12 - Full-Scale Anaerobic Co-digestion of Organic Waste and Municipal Sludge

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<ul><li><p>7/27/2019 12 - Full-Scale Anaerobic Co-digestion of Organic Waste and Municipal Sludge</p><p> 1/6</p><p>Available at www.sciencedirect.com</p><p>http://www.elsevier.com/locate/biombioe</p><p>Full-scale anaerobic co-digestion of organic waste and</p><p>municipal sludge</p><p>Gregor D. Zupancica,, Natasa Uranjek-Zevartb, Milenko Rosa</p><p>aNational Institute of Chemistry, Hajdrihova 19, PO Box 660, SI-1001 Ljubljana, SloveniabMunicipality of Velenje, Koroska 37/b, 3320 Velenje, Slovenia</p><p>a r t i c l e i n f o</p><p>Article history:</p><p>Received 8 May 2006</p><p>Received in revised form</p><p>6 July 2007</p><p>Accepted 10 July 2007</p><p>Available online 20 August 2007</p><p>Keywords:</p><p>Anaerobic digestion</p><p>Biogas production</p><p>Organic waste</p><p>Sludge digestion</p><p>a b s t r a c t</p><p>A full-scale experiment on the anaerobic co-digestion of organic waste from domestic</p><p>refuse (swill) and municipal sludge is described. In a wastewater treatment plant of 50,000</p><p>population equivalents, two conventional mesophilic digesters with a combined volume of</p><p>2000m3 and 20 days hydraulic retention time were used. The digesters usual influent is</p><p>waste sludge from wastewater treatment plants (a mixture of primary sludge and waste</p><p>activated sludge) with an average organic loading rate of 0.8 kgm3d1 of volatile</p><p>suspended solids. In the experiment, organic waste was added to the digester influent to</p><p>increase the organic loading rate by 25% to 1.0 kg m3d1 of volatile suspended solids. Biogas</p><p>quantity increased by 80% and specific biogas production increased from 0.39 m3kg1</p><p>volatile suspended solids inserted prior to the experiment to over 0.60 m3kg1 volatile</p><p>suspended solids inserted, peaking at 0.89 m3kg1 volatile suspended solids inserted. The</p><p>excess biogas was used in a boiler and a 50 kW combined heat and power engine. Electrical</p><p>energy production increased by 130% and heat production increased by 55%. Volatilesuspended solids degradation efficiency increased from 71% to 81% with no increase of</p><p>volatile suspended solids in the digester effluent. Virtually all of the organic waste was</p><p>degraded.</p><p>&amp; 2007 Elsevier Ltd. All rights reserved.</p><p>1. Introduction</p><p>Anaerobic digestion has the potential for treatment of many</p><p>kinds of organic waste (OW) mixtures, mostly in combination</p><p>with municipal sludge. In recent years, such research has</p><p>received much attention [13] due to its potential for</p><p>increased output of biogas (renewable energy) in digestion</p><p>plants and some economic benefits in OW disposal. In the</p><p>past, OW of domestic refuse (swill) has usually been a food</p><p>source for domestic animals, mostly pigs. As a food source it</p><p>was recognised as a possible source of pathogenic hazard [4]</p><p>and was therefore banned for such use. This caused</p><p>accumulation of increased quantities of OW, which are</p><p>disposed of by landfilling. Such handling is prohibited in</p><p>Slovenia by a decree on waste handling and pollution [5]. The</p><p>alternatives offered are processing by anaerobic digestion or</p><p>composting. Incineration is also an alternative, but due to the</p><p>high moisture content, energy recovery is poor and such</p><p>treatment is not very imaginative. Anaerobic digestion is</p><p>therefore the most cost-effective way to efficiently process</p><p>wet OW for energy recovery [2]. Many authors have conducted</p><p>research in this field in recent years. There are many possible</p><p>ways of successfully digesting OW of any kind, ranging from</p><p>conventional mesophilic digestion, where the organic loading</p><p>rate (OLR) is up to 3.7kg m3d1 of volatile suspended solids</p><p>(VSS) [3], to two-stage digesters. Sosnowski et al. [6] and</p><p>Gomez et al. [7] reported successful operation with an OLR of</p><p>3.34.3kgm3d1 of VSS. Gallert et al. [8] reported operating a</p><p>ARTICLE IN PRESS</p><p>0961-9534/$ - see front matter&amp;</p><p>2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.biombioe.2007.07.006</p><p>Corresponding author. Tel.: +3861 4760249; fax: +3861 4760300.E-mail address: gregor.zupancic@ki.si (G.D. Zupancic).</p><p>B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 1 6 2 1 6 7</p>http://localhost/var/www/apps/conversion/tmp/scratch_10/dx.doi.org/10.1016/j.biombioe.2007.07.006mailto:gregor.zupancic@ki.simailto:gregor.zupancic@ki.sihttp://localhost/var/www/apps/conversion/tmp/scratch_10/dx.doi.org/10.1016/j.biombioe.2007.07.006</li><li><p>7/27/2019 12 - Full-Scale Anaerobic Co-digestion of Organic Waste and Municipal Sludge</p><p> 2/6</p><p>single-stage digester with OLR values as high as 8.5 kg m3d1</p><p>of total chemical oxygen demand (TCOD).</p><p>In the municipality of Velenje, about 1200 m3 of wet OW</p><p>(250 tonnes of dry matter) are collected annually. Instead of</p><p>dumping this waste on a sanitary landfill, its potential for</p><p>biogas production was quickly realised and a 15-month full-</p><p>scale pilot project was started to test the possibilities of OW</p><p>co-digestion with municipal sludge. Our digesters are de-</p><p>signed to process the OLR of 1.01.5 kgm3d1 of VSS;</p><p>therefore we had plenty of deviation to multiply the load</p><p>with OW several times. The aim of the work was to</p><p>investigate the possibilities for increasing the portion of</p><p>renewable energy by adding the value to OW residues using</p><p>anaerobic digestion as well as reduction of CO2 emission by</p><p>replacement of fossil fuels (mostly natural gas) in the waste-</p><p>water treatment plant (WWTP) where the experiment was</p><p>conducted with biogas.</p><p>2. Materials and methods</p><p>The municipality of Velenje operates a WWTP of 50,000</p><p>population equivalents (PE) with two mesophilic anaerobic</p><p>digesters of a combined volume of 2000 m3. The digesters are</p><p>fed with municipal sludge from the WWTP semi-continu-</p><p>ously every 3 h from a sludge thickener. The VSS concentra-</p><p>tion in sludge ranges from 10 to 20gl1, total suspended</p><p>solids (TSS) concentration from 20 to 30gl1, and TCOD of</p><p>sludge between 18,000 and 30,000mg l1. Sludge is a mixture</p><p>of primary sludge (PS) and waste activated sludge (WAS). The</p><p>average ratio is 60% of PS to 40% of WAS. The hydraulic</p><p>retention time is 20 days. Biogas produced in the digesters is</p><p>collected in a biogas storage unit and used online in a biogas</p><p>boiler initially to cover all heat demands of all WWTP</p><p>premises, and any surplus is used in a 50 kW combined heat</p><p>and power (CHP) engine. The digesters and power set-up are</p><p>shown in Fig. 1.</p><p>TCOD, TSS and VSS of OW and municipal sludge (influent</p><p>and effluent) were monitored and analysed using standard</p><p>methods [9]. The average values of OW influent are shown in</p><p>Table 1. Total influent load is shown in Table 2 and influent</p><p>composition in Fig. 2. Biogas and pH were also continuously</p><p>measured and monitored. We also monitored the electrical</p><p>power output of the CHP engine and the heat power output of</p><p>the biogas boilers and CHP combined. The degradation</p><p>efficiency presented in this paper is calculated from influent</p><p>solids and dewatered effluent solids. Solids in the digester</p><p>overflow and water from dewatering are not accounted for in</p><p>the degradation efficiency. These unaccounted solids are</p><p>returned to the influent of the WWTP.</p><p>Normal digester operation is with municipal sludge only</p><p>(a mixture of PS and WAS). The experiment involving addition</p><p>of OW was conducted from January 2004 to March 2005. OW</p><p>from domestic refuse was collected from households in the</p><p>local area and brought to the WWTP two to three times</p><p>weekly. Our aim was to slowly raise the digester OLR to</p><p>achieve a steady state in 56 months. Therefore, 3 m3 of OW</p><p>was fed to the digester according to OLR two to three times</p><p>per week from January 2004 to August 2004. From August 2004</p><p>to March 2005 the digester was fed with more OW (up to 6 m3</p><p>per batch) to achieve a steady OLR, because the WWTP</p><p>produced less sludge. The OW was fed to the digester at once</p><p>in a batch. Prior to the experiment, the average OLR was</p><p>0.76kgm3d1 of VSS (0.9kgm3d1 of TCOD). We decided to</p><p>plan the OLR increase gradually by 40%, since a digester</p><p>overload and possible breakdown was just not affordable in a</p><p>fully operating WWTP plant.</p><p>3. Results and discussion</p><p>Fig. 2 shows the VSS content of influent and effluent in the</p><p>digester. We gradually increased the VSS load by 30% from the</p><p>start of the experiment in January 2004 until September 2004.</p><p>In the effluent there was no significant change; therefore, we</p><p>can conclude that practically all of the OW was degraded.</p><p>With such a low OLR (Fig. 3) such a result can be expected.</p><p>Table 1 shows that for most of the time over 90% of the OW</p><p>influent is volatile, most probably biodegradable, which is</p><p>confirmed by the degradation efficiency. In the year 2003 the</p><p>average degradation efficiency was 71%, while at the time of</p><p>the experiment with OW (January 2004March 2005) it was</p><p>81%. After finishing the experiment the degradation efficiency</p><p>again decreased to 73.5%.</p><p>Fig. 3 shows the OLR and biogas production. The average</p><p>OLR in 2003 was 0.9kgm3d1 of TCOD (0.76kgm3d1 of</p><p>VSS). At the time of the experiment we gradually increased</p><p>the OLR to 1.44kgm3d1 of TCOD (1.01kgm3d1 of VSS).</p><p>After the end of the experiment OLR decreased below</p><p>0.6kgm3d1 of TCOD (0.5kgm3d1 of VSS). The specific</p><p>biogas productivity (SBP) prior to the experiment was</p><p>0.39m3kg1 VSS inserted. According to the OLR, biogas</p><p>quantity increased on starting to add OW by 80%. SBP slowly</p><p>increased to over 0.60m3kg1 (peaking in January 2005 at</p><p>ARTICLE IN PRESS</p><p>Nomenclature</p><p>BPR biogas production rate, m3 per m3 of the digesterper day (m3m3d1)</p><p>CHP combined heat and powerTCOD total chemical oxygen demand (mg l1)HRT hydraulic retention time, dOLR organic loading rate, kg of TCOD or VSS per m3 of</p><p>the digester per day (kg m3d1)</p><p>OW organic wastePE population equivalentPS primary sludgeSBP specific biogas productivity, m3 per kg VSS</p><p>inserted (m3kg1)</p><p>TSS total suspended solids (mg l1)VSS volatile suspended solids (mg l1)WAS waste activated sludgeWWTP waste water treatment plant</p><p>B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 1 6 2 1 6 7 163</p></li><li><p>7/27/2019 12 - Full-Scale Anaerobic Co-digestion of Organic Waste and Municipal Sludge</p><p> 3/6</p><p>0.89m3kg1). BPR increased from 0.32 m3m3d1 prior to the</p><p>experiment to 0.67 m3m3d1 in February 2005. Interestingly,</p><p>after finishing the experiment in March 2005, biogas values</p><p>did not return to the values before the experiment. SBP</p><p>increased dramatically and BPR decreased slightly, but it</p><p>remained significantly higher than the values in 2003 (by</p><p>60%). After we stopped feeding the digester with OW at the</p><p>end of March 2005, it took about 30 days for the biogas</p><p>production to start decreasing. At this point, all of the OW</p><p>was most probably degraded. However, it seems that</p><p>the activity of the digester biomass (which is reflected in</p><p>the SBP) needed an additional 5 months to decrease to the</p><p>initial value of 2003. Throughout the experiment, the pH in</p><p>the digester was monitored. The values were always between</p><p>7.1 and 7.5.</p><p>Fig. 4 shows the daily quantity of biogas produced and the</p><p>power output of the boiler and CHP engine. A 40% higher OLR</p><p>resulted in 80% more biogas. The power set-up is designed to</p><p>ARTICLE IN PRESS</p><p>Table 1 Characteristics of OW influent (average monthly)</p><p>Date COD(mgl1) Averagequantity (m3d1) VSS load(kgd1) TSS(g l1) VSS(g l1) Ratio VSS/TSS(%)</p><p>January 2004 199,600 1.00 187 197 187 95</p><p>February 2004 196,950 1.03 173 221 206 93</p><p>March 2004 298,800 1.17 277 247 237 96</p><p>April 2004 189,000 1.29 228 188 177 94</p><p>May 2004 144,500 1.30 150 125 115 92</p><p>June 2004 298,500 1.30 212 230 220 96</p><p>July 2004 309,550 1.07 254 248 237 96</p><p>August 2004 290,800 1.48 253 178 171 96</p><p>September 2004 268,800 1.83 401 240 219 91</p><p>October 2004 219,800 1.80 310 184 172 93</p><p>November 2004 239,750 2.01 384 224 191 85</p><p>December 2004 239,150 2.03 369 194 182 94</p><p>January 2005 146,150 1.93 185 102 96 94February 2005 223,300 2.08 343 176 165 94</p><p>March 2005 184,350 1.08 185 182 171 94</p><p>Fig. 1 Digesters and power set-up.</p><p>B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 1 6 2 1 6 7164</p></li><li><p>7/27/2019 12 - Full-Scale Anaerobic Co-digestion of Organic Waste and Municipal Sludge</p><p> 4/6</p><p>use gas in the boiler first and surplus in the CHP engine.</p><p>Therefore, it is to be expected that in winter months,</p><p>electrical power would be rarely produced, as shown in</p><p>Fig. 4, with production occurring only in the warmer months.</p><p>During the experiment, 45% more heat energy and 130% more</p><p>electrical energy was produced. It is also observed that during</p><p>the period from June to November 2004 the CHP engine was in</p><p>operation over 95% of the time. It has never happened before</p><p>during WWTP operation that the CHP would be fully</p><p>operational for such a long period. Even after November</p><p>2005, the CHP engine was operating more often than in</p><p>previous winter seasons. After finishing the experiment,</p><p>electrical power production decreased to levels similar to</p><p>those prior to the experiment. There is, however, a break in</p><p>electrical power production in May 2004, which was the result</p><p>of engine maintenance.</p><p>On completing the experiment, our opinion, as well as that</p><p>of many other authors [10,11], is that anaerobic digestion is</p><p>the solution to handling OW. All the results clearly show that</p><p>digesting OW (swill) is very beneficial. There are almost no</p><p>residual solids and degradation of OW VSS is very close to</p><p>100%. This is also reflected in increased biogas production.</p><p>ARTICLE IN PRESS</p><p>Table 2 Average monthly total VSS and TCOD load of sludge (PS+WAS+OW)</p><p>Date TotalVSS</p><p>inserted(kgd1)</p><p>TotalCOD</p><p>inserted(kgd1)</p><p>Date TotalVSS</p><p>inserted(kgd1)</p><p>TotalCOD</p><p>inserted(kgd1)</p><p>Date TotalVSS</p><p>inserted(kgd1)</p><p>TotalCOD</p><p>inserted(kgd1)</p><p>Jan-03 1399 1588 Jan-04 1226 2124 Jan-05 1434 2362Feb-03 1668 1892 Feb-04 1805 2157 Feb-05 1835 3491</p><p>Mar-03 1366 1550 Mar-04 1788 2273 Mar-05 1455 1742</p><p>Apr-03 1563 1774 Apr-04 1608 2257 Apr-05 1792 2726</p><p>May-03 1761 1998 May-04 2012 2574 May-05 1891 2682</p><p>Jun-03 1516 1720 Jun-04 1703 2873 Jun-05 1262 1553</p><p>Jul-03 1481 1680 Jul-04 1964 3350 Jul-05 992 1184</p><p>Aug-03 1598 1855 Aug-04 1881 2842 Aug-05 750 868</p><p>Sep-03 1445 1640 Sep-04 1569 2271 Sep-05 1020 1233</p><p>Oct-03 1307 1483 Oct-04 1445 2158 Oct-05 1434 1597</p><p>Nov-03 1680 1989 Nov-04 1538 2370</p><p>Dec-03 1440 1919 Dec-04 1891 2687</p><p>Fig. 2 Influent and effluent VSS quantity and composition.</p><p>B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 1 6 2 1 6 7 165</p></li><li><p>7/27/2019 12 - Full-Scale Anaerobic Co-digestion of Organic Waste and Municipal Sludge</p><p> 5/6</p><p>OW is disposed of (virtually removed) and it is regenerated as</p><p>energy very efficiently.</p><p>4. Conclusions</p><p>A full-scale experiment on co-digestion of OW of domestic</p><p>refuse (swill) with municipal sludge is presented. Results have</p><p>shown that anaerobic digestion is the solution to handling</p><p>OW (swill) and above all it is very beneficial with little adverse</p><p>impacts on the environment. The experiment gave the</p><p>following results:</p><p> Virtually complete degradation of OW. The results showed</p><p>no increase in effluent VSS during the experiment and</p><p>degradation efficiency increased from 71% to 81%.</p><p> 80% increased biogas quantity. BPR increased from 0.32 to</p><p>0.67m3m3d1. SBP increased from 0.39 to a peak of</p><p>0.8...</p></li></ul>