Co-digestion of biowaste and commercial waste with agricultural residues Experiences from Castelleone (Italy)

Transcription

1 Co-digestion of biowaste and commercial waste with agricultural residues Experiences from Castelleone (Italy) Paolo Bozano*, Valeria Nosiglia*, Giorgio Vitali* * Biotec Sistemi S.r.l., via Privata Galla 4, Serra Riccò, Genova Italy. paolo.bozano@biotecsistemi.it SUMMARY: It has been proven that the co-digestion of biowaste and commercial residues with agricultural residues brings biological and operational benefits to the anaerobic digestion process. Furthermore, an anaerobic co-digestion plant is a good example of integration between private and public interests, leading to different environmental, economical and social advantages for both farmers and waste treatment companies. In order to guarantee a good codigestion process performance it is essential to remove efficiently the impurities contained within biowaste and commercial residues, thus producing high amount of biogas and high quality fertilizer associated with almost 100% plant availability. Castelleone -IT- co-digestion plant started in 2010 and is authorized to treat t/y ( t/y of biowaste coming from source separated collection; t/y of commercial residues, t/y of agricultural substrates and t/y of cattle and pig slurry), generating kwel and producing approx t/y of digestate, successfully used as fertilizer in farmers field after proper sanitation. In this paper, plant design and operational results are presented. 1. INTRODUCTION Co-digestion is the simultaneous digestion of a mixture of two or more substrates (Wu, 2007). Usually, co-digestion improves the biogas yields from anaerobic digester due to positive synergisms established in the digestion medium and to the supply of possible missing nutrients by the co-substrates (Mata-Alvarez et al., 2000). Co-digestion of biowaste or commercial residues with agricultural residues gives the following benefits to the stakeholders: to the farmers the possibility to improve the profitability of their biogas plants and a source for free fertilizers; to the waste treatment companies a location where a waste treatment plant will be accepted and the link to the end-user of the fertilizer/digestate produced in the plant; to the authorities/community a good location for a waste treatment plant and the certainty that the digestate produced will be recycled as fertilizer. Thus, it s likely that the plant will be effectively well managed and optimized as local farmers are directly involved and interested in good plant performances; the optimization of the overall environmental impact combining management of waste, manure and fertilizer in Proceedings San Servolo, Venice, Italy; November by CISA Publisher, Italy

2 a single plant equipped with a pipe network for manure and fertilizer handling without the need of road transportation. It must be underlined that different inorganic materials and impurities are contained in biowaste and commercial residues, with continuously varying concentrations and characteristics. Thus, as a matter of fact, experience shows that the waste composition defined in the tenders differs from the real composition analyzed during start-up or acceptance tests of these plants, in particular the amount of impurities to be removed is usually higher than the tendered one and also the total solid content (Nimmrichter, 2012). The presence of inorganic and inert materials, as well as of non defibered organic material, in anaerobic digestion systems can rapidly leads to both biological (biomass inhibition) and mechanical problems (pipe clogging, pump wearing, material sedimentation-floatation inside digesters-tanks) causing low biogas production, low digestate quality, affecting negatively to the fertilizer and compost characteristics (Bozano, 2012). These problems can lead even to the failure of the plant, implying economical and environmental problems. Hence, it is essential to remove efficiently the impurities contained inside the biowaste in the pre-treatment phase in order to guarantee a high reliability of the anaerobic digestion process and to meet the digestate quality requirements. The anaerobic digestion plant in Castelleone, which is in operation since April 2010 with manure and from July 2010 also with biowaste and commercial residues, is a successful example of codigestion plant. 2. PLANT DESCRIPTION The co-digestion plant in Castelleone has been designed and constructed by Biotec Sistemi on the basis of the BTA Process. The plant is owned by a public company and three local farmers, who provide manure and receive fertilizer from the plant. The plant in managed by the public company. The plant is in operation for 310 days per year, 12 hours per day for the reception and pretreatment of the co-substrates and 365 day per year, 24 hours per day for the biological step. The plant has been designed for the treatment of approx tons per year of the following substrates: Source separated organic waste (SSOW) t/year Commercial residues t/year Agricultural substrates, mainly corn (maize) and triticale silage t/year Cattle and pig slurry t/year The commercial residues do include residues like: Residues from the dairy and baking industry Packed residues from the food industry Residues from the processing of edible oils and fats

3 Figure 1. Castelleone plant layout: (1) reception; (2) manure storage; (3) agricultural substrates storage, (4) pretreatment and sanitation; (5) anaerobic digestion; (6) cogeneration (7) biofilter; (8) digestate storage; (9) rejects storage; (10) biogas flare 2.1 Flexibility in the reception of the residues A co-digestion plant should have a high flexibility for the reception of different types of waste. In Castelleone, there are the following alternatives: Cattle and pig manure are pumped from five farms close to the plant and which are connected to the plant with a network of pipes. Manure is pumped from the farms to an intermediate pumping station, and then pumped with a single line to a storage tank inside the plant. From this slurry tank with a volume of approx. 200 m³, the slurry is pumped into the primary digesters; Silages are stored, once a year, in two storage cement trenches with the capacity of t. The silage is taken from the trenches through a wheel loader and charged to a feeder with a capacity of 32 m 3 and two independent screw conveyor systems which feed the silage independently into the two primary digesters. The liquid commercial residues are pumped from the tank lorries into two insulated and heated storage tanks with a volume of 40 m 3 each, from where they are pumped to the primary digesters. The SSOW and the solid commercial residues are discharged on a flat bunker in the waste reception building (max storage volume 180 m 3 ). A front loader feeds the waste into a hopper with a belt conveyor system which transports these residues to the pre-treatment step. 2.2 BTA Hydromechanical Pre-treatment The pre-treatment installed is based on the BTA consists of two main steps: Hydromechanical Pre-treatment, which BTA Waste Pulper is a wet pretreatment system working in batch load. Feedstock is added to pre-filled process water into the BTA Waste Pulper. A mixer generates a vigorous turbulence which promotes a very selective modification of the waste fed: only non-soluble organic components are reduced to fibers by hydraulic shearing forces and brought into suspension. During this first step, in the BTA Waste Pulper the digestible organics are defibered and dissolved, the organic suspension obtained is pumped through an internal sieve

4 with 10 mm mesh size toward the GRS. After suspension extraction, process water is added to dilute and wash contaminants bigger than 10 mm remained within the Pulper. In this way, it s created a low viscosity medium that facilitates a further impurities removal as heavy materials (density > 1 t/m 3 ) settle and light materials float. Thus, there are three streams outcoming from BTA Waste Pulper: raw organic suspension; light fraction (plastics, foils, textiles, wood, etc.) and heavy fraction (stones, bones, batteries, core fruits etc.). Both light and heavy fractions are flushed and washed with recirculated process water. The suspension still contains sand, glass, shellfish, egg, shell pieces and similar smaller than 10 mm which must be removed in order to avoid the above described problematic, such as equipment wear, pipe clogging and settling inside the digester. BTA Grit Removal System (GRS): the grit fraction is efficiently removed in the BTA Grit Removal System (GRS), which basically consists in a storage tank, a hydro-cyclone, a classifying screw and a grit box. Grit-enriched sludge is discharged by the hydro-cyclone s centrifugal forces as underflow into the classifying tube, where it sediments in the grit box placed below. Simultaneously, the content of organic particles in the grit is reduced by elutriation. The separated impurities are discharged in intervals from the grit box to a classifying screw downstream, where the smallest organic particles are removed and the grit is dewatered and collected in a container. The organic suspension, free from contaminants, is pumped to the buffer reactor, which ensures a following constant feeding of the A.D. reactor. All the BTA Hydromechanical Pre-treatment is accomplished with recirculated process water taking advantage from waste water content: part of the digestate is dewatered and merged with water coming from the Hydromechanical Pre-treatment (light fraction, heavy fraction and sand dewatering), filtered, and re-used for the BTA Waste Pulper and plant s purposes. Figure 2. BTA pretreatment scheme.

5 2.3 Sanitation step In order to allow the use and spreading of the digestate on the farmland, the suspension is, according to current law, subjected to a sanitation phase, necessary to ensure the elimination of pathogens. Sanitation of the suspension is accomplished by heating it at 70 C and maintaining the suspension at that temperature for one hour. The third requirement, a particle size smaller than 12 mm, is guaranteed by the mesh size (10 mm) of the internal sieve in the BTA Waste Pulper. Sanitation is executed in a batch operation in three tanks. While one tank receives the cleaned and heated suspension, the second one is mixing and ensuring 1 h of retention and the third one is being emptied. The suspension is heated in tube heat exchangers using the heat recovered from the suspension in the output of the sanitation, which is then cooled down to a temperature close to 38 C, as the mesophilic digestion process requires. The heat comes from the cooling system of the co-generator engines. After the sanitation, the suspension is pumped to the primary digesters, where it is mixed with the other liquid streams which don t need a pre-treatment. Figure 3. Castelleone pre-treatment area - starting from left: two GRS storage tank, sanitation tanks and BTA Waste Pulper. 2.4 Anaerobic Digestion The plant in Castelleone has four insulated concrete digesters (two primary and two secondary) with a capacity of m 3 each. They are mixed by means of lateral Biotec paddle stirrers, which ensure a constant and perfect mixing of the suspension. The anaerobic digestion is carried out in the mesophilic range. The biogas is stored in gas meters (elastic membranes) above each digester. The maximum storage volume per digester is 500 m 3. Through controlled dosing of air inside the digesters, the hydrogen sulfide (H2S) is converted by microorganisms in elemental sulfur, which enters in solution in the suspension.

6 Figure 4. Castelleone plant - anaerobic digestion reactors 2.5 Gas Valorization The biogas produced is send to two CHP units with a total electrical power installed of 1,67 MWel. The buffering volumes of silages, suspension, liquid waste and manure allow to remote control such streams to the digesters with the goal to operate the CHPs very close to maximum load. 2.6 Use of digestate The digestate is partially pumped to a solid-liquid separation unit to separate the filtrate from the suspension. The filtrate is recycled to the wet pre-treatment section as process water. The thickened flow and the digestate are pumped to four storage tanks. The first tank is covered and connected to the biogas collection system, in order to reduce odors and to collect any residual biogas. The digestate is pumped from the storage tanks to the five nearby farms using the same network of pipes of the manure and then it is used as fertilizer. The farms use also their pre-existing manure storage tanks to reach the required storage capacity of 6 months. 2.3 Exhaust air treatment The reception and pre-treatment halls as well as the main equipment and tanks are maintained in underpressure by means of a ventilators and air ducts ensuring approximately 4 volumes recycle per hour. The exhaust air is then treated in a scrubber and a biofilter to capture and biologically degrade process pollutants, hence minimizing smell production.

7 3. OPERATION DATA 3.1 Plant feeding Since July 2010 the plant treats the Source Separated Organic Waste (SSOW) collected in surrounding municipalities; also wastes produced in other Italian communities are treated in the plant as waste amount from local communities is not enough. So far an average of t/month of SSOW have been fed. During 2011, tons of SSOW, tons of commercial residues, tons of agricultural substrates and tons of manure were fed. Commercial residues feeding is highly variable (from a max. of t/month to a minimum of 79 t/month) depending on the availability of this stock. Also the type of this feedstock is changeable and includes: milk, beer, pasta, cheese, biscuits, bread, oil, rice, snacks, preserve of fruits, creams, chips, chocolate Easter eggs, etc. As BTA Waste Pulper is able to separate organic content from inorganic (such as plastic, glass, wood), commercial residues (see commercial residues picture in figure 2) are directly fed with their packaging into the Pulper. The liquid products (beer, oil, etc.) are directly fed inside the digesters or previously stocked in dedicated tanks (as visible in figure 4). Cattle and pig slurry feeding during latest months of 2011 stabilized at around t/month (design basis t/month). Agricultural substrates (mainly triticale silage) feeding is tendentially minimized as these feedstock are paid and represent a cost. In 2011 the average Hydraulic Retention time (HRT) has been 23 days and volumetric loading rate was estimated to be around 2,9 kgsv/m 3 d. Figure 5. Castelleone A.D. plant feeding Assuming a TS content of 30% for SSOW, 20% for Commercial redisidues, 35% agricultural substrates and 4,4% for cattle and pig slurry, it was possible to estimate the TS distribution of the different feedstock fed in 2011 in figure 6.

8 Figure 6. Castelleone A.D. plant feeding in percentage, considering overall and TS amount 3.2 Plant products In tons of digestate were produced and re-used as fertilizer in farmer s fields. Pretreatment rejects were on average 11 % (8,8% light fraction, 2,4% heavy and grit fraction) of the material fed to the pre-treatment (SSOW + Commercial Residues). In autumn 2011 Biotec has introduced a new dewatering system of the light fraction which has increased the light fraction TS, therefore the amount of rejects in 2012 is expected to be lower. Light fraction is mainly made up by plastics and organic materials not defibered within the Pulper as they contain lignin which is not defibered nor anaerobically degradable. The last consideration is valid also for Mater-B bags, which are degrabable but only under aerobic conditions, while in anaerobic plants this material can cause mechanical problems; Mater-B bags have proven to be very well suitable for the plant as they are efficiently removed. Rejects are stored and disposed in landfill. Figure 7. Castelleone pre-treatment feeding and rejects production

9 3.3 Energy production In MWhel, corresponding to an average energy power generation of 1,48 MWel, were produced. During the firsts months of 2012, the average power generation was 1,59 MWel. In 2011 the two cogeneration units worked for hours, corresponding to 93% of the available yearly hours, while in the firsts months of 2012, the cogeneration engines worked for the 98% of the available yearly hours, producing 95% of the maximal electrical power energy that the engines can generate operating 24 h/d at full load. Plant electrical energy consumption was 23.5% of the overall the plant energy production in 2011, during 2012 this percentage decreased to 21.8%. Figure 8. Energy production and consumption and % of cog. operating hours on total. In 2011, the overall specific energy production was 194 kwh per ton of material fed, assuming that all the biogas produced was converted in energy and not flared. Focusing on the contribute of each single feedstock on the overall biogas and energy production, the specific biogas production of agricultural substrate (mainly maize) and cattle/pig slurry is quite easily predictable, while it s difficult to estimate SSOW and commercial residues contribution. In particular, SSOW composition and moisture is heavily variable depending on season, collection area and methodology; commercial residues includes a large variety of products with a wide spectrum of characteristics. In order to estimate the contribution of commercial residues and SSOW altogether, the values reported in table 1 have been considered. These values are taken from literature and experience. Table1: substrates characteristics Agricultural substrates Cattle and pig slurry ST 35,0% 4,4% SV/ST 92% 79% Nm 3 /tsv % CH 4 51% 60% kwh/nm 3 5,1 5,9 kwh/t fed

10 With the assumptions showed in Table 1 and considering a cogeneration engine energy conversion efficiency of 39%, in 2011 an average specific energy production of 422 kwh per ton of commercial residues and SSOW fed was estimated. In 2011 the specific biogas production of SSOW and commercial residues was 185,4 Nm 3 /t, with a methane content of 60%, corresponding to a methane production of 111 Nm 3 per ton of SSOW and commercial residual fed. Figure 9. Single feedstock contribute on overall energy production 3. CONCLUSIONS The co-digestion of biowaste and commercial residues with agricultural residues is a good example of integration between private and public interests, leading to different environmental, economical and social advantages for environment, farmers and waste treatment companies. The anaerobic co-digestion plant of Castelleone treats biowaste, commercial residues, agricultural substrates and cattle and pig manure. In 2011 and 2012 an average energy power generation of respectively 1,48 MWel and 1,59 MWel were produced by the cogeneration units, which worked for 93% of the time in 2011 and 98% in the firsts five months of A specific energy production of 422 kwh and 185,4 Nm 3 of biogas per ton of SSOW and commercial residues fed was estimated. Castelleone plant is a successful example of anaerobic co-digestion plant, combining environmental, public and private interests. REFERENCES Bozano P., Giorgio Vitali, Valeria Nosiglia (2012). Anaerobic digestion of municipal solid waste: an example of successful revamping applying single stage wet system. SIDISA 2012 proceeding acts. Nimmrichter R., Rahn T., Schulte S., Bozano P.(2012). The importance of an efficient removal of impurities from organic material before its anaerobic digestion at Mechanical Biological Treatment Plants for Municipal Solid Waste. Mata-Alvarez, J.; S. Mace; P. Llabres Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresource Technology 74: 3-16 Wu, Wei Anaerobic co-digestion of biomass for methane production: recent research achievements. Iowa State University.