Evaluation of the Anaerobic Co-Digestion of Sewage Sludge and Tomato Waste at Mesophilic Temperature

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<ul><li><p>Evaluation of the Anaerobic Co-Digestion of SewageSludge and Tomato Waste at Mesophilic Temperature</p><p>Siham Belhadj &amp; Yassine Joute &amp; Hassan El Bari &amp;Antonio Serrano &amp; Aida Gil &amp; Jos . Siles &amp;Arturo F. Chica &amp; M. ngeles Martn</p><p>Received: 24 September 2013 /Accepted: 10 February 2014# Springer Science+Business Media New York 2014</p><p>Abstract Sewage sludge is a hazardous waste, which must be managed adequately.Mesophilic anaerobic digestion is a widely employed treatment for sewage sludge involvingseveral disadvantages such as low methane yield, poor biodegradability, and nutrient imbal-ance. Tomato waste was proposed as an easily biodegradable co-substrate to increase theviability of the process in a centralized system. The mixture proportion of sewage sludge andtomato waste evaluated was 95:5 (wet weight), respectively. The stability was maintainedwithin correct parameters in an organic loading rate from 0.4 to 2.2 kg total volatile solids(VS)/m3 day. Moreover, the methane yield coefficient was 159 l/kg VS (0 C, 1 atm), and thestudied mixture showed a high anaerobic biodegradability of 95 % (in VS). Although theammonia concentration increased until 1,86423 mg/l, no inhibition phenomenon was deter-mined in the stability variables, methane yield, or kinetics parameters studied.</p><p>Keywords Sewage sludge . Tomatowaste .Mesophilic anaerobic co-digestion .Methaneyield .</p><p>Organic loading rate</p><p>Introduction</p><p>Water treatment systems started to become popular in the 1990s as a consequence of a higherand higher influence of environmental consciousness in society reaching an 80 % in Europeregarding wastewater [1]. Wastewater plants generate a high level of potentially dangeroussludge. At the end of the 1980s, sludge started to become a problem as it increased considerably,so water treatment was a matter to bear in mind. In particular, sludge production has increased a28 % in the last 25 years in Europe [2]. Biological sludge is a waste that ought to be managed</p><p>Appl Biochem BiotechnolDOI 10.1007/s12010-014-0790-9</p><p>S. Belhadj : Y. Joute : H. El BariLaboratory of Environmental Biotechnology and Quality, Faculty of Sciences, University Ibn Tofail(Morocco), BP 133, Kenitra, Morocco</p><p>A. Serrano : A. Gil : J. . Siles : A. F. Chica :M. . Martn (*)Department of Chemical Engineering, Campus Universitario de Rabanales, University of Cordoba (Spain),Edificio Marie Curie (C-3), Ctra. N IV, Km 396, 14071 Cordoba, Spaine-mail: iq2masam@uco.es</p></li><li><p>properly due to the high volume generated (8.3 million tons of dry solid matter in 2006), as wellas the elevated concentration in heavy metals, organic polluting agents, and pathogens; thereby, itconstitutes a problem for the environment and a risk for the human health [3].</p><p>The traditional technique used for the sludge treatment has been to deposit the waste inlandfill sites [4]. Nowadays, the directive tends to propose new treatment technologies such asincineration, autothermal thermophilic aerobic digestion (ATAD), composting or anaerobicdigestion [5]. The techniques used in Europe vary depending on the legislation and themanagement strategies of each country. More than two thirds of the total volume of sludgeis used in the agriculture in Spain and Ireland, whereas in Holland, Germany, and Belgium,incineration is used as the main technique. Other countries, such as Greece and Malta, stilldeposit the waste in landfills, as well as other developing countries [6].</p><p>This technique is one of the most extended options, although it involves wasting organicmatter and nutrients that can be found in residues [7]. Besides, green house gasses aregenerated in the landfills, such as CO2 and CH4, as well as unpleasant odor compounds witha consequent impact on the environment and the human health. For that reason, the Directive1999/31/EC [4] on landfills settles an objective on reducing the municipal biodegradable wastedeposited by 35 % for 2016.</p><p>Incineration is a technique that permits generating energy and reducing the volume ofwaste. However, the sewage has a high content in water, so it makes it a barely viabletechnique. Likewise, the gas produced during the incineration could contain environmentallyhazardous compounds such as dioxins, SO2, NOx, which could cause acid rain and environ-mental pollution [8]. On soils that are poor in organic matter, sewage treatment is a frequentorganic solution, as well as the composting treatment and its use in agriculture, although itinvolves drawbacks such as the presence of heavy metals, specially cadmium, and othercompounds that could have an impact on human health [9]. Regarding ATAD, this emergingtechnology presents drawbacks such as an elevated consumption of oxygen and energy [10].</p><p>Anaerobic digestion is a broadly used technology, which is viable to stabilize and treat thesewage sludge and further organic waste [11]. This technique is characterized by low levels ofbiological sludge generation, low nutrient requirements, high efficiency, the production ofmethane, which can be used as an energy source, and stabilized fertilizers, recovering N and Pby the soil [7, 12]. However, the single anaerobic digestion of sewage sludge implies a lowbiodegradability and methane production rate [13].</p><p>On the other hand, co-digestion implies digesting several substrates to improve the viabilityof the anaerobic digestion. It is a long-established process in Europe, as Germany andScandinavia were pioneers and have 20 years experience in the process now [14, 15]. Thistechnique allows improving the methane production and diluting some inhibitory compoundsthat are present in digested organic residues [16, 17]. Jansen, Gruvberger, Hanner, Aspegren,and Svrd [18] described an increase in the methane production of a 21 % in the mesophilicanaerobic digestion of sewage sludge by adding a 20 % of food waste in total volatile solids(VS). Another co-substrate that is extensively employed at full scale is the organic fraction ofmunicipal solid waste (OFMSW), which allows increasing the methane yield since OFMSWprovides carbon and compensates the excess of nitrogen from sewage sludge [18].</p><p>Amongst the possible co-substrates for the sewage sludge digestion, agricultural residuesare an interesting alternative compared to other possibilities, such as purines, due to theircomposition and widespread location [19]. An example would be fruit and vegetable waste inMediterranean areas. The production of tomatoes is worldwide (reaching an internationalproduction of 159 million tons per year). China is the leading producer with a 30 % (48 milliontons per year). The next in the top list of producers is India, with a 10 % of production. Othercountries, such as Morocco, have a 0.7 % (1 million tons) [20]. However, tomato</p><p>Appl Biochem Biotechnol</p></li><li><p>has a high level of humidity and a high content in nitrogen and cellulose (12.5 %),hemicellulose (7.9 %), lignin (1.4 %) and a low content in heavy metals [21]. Tomatowaste would be interesting to combine with sludge, as it would dilute the content inheavy metals it has, and it would allow improving a balance in the main nutrients such ascarbon, nitrogen, and phosphorus.</p><p>In this research study, anaerobic co-digestion with residual tomato in mesophilic conditionshas been proposed to improve sewage sludge treatment. The addition of residual tomatogenerated in farms or green houses allows treating and revaluing this residue, avoiding theenvironmental impact that storing in landfills provokes. This research may be considered ofspecial interest for rural areas and developing countries where both residues are generated.</p><p>Materials and Methods</p><p>Experimental Setup</p><p>The experimental setup used for the anaerobic co-digestion consisted of two 3.5-l Pyrexcomplete mixing reactors working in parallel under mesophilic temperature (35 C). It workedin semicontinuous mode and with recirculation of the solid fraction of the digestate (whichincluded microorganisms and nonbiodegraded substrate). The reactors were equipped withfour connections in order to load feedstock, ventilate the biogas, inject inert gas (nitrogen) tomaintain the anaerobic conditions and remove effluent. The content of the reactors wasmechanically stirred and temperature was maintained by means of a thermostatic jacketcontaining water at 37 C under mesophilic conditions. The volume of methane producedduring the process was measured using 2-l BoyleMariotte reservoirs connected to eachreactor. To remove the CO2 produced during the process, tightly closed bubblers containinga NaOH solution (6 N) were connected between the two elements. The volume of methanedisplaced an equal measurable volume of water from the reservoirs. This volume was correctedin order to remove the effect of the water steam pressure, and the measured methane was thenexpressed at standard temperature and pressure conditions (STP: 0 C and 1 atm).</p><p>The reactors were inoculated with methanogenically active granular biomass obtained froma full-scale anaerobic reactor used to treat brewery wastewater from the Heineken S.A. Factory(Jaen, Spain; total mineral solids (MS) 14,945 mg/kg, VS 53,680 mg/kg). The inoculum wasselected on the basis of its high methanogenic activity [22]. The methane production rateobserved in the employed inoculum reached a value of 58 mlSTP CH4/g added chemicaloxygen demand (CODadded) h.</p><p>Substrate</p><p>The raw materials used as substrate were sewage sludge and tomato waste. The sewage sludgewas collected from an aerobic reactor from a wastewater treatment plant of the city of PuenteGenil, Spain. The sewage sludge was dehydrated in the plant by centrifugation after itscollection. A previous coagulation and flocculation was carried out to facilitate this stage.The tomato waste was obtained from a local marketplace, which dismisses the tomatoes thatare considered unfit for human consumption. Tomato waste presenting a COD:N:P ratio of119:1:1 was previously blended, homogenized, and conserved under freezing conditions. Themain analytical characteristics of the substrates are shown in Table 1.</p><p>The substrate mixture studied consisted of sewage sludge and tomato waste mixture at aratio of 95:5, respectively, in wet basis. The mixture was blended to facilitate handling and the</p><p>Appl Biochem Biotechnol</p></li><li><p>feeding process of the digesters, thus improving the homogenization of the mixtures andavoiding organic overload as previously described by other authors [23]. The studied propor-tion corresponds with the generation ratio of both substrates in the studied area. The mainanalytical characteristics of the mixture are also shown in Table 1.</p><p>Anaerobic Digesters: Experimental Procedure</p><p>The reactors were initially loaded with 7 g VS/l of anaerobic granular sludge as inoculum.Likewise, nutrients (mainly nitrogen and phosphorus) and the trace element solutions de-scribed by Fannin [24] and Field, Sierra-Alvarez, and Lettinga [22] were added when thesludge was loaded in order to reach a nutrient balance close to 300:5:1 for the correct start-upof the process as described by Aiyuk, Forrez, Lieven, van Haandel, and Verstraete [25]. Bothsolutions are very important for activating bacterial growth and metabolism at the beginning ofthe process. Additionally, 1 g KHCO3/l was added to increase the buffer capacity in thereactors during the initial phases of the process.</p><p>In order to bio-stimulate the biomass prior to the experiments, the reactors were first fed with asynthetic solution composed of glucose, sodium acetate, and lactic acid at concentrations of 50 g/l,25 g/l, and 21ml/l, respectively. During this initial period, the organic load added to the reactors wasgradually increased from0.50 to 1.00 gCOD/l, with intervals of 0.25 gCOD/l, over a 15-day period.</p><p>Subsequently, biomass acclimatization was carried out. The reactors were then fed with loadsof 1.00 g COD/l, in which the percentage of the waste mixture in the feeding was increased from25 to 100 % after several loads. During this acclimatization period, the volume of methane wasmeasured as a function of time. The maximum duration of each assay in this stage was 26 h andcorresponds to the time interval required for the maximum gas production and substrate removal.</p><p>Subsequently, during each set of experiments with the waste mixture, the organic loadadded to the reactors was gradually increased from 0.5 to 3.0 g VS/l, which corresponds withan organic loading rate (OLR) interval from 0.4 to 2.2 kg VS/m3 day; each load was carriedout at least in duplicate. In all cases, the volume of methane was measured as a function oftime, and samples were taken and analyzed before and after feeding. Furthermore, the resultshave been referred to the allowed OLR due to its wide use in this research field. To determinethe allowed OLR, we considered the time required to reach 95 % of the total methaneproduction for each load added to the reactors.</p><p>Chemical Analyses</p><p>The following parameters were determined in the effluents of each load: pH, COD (milligramO2 per kilogram), total solids (TS, milligram per kilogram), MS (milligram per kilogram), VS</p><p>Table 1 Analytical characterization of tomato waste, sewage sludge and tomato-sewage sludge mixture (wetbasis)</p><p>Tomato waste Sewage sludge Tomato waste-sewage sludge mixture</p><p>Moisture (%) 94.40.9 88.42.3 89.00.1</p><p>CODt (grams O2 per kilogram) 872 2106 2041</p><p>N-NH4+ (grams per kilogram) 1.110.10 10.511.69 8.650.15</p><p>PT (grams per kilogram) 0.730.09 6.870.32 6.560.18</p><p>COD/N-NH4+ 78 23 24</p><p>COD:N-NH4+:PT 119:1:1 31:2:1 31:1:1</p><p>Appl Biochem Biotechnol</p></li><li><p>(milligram per kilogram), volatile acidity (VA, milligram acetic acid per liter), alkalinity (Alk,milligram CaCO3 per liter), ammoniacal nitrogen (N-NH4</p><p>+, milligram per liter), and solublephosphorus (Psoluble, milligram per liter). All analyses were carried out in accordance with theStandard Methods of the APHA [26]. On the other hand, the same parameters and the moistureand total phosphorus (PT, in gram per kilogram, were analyzed to characterize the sewagesludge, the tomato waste, and their mixture, following the test methods for the examination ofcomposting and compost developed by the US Department of Agriculture and the USComposting Council [27].</p><p>Software</p><p>Sigma-Plot software (version 11.0) was used to create graphs to perform the statistical analysisand to fit the experimental data presented in this work.</p><p>Results and Discussion</p><p>Stability, methane yield coefficient, biodegradability, process kinetics, and inhibitory com-pounds were studied to evaluate biomethanization of sewage sludge and tomato mixture.</p><p>Monitoring Parameters and Stability</p><p>The stability of the process has been monitored through the pH, volatile fatty acids (VFA)concentration, and alkalinity (Alk) in the effluents of the reactors at the end of each load. TheVFA and Alk has been evaluated according to the VFA/Alk ratio described by the WaterPollution Control Federation [28], which established a value of 0.30 as a limit for stable workin anaerobic reactors. Figure 1 shows the evolution of the pH and the VFA/Alk rat...</p></li></ul>

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