3.1 Process conditions
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].
Table 2
Synthesis of characteristics of the composting process in the experiment
Treatment
|
Mesophilic phase duration (d)
|
Thermophilic phase duration (d)
|
Tmax
(°C)
|
Time to reach Tmax (d)
|
Cooling phase duration (d)
|
VS reduction (%)
|
Added water (L)
|
TA
|
1
|
27
|
62.6
|
6
|
46
|
40.4 (2.2)
|
148.3
|
TB
|
1
|
33
|
68.6
|
12
|
40
|
38.5 (7.6)
|
120.0
|
Note: Standard deviation is in parenthesis. Tmax: Maximum temperature. VS: Volatile Solids |
Both treatments had a rapid increase on temperature, achieving thermophilic phase in day 1 of the process, similar to findings 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 influence of this modification 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.
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 influence 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 ratifies 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 first phases, and the decomposition of recalcitrant organic matter (lignin and cellulose) in the final phases [29].
In both treatments, the temperatures were above 50 °C for more than three consecutive days, promoting the material sanitization [30]. Furthermore, TB fulfill 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 NH3 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 first 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 influence of TSC in accelerating the degradation of lignocellulosic substances in the composting process was not observed. This was ratified on the final 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 significant 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 CO2 [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 significant 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 humification 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 NTotal 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].
3.2. Product quality
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.
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 significant 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 CO2 during the degradation of organic waste [44]. Furthermore, PR can also reduce the pH for the adsorption of NH3 and cations [5, 45].
Table 3 End-product quality characteristics for both treatments
Parameters
|
Units
|
TA (n=3)
|
TB (n=3)
|
NTC 5167
|
Hernández et al. [16]
|
Oviedo-Ocaña et al. [1]
|
Boldrin et al. [46]
|
Zhang and Sun [6]
|
Moisture
|
%
|
45.70 (3.59)
|
44.20 (1.13)
|
<35
|
33.87
|
54.6
|
29 – 44
|
--
|
pH
|
Units
|
7.39 (0.17)
|
7.34 (0.57)
|
>4 - <9
|
7.51
|
7.60
|
--
|
7.9
|
Total organic carbon
|
% dw
|
28.16 (0.85)
|
29.04 (1.58)
|
>15
|
20.87
|
23.1
|
10 – 19
|
25
|
Total Nitrogen
|
% dw
|
1.76 (0.32)
|
1.55 (0.31)
|
>1
|
1.37
|
2.35
|
0.7 – 0.9
|
3.0
|
C/N
|
-
|
15.97 (4.09)
|
18.68 (7.19)
|
-
|
14.9
|
--
|
11 – 27
|
--
|
Cation exchange capacity
|
cmol kg-1
|
38.13 (6.71)
|
32.70 (7.05)
|
>30
|
32.77
|
--
|
--
|
--
|
Electric conductivity
|
dS m-1
|
0.93 (0.23)
|
0.62 (0.14)
|
-
|
0.21
|
1.66
|
--
|
2.61
|
Water retention capacity
|
%
|
157.00 (19.26)
|
141.53 (11.99)
|
>100
|
237.4
|
--
|
--
|
--
|
Density
|
g cm-3
|
0.50 (0.11)
|
0.46 (0.04)
|
< 0,6
|
--
|
--
|
--
|
--
|
Total Phosphorous
|
% dw
|
4.75 (2.81)
|
4.19 (0.44)
|
>1
|
0.56
|
0.8
|
0.15 – 0.23
|
0.3
|
Ash
|
% dw
|
52.10 (4.09)
|
55.40 (7.92)
|
< 60
|
51.60
|
--
|
72 – 79
|
--
|
Lignin
|
% dw
|
30.30 (2.28)
|
28.60 (1.67)
|
--
|
--
|
--
|
--
|
--
|
Note: n: number of samples; dw. dry weight; Standard deviation is in parenthesis; all means (compared in the same row) are statistically similar at p < 0.05
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 significant differences. Therefore, it is not possible to attribute this smaller content as an influence 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 significance. This smaller concentration is possibly linked to a higher N volatilization during the first two phases of the process. This is evident in the high temperatures in TB during the first 10 days combined with alkaline pH that promotes NH3 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 NH3 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 significant 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 first 10 days, which are reflected in mass loss in the form of CO2 [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 humification 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 significant differences, there were higher values in TA compared to TB possibly due to an increase in the humification 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 significant 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 significant 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 first two phases of the process in the traditional composting.