Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 MONITORING THE PERFORMANCE OF TWO MUNICIPAL SOLID WASTE MECHANICAL AND BIOLOGICAL PRETREATMENT FACILITIES IN GREECE A. EVANGELOU 1, E. KOTSIARI 1, S. GERASSIMIDOU 1, N.MAVRAKIS 2 and D. KOMILIS 1 1 Laboratory of Solid and Hazardous Wastes, Dept. of Environmental Engineering, Democritus University of Thrace, Xanthi, 671 00, 2 Mesogeos S.A. e-mail: dkomilis@env.duth.gr EXTENDED ABSTRACT Despite the numerous laboratory or pilot scale experiments on MSW treatment processes, measurements from field scale MSW treatment facilities have major practical merit. Objective of the work was to monitor the performance of two new large scale commingled Municipal Solid Waste (MSW) mechanical and biological pretreatment (MBT) facilities in Greece, namely: i) an aerobic biological pretreatment (composting) facility (PLANT A) located in Kefallonia, and ii) a biodrying facility (PLANT B) located in Heraklion, Crete. Monitoring data from a 1.5-years sampling period are presented for both plants. Both facilities started operation on 2009 in Greece. Plant A accepts 13000 t/y of commingled MSW, which are shredded, screened and then directed to four (4) aeration cells (biocells). A 28-d active aeration retention time per cell is achieved followed by a 4 week curing period in open piles. The pretreated end product is currently discarded to the local landfill. Plant B accepts approximately 75000 t/y of commingled MSW which are first shredded and are then placed in 24 15-day retention time aeration beds. Material is not wetted during the process and negative aeration (sucking) is applied to dry the wastes. Magnetic separation of ferrous material is applied at the end of the process. Then, wastes are baled with wires prior to their disposal to the local landfill. A future goal is the incineration of the end product. Sampling of wastes at plant A was performed on the raw refuse, on the product exiting the biocells and at 4 stages during the curing period. Material characterization comprised: moisture (M) content, organic matter (OM) content, total organic carbon content, total nitrogen content, and microbial respiration activity (MRA) as this was measured via the oxygen consumption and the C-CO 2 generation after 7 days. Results show that OM started from around 68% to 75% dw and reduced to around 61% to 64% dw after 8 weeks of combined composting/curing. Material exiting the biocells had a moisture content of less than 30% indicating a moisture limitation during the active composting process. The static respiration indexes indicated that some stabilization occurred during the process, but the final material cannot be characterized as stable compost. The performance of plant B was studied through measurements of moisture, organic matter, total organic carbon, total nitrogen, higher calorific value and MRA indexes at the inlet and outlet of the facility. Results show that initial and final moisture contents were approximately 50% and less than 20% ww, respectively. Lower heating values at the inlet and outlet were approximately 5.5 and 10 MJ/wet kg, respectively. The OM content significantly reduced during the process from a range of 63%-77% dw (inlet) to a range of 61%-70% dw. Stabilization took place for some samples within the biodrying facility. Keywords: biodrying, composting, mechanical biological pretreatment, microbial respiration activity, municipal solid wastes.
1. INTRODUCTION The main municipal solid waste (MSW) management technology in Greece is sanitary landfilling. Since the European legislation requires extensive treatment prior to landfilling, some MSW pretreatment facilities recently started to emerge. In Greece, there are currently 75 sanitary landfills, 2 mechanical and biological aerobic pretreatment (MBT) facilities, one biodrying facility, one aerobic biological pretreatment (composting) facility, and 31 material recovery facilities for source separated MSW (clean MRFs). Objective of the work was to monitor the performance of two new (built on 2009) commingled MSW MBT facilities in Greece, namely: i) an aerobic biological pretreatment (composting) facility (PLANT A) in Kefallonia, and ii) a biodrying facility (PLANT B) in Crete. Data from a 1.5-years operation period are presented for both plants. The performance of the above facilities was assessed via measurements of moisture, organic matter, C, N, calorific value and various indexes of microbial respiration activity at several stages of the process. 2. DESCRIPTION OF FACILITIES 2.1. Composting facility (Plant A) The aerobic biological pretreatment (composting) facility in Kefallonia (Plant A) treats 13000 t/y of the undersized fraction of commingled MSW. MSW are first shredded and screened, in order to reduce their size, and are then directed to four biological aeration cells (biocells) of equal capacity. The 4 aeration biocells provide a 28 day retention capacity each. Air is pumped into the wastes (forced aeration) via perforated pipes in the bottom of the cells. Two probes are inserted within the waste mass in each cell to monitor temperature. Other sensors measure relative humidity inside each biocell. The air flowrate per cell ranges from 1700 m³/h to 8500 m³/h so that to reach and maintain internal temperature above 55 o C for at least 72 hours. As soon as the above criterion is satisfied, air flowrate is adjusted at 1700 m 3 /h, if the temperature falls below 55ºC, and at 8500 m 3 /h, if the temperature exceeds 55ºC. After 28 days of active composting, the endproduct is cured for 4 weeks in open air static piles. MSW samples in Plant A were collected before and after the loading of each biocell (i.e., at 0 and 28 days, respectively), as well as during the four weeks curing time (i.e., at 35, 42, 49 and 56 days). Approximately 3 to 5 kg (wet weight) of sample was collected using a sequential quartering process at each of the aforementioned composting process stages and was shipped to the university laboratory in plastic buckets for analysis. The 4 cells will be identified as N1, N2, N3 and N4 hereon. 2.2. Biodrying facility (Plant B) Commingled MSW were sampled from the inlet and outlet of a biodrying facility (Plant B). The facility receives daily 250 tonnes of commingled MSW ( 75000 t/y). After entering the plant, MSW are first shear-shredded and are then placed into 24 biodrying cells. Each drying cell achieves a retention time of 15 days. Air is supplied with negative aeration through the waste mass to remove water via advection and to promote some aerobic degradation. Perforated pipes in the bottom of the cells direct the aeration stream to an open-air biofilter that contains shredded and composted branches/wood chips and some organic compost. At end of the process, MSW from each bed pass through a magnet, to remove ferrous material, then baled and finally landfilled. According to the literature, biodrying facilities aim to reduce the moisture content of wastes, whilst some minor amounts of carbon may be lost. It is noted that extensive stabilization of the organics is
not desired within a biodrying facility, since this may lead to a reduction of the energy content of the bio-dried wastes that are commonly directed to an incineration facility. Eighteen inlet and eighteen outlet MSW samples were collected over 8 sampling events in 2010 and 2011. The sampling interval between each pair of inlet and outlet samples was 15 d, which corresponded to the biodrying retention time within the facility. The sampling at the inlet of the facility took place randomly right after MSW shredding with the aid of a grab. Sampling at the outlet of the facility took place after the removal of the ferrous metals by destroying a few bales. Between 10-15 kg (wet weight) were selected from each location (inlet, outlet) via manual quartering and shipped to the university laboratory in plastic buckets. Approximately 40 50% of the total wet amount that reached our laboratory was selected using the same quartering process, and was divided into sub-samples for analysis. 3. ANALYTICAL METHODS The measurements performed for all samples were: moisture content (M, total carbon (C) content, total nitrogen (N) content, organic matter (OM) content and microbial respiration activity (MRA). Moisture content was measured at 75 C till constant weight. Grinding of the dried material was performed with a RETSCH cutting mill (1.5 mm mesh size). For both facilities (plant A and plant B), inorganic components (stones, metals and glass) that were present in the initial sample were manually removed after drying and prior to grinding. Therefore, only the organic fraction (including plastics) of the commingled MSW was ground and analyzed. OM was measured via the loss on ignition at 550 C for 2h. C and N were measured using an elemental analyzer (CE Instruments, CHNS-O Model EA- 1110) according to Komilis et al. (2012). Microbial respiration was measured via a static manometric respiration test described in Komilis et al. (2011) using 50 g of wet samples from which plastics had been manually removed (i.e. microbial respiration indexes are expressed per dry kg of biodegradable wastes). The MRA indexes were: the maximum oxygen consumption rate recorded over a 24h period (SRI 24), the total O 2 consumption after 7 days (CRI 7) and the total C-CO 2 generation after 7 days. ph (only for the composting plant samples) was measured according to standard methods. The energy content (higher heating value, HHV) was measured for the MSW (incl. plastics) from the biodrying plant using a Parr plain jacket bomb calorimeter (Model 1341EE) equipped with a 1108 Oxygen Bomb (Komilis et al., 2012). The lower heating value (LHV in MJ/kg on a ww basis), was calculated using the HHV d (in MJ/ dry kg) and the moisture content (M) using the formula: LHV = HHV d*(1-m) 2.45 * M. 4. RESULTS AND DISCUSSION 4.1. The composting facility in Kefallonia (Plant A) Figure 1 presents the times series of 4 parameters for each composting cell of Plant A. Measurements correspond to days 0 (raw refuse), 28 (end of active composting), 35 (1 st week of curing), 42 (2 nd week of curing), 49 (3 rd week of curing) and 56 (4 th week of curing, final product). According to Figure 1, data were distributed normally in most cases. One MRA index (CRI 7) was highly variable for the waste samples of biocells N1 and N3. In the case of N2, however, CRI 7 reduced significantly from 17 g O 2/dry kg (day 0) to 10 g O 2/dry kg, after 2 months of composting and curing. In the case of N4, CRI 7 reduced significantly from 16 to 8 g O 2/dry kg. In cells N1 and N3, final average CRI 7 values were 15 and 13 g O 2/dry kg, respectively, but these were statistically similar to the CRI 7 values of the starting materials.
Figure 1. Properties of the solid material of Plant A at several stages of composting (means on each boxplot that do no share letter are statistically different at p<0.05 based on Tukey s pairwise comparison test).
All cumulative oxygen consumption curves revealed a two-stage decomposition rate (see Figure 2). According to Komilis et al. (2011), CRI 4 and CRI 7 values for moderately and very stable composts should be less than 5 and 6 g O 2/dry kg, respectively. A typical cumulative oxygen consumption profile for raw and cured MSW from cell N4 in plant A is shown in Figure 2. CRI (g O2 / dry kg) 16 14 12 10 8 6 4 2 0 0 days 56 days 0 1 2 3 4 5 6 7 Days Figure 2. Typical static cumulative respiration indexes (CRI) for raw (0 d) and finished (56 d) products from the composting facility (Plant A). Statistically significant diminishing trends are apparent in the C/N ratio for cells N2, N3 and N4. A reduction of C/N from around 30-37 to around 20-23 is observed in all cases. Although a single C/N value is not a reliable stability indicator, some researchers have sometimes proposed an upper value of 10 for stable composts. Organic matter shows a large variability at each sampling event, which is a result of the heterogeneity of the wastes (since plastics are included in the OM measurement). Yet, a statistically significant diminishing trend is visible in all 4 biocells. OM started from around 68% to 75% dw and reduced to around 61% to 64% dw. Note that the OM contents of composted MSW from 3 large aerobic MBT facilities in Greece that have been analyzed in our laboratory were 31%, 41% and 43% dw. ph has a clear increasing trend throughout the process, which is common in all composting processes. Final ph values of the cured end products ranged from 7 to 8.4, which are expected values for composted MSW. Table 1 presents additional properties of the biodrying materials. According to Table 1, the mean moisture contents of the composted and cured products were all below 30% ww; this is a considered a limiting factor for MSW aerobic degradation processes. This limitation probably explains the high MRA of the material at all composting stages. For example, SRI 24 of the cured end-products is above the proposed upper limit of 130 mg O 2/dry kg/h for moderately and very stable composts (Komilis et al., 2011). The 7-d CO 2 generation is also shown in Table 1. All C-CO 2 indexes were greater than around 8 g CO 2/dry kg at all composting/curing stages, with a proposed upper limit for organic composts being around 4-4.5 g C-CO 2/dry kg (Komilis et al., 2011). As an example, microbial static respiration activity indexes (7-d CO 2 and CRI 7) of 3 aerobic MBT derived composts in Greece were: a) 0.8 g C/dry kg and 1.9 g O 2/dry kg (MSW compost remained in piles for more than 5 years), b) 2.1 g C/dry kg (CRI 7 not available), and c) 4.0-4.7 g C/dry kg and 10-12 g O 2/dry kg (Komilis and Kletsas, 2012; Komilis et al., 2011).
0 56.6 a ± 8.5 Table 1. Characterization of the waste samples from the composting facility* Days Moisture (% ww) SRI 24 (mg O 2/dry kg- C-CO 2 (g/dry kg) Carbon (% dw) hr) 401 a ± 166 (n=17; N=31) 12.8 a ± 7.7 (n=17; N=34) 37.9 a ± 2.6 (n=17; N=83) Nitrogen (% dw) 1.09 a ± 0.30 (n=17; N=80) 28 31.3 b ± 12.4 268 b ± 91 (n=17; N=28) 8.9 b ± 4.3 (n=17; N=33) 34.8 b ± 4.1 (n=17; N=79) 1.50 b ± 0.55 (n=17; N=78) 35 26.1 b ± 16.7 221 b ± 69 (n=17; N=28) 7.4 b ± 3.9 (n=17; N=34) 35.6 b ± 4.2 (n=17; N=82) 1.42 b ± 0.40 (n=17; N=80) 42 26.2 b ± 15.6 224 b ± 57 (n=17; N=31) 8.0 b ± 4.1 (n=17; N=33) 35.4 b ± 3.7 (n=17; N=78) 1.44 b ± 0.29 (n=17; N=78) 49 27.5 b ± 18.2 (n=17; N=48) 233 b ± 77 (n=17; N=27) 8.4 b ± 4.8 (n=17; N=31) 35.5 b ± 3.5 (n=17; N=75) 1.55 b ± 0.47 (n=17; N=74) 56 27.3 b ± 16.1 202 b ± 67 (n=17; N=29) 9.4 ab ± 5.3 (n=17; N=33) 35.1 b ± 4.1 (n=17; N=81) 1.61 b ± 0.54 (n=17; N=78) *: means ± standard deviations; (n=sampling events; N=total number of replicates used to calculate the mean); means on the same column that do not share a letter are statistically different at p<0.05 based on Tukey s pairwise comparison test. 4.2. The biodrying facility in Crete (Plant B) Results for the biodrying facility are shown in Table 2 and Figure 3. According to Table 3, Plant B achieves an adequate moisture reduction. A statistically significant reduction of SRI 24 is observed indicating that some stabilization occurs within the plant. On the other hand, this is not confirmed by another microbial respiration activity index (C-CO 2), for which the differences between the inlet and outlet were not statistically significant. The C and N contents of the starting and end-products are also not statistically significant at p<0.05. Table 2. Characterization of the biodrying starting and end-products* Source Moisture (% ww) SRI 24 (mg O 2/dry C-CO 2 (g/dry kg) Carbon (% dw) IN 51.4 a ± 6.5 (n=18; N=86) kg-hr) 218 a ± 35 (n=3; N=15) 5.3 a ± 1.7 (n=3; N=13) 36.7 a ± 7.3 (n=9; N=47) Nitrogen (% dw) 1.27 a ± 0.32 (n=9; N=45) OUT 17.9 b ± 5.4 (n=18; N=92) 124 b ± 24 (n=4; N=22) 4.6 a ± 0.8 (n=4; N=12) 35.6 a ± 6.2 (n=10; N=53) 1.39 a ± 0.37 (n=10; N=58) *: values shown are averages ± standard deviations; (n=sampling events; N=total number of replicates used in the analysis); means on the same column that do not share a letter are statistically different at p<0.05 using Tukey s test. According to Figure 3, in the case of the November 2011 and December 2011 samples, the outlet had statistically lower CRI 7 indexes (i.e. microbial respiration) compared to that of the inlet. However, no such differences were calculated for the February 2011 paired samples.
Figure 3. Characteristics of the starting and end-products of the biodrying facility (Plant B).
A large variability was recorded in the C/N ratios of both the starting and end-products. C/N ratio varied from 16±3.5 (outlet sample) to 41±5.8 (outlet sample). Only in one case (Feb 2011), the outlet sample had a significantly higher C/N (41±5.8) compared to the inlet (28±4.8). In most cases, the OM content reduced during the process from a range of 63%-77% dw (inlet) to a range of 61%-70% dw. In a few cases, the OM reductions during the process were statistically significant at p<0.05 (Oct 10, Feb 11, Sep 11, Dec 11). According to Figure 3, there was a statistically significant increase of the LHV of the endproduct compared to the raw refuse, which confirms the successful operation of the plant with regard to MSW energy content performance. LHV increased from 6.4±1.6 MJ/wet kg (inlet, N=11) to 11.6±1.5 MJ/wet kg (outlet, N=11). Figure 4 shows the LHV (MJ/wet kg) of MSW in Plant B as a function of the OM content and the moisture content (ww basis) of the wastes. According to Figure 4, the highest LHV is achieved at moistures below 25% ww and at OM contents above 70% dw. LHV (MJ/wet kg) Volatile matter (% dw) - incl.plastics 78% 76% 74% 72% 70% 68% 66% 64% 62% 12 10 20% 12 11 Outlet samples 9 Inlet samples 30% 40% Moisture % (ww) 8 7 6 50% 5 5 5 6 5 60% Figure 4. Contour plot depicting the effect of moisture and organic matter (incl. plastics) on the lower heating value (LHV) of MSW (dots indicate actual measurements). 5. CONCLUSIONS 1. The composting facility produces a semi-stabilized end product that requires further treatment to be rendered compost. 2. The biodrying facility achieves a statistically significant moisture reduction and a statistically significant lower heating value increase. ACKNOWLEDGMENTS This work was funded by Mesogeos S.A. that constructed and operates the 2 MBT facilities described in this work. REFERENCES 1. Komilis D., Evangelou A., Giannakis G. and Lymperis C. (2012), Revisiting the elemental composition and the calorific value of the organic fraction of municipal solid wastes. Waste Manage., 32, 372-381. 2. Komilis D. and Kletsas C. (2012), Static respiration indices to investigate compost stability: Effect of sample weight and temperature and comparison with dynamic respiration indices. Biores. Tech., 121, 467-470. 3. Komilis D.P., Kontou I. and Ntougias, S. (2011), A modified static respiration assay and its relationship with an enzymatic test to assess compost stability and maturity. Biores. Tech., 102, 5863-5872.