a methodology for optimising feed composition for anaerobic co-digestion of agro-industrial wastes

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    Organic wasteBatch assayLinear programming

    forlinetraterdend bssay

    (16.4 L CH4/kg COD d) was obtained by a mixture containing 88% pig manure, 4% sh waste and 8% bio-

    f organith 120t reprereatme

    2007; Weiland, 2000), and there is an increasing interest, mainlyin Europe, in using this technology for bioenergy production. Onthe other hand, it is well known that organic waste anaerobicdigestion produces a new semi-liquid waste: digestate, whichcan be used in agriculture after doing a stabilisation or compostprocess.

    Co-digestion is dened as the anaerobic treatment of a mixtureof at least two different waste types with the aim of improving the

    1993; Chen et al., 2008; Hansen et al., 1998).Alkalinity is necessary to avoid decreasing pH due to accumula-

    tion of volatile fatty acids when applying a high organic load.Anaerobic digesters work in a wide variety of alkalinity valuesdepending on the substrate to be degraded. These values rangefrom 2000 to 18000 mg CaCO3/L (Cuetos et al., 2008; Gelegeniset al., 2007a; Murto et al., 2004; Mshandete et al., 2004).

    Moller et al. (2004) studied the specic methane productivity ofdifferent types of manure in batch tests. The specic methanepotentials measured were 148 41, 356 28 and 275 36 L CH4/kg VS (volatile solids) for cattle, pig fattener and sow manure,

    * Corresponding author. Tel.: +34 981563100x16016; fax: +34 981528050.

    Bioresource Technology 101 (2010) 11531158

    Contents lists availab

    T

    elsE-mail address: juanantonio.alvarez@usc.es (J.A. lvarez).2006). The idea of co-digestion offers several possible ecological,technological and economical advantages, so it can improveorganic waste treatment through anaerobic digestion. Anaerobicco-digestion can increase CH4 production of manure digesters by50200%, depending on the operating conditions and the co-sub-strates used (Amon et al., 2006; Callaghan et al., 1999; Ferreiraet al., 2007; Murto et al., 2004; Soldano et al., 2007). Currently,there is a increasing number of full-scale co-digestion plants treat-ing manure and industrial organic wastes, mainly in Denmark andGermany (Angelidaki and Ellegaard, 2003; Raven and Gregersen,

    ents, C:N ratio, pH, inhibitors/toxic compounds, biodegradable or-ganic matter and dry matter (Hartmann et al., 2003). Optimumvalues of C:N and COD:N ratios of 20 and 70, respectively, havebeen suggested for the stable performance of anaerobic digestion(Burton and Turner, 2003; Chen et al., 2008). However, lower val-ues of C:N ratios (between six and nine) have been reported assuitable for the anaerobic digestion of nitrogen-rich waste(Mshandete et al., 2004). Threshold limits of free ammonia and to-tal ammonia of 1.1 and 4 g N/L, respectively, in swine and cattlemanure digestion have been reported (Angelidaki and Ahring,1. Introduction

    Although anaerobic digestion oestablished technology in Europe, wing about 4 million tons per year, i27.5% of all of the biological waste t0960-8524/$ - see front matter 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.09.061diesel waste.Linear programming was proved to be a powerful, useful and easy-to-use tool to estimate methane

    production in co-digestion units where different substrates can be fed. 2009 Elsevier Ltd. All rights reserved.

    ic solid wastes is anfull-scale plants treat-sents on average, onlynt processes (De Baere,

    efciency of the anaerobic digestion process. Therefore, it is veryimportant to establish the best blend in order to maximise meth-ane production, avoid inhibition processes and make protablebiogas plants.

    The main issue for co-digestion process lies in balancing severalparameters in the co-substrate mixture: macro- and micronutri-Keywords:Anaerobic co-digestion

    The highest biodegradation potential (321 L CH4/kg COD) was reached with a mixture composed of 84%pig manure, 5% sh waste and 11% biodiesel waste, while the highest methane production rateA methodology for optimising feed compof agro-industrial wastes

    J.A. lvarez *, L. Otero, J.M. LemaDepartment of Chemical Engineering, School of Engineering, University of Santiago de C

    a r t i c l e i n f o

    Article history:Received 9 June 2009Received in revised form 17 September 2009Accepted 18 September 2009Available online 14 October 2009

    a b s t r a c t

    An optimisation protocolwastes was carried out. Amaximising the total subs(L CH4/kg substrate d). In omanure, tuna sh waste astudies in discontinuous a

    Bioresource

    journal homepage: www.ll rights reserved.ition for anaerobic co-digestion

    ostela, Ra Lope Gmez de Marzoa, 15782 Santiago de Compostela, Spain

    maximising methane production by anaerobic co-digestion of severalar programming method was utilised to set up different blends aimed atbiodegradation potential (L CH4/kg substrate) or the biokinetic potential

    r to validate the process, three agro-industrial wastes were considered: pigiodiesel waste, and the results obtained were validated by experimentals.

    le at ScienceDirect

    echnology

    evier .com/locate /bior tech

  • Although there is a sufcient methodology for determining bio-methanation potential, most of the approaches used to date are

    for storage until characterisation and triturated until homogenised.Biodiesel waste (BW) was sampled from a biodiesel factory. It con-

    echtained mainly glycerine (glycerol) produced in the transesterica-tion of triglycerides with methanol and sodium hydroxide togenerate biodiesel (methyl esters).

    Granular biomass from a pilot hybrid reactor (UASB-FA) treat-ing wine waste and from an IC reactor treating brewery wastewa-ter was used as inoculum in the biodegradation assays andbiokinetic assays, respectively. During biodegradation assays,anaerobic hybrid reactor was washed and its biomass was lost,so a similar granular inoculum had to be used to carry out bioki-netic assays and the IC reactor biomass was selected. The specicmethanogenic activities of each inoculum were 0.15 and0.10 g CH4-COD/g SSV d, respectively.

    2.2. Analytical methods

    Standard methods (1995) were applied for pH measurementsand for determination of COD, TS, VS, TKN-N, NH4 N and TA (totalalkalinity). Biogas composition (N2, CH4, CO2 and H2S percentage)was analysed by gas chromatography (HP, 5890 Series II) equippedwith a thermal conductivity detector (Molina et al., 2008). Volatilefatty acids (VFA) were determined by gas chromatography (HP,5890A) equipped with a ame ionisation detector (Molina et al.,2008). Samples were previously centrifuged (5 min, 3500 rpm),and the supernatant ltrated through 0.45-lm cellulose lters. To-based on experimental studies concerning the behaviour of differ-ent feedings with different properties of raw waste.

    The aim of this work is to develop a methodology useful fordetermining the most adequate ratios of different co-substratesthat provide an optimised biodegradation potential or biokineticmethane potential.

    For this propose, a linear programming optimisation methodbased on determining restrictions (minimum and maximum val-ues) on several characteristics of the mixture has been developed.In order to validate the methodology, three types of wastes withquite different characteristics (pig manure, sh waste and biodieselwaste) were considered as co-substrates.

    2. Methods

    2.1. Waste and inoculum origin and collection

    Pig manure (PM) was taken from a sewer of a 150-pig fattenerand sow farm, which collects both faeces and urine. It was stored at4 C until characterisation. PM samples were homogenised andsieved to 2 mm. Fish waste (FW) from a canning industry consistedof heads, tails, sh bones and viscera of tuna sh. FW was frozenrespectively. The manure specic methane potential has been im-proved by co-digestion with other substrates: sewage sludge (Mur-to et al., 2004), fruit and vegetable waste (Ferreira et al., 2007),energy crops (Lehtomki et al., 2007), glycerine (Amon et al.,2006) and the organic fraction of municipal solid waste (Hartmannand Ahring, 2005).

    Waste biomethanation potential depends on the concentrationof the three main organic components: proteins, lipids and carbo-hydrates, and a substrate characterisation is required to predictmethane production (Gelegenis et al., 2007a,b; Maya-Altamiraet al., 2008; Neves et al., 2008; Shanmugam and Horan, 2009).

    1154 J.A. lvarez et al. / Bioresource Ttal lipid content was determined using a Standard Soxhlet method(Standard methods, 1995). Proteins were calculated from organicnitrogen composition. Carbohydrates were estimated as theremaining fraction of VS or COD after proteins and lipids weredetermined.

    2.3. Linear programming

    Computational software used to solve linear programmingproblems is actually a strength technological alternative whichfacilitates feasibility studies. Specically, ExcelTM Solver is an ade-quate tool of relatively easy initial programming and versatile pos-terior usage to apply for different problems solution. So, solvermethod from ExcelTM was chosen as linear programming tool in thiswork.

    The reported method consists of maximising an objective func-tion, taking into account several restrictions that need to be ful-lled. Two different objective functions have been considered:the total substrate biodegradation potential (L CH4/kg wet weight(WW) of the substrate); taking into account substrate transforma-tion efciency and the biokinetic potential (L CH4/kg WW d); tak-ing into account kinetic capacity of the anaerobic process. Thetheoretical biodegradation potential value was calculated fromthe COD content of the different wastes using a factor of350 L CH4/kg COD removed (Angelidaki and Sanders, 2004). Thetheoretical biokinetic potential was calculated by considering thefollowing values: 35 L CH4/kg lipid d; 42 L CH4/kg protein

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