more sensitive process, with process overload occurring at a

    lower organic loading rate (OLR). In the second investigation, pig manure, slaughterhouse waste, vegetable waste and various kinds of

    industrial waste were digested. This resulted in a highly buffered system as the manure contributed to high amounts of ammonia. However, it

    is important to note that ammonia might be toxic to the micro-organisms. Although the conversion of volatile fatty acids was incomplete the

    processes worked well with high gas yields, 0.81.0 m3 kg21 VS.

    q 2004 Elsevier Ltd. All rights reserved.

    Keywords: Alkalinity; Anaerobic; Biogas; Co-digestion; Manure; Sewage sludge; Slaughterhouse waste; Vegetable waste

    1. Introduction

    The EU countries have agreed on a directive stating that

    the amount of biodegradable organic waste that is deposited

    in landfills should be decreased by 65% by July 2016

    (Council Directive 1999/31/EC on the landfill of waste,

    1999). The Swedish goal is stricter: no biodegradable waste

    should be landfilled after 2005 (SFS, 2001) and a tax of 25

    Euro per ton of biodegradable material deposited in landfills

    was introduced in 2000. While most of the municipalities inSweden regard incineration as the main alternative, it is also

    important to investigate and improve techniques for the

    biological treatment of organic waste.

    Anaerobic digestion has many environmental benefits

    including the production of a renewable energy carrier, the

    possibility of nutrient recycling and reduction of waste

    volumes (Ghosh et al., 1975; Hawkes and Hawkes, 1987;

    van Lier et al., 2001). Many kinds of organic waste have

    been digested anaerobically in a successful way, such as

    sewage sludge, industrial waste, slaughterhouse waste, fruit

    and vegetable waste, manure and agricultural biomass.

    The wastes have been treated both separately and in co-

    digestion processes (Callaghan et al., 2002; Claassen et al.,

    1999; Gunaseelan, 1997). Our knowledge about the

    anaerobic digestion process is increasing. Nevertheless,

    studies are needed to investigate the effects of variations in

    the input to a digester, and how the waste composition

    influences the overall stability of the process.

    There is a long tradition of treating sewage sludge

    anaerobically at wastewater treatment plants to reduce the

    volume of sludge, but the process has not been focused onoptimal biogas production. Anaerobic digesters are often

    very simple in construction and the process is poorly

    monitored. As a result, they are often run at a low OLR to

    avoid overload. In a society where landfilling of organic

    waste is prohibited or limited it would be of interest to use

    the already existing biogas plants for waste treatment.

    Co-digestion of suitable organic waste with municipal

    sludge would provide a means of using the extra capacity of

    the anaerobic digesters.

    The main steps in anaerobic digestion are hydrolysis,

    acidogenesis, acetogenesis and methanogenesis (Gujer and

    Zehnder, 1983). Protein- and carbohydrate-degrading bac-

    teria grow rapidly, and these kinds of substrates are

    rapidly fermented, with a retention time of less than a day

    0301-4797/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

    doi:10.1016/j.jenvman.2003.11.001

    Journal of Environmental Management 70 (2004) 101107www.elsevier.com/locate/jenvman

    * Corresponding author. Tel.: 46-46-2228193; fax: 46-46-2224713.

    E-mail address: marika.murto@biotek.lu.se (M. Murto).

    http://www.elsevier.com/locate/jenvmanhttp://www.elsevier.com/locate/jenvman
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    (Bryant, 1979). If the substrate is easily hydrolysed, the last

    degradation step is often rate limiting since methanogens

    grow more slowly than the acidogens upstream in the

    degradation chain. This can give rise to negative effects in

    the case of organic overload or exposure to toxic

    compounds that may induce a build-up of the metabolic

    intermediates, mainly volatile fatty acids (VFAs) (Rozzi,

    1991). The acid-consuming methanogenic species are more

    inhibited by a decrease in pH than are the acid-producing

    species (Anderson and Yang, 1992). This causes further

    acid accumulation and eventually leads to process failure.

    The resistance to a pH-change in the digester liquid

    depends on the buffering capacity, which is mainly

    comprised of the bicarbonate/carbon dioxide buffer

    (Rozzi, 1991). If other ions are present they also

    contribute to the alkalinity. For example, when proteinsare degraded, ammonium is released forming ammonium

    bicarbonate, which results in additional buffering of the

    digester liquid (Gallert et al., 1998; Nyns, 1986) and

    thereby gives higher resistance to organic overload.

    However, the anaerobic degradation process may be

    inhibited by high amounts of ammonia (Hansen et al.,

    1998). The toxicity is related to temperature and the

    pH-dependent concentration of free ammonia (Gallert

    et al., 1998). In unadapted cultures, a free ammonia level

    of 0.15 g l21 can cause growth inhibition (Braun et al.,

    1981). If the culture has undergone gradual adaptation, a

    level of up to 1.1 g l21 free ammonia can be tolerated

    and it has been reported that the aceticlastic methanogensare most sensitive to ammonia toxicity (Hansen et al.,

    1998).

    In co-digestion, it is important to consider the effects of

    the different incoming waste streams. Better handling and

    digestibility can be achieved by mixing solid waste with

    diluted waste. Furthermore, successful mixing of different

    wastes results in better digestion performance by improving

    the content of the nutrients and even reduces the negative

    effect of toxic compounds on the digestion process. Pig and

    poultry manure have high amounts of ammonia (4 g l21 as

    ammonia-N). These are preferably co-digested with waste

    that has high carbon content to improve the C/N ratio.Sievers and Brune (1978) have reported that the C/N ratio

    should be 16/1 for optimal operation.

    The number of full-scale co-digestion plants is increasing

    and there are many full-scale digesters in operation

    co-digesting manure and industrial organic waste (Danish

    Energy Agency, 1995; Hedegaard and Jaensch, 1999).

    This paper reports on two investigations: co-digestion of

    sewage sludge and potato processing industrial waste, and

    co-digestion of manure, slaughterhouse and agricultural

    waste, both performed in laboratory-scale reactors. The aim

    was to investigate how the co-digestion of the different

    kinds of waste affected the conditions in and performance of

    the anaerobic digestion process.

    2. Materials and methods

    2.1. Co-digestion of sewage sludge and potato processing

    industrial waste

    The model for the first study was a full-scale anaerobic

    digester co-digesting sludge from wastewater treatment

    with starch-rich waste from a potato processing facility. The

    full-scale plant consists of two serially connected meso-

    philic reactors of 3500 m3 each. The substrate for this plant

    has, on average, a total solids (TS) content of 3.4%, with

    85% of this being volatile solids (VS). The main volumetric

    contribution to the plant is excess sludge from municipal

    wastewater treatment (64% of the volumetric flow rate).

    However, in terms of organic material the main constituent

    is starch-rich waste from a food industry facility (72% of the

    VS). The average OLR is 1.4 kg VS m23 d21 and the

    hydraulic retention time (HRT) is 20 days.

    2.1.1. Reactor design

    In the laboratory-scale study, the experimental set-up

    consisted of a jacketed glass reactor (35 8C) with a volume

    of 500 ml, sealed with a rubber stopper. A magnetic stirrer

    was used for mixing. The mixed substrate was fed from a

    cooled vessel (4 8C) once per day into the reactor. The

    amount of gas was measured according to the water

    displacement principle. Four reactor set-ups were used in

    parallel.

    2.1.2. Inoculum and feedstocksThe inoculum for the reactors was taken from the full-

    scale anaerobic digester described above. The two sludge

    fractions, primary sludge and excess activated sludge, and

    the starch-rich food industrial waste were collected at

    Table 1

    Composition of feedstocks as a percentage of the volume and organic material fed to the four reactors co-digesting sewage sludge and potato processing

    industrial waste

    Reference and reactor 1 Reactor 2 Reactor 3

    (% vol) (% VS) (% vol) (% VS) (% vol) (% VS)

    Food industrial waste 36 72 44 80 49 84

    Primary sludge 11 9 9 6 8 5Excess activated sludge 53 19 47 14 43 11

    M. Murto et al. / Journal of Environmental Management 70 (2004) 101107102

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    the full-scale biogas plant, dispersed with a blender, and

    stored at 220 8C until use.

    2.1.3. Experimental procedure

    During a start-up period of 40 days the reference reactor

    and reactors 1 3 were run at an OLR of 1.5 kg VS m23 d21

    and with a substrate composition as in the full-scale

    anaerobic digester (Table 1). The substrates for the referencereactor and reactor 1 had this composition during the whole

    study. The reference reactor, used as a control, was

    maintained at this OLR throughout the study to verify that

    the substrate remained unchanged during storage. On day 40,

    the composition of the substrates for reactors 2 and 3 was

    changed, in that the fraction of starch-rich sludge was

    increased (Table 1). The OLR was then increased stepwise in

    the reactors 13 by decreasing the HRT until failure of the

    process. The reactors were maintained at each OLR for a

    minimum of three HRTs.

    2.2. Co-digestion of manure, slaughterhouse

    and agricultural waste

    The model for the second investigation was a planned

    full-scale co-digestion plant. The full-scale anaerobic

    digester was intended to treat around 45,000 tonnes of

    organic waste per year. The base fractions constituted of

    pig manure (35,000 tonnes per year) and various

    industrial waste (7000 tonnes), to which it was possible

    to add two other waste fractions, slaughterhouse waste

    (5000 tonnes) and restaurant, fruit and vegetable waste

    (2000 tonnes), to obtain a more favourable carbon/nitro-

    gen ratio in the feedstocks.

    2.2.1. Reactor design

    The experimental set-up consisted of a cooled

    substrate vessel (4 8C) and a 3-litre jacketed glass reactor

    (35 8C). An impeller (200 rpm) was used for mixing and

    was turned off every second hour for 15 min. The

    substrate was fed into the reactor once every 4 h with a

    peristaltic pump. The produced gas was collected in agas-tight bag and the volume was measured with a wet-

    type precision gas meter (Schlumberger, Karlsruhe,

    Germany). Three reactor set-ups (reactors A C) were

    used in parallel.

    2.2.2. Inoculum and feedstocks

    The inoculum was taken from a full-scale anaerobic

    digester in Karpalund, Kristianstad, Sweden, where manure

    and slaughterhouse waste are co-digested with small

    amounts of household waste and industrial waste.

    The waste fractions were collected from local

    industries. The composition of the mixture of industrial

    was te i s given i n Tabl e 2. The three differentcombinations of feedstocks used in the experiments are

    given in T able 3. The different substrates were

    homogenised, mixed and stored in bottles at 220 8C

    until use. The substrate was thawed, and sanitised for 1 h

    at 70 8C before use to mimic the procedure at full-scale

    operation.

    The characteristics of the separate waste fractions are

    listed in Table 4, and those of the three different substrate

    mixtures in Table 5.

    2.2.3. Experimental procedure

    During a start-up period of 30 days, the HRT was set

    at 50 days and thereafter it was decreased to around 30

    days, giving OLRs of 2.6, 3.1 and 3.7 kg VS m23 d21 for

    reactors A, B and C, respectively. When the HRT was

    decreased reactor C became unstable and foam was

    Table 3

    Compositions of feedstocks co-digested in the three reactors, AC

    Reactor A (% vol) B (% vol) C (% vol)

    Mixture of industrial waste 17 17 17

    Pig manure 83 71 66Slaughterhouse wastea 12 12

    Restaurant, fruit and vegetable waste 5

    a 50% sludge, 25% rumen and intestinal contents and 25% manure.

    Table 2

    Composition of the mixture of industrial waste fed to reactors AC

    Waste fraction (wt%)

    Grease trap residues 87

    Confectionary waste 7

    Dairy product waste 2

    Bakery waste 3

    Fodder/mill waste 1

    Table 4

    Characteristics of the waste fractions

    pH Total solids

    (%)

    Volatile solids

    (% of TS)

    Total nitrogen

    (% of TS)

    Total carbon

    (% of TS)

    Phosphorus

    (% of TS)

    C/N ratio

    Mixture of industrial waste 5.4 19 93 0.4 20 0.9 49

    Pig manure 7.2 9 76 7.4 40 2.1 5

    Slaughterhouse waste 5.9 13 96 1.0 60 0.3 58Restaurant, fruit and vegetable waste 4.5 21 95 3.8 49 0.4 13

    M. Murto et al. / Journal of Environmental Management 70 (2004) 101107 103

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    produced. The foam caused clogging in the reactor,

    which was then reconstructed, allowing a larger head-

    space, and restarted. The OLR had to be decreased to2.6 kg VS m23 d-1 (HRT 36 days) before the foaming

    stopped, and the reactor was operated at these conditions

    throughout the remaining time of the study.

    2.3. Analytical methods

    The partial alkalinity (PA), total alkalinity (TA), VFA

    concentrations measured by high performance liquid

    chromatography (HPLC), TS, VS and gas composition

    were measured by the methods described in (Bjornsson

    et al., 2000). Samples were centrifuged (3,000 g) for

    3 min and the supernatant was used for alkalinity and VFAmeasurements. The VFA samples were acidified and stored

    at 220 8C. They were then filtered (0.45 mm Minisart,

    Sartorius AG, Gottingen, Germany) before analysis. The

    VFAs are given only as the total VFAs (TVFAs) expressed

    as g acetic acid (HAc) l21.

    A number of chemical characteristics of the feedstocks

    were determined by AgroLab AB, Kristianstad, Sweden,

    as follows: total nitrogen, (Swedish Standard SS

    028101:1-92 mod), ammonium nitrogen (KLK nr7 1950

    mod), total carbon (M-1011) and phosphorus (SS

    028150, IC-AES).

    3. Resu...

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