Investigation of Biological Processes Aimed at Improving the Quality of Compost from Bio-Waste

Exclusively aerobic and integrated anaerobic with post-aerobic treatments of bio-waste generated in a given waste management area were compared with respect to the quality of the final compost. Pilot-scale apparatuses were used for reproducing static aerated pile composting and solid anaerobic digestion batch (SADB) for the anaerobic pre-treatment. Compost was assessed according to the organic carbon content and humic and fulvic acid concentrations. Different runs for the integrated anaerobic and post-composting treatment were also performed to evaluate the effect of SADB inoculation. Inoculated SADB results in a very intensive pre-treatment of the bio-waste leading to the production of a compost with a lower organic carbon content < 24% TS. On the contrary the compost arising from the integrated anaerobic and post-composting process in which SADB was not inoculated gave the maximum organic carbon content, up to 30% TS. Similarly the compost generated by the latter process had a higher degree of humification compared to the exclusively aerobic treatment.


Introduction
Bio-waste, the organic fraction (OF) of municipal solid waste and fruit and vegetable waste, is the largest fraction (20% up to 60%) of the municipal solid waste (MSW) generated yearly throughout the world [1,2]. If properly processed it can be an important means for contributing to the restoration of the carbon sink in soils and for substituting mineral fertilizers [3][4][5][6]. On the other hand, if not properly managed bio-waste can be a relevant source of gaseous and liquid emissions with a high pollution potential [7][8][9][10].
Bio-waste can be successfully recovered by composting, resulting in a soil improver/compost that has been favorably accepted in many areas. In the EU27 about 50% of the entire organic waste generated, corresponding to about 35,000,000 mg, is currently composted [11]. In the USA the amount of waste composted in 2013 was about 25,000,000 mg [12]. Composting, however, requires a rather high energy consumption (i.e., 30-50 kWh) and has direct and indirect emissions [13,14]. Another rather diffused technique for recovering material (e.g.. nutrients and organic carbon) and energy from biowaste is anaerobic digestion (AD) [15,16]. AD can also play an important role in the waste management sector for contributing to the achievement of EU 2020 goals [17,18] and for improving the energy efficiency of the whole biological treatment of bio-waste. Full-scale AD technologies currently in use are mainly wet and dry ones [19], equipped with continuous flow digesters, able to process substrates with a total solids (TS) concentration < 15% w/w and < 25% w/w, respectively. Before being processed in such facilities, bio-waste requires more or less important pre-treatments, such as mechanical sorting, shredding, metal separation, moisture increase, and pulping, which are costly and complex operations affecting the viability of AD [20]. On the other hand, the main outputs from AD are a biogas rich mainly in methane (i.e., about 60% v/v) and carbon dioxide (i.e., about 40% v/v) and a digestate rich in nutrients and organic carbon with a moisture content (MC) usually > 80% w/w. Digestate with such a high MC is another important technically and economically negative aspect when using AD for waste treatment in many EU areas [21]. In these cases the digestate generally undergoes a preliminary solid/ liquid separation process before successive recovery operations. The liquid fraction is generally processed in wastewater treatment plants, whereas the solid fraction is composted. Besides being a further cause of cost increase, the dewatering process can remove up to 80% of the nutrients from the solid fraction of the digestate, consequently reducing their concentration in the final soil amendment [22]. As already demonstrated in previous studies, the Solid Anaerobic Digestion Batch (SADB), which operates with TS up to 40% w/w, can significantly reduce these problems even if biogas yields are lower [9,15,23,24]. The digestate, the liquid fraction generated during SADB, is generally < 10% w/w of the amount of substrate processed [15], whereas the solid fraction is directly treatable through a postcomposting phase. A relevant question regarding the batch mode is the use of inoculation. In a previous study [25] reported that the best compromise between energetic and economic performances of SADB was achieved for an inoculum to fresh OFMSW ratio of about 1:1 by weight. As reported by several authors, the post-composting treatment is a mandatory step for reducing the residual phytotoxicity of the digestate and for achieving a high quality and stabilized soil amendment [5,[26][27][28].
Compost quality includes several chemical and physical properties such as pollutant concentration, impurities, residual biological stability, nutrients and organic carbon content [29,30]. The organic carbon content is of particular interest also for CO 2 storage [31]. Two processes affect organic matter during biological treatments: mineralization (organic carbon is converted mainly to CO 2 and/ or CH 4 ) and humification (humus is generated). The humification process, typically occurring under natural soil conditions [32], has started and creates new stabile organic substances (i.e., humiclike substances), with good soil amendment properties. Humus is a composition of humins, humic acids (HA) and fulvic acids (FA). For this reason, biological treatments aimed at material recovery from the bio-waste should generate compost with a high, but also a stable organic matter content. Evaluation of the organic matter together with the HA and FA amounts has been found to be a suitable approach for assessing the quality of compost [29,30,33].
In assessing the amendment properties of the digestate from wet AD of the OFMSW, Tambone, et al. [34] reported a high level of biological stability compared to the fresh material, due to the high degradation of total organic carbon (TOC). Marcato, et al. [35] reported a relevant reduction in digestate biological reactivity compared to the feedstock, and generation of humified substances was limited as well. In another work Tambone, et al. [36] reported a comparative study between digested sludge and compost from OFMSW for assessing, respectively, soil improvement and fertilizing properties. The main findings showed that the digestate had a higher concentration of N (%TS) and K 2 O (%TS), but a lower concentration of TOC (%TS) compared to the compost. Also in these cases the digestate arose from wet AD.
An exclusive aerobic treatment of OFMSW was investigated by Ruggieri, et al. [37]. They reported an organic matter concentration in the final compost of about 50%TS.
In analyzing the wet anaerobic digestion of food industry waste Ruffino, et al. [38] reported a TS and VS depletion up to 100%, whereas the initial MC of different substrates ranged from about 31% w/w up to about 94% w/w. Novarino and Zanetti [39] reported a TS content in the digestate from wet AD of extruded OFMSW ranging from 3% w/w to 10% w/w, whereas VS reduction ranged from 60% w/w to 70% w/w.
In analyzing the compost obtained from more than 200 facilities, Binner, et al. found a HA content from about 5% TS up to 47% TS. Higher values were achieved through co-composting of nonintensively anaerobically pre-treated bio-waste and stabilized sewage sludge. All these findings demonstrate the relevant role played by both the processes and the feedstock used on the final concentration of HA and FA and hence on the quality of the compost.

Aim of the study
The literature surveyed points out that there is a lack of information on how SADB pre-treatment performs in the production of compost from the OFMSW. Hence, the present study is aimed at investigating this aspect by the aid of a suitably designed experimental pilot-scale apparatus. Integrated SADB with post-composting (PC) and exclusively composting (COMP) processes were compared and the quality of the final amendment was assessed mainly by the content of TOC, HA and FA. The effect of the inoculation of SADB was also investigated.

Pilot-scale apparatus and runs procedure
Three separate pilot-scale apparatuses were used: one for the SADB test and two for the aerobic processes. The pilot-scale apparatus used in the analysis of the SADB was the same as reported in Di Maria, et al. [16,27]. Briefly it consisted of a 100 liter gas tight anaerobic reactor with a removable top ( Figure 1a). The temperature in the bio-waste mass was maintained at 35 ± 2°C by the aid of a thermal heating jacket wrapping the digester and by a 2 cm thick insulated layer. The temperature was continuously monitored with a resistance temperature probe inserted inside the processed substrate. During each test a temporized pump withdrew a given rate of liquid digestate from the reactor bottom and spread it on the bio-waste on the reactor top. Gas produced during the process was continuously withdrawn a) b)  Aerobic processes were conducted in a gastight HDPE cylindrical reactor, 30 cm in diameter and 100 cm high (Figure 1b) equipped with a removable top and bottom, reproducing static aerated pile composting. Each cylinder, one for the SADBPC and one for the COMP test, was filled with the solid digestate generated by the SADB and with fresh bio-waste, respectively. 5 Nm 3 of air per kgVS of substrate of process air, delivered by a compressor, were injected at the reactor bottom and withdrawn from the reactor top. A pressure reduction system coupled with a flow meter/regulator was used for setting an adequate amount of air, whereas the process temperature was monitored by the aid of a thermocouple inserted directly in the material processed.
The bio-waste exploited for the tests was the OFMSW. Before the activation of each run, about 100 kg of OFMSW arising from the source segregated collection from a given waste management district in central Italy, were withdrawn directly from the collection vehicles.
After preliminary, manual removal of impurities and bulky components, the OFMSW was mixed and two large samples > 20 kg were generated. One sample was processed exclusively by the aerobic apparatus (COMP), whereas the second sample was first processed by the SADB and successively by the aerobic apparatus (SADBPC). Both tests started simultaneously and lasted for 90 days. In the integrated SADBPC, OFMSW was first processed anaerobically for 30 days and then the solid digestate was further treated aerobically for 60 days. Six parallel runs were performed. The first three SADBPC runs were performed without inoculum. In the second three tests, the SADB was inoculated (I) by mixing 1:1 by weight the fresh OFMSW with the digestate generated from the previous SADB run. During the test the following sampling procedure was adopted: 1. One sample of fresh OFMSW at the beginning of each test.
2. For integrated SADBPC one sample at the end of the anaerobic phase and a sample every 15 days during the PC.
3. For COMP a sample every 3 days during the first 15 days, a sample every 7 days for the successive 15 days and the a sample every 15 days until the end of the run.
Due to the length of the entire run time, some differences with the scheduled sampling procedure occurred during weekends or holidays. Each sample, of about 0.5 kg, was first homogenized and blended before chemical characterization.

Chemical characterization
Total solids (TS), moisture content (MC) and volatile solids (VS) were determined according to standard methods [40]. pH was determined on a 1:10 w/v solid/water suspension with a Delta Ohm HD2305.0 instrument. Total organic carbon (TOC) content was determined according to the Springer-Klee wet dichromate oxidation method [41]. Total Kjeldahl-N was determined according to macro-Kjeldahl distillation methods [42]. Humic-like substances from the digestate were extracted and purified according to Ciavatta, et al. [43]. Briefly, this method involved extraction of the humic-like substances with a 0.1 M NaOH and 0.1 M Na 4 P 2 O 7 solution (1:10 w/v soil to solution ratio) under N 2 at 65°C for 24 h. The suspensions were centrifuged at 12000 rpm for 20 min, and the supernatants were filtered through a 0.45 μm membrane filter. An aliquot of the extracts was acidified to pH 2 with concentrated H 2 SO 4 to separate HA from FA. Coagulated HA were collected, while the supernatants containing the FA were further purified on 10 to 12 cm 3 of insoluble polyvinylpyrrolidone resin (Aldrich, Germany) previously equilibrated in 0.005 M H 2 SO 4 [44]. The eluate contained the nonhumified fraction (NH), characterized by the presence of organic compounds such as carbohydrates, free amino acids, and peptides, which are co-extracted in alkaline solutions [45]. The organic C concentration of the filtered alkaline extract (total extractable C, TEC), as well as that of the NH fraction, was determined using Pt-catalyzed, high-temperature combustion (680°C) followed by infrared detection of CO 2 (TOC-5050, Shimadzu Corp., Tokyo, Japan). HA + FA was obtained by taking the difference between TEC and NH. The degree of humification (DH %) was also calculated as the percentage of the ratio (HA + FA)/TEC. All the analyses were carried out in triplicate.

Results and Discussion
Chemical characterization of the OFMSW (Table 1) showed values in line with those reported by other authors. MC ranged from about 50% w/w up to 65% w/w, whereas the VS concentration ranged from about 60% TS up to 77% TS. The organic carbon concentration varied from about 30% TS up to 38% TS, whereas TKN varied from 0.86% TS to 1.99% TS. The HA+FA concentration ranged from about 23% TOC to about 30% TOC with a DH generally lower than 60% with the exception of sample n° 1, which had a DH of about 72%. As expected all the samples had quite acidic pH values with the exception of sample n° 5. For biogenic waste from separate collection, Smidt, et al. [30] reported a TOC ranging from 26% TS to 47% TS, with a corresponding VS concentration ranging from 60% TS to 85% TS. On the other hand a MC of 73% w/w, a VS concentration of 93% TS, a TOC of 29% TS and a TKN of 2.7% TS were reported by Di Maria, et al. [46] for the source-segregated organic fraction of municipal solid waste. Similar results were also reported by Massaccesi, et al. MC and TOC, reported by Sanchez-Mondero, et al., for municipal bio-waste were 59.4% w/w and 35.4% TS, respectively, with a pH of 6.8.
Temperature trends and levels during the COMP indicated that the larger fraction of rapidly degradable organic compounds was oxidized in the first three weeks of the process (Figure 2a and Figure  3a). In the remaining period the temperature was quite constant and similar to the ambient one. Similarly, the quite uniform values for the temperature of the post-composting of the digestate (Figure 2b and Figure 3b) indicate that the SADB pre-treatment was as efficient in the degradation of the organic matter as the COMP process. In particular SADB-I was more efficient than SADB in the mineralization of the organic compounds into biogas, with the exception of test n°6 (Figure 2c, Figure 2d, Figure 3c and Figure 3d). In fact, on average, SADB-I generated about 300 NL/kgVS versus about 267 NL/kgVS     to the one generated by the corresponding COMP tests (Figure 4e and Figure 4f). Even with the higher C/N ratio the DH% (Table 2) was higher for the compost produced by the former tests. Opposite results were obtained by the test in which SADB was inoculated (Figure 5e, Figure 5f and Table 2).
Ammonium generation is also at the basis of the pH increase determined during both the exclusively aerobic and the integrated tests. The intense microbial activity and TOC degradation during the first weeks of COMP led to the formation of ammonia as a consequence of organic nitrogen ammonification [49]. Then the solubilization of ammonia led to the formation of ammonium and a consequent increase in the pH. There was a similar effect also for the integrated runs. In fact, as is known, anaerobic digestion is very efficient in mineralizing organic nitrogen into ammonia leading, also in this case, to pH increase.
These findings indicate that the quality of the amendment in terms of TOC, HA and FA content was higher for the integrated SADBPC treatment with respect to both SADBPC-I and COMP. The main components of HA and FA are organic carbon and nutrients (e.g., N), which are compounds in a stable form useful for crops and for improving the agricultural properties of soils [29,50]. The final concentration of HA and FA was influenced by several factors among which were their initial concentration in the fresh OFMSW, their compositions, a well-balanced ratio between reactive and less reactive components and the nature and intensity of the processes.  demonstrating that mineralization of organic compounds into CO 2 and CH 4 was enhanced by the amount of inoculum introduced in the digester.
The higher efficiency of SADB-I in the degradation of the organic compounds was also confirmed by the evolution of the TOC concentration during the tests. After 30 days SADB gave a digestate with a TOC content higher than that of the corresponding COMP process (Figure 4a and Figure 4b), whereas opposite results were obtained for the SADB-I runs (Figure 5a and Figure 5b). The higher value for the TOC concentration detected for the integrated SADBPC was also confirmed at the end of the 90 th day ( Table 2). In general the nitrogen concentration, expressed as %TS, increased when organic matter losses were greater than the NH 3 ones [33]. SADB mineralized a large fraction of organic nitrogen to ammonia that can also be leached by the liquid digestate, modifying its content in the solid digestate. The combination of these two phenomena explains the trends of TKN reported in (Figure 4c, Figure 4d, Figure 5c and Figure 5d).
Due to the higher leachability of ammonium with respect to the organic matter, the C/N ratio can be increased at the end of the SADB pre-treatment (Figure 4f). On the other hand during both COMP and post composting, there was a continuous decrease in the C/N ratio (Figure 4e, Figure 4f, Figure 5e and Figure 5f). The lower TOC degradation together with higher nitrogen volatilization leads to a higher C/N ratio for the compost at the end of SADBPC with respect As demonstrated by Binner et al. the lack of well-balanced reactive and less reactive organic compounds limits the humification process.
This means that the solid digestate from SADB was characterized by a very favorable balance between the content of less and more reactive organic compounds compared to the digestate from the SADB-I. Consequently the humification process was enhanced compared both to SADBPC-I and COMP ( Table 2). The ability of intensive anaerobic digestion to mineralize the organic matter with limited effect on the humic generation process was also reported by Marcato, et al. and Tambone, et al. (2009Tambone, et al. ( , 2010. In analyzing more than 200 composting and integrated anaerobic and aerobic treatment facilities for bio-waste, Binner, et al. reported that compost with the higher HA concentration was obtained by anaerobic pre-treatment lasting for a short period (i.e., less intensive). Bernal, et al. reported a  The concentrations of HA reported in the literature for anaerobic and/or aerobic treatment of different substrates are reported in table 3. There were higher HA concentrations for compost obtained after the aerobic treatment of a 1:1 mixture by weight of bio-waste (i.e., reactive) and yard waste (i.e., less reactive) [29]. Positive effects on the HA generation of well-balanced mixtures was also confirmed for the compost obtained after the post-composting of digestate with fresh bio-waste [30]. In particular SADBPC analyzed in this research   (Table 3).

Conclusions
Biological processes can play an important role in improving the quality of the final amendment obtainable from bio-waste (i.e., content of organic carbon and of humic compounds). Less intensive treatments such as static instead of turned aerated plies or anaerobic pre-treatments lasting for short periods before post-composting were able to enhance the humification process, increasing the quality of the final compost. Among the different key factors able to enhance the generation of humic compounds during composting, a balanced presence of less and more reactive compounds in the material plays an important role. The solid anaerobic digestion batch (SADB) process showed suitable features as a pre-treatment for achieving this goal. In fact, the partial inhibition due to the high concentration of volatile solids together with the absence of inoculum reduces the intensity of SADB in the degradation of organic compounds. Consequently the digestate was characterized by a balanced concentration of less and more reactive organic materials able to enhance the humification process during the successive post-composting treatment.
The present study showed how a non-intensive anaerobic digestion pre-treatment results in a digested bio-waste with a balanced concentration of less and rapidly biodegradable organic compounds able to increase the quality of the final compost after a post-composting phase. Increase in TOC, HA and FA content together with the possibility of recovering renewable energy makes the SADB and post-composting a suitable treatment for reducing GHG emissions and contributing to the restoration of the carbon sink in soils.