1.3.1 Characteristics of beverage sludge and soil
Bulk density and specific gravity are noticeable physical properties for agriculture which are factors for the healthy growth of plants. The bulk density of beverage sludge and soil was found 1.24 g cc-1 and 1.89 g cc-1 respectively (Table 2).
Table 2 Physical, physiochemical, and organic status of beverage sludge and soil
Parameter
|
Unit
|
Beverage Sludge
|
Soil
|
Bulk density
|
g cc-1
|
1.24
|
1.89
|
EC
|
μS cm-1
|
1312
|
255
|
pH
|
-
|
8.1
|
6.8
|
Organic content (OC)
|
%
|
37.39
|
0.67
|
According to USDA bulk density greater than 1.80 g cc-1 affects root growth adversely and less than 1.60 g cc-1 is favorable for most plant growth [28]. In this study, bulk density of beverage sludge assertively complies with USDA-NRCS recommendations whereas it is slightly higher in case of soil. On the other hand, specific gravity of beverage sludge is 40.43% lighter than utilized soil in weight. Lower specific gravity than conventional value indicates the possibility of organic content. The EC of beverage sludge and soil was found 1312 and 255 μS cm-1 respectively while EC of a good cultivable medium ranges from 200 to 1200 μS cm-1 [29], [30]. In this study, EC of beverage sludge was slightly higher than the standard range, which is not significant. On the other hand, the OC of beverage sludge and soil was found to be 37.37% and 0.67% respectively. Undoubtedly, OC of beverage sludge was found in an enriched state compared to soil whereas 3.4% is the minimum recommended amount [9]. Essential soil nutrients (NPK) and other important nutrients (S, Fe, Mn, Ca, Mg and Zn) concentration are presented in Table 3.
Table 3 Crop growing nutrients in beverage sludge
Nutrient/ heavy metal
|
Unit
|
Beverage sludge
|
Soil
|
Total nitrogen, N
|
%
|
1.25
|
0.074
|
Phosphorus, P
|
%
|
0.87
|
0.63
|
Potassium, K
|
%
|
0.47
|
0.52
|
Sulfur, S
|
%
|
0.76
|
0.072
|
Iron, Fe
|
mg kg-1
|
11.80
|
10.00
|
Manganese, Mn
|
mg kg-1
|
11.40
|
13.08
|
Calcium, Ca
|
mg kg-1
|
12.63
|
10.52
|
Magnesium, Mg
|
mg kg-1
|
4.88
|
3.10
|
Zinc, Zn
|
mg kg-1
|
57.20
|
50.00
|
NPKS of beverage sludge were found 1.25%, 1.15%, 1.88% and 0.76% respectively where 0.074%, 0.64%, 0.47% and 0.0072% were found in soil. N and P functions for plant growth while K is responsible for osmosis and enzymatic activities with S being helpful for photosynthesis. Usually, nutritious chemical fertilizers like DAP, MOP, urea, and TSP are being used in soil to increase the amount of NPKS. Fe, Ca, Mg and Zn were found in a higher level than soil except Mn (Table 3). It is worth noting that the nutrient of beverage sludge is much better for cultivation compared to soil where fertilizer might be needed in terms of soil only.
However, beverage sludge can be applied to cultivable land because it is rich in organics and other plant growing nutrients. There are little chances of heavy metal presence because most ingredients being used are of food grade variety.
1.3.2 Effects of beverage sludge on growth and harvest of cultivates
The germination rate of Indian spinach and Okra was found to be 100%. Weekly periodical growth was observed from the 1st day after sowing (DAS) till the 1st harvest. Periodical growth variation has been compared with control treatment (T1R1) where beverage sludge was not mixed. The length or height and number of leaves are illustrated in Fig. 1 which is the average of six replications.
Length of Indian spinach and height of Okra gradually increased over time with treatment T5R5 (80% beverage sludge and 20% soil). The number of leaves of both cultivated plants has been found to increase with the increase in the amount of beverage sludge being used in pots. There is a proportional relation between the number of leaves and plant height. Notably, growth is a little bit lower than the maximum growth (T5R5) at T6R6 where only beverage sludge was used for both cultivates. The amount of media, excessive nutrients and media texture may be the reason for that. However, growth of all treatments was found higher compared to control treatment (T1R1) and the maximum growth was observed with 80% beverage sludge containing treatment.
From a previous study, the maximum length of Indian spinach was found 30 cm at 60 DAS when it was nourished by biogas plant residues (BPR) which was 64.5% higher than the control treatment [31]. This study of beverage sludge results in 42 cm as growth (length) at 38 DAS. In that case it is quite clear that beverage sludge is much healthier than BPR to be used in agricultural soil. On the other hand, sewage sludge was used in soil to study the effects on Okra in another study [32]. Number of leaves of the Okra plant was 24 when it was grown in 40% sewage sludge at 65 DAS but 23 leaves were totaled at 49 DAS using 60% sewage sludge. That means, an increase in the volume of sewage sludge reduces growth. Here, replacing 80% soil by beverage sludge causes 25 leaves at 49 DAS (Fig. 1). Beverage sludge causes decreased growth patterns if more than 80% sludge on the media.
Yield is an agricultural output, an important parameter to identify agricultural produce. In this study, Indian spinach and Okra were harvested at 38 DAS, 44 DAS, 65 DAS and 78 DAS. The yield illustrated in Fig. 4 is the summation of four harvests and averaged from replications. Weight of stems & leaves of Indian spinach and fruits of Okra determined in harvests.
Yield of both cultivated vegetables has been found to increase with the increase in the volume of beverage sludge. The maximum yield of Indian spinach and Okra was observed with treatment T5R5 (where 80% soil of the pot was replaced by beverage sludge). 10.88 kg sqm-1 yielded from Indian spinach whereas yield was only 1.16 kg sqm-1 in the control treatment (T1R1). On the other hand, the highest yield of Okra was 498.9 g per plant (14 fruits) with T5R5 but 302 g per plant (8 fruit) yielded from control treatment (T1R1). The yield with T6R6 was 10.83 kg sqm-1 and 324.4 g per plant (12 fruit) for Indian spinach and Okra respectively. That means, the agricultural output from 100% beverage is a little bit lower than the combination of 80% sludge and 20% soil. Previously, BPR were used for fertile cultivation of Indian spinach and the maximum yield was found to be 3.63 kg sqm-1 [31]. Sing and Agarwal studied Okra where 111 g per plant and 150 g per plant were found as the yield from the treatment of 20% and 40% sewage sludge with soil, respectively [32].
1.3.3 Food values and heavy metal uptake
Food values i.e., ascorbic acid (C6H8O6) or vitamin C, β- carotene (C40H56) and protein have been tested from the edible parts of Indian spinach and Okra. The samples were prepared from the 1st harvest.
From the analysis of food values i.e., ascorbic acid, β- Carotene and protein of Indian spinach and Okra, it is observed that food values increased with the increase in the volume of beverage sludge (Fig. 5). Higher organic content of beverage sludge may be a reason for that because organic matter is a reservoir of nutrients that can be released over time. Ascorbic acid, β-carotene and protein contents of control treatment (T1R1) were found 32.93 mg 100g-1, 37.24 μg g-1 and 0.83 g 100g-1 respectively for Indian spinach whereas the maximum detected as 76 mg 100g-1, 55.92 μg g-1and 3.25 g 100g-1 respectively with T6R6. Similarly, the maximum content of ascorbic acid, β-carotene and protein from Okra was found 38.80 mg 100g-1, 56.20 μg g-1 and 2.34 g 100g-1 respectively. Notably, food value increased up to 100% beverage sludge whereas maximum yield was found with the treatment of 80% beverage sludge and 20% soil (T5R5).
According to the USDA, typically 28.1 mg 100g-1ascorbic acid, 56.26 μg g-1 β-carotene and 2.86 g 100g-1 protein are present in Indian spinach [33]. In terms of typical food values of Okra 23 mg 100g-1ascorbic acid, 41.60 μg g-1 β-carotene and 1.93 g 100g-1 protein exist in Okra fruit [34].
Table 4 Effects of beverage sludge on nutrients contents of the cultivates
Plant
|
Nutrients
(mgkg-1)
|
T1R1
|
T2R2
|
T3R3
|
T4R4
|
T5R5
|
T6R6
|
Typical valuesa, b
|
Indian spinach
(Basella alba)
|
Fe
|
17.56
|
18.21
|
18.75
|
20.45
|
21.11
|
21.25
|
27.1
|
Ca
|
570.2
|
627.8
|
667.6
|
734.5
|
920.8
|
1013.6
|
990
|
Mg
|
411.07
|
497.5
|
641.8
|
713.3
|
848.2
|
879.8
|
790
|
K
|
3120.6
|
3040.2
|
2810.5
|
2680.5
|
2360.7
|
1750.4
|
5580
|
P
|
402.8
|
413.4
|
427.7
|
441.3
|
463.7
|
528.9
|
490
|
Zn
|
8.2
|
13.4
|
22.2
|
19.7
|
22.9
|
23.1
|
5.3
|
Okra
(Abelmoschus esculentus)
|
Fe
|
4.83
|
5.39
|
5.97
|
6.59
|
7.12
|
7.64
|
6.2
|
Ca
|
496.2
|
523.8
|
595.2
|
631.7
|
714.5
|
788.5
|
820
|
Mg
|
365.7
|
398.4
|
434.3
|
467.7
|
495.6
|
526.4
|
570
|
K
|
2856.4
|
2848.8
|
2837.4
|
2828.6
|
2815.2
|
2789.4
|
2990
|
P
|
476.2
|
489.5
|
504.4
|
528.4
|
535.6
|
586.1
|
610
|
Zn
|
13.8
|
18.4
|
21.2
|
20.1
|
18.9
|
18.4
|
5.8
|
Sources: a, bUSDA central food database [33], [34]
Nutrients i.e., Fe, Ca, Mg, K, P and Zn were determined using the standard method described in the methodology. The positive effect of beverage sludge on nutrient values of cultivates is easily understandable here (Table 4). All concentrations of nutrients have been found to increase with the increasing amount of beverage sludge in pot except potassium (K). Measured nutrients were compared with the typical value of nutrients and found to be the same as the typicals [33], [34]. The important observation is that beverage sludge leads to a good quality crop without chemical fertilizer being needed. However, the more likely reason behind the uptake sequence of nutrients by Indian spinach and Okra is the contents of beverage sludge and the soil itself (Table 3). In short, uptakes gradually increased as the elements of media were increased. However, the substitution of beverage sludge by chemical fertilizer may be an optimum solution to reduce environmental pollution alongside increased food production.