Co-composting of green waste, food waste and phosphoric rock. Comparison of two-stage composting with traditional composting.


 Green waste (GW) co-composting has limitations due to the content of slowly degradable compounds (i.e. lignocellulosic substances). The introduction of amendment and bulking materials has improved organic matter degradation and end-product quality. However, recent studies have included additional strategies such as two-stage composting (TSC). This research evaluates the effect of TSC on the process and end-product quality of co-composting of GW, food waste, sawdust and phosphoric rock. In our knowledge, TSC has not been studied together with strategies like the addition of co-substrates such as FW and phosphoric rock to improve GW composting. A pilot-scale study was developed using two triplicate treatments: TA: TSC and TB: Traditional composting. The two treatments used the same mixture (wet weight): 46% GW, 19% unprocessed food waste, 18% processed food waste, 13% sawdust, and 4% phosphoric rock. TB had a higher degradation rate of organic matter during the mesophilic and thermophilic phases compared to TA. This is related to the higher temperatures for longer periods during these two phases, with a higher degradation of volatile solids. Nonetheless, during the cooling and maturation phases, the two treatments had similar behavior on temperature, pH, and electrical conductivity with ash and lignin contents without significant differences at the end of the process. End-products in both treatments lacked statistically significant differences and fulfill quality criteria for use as soil improvers. However, end-products from traditional composting had lower nutrient content (NTotal and PTotal) that can be associated with nitrogen volatilization or the use of nutrients by the microorganisms during the active process phases. These results indicate that at the end of the process, there were no significant differences in the total processing time, degradation rate or end-product quality among TSC and traditional composting. Evaluation of complementary strategies must continue to improve GW composting.

nitrogen and potassium. In addition, co-composting of GW and FW simultaneously addresses the management of these two types of waste which together represent the higher proportion of MSW in developing countries [19].
Previous studies that used processed food waste (PFW) and unprocessed food waste (UPFW) [1,16] on GW composting show a reduction of processing times in the mesophilic and thermophilic phases compared to composting of GW only, and improved end-product quality that better ful lls the Colombian quality standards for biofertilizers. However, the cooling and maturation phases (i.e. when the degradation of lignocellulosic substances occur) have shown temperature behavior and processing times similar to the composting of only GW. These studies show the need to introduce additional strategies that contribute to increase the degradation rate of lignocellulosic substances and help to reduce the total processing time.
Operational modi cations to the process are another strategy to improve GW composting [4,10]. One of these strategies is two-stage composting (TSC). This method results in two peaks in the composting temperature (at 55-60 °C or even higher) and a longer thermophilic period. As a consequence, the production of a mature and stable compost requires only 30 days rather than the 90-270 days required for traditional composting [10]. However, in our knowledge, TSC has not been studied together with strategies like the addition of co-substrates such as FW and PR to improve GW composting.
This research evaluates the effect of the TSC on the process and quality of end-product in the co-composting of GW, FW, sawdust (SW) and PR, considering that: i) the mixture of GW and FW has been bene cial for the process and end-product quality of composting [1]; ii) SW have proved to be a bulking agent appropriate for GW composting [4]; iii) phosphorous must be used in the co-composting of GW and FW due to their de ciencies in the product [5,9,16]; and iv) additional strategies for co-composting need to be assessed such as TSC [4,10].

Experimental setup
We developed a pilot-scale composting experiment using two treatments to assess the effect of TSC on the process and end-product quality of co-composting of GW, FW, PR and SW. Treatment A (TA) was TSC and Treatment B (TB) was Traditional Composting. The two treatments had triplicate experimental units of 200 kg. The treatments had the same mixture of materials (wet weight): 46% GW, 19% UPFW, 18% PFW, 13% SW, and 4% PR. The mixture was de ned based on previous studies [1,16], considering two criteria: i) predominance of GW on the mixture, and ii) C/N ratio around 20.
FW was introduced as amendment material to provide readily degradable organic matter (i.e. simple carbohydrates) and nitrogen.
PR was added as bulking agent and material amendment, providing porosity and phosphorus. The amount added was considered taking into account previous studies [5,9,16].
The SW was added as a source of carbon and as a bulking agent. GW was obtained from the maintenance of green areas of a university campus and had the following physical composition: 35% leaves, 26% grass clippings, 20% soil extract, 9% tree branches, 3% fruit, 1% roots and 6% nonbiodegradable materials. Before the experimental setup, non-biodegradable materials were removed (e.g. stones, plastics).
GW was stored for one or two weeks. UPFW and PFW were source-separated and were collected, using composite sampling, from a university restaurant, where approximately 3,000 lunches per day are normally prepared. Both substrates were stored for three days. PR and SW were provided by local suppliers. GW and FW were manually crushed to achieve particle size between 5 and 7 cm. Later, they were manually mixed and homogenized using shovels.
The experiment was developed in the campus of Universidad Industrial de Santander (Bucaramanga, Colombia) (average temperature of 24 °C). The experimental setup was developed in a covered area with concrete oor. All piles were run simultaneously to maintain similar environmental conditions during the experiments.
According to previous studies [16], TSC was carried out in the following way: i) in the rst stage, wooden containers (0.55 × 1.3 × 1.25 m) were used to con ne the mixture, the containers had holes of 5 cm diameter and four perforated pipes (1.4 m high), both to maintain the necessary aerobic conditions for the process. This stage ended when the rst thermophilic phase was completed; ii) in the second stage, the material was removed from the containers and piles in conical heaps. In the second stage, a second thermophilic phase was expected, and the completion of the process when the material reached ambient temperature [10]. During the process the material was removed from the containers on the tenth day to start the second stage of the process. Traditional composting was carried out in conical heaps of 1 m height.

Process monitoring
The parameters monitored were: temperature, pH, electrical conductivity (EC), moisture, oxygen concentration, volatile solids (VS), germination index (GI) and self-heating. Monitoring started right after the preparation of the piles. Temperature was measured daily on the compost pile centroid, using a 60-cm thermometer (i.e. digital thermometer K-Type HI935005N with high accuracy (± 0.2%)). Subsamples taken from four opposite locations in each compost pile were combined to form a 200 g sample that was speci cally used for pH, EC, moisture and VS measurements [20,21]. pH and EC were measured at least three times a week for the rst two weeks and then twice a week at least, until the end of the monitoring. These parameters were potentiometrically measured to an aqueous extract obtained from a stirred mixture of the sample and distilled water (1:10 w/v). Measurement was carried out using a desk pHmeter and EC ionometer, sensIONTM + MM374. Moisture was measured by drying in an oven a 50 g sample at 105 °C for 24 h; this parameter was measured three times a week up to day 42, and later twice a week up to the end of the process. VS were measured, by burning a dried sample at 550 °C in an oven for 4 h. The Total Organic Carbon (TOC) was calculated from the ash content [22].
Pile monitoring was performed until one of the piles reached ambient temperature (24 ± 2 °C) (i.e. day 73). Before to the completion of the experiment, on-site self-heating tests were performed to determine if temperature rises occurred when piles were moisturized [23]. In addition, stability was evaluated by the self-heating test using Dewar asks of 1.5 L of volume according to Brinton et al. [24].
Maturity evolution was established through germination test during the process. The germination index was determined by the methodology established by Varnero et al. [25]. A fresh sample was extracted with distilled water at compost to water ratio 1:10 (w/v), shook, allowed to stand for 3 h and then ltered. Subsequently, 10 mL of extract were placed in 9 cm Petri dishes containing ten seeds of radish on lter paper. Experiments were conducted in triplicates and distilled water was used as control. This test was performed on the days 47, 53 and 60.
Oxygen was provided to the piles through manual turning in the two treatments. Daily turning frequency was used for the two treatments up to day eight of process. Later, turning was performed according to the process requirements (i.e. every two days up to day 25, and then, every three days up to day 37, and lastly once a week until day 73). The Oxygen concentration (OC) was controlled using a probe CM37 (i.e. twice a week until day 37 and once a week until the end of the process).

End-product quality evaluation
At the end of the composting process, manual sieving of the products was carried out using a 1.25 cm sieve. A representative sample of each experimental unit was taken to carry out product quality analysis. Samples were analyzed at the laboratory of the Interdisciplinary Group of Molecular Studies from Universidad de Antioquia, following the methods described in the Colombian Technical Norm (NTC) 5167 [26]. Table 1 presents the parameters analyzed and the techniques used. Finally, the characteristics of the products obtained were analyzed and compared with the standards from NTC 5167 for organic products used as fertilizers and soil amendments or conditioners. The Lignin content was quanti ed using the Neutral detergent ber (NDF), acid detergent ber (ADF) and acid detergent lignin (ADL), considering the procedures established by Van Soest et al. [27].  Figure 1 presents the temperature in the two treatments. The time to reach the maximum temperature, the duration of the thermophilic phase and the time to reach ambient temperature are presented in Table 2. Temperature increases due to microbial activity. [28]. In both treatments, the typical sequential phases: mesophilic, thermophilic, cooling and maturation were observed (see Fig. 1). Besides, TA had two thermophilic peaks after the extraction of the mixture from the container (i.e. day 10). These peaks are characteristic of TSC [4,10]. Both treatments had a rapid increase on temperature, achieving thermophilic phase in day 1 of the process, similar to ndings from other GW co-composting studies [1,5]. This rapid temperature increase evidences adequate conditions for the process regarding the studied substrate mixture (i.e. pH, moisture, TOC, nutrients, porosity). In day 2, the temperature increase was higher in TA (54 °C) compared to TB (46 °C), showing the in uence of this modi cation in the process (i.e. possibly associated to the reduced heat lost in the mixture inside the container). However, from day three to day 20, the temperature in TB was higher than in TA, showing a higher organic matter degradation rate in this period. Despite having a higher reduction in VS by day 24 in TB compared to TA (i.e. 25.9% in TB and 25.5% in TA) the difference is not considered substantial.

Process conditions
After day 3, continuous oscillations of temperature occurred in both treatments. However, in TA there was a pronounced reduction after day 5 and until the material was removed from the container (day 10), with a new temperature increase. Thus, the two typical thermophilic temperature peaks for the TSC were observed [4,10]. By day 12, the two temperature peaks and the subsequent decrease occurred in both treatments. The thermophilic phase lasted 27 d in TA and 33 d in TB, showing the in uence of the process type on the degradation of the organic matter readily degradable (i.e. mesophilic and thermophilic phases). It is important to emphasize that by day 32, there was a reduction in VS in 29.0% in TB and 27.2% in TA, which rati es the higher biological activity and thus, the higher transformation of organic matter in the traditional composting compared to TSC. As expected, in both treatments there was a higher degradation rate on the mesophilic and thermophilic phases compared to the cooling and maturation phases. This is due to the decomposition of the readily degradable organic matter in the rst phases, and the decomposition of recalcitrant organic matter (lignin and cellulose) in the nal phases [29].
In both treatments, the temperatures were above 50 °C for more than three consecutive days, promoting the material sanitization [30]. Furthermore, TB ful ll the recommendations from Böhm [31], who indicates that temperature should be at least higher than 55 °C at least for two weeks (TB:16 d and TA: 12 d). The higher temperature values in TB are ascribed mainly to the aeration conditions in the containers. Although, the temperature increase was faster in TSC, higher temperatures maintained longer were observed in traditional composting, showing higher organic matter degradation rates in the thermophilic and mesophilic phases (i.e. evidenced in the VS reduction).
The favorable conditions for degradation in both treatments can be connected to the incorporation of PR, characterized for promoting the generation of heat due to the increase in porosity and the provision of the oxygen required for aerobic degradation [5].
On the other hand, during the thermophilic phase, higher nitrogen losses could occur due to NH 3 volatilization (i.e. associated to high temperatures and alkaline pH), mainly in TB (i.e. turning also promotes this loss), as observed in the end-product quality records (see Table 3) [32,33]. In this case, TSC reduced nitrogen volatilization possibly associated with the storage of the material within the containers during the ten rst days.
The temperature in the two treatments had similar values in the cooling phase, especially after day 36, with variations associated with pile turning. This behavior can indicate that decomposition of hard to degrade substances in this phase was similar in both treatments. Thus, the in uence of TSC in accelerating the degradation of lignocellulosic substances in the composting process was not observed. This was rati ed on the nal average values of the VS reduction, that was 40.4% in TA and 38.5% in TB, showing similar degradation rates (i.e. there were not statistical signi cant differences).
Regarding pH (see Fig. 2), the mixture of substrates in both treatments showed an initial pH with slightly acidic values, due to the previous degradation of some materials (e.g. UPFW-4.3 units and PFW-4.1 units) which had 3 storage days before the experiment start. This degradation leads to the formation of volatile fatty acids which reduce substrate pH and could affect the process start [34]. However, in both treatments pH increased rapidly as a consequence of the transformation of organic matter in organic acids (intermediate byproducts of the microbial decomposition of sugars, starch and lipids) and later the volatilization of products such as CO 2 [35]. Small variations in the pH were observed during the process associated with its conditions of moisture and oxygenation. However, the pH in the process was between 8 and 9 units as suggested by some authors for optimal microbial degradation [36]. At the end of the process, the pH had average values of 7.3 units in both treatments (i.e. there were not statistical signi cant differences). Figure 3 presents the average EC in both treatments. EC indicates the presence of salts due to the content of sodium, chloride, potassium, nitrate, sulfate and ammonium salts, which in high concentrations could inhibit plant growth [37]. The relatively high EC values from the start of the process could be associated to the presence of soluble salts (i.e. phosphates) due to the addition of PR. The small increase observed at the end of the process could be connected to the effect of the material degradation with the processes of humi cation and nutrients liberation (i.e. nitrates). Higher values of EC were observed in TSC compared to traditional composting possibly linked to a higher concentration of phosphates in the material (see Table 3). In the case of TB, the decomposition rate of the readily degradable organic matter in the early stages of the process led to a higher nutrient liberation from the start of the process (i.e. lower values of N and P in the products).
As indicated by Tiquia [38], salinity could be connected to the relatively high content of N Total in the end-product as reported for TA. In both treatments, the ranges recommended by Dimambro et al. [39] to avoid toxicity for plants and crops were not surpassed.
The seed germination test has been widely used for evaluating compost quality, since the application of an unstable and immature compost could inhibit the germination of seeds, reduce plant growth and damage the crops [40,41]. A germination index (GI) higher to 80% indicates that compost is not phytotoxic [25,42]. In both treatments, the products were mature in the three measurements developed (i.e. days 47, 53 and, 60), with higher values at the end of the process in TB piles (GI = 160%) compared to TA piles (GI = 150%). This could be associated with the lower content of salts or phytotoxic substances measured through the EC parameter [40]. Table 3 presents the information on end-product quality for both treatments. In addition, it includes a comparison of quality standards according to the NTC 5167 and other studies about GW co-composting.

Product quality
Regarding moisture, both treatments had values about those established by the NTC 5167. These relatively high values are associated with the moisturizing carried out 10 days before the end of the process in all piles. Likewise, the values agreed with those reported in previous studies [1,16]. Comparing end-product moisture in both treatments there were no statistically signi cant differences.
The values of pH were within the range established by the NTC 5167 for the use of products as soil improvers. The pH was slightly alkaline (around neutral values), which favors their application in acid soils facilitating carbon mineralization, generation of OH − ions and introduction of basic cations such as K + [43]. The results obtained are similar to those reported for co-composting processes of GW and FW [1,16]. The smaller values of pH found in this study could be related to the addition of PR, since its dissolution improves the formation of organic acids of low molecular weight and the generation of CO 2 during the degradation of organic waste [44]. Furthermore, PR can also reduce the pH for the adsorption of NH 3 and cations [5,45]. Regarding TOC, both treatments showed higher values compared to those required by the Colombian Norm as soil improver [26]. These higher values could be connected to the carbon content still available to transform due to the presence of lignocellulosic compounds in the GW. Similar values were reported in previous studies from the authors [1,16]. The products did not have a statistical signi cant differences. Therefore, it is not possible to attribute this smaller content as an in uence of TSC in the process.
Nitrogen had values higher than 1% in both treatments, which is favorable for product use in agricultural activities. There was a smaller concentration of N in TB compared to TA, but lacked of statistical signi cance. This smaller concentration is possibly linked to a higher N volatilization during the rst two phases of the process. This is evident in the high temperatures in TB during the rst 10 days combined with alkaline pH that promotes NH 3 volatilization during turning [34].
On the other hand, relatively high values of N in both treatments could be connected to the high porosity that PR could provide to absorb NH 3 and improve N conservation during the process [47,48].
Ash content was lower than 60% in both treatments, according to the requirements of the NTC 5167, and lacked signi cant statistical differences comparing TA and TB. However, TB had relatively higher values compared to TA which can be linked to the intense organic matter degradation during the rst 10 days, which are re ected in mass loss in the form of CO 2 [49].
This high degradation can also be associated with the addition of PR in both treatments that provide nutrients and energy for microorganisms, accelerating transformation processes [5]. The addition of PR increased the quantity of inorganic material in both end-products, associated to phosphorous mineralization, and thus, intervene in the reported ash content in both treatments in contrast with our previous studies where this amendment material was not introduced [1].
The Cation Exchange Capacity (CEC) is used to evaluate the humi cation degree and nutrient retention capacity of compost. In both treatments, CEC was higher compared to the standard of NTC 5167 for soil improvers [26]. Despite the fact, there was not statistical signi cant differences, there were higher values in TA compared to TB possibly due to an increase in the humi cation processes in GW during the cooling and maturation phases, evidenced in a lower OC content at the end of the process. The results show that in both treatments, the end-products are able to improve the water and nutrient retention capacity of soils [4,5].
The water retention capacity for a mature product must be higher to 100% [26] or 75% wet weight [50]. This parameter allows stablishing the product capacity to retain moisture, which is fundamental during the use of the product for agricultural purposes. The treatments had higher values compared to those established by the literature without statistical signi cant differences comparing TA and TB. The high water retention capacity in these products is connected to their low density (i.e. density was 0.50 g cm − 3 for TA and 0.46 g cm − 3 for TB) and high porosity of materials such as GW, and to the processes of transformation and mineralization of organic matter.
Finally, the phosphorous content in TA and TB was higher to the standard from the NTC 5167 for soil improvers [26].
According to Khan and Joergensen [51], the solubilization of PR could increase microbial biomass to release inorganic phosphorous. The high porosity of PR could also provide and habitat to the high microbial biomass [9,52]. P is a central component of the energy-carrying molecule (adenosine triphosphate, ATP) in all cells; increased P availability resulting from PR addition may increase the formation of ATP during microbial activity and reproduction, and therefore enhance the decomposition of organic waste [5,53]. Therefore, an increase on the microbial biomass during composting could contribute with an increase in the organic content of C and P in the end-product, as evidenced in the results from both products. Despite the fact, there were not statistical signi cant differences between the treatments, there were smaller values of P in TB, which can be associated to a more intense activity of organic matter degradation during the rst two phases of the process in the traditional composting.

Conclusions
The conclusions of this work are: Traditional composting had higher biodegradation rates during the mesophilic and thermophilic active composting phases than the Two Stage Composting (i.e. higher temperatures reached for longer periods and a higher reduction of volatile solids). However, during the cooling and maturation phases, both treatments showed similar process conditions as these were judged by the temperature, volatile solids, pH, and electrical conductivity.
The twelve parameters measured here (moisture, pH, electric conductivity, total organic carbon, total nitrogen, C/N ratio, cation exchange capacity, water retention capacity, density, ash, total phosphorous, and lignin) on the endproducts resulting from both the two-stage composting and traditional composting did not statistically differ. Both end-products could be used as soil improvers.
Two-stage composting of green waste, food waste, phosphoric rock and sawdust at the end of the process did not in uence of the processing time, degradation rate or end-product quality compared with traditional composting. Additional complementary strategies must be evaluated to improved GW composting.

Declarations
Availability of data and materials All data generated or analyzed during this study are within the submitted manuscript.  EC behavior in both treatments (number of replications=3)