Abstract

Key Words

Oxygen depletion, acidification, aquatic life, odour.

Introduction

India is a major producer of sugar in the world, and sugar industry offers employment potential and contributes substantially to economic development. There are about 579 sugar mills and 285 distilleries in India. Apart from the sugar and alcohol, these factories generate many by-products and waste materials. For example, more than 5 million tones of solid waste (pressmud) are being produced from sugar industries. In addition, the industries also generate about 7.5 million tones of molasses and 45 million tones of bagasse as valuable by-products (Rajukkannu and Manickam 1997).

Molasses is used as a raw material in the distillery industries where alcohol is produced from two types of fermentation processes, Praj type and Alfa Laval distillation. In Praj type, for one liter of alcohol produced about 12-15 liters of waste water (spentwash) are generated, whereas, in the Alfa Laval continuous fermentation and distillation process only 7-8 liters of waste water per liter of alcohol are produced as it uses boilers for concentrating the effluents. Currently, about 40.72 million m³ of spentwash is generated annually from distilleries alone in India.

The spentwash is acidic (pH 3.94 to 4.30) and loaded with organic and inorganic salts, resulting in high EC (30 - 45 dS/m). Being plant originated, the spentwash also contains considerable amounts of plant nutrients and organic matter (Table 1). Nitrogen (N) content in spentwash ranges from 1660 to 4200 mg/l, phosphorus (P) from 225 to 3038 mg/l and potassium (K) from 9600 to 17475 mg/l. Calcium (Ca), magnesium (Mg), sulphate (SO₄) and chloride (Cl) are also present in appreciable amounts. Thus, it can effectively be used as a source of plant nutrients and as soil amendment. Recently, the presence of appreciable amounts of plant growth promoters viz., gibberellic acid (GA) and indole acetic acid (IAA) have also been detected which further enhances the nutritient value of spentwash (Murugaragavan 2002). The high concentration of Ca (2050 – 7000 mg/l) in spentwash may have the potential in reclaiming the sodic soils similar to that of gypsum effect.

Table 1. Some important chemical characteristics of spentwash.

Parameters	Range values*
pH	3.9 – 4.3
EC (dS/m)	30.5 – 45.2
Biological Oxygen demand	46100 – 96000
Chemical oxygen demand	104000 – 134400
Total dissolved solids	79000 – 87990
Nitrogen	1660 – 4200
Phosphorous	225 – 3038
Potassium	9600 – 17475
Calcium	2050 – 7000
Magnesium	1715 – 2100
Sodium	492 – 670
Sulphate	3240 – 3425
Chloride	7238 – 42096
SAR	5.0 – 7.3
Zinc	3.5 – 10.4
Copper	0.4 – 2.1
Manganese	4.6 – 5.1
Gibberellic acid	3246 – 4943
Indole acetic acid	25 – 61

(*All values are in mg/l unless otherwise stated)
Sources : Rajukkannu and Manickam (1997); Valliappan (1998); Murugaragavan (2002)

Though the spentwash generally does not contain any toxic metals, but is characterized by a high BOD and COD levels (Table 1). Therefore, high BOD, COD and other organic compounds like phenols, lignin and oil and greases in spentwash are likely to deteriorate soil and environmental health. The unpleasant odour due to the presence of skatole, indole and other sulphur compounds which are not effectively decomposed by yeast, or methanogenic bacteria during distillation is also an issue of public concern. The beneficial effect of spentwash on crop production is well documented (Joshi et al.1996; Rajukkannu and Manickam 1997). However, information is scarce on its usage as a soil amendment in dryland soils and its possible pollution potential. Therefore, this paper aims to examine the potential beneficial and detrimental effects of distillery spentwash application in dryland agriculture under Indian conditions.

Experimental conditions

The results from laboratory, pot and field experiments conducted by Murugaragavan (2002), Malathi (2002) and Saliha (2003), to assess the potential of spentwash use in agriculture are summarised in this paper.

Effect of spentwash in seed hardening

Seed hardening is a specific treatment given to seeds before sowing to withstand adverse soil moisture conditions. It is being recommended for better establishment of seeds in rainfed agriculture. The physiological and biochemical nature of the seed is modified due to seed hardening treatment mainly to resist drought at the time of sowing. Seed hardening is normally practiced with water and chemicals such as 0.5% KH₂PO₄, 2% KCl, and 0.01% of ZnSO₄ etc. Murugaragavan (2002) compared the efficacy of spentwash in seed hardening of ragi (Eluesine coracana), groundnut (Arachis hypogaea), gingelly (Sesamum indicum), sorghum (Sorghum bicolour) and green gram (Vigna radiata) at different concentrations with existing seed hardening treatment. The seeds were soaked with 10 or 20% solution of spentwash at a seed:solution ratio of 1:1 for 16 hrs (ragi and sorghum), 12 hrs (gingelly), 6 hrs (groundnut) and 3 hrs (green gram) and air-dried to initial moisture content (8 – 10%). The seeds were sown in 2.5 kg soil in the sintex pots. Germination, root and shoot length, biomass production and vigour index were measured.

Pollution potential of spentwash

Soil leaching experiments were conducted to examine the mobility and transport of salts and the possible groundwater contamination with spentwash application (Malathi 2002). The soil columns (cylindrical PVC tubes with an internal diameter of 7.0 cm) were constructed containing layers of soil with different bulk densities viz., 1.4 (subsurface) and 1.3 (surface) Mg/m³. The spentwash was applied at a rate equivalent to 0, 25, 125, 250 and 500 m³/ha with and without organic amendments viz., farm yard manure (FYM @ 12.5 t/ha), green leaf manure (GLM, daincha – Sesbania aculeata @ 6.25 t/ha) and bio-compost (BC @ 3 t/ha). Ten days after incubation leaching was carried out with water and the leachates collected and analysed for pH, EC, Na⁺, Ca²⁺, Mg²⁺, Cl^- and SO₄^2- contents. Four weeks after leaching 10-15 seeds of ragi were sown in all columns and the germination and the vigour index examined. After ten days the columns were dismantled and soils from each layer removed and analysed for pH, EC, Na⁺, Ca²⁺, Mg²⁺, Cl^- and SO₄^2- contents.

Bio-toxicity of spentwash

The effect of groundwater contamination of spentwash on aquatic life was investigated through a simple bio-toxicity study on fingerlings of a fresh water fish species viz., Cyprinus carpio var. communis (Malathi 2002). The fingerlings were acclimatized to laboratory conditions for about 3 weeks before experimentation. The spentwash was added at different concentration ranging from 0.1 to 10% (V/V). A static bioassay technique was employed considering its simplicity and accuracy. The survival/mortality of fingerlings was recorded and the LC₅₀ (Lethal concentration estimated to produce mortality in 50 per cent of the test population over a period of 24 hours) calculated (Nagarajan and Shasikumar 2002).

Effect of spentwash on selected soil properties, seed germination and crop yields

The effect of spentwash at rates equivalent to single application of 0, 25, 50, 125, 250 and 500 m³/ha with and without organic amendments viz., FYM (12.5 t/ha), GLM (daincha @ 6.25 t/ha) and BC (3 t/ha) on selected soil properties (EC, pH, N, P, K, organic carbon, and exchangeable sodium percentage, ESP), and on production of selected dryland crops (ragi, groundnut, gingelly, sorghum, rice and green gram) were examined by Murugaragavan (2002) and Saliha (2003).

The soils used in the pot and field experiments were either vertisol (a deep moderately well drained, calcareous clay soil belongings to fine montmorillonite vertic ustropepts, Entic chromustert in USDA classification), or an alfisol (a deep well drained, loam soil belonging to fine loamy textured Typic rhodustalfs) or a sodic soil (with a high pH >9.8). These represent the major soil groups in the dryland areas of Theni district of Tamil Nadu, India. The climate was hot, dry and variable with temperature ranging between 31.7^o C in summer and 20^o C in winter with an average rainfall of 850 mm. Cotton (Gossypium vitifolium), pulses (e.g green gram), millets (e.g Pennisetum glaucum) and small millets (ragi) are grown as main crops in these dry tracts. These soils are generally low in organic carbon and plant available nutrients.

Results

Effect of spentwash in seed hardening

It has been consistently shown that the seed hardening with the spentwash at a concentration of 10 and 20% (V/V) markedly improved the germination of ragi groundnut, gingelly, sorghum and green gram by 16, 30, 28, 27, and 28%, respectively, over the control. Similarly, such seed treatment with spentwash was also found to improve the root length (420%), plant height (500%), biomass production (161%) and vigour index (315%). Marked increase in N (11-13%), P (17-20%) and K (16-27%) contents of crops was also recorded due to spentwash treatment over the control. This effect was more pronounced in green gram than other crops. The seed hardening with spentwash at higher rate (20%) was found more effective than the lower rate (10%) and other chemical treatments in improving the growth parameters. These effects could be ascribed to the nutrients and the growth promoters like GA and IAA present in the spentwash.

Bio-toxicity of spentwash

The results revealed that with increasing concentration of spentwash, the salt loading in test solution increased several fold (EC >4.8 dS/m). It resulted acidification (pH<6.4) and depletion of dissolved oxygen (DO <2.25 mg/l) content of the test solution. A high rate of mortality of fingerlings was observed with the increase in the concentration of spentwash. The estimated LC₅₀ for distillery spentwash was found to be 0.5%. The destruction of aquatic life in water bodies like river Ganga and Gomti due to indiscriminate disposal of spentwash was reported in India (Joshi 1988).

Effect of spentwash on selected soil properties

The effect of spentwash on selected properties of sodic soil in the presence and absence of organic amendments is presented in Table 2. The spentwash application significantly reduced the pH of sodic soil. Increase in the rate of spentwash application resulted a notable decrease in the pH of soils and such effect was more pronounced in the presence of organic amendments (Saliha 2003). The decrease in soil pH may be attributed to the acidic nature of the spentwash and the release of organic acids during the decomposition. The electrical conductivity of soil increased markedly due to accumulation of salts from spentwash. The concentration of plant available nutrients (KMnO₄-N, NaHCO₃-P, and NH₄OAc-K) in soil increased substantially. Application of organic amendments appeared to have an additive effect in improving the efficacy of the spentwash and followed BC > GLM > FYM. Significant increase in organic carbon content of soils was also observed which could be ascribed to the addition of organic matter through spentwash application. Similar effect was also observed in vertisol and alfisol (Murugaragavan 2002). Marked reduction in ESP of sodic soil was also observed due to spentwash application (Table 2). The exchangeable Ca present in the spentwash might have replaced the Na⁺ from the exchange sites in the soil and eventually reduced the ESP. Data (not presented) on exchangeable Ca²⁺ and Na⁺ in soil further confirmed these observations.

Table 2. Effect of spentwash application on selected characteristics of sodic soil (Saliha 2003).

Soil characteristics	Control	SW alone	SW + FYM	SW + GLM	SW + BC
pH	9.24	7.73	7.64	7.52	7.43
EC (dS/m)	0.84	11.0	10.2	10.5	12.4
Organic carbon (%)	0.26	0.67	0.71	0.70	0.71
KMnO4-N (kg/ha)	185	235	251	258	264
NaHCO3-P (kg/ha)	16.7	24.6	27.0	28.5	30.2
NH4OAc-K (kg/ha)	265	2725	2965	2850	3560
ESP	27.0	21.00	15.6	17.4	14.7

SW =Spentwash (@ 500 m³/ha); FYM=Farm yard manure (@ 12.5 t/ha); GLM=Green leaf manure (@ 6.25 t/ha); C=Biocompost (@ 3 t/ha)

In the case of vertisol and alfisol Murugaragavan (2002) observed increase in SAR and ESP due to application of spentwash. However, these values (SAR <3.62; ESP<4.8) were well below the threshold levels, suggesting even at higher rate the spentwash application is unlikely to cause any sodicity problem in these soils. Although initial enhancement in enzymes (phosphatase, dehydrogenase, and urease) and microbial (actinomycetes, yeast and bacteria) activities was evident in soil amended with the spentwash, no marked effect was observed at the end of 60 days incubation (Murugaragavan 2002).

The results of pot and field experiments showed marked improvement in soil fertility as evident from the increase in available N, P, and K. From this study, it could be inferred that the spentwash at 25 m³/ha may have potential in improving the productivity of vertisol without affecting the soil health. Whereas, in alfisol still lower dose may be required. However, long term field experiments are needed to confirm these results.

Effect of spentwash on seed germination and crop yields

Data from pot experiment demonstrated that the spentwash >50 m³/ha was found detrimental for the germination and establishment of green gram in vertisol, whereas, in alfisol even at a rate of 25 m³/ha, the spentwash was found to inhibit the germination and growth of green gram. However, in vertisol the germination, growth, nutrients contents and yield of green gram were significantly improved with spentwash application at 25 m³/ha. Differential crops response to spentwash application was also evident. For example, in a field experiment with rice (Oryza sativa) grown on sodic soil Saliha (2003) observed that the application of spentwash at a rate of 125 m³/ha followed by four leaching resulted significantly higher grain yield than at 250 and 500 m³/ha (Table 3).

Table 3. Effect of spentwash with and without organic amendments on grain yield of rice (kg/ha) (Saliha 2003).

Treatments	Levels of spentwash application (m3/ha)
	0	125	250	500
Control FYM GLM BC	1270 1915 1960 2238 manure (M)	2150 2366 2575 2480 spentwash (S)	945 1110 1188 1210 M x S	710 854 826 752
CD(P=0.05)	66.5	50.1	112.2

Effect of spentwash on salt transport and movement

Data from the soil leaching experiments showed that large amounts of soluble cations were found leached from soils amended with the spentwash (Table 4 and 5). Increase in the rate of spentwash application had markedly enhanced the leaching of cations. Calcium was the dominant cation leached from calcareous vertisol (Table 4), whereas, greater amount of Na⁺ was found leached from high pH sodic soil (Table 5) reflecting the exchange reactions of spentwash-Ca²⁺ with Na⁺ on the soil exchange sites. This suggests that the spentwash may have potential in ameliorating the sodic soils.

Table 4. Effect of spentwash and organic amendments on total amount (mg) of cations and anions leached from calcareous vertisol (Malathi 2002).

Cations/anions	Control	SW alone	SW + FYM	SW + GLM	SW + BC
Ca	910	3007	3511	3840	4283
Mg	132	351	315	224	479
Na	36	126	140	151	215
K	5	11	9	17	17
Cl	495	1739	1887	1984	2025
SO4	1351	3493	2329	1827	3502

SW = Spentwash (@ 500 m³/ha); FYM=Farm yard manure (@ 12.5 t/ha); GLM=Green leaf manure (@ 6.25 t/ha); BC=Biocompost (@ 3 t/ha)

Application of organic amendments was found to enhance the leaching of cations from the spentwash amended soils, mainly, by improving the hydraulic conductivity and other physical conditions of soils. Application of spentwash both in the presence and absence of organics resulted in large amounts of Cl^- and SO₄^2- leached from soils. Chloride and SO₄^2- leaching was relatively greater from sodic soil than calcareous vertisol. While the potential salinity of leachate from sodic soil was less than vertisol, the SAR was found higher with sodic soil than vertisol.

Table 5. Effect of spentwash and organic amendments on total amount (mg) of cations and anions leached from sodic soil (Malathi 2002).

Cations/anions	Control	SW alone	SW + FYM	SW + GLM	SW + BC
Ca	574	1523	1569	2205	1915
Mg	203	1403	1872	1365	1263
Na	1026	2044	1869	2135	2412
K	5	28	28	27	25
Cl	1372	2856	3381	2699	2268
SO4	1050	1953	3206	3395	4291

SW = Spentwash (@ 500 m³/ha); FYM=Farm yard manure (@ 12.5 t/ha); GLM=Green leaf manure (@ 6.25 t/ha); BC=Biocompost (@ 3 t/ha)

The leachates from both vertisol and sodic soil amended with spentwash had exceptionally high organic loading as indicated by BOD and COD (Table 6). This demonstrates the adverse effect of the spentwash application on groundwater contamination. Though not of similar magnitude, field observation of the open well waters around spentwash applied fields also had revealed deterioration of water quality due to high BOD, COD and excessive salt content.

Table 6. BOD and COD (mg/l) of leachate collected from vertisol and sodic soil (Malathi 2002).

Treatment	Vertisol		Sodic Soil
	BOD	COD	BOD	COD
Control	153	137	124	194
Spentwash@125m3/ha	8331	24136	7695	21200
Spentwash@125m3/ha + FYM	6921	15227	5368	11090
Spentwash@125m3/ha + GLM	10342	16887	8352	12589
Spentwash@125m3/ha + BC	12356	18043	9588	15421
CD(P=0.05)	453	182	431	1104

Even after seven leaching events soils had large accumulation of salts from spentwash application both in the presence and absence of any organic amendments. The accumulation of cations followed: K⁺ > Ca²⁺ > Na⁺ > Mg²⁺ in vertisol, and K⁺ > Ca²⁺ = Na⁺ > Mg²⁺in sodic soil. While large amount of Cl^- was found accumulated, only traces of SO₄^2- were measured in both soils. Even after seven leaching events, the establishment of ragi, as measured by germination and vigour index, was markedly suppressed due to the application of large amounts of spentwash. However, its combined application with organic amendments particularly GLM and FYM improved the germination and the vigour index.

Conclusions

Distillery spentwash is acidic and contains high levels of soluble salts. Among the plant nutrients, K is found in higher amounts followed by N and P. The presence of Ca in considerable amounts makes the spentwash a potential amendment for reclaiming sodic soils. The beneficial effect of spentwash on crop production was exerted only at lower rate of application. The presence of appreciable amounts of plant growth promoters viz., GA and IAA adds additional value to the spentwash. Although the spentwash application increased the soil salinity induced by accumulation of salts, it had markedly improved the fertility status of dryland soils. Enhancement in the activities of enzymes and microbes was also evident in soils amended with the spentwash. Seed hardening with 20% spentwash was found very effective in improving the germination, and growth parameters in certain dryland crops. However, exceptionally high organic loading is likely to diminish the nutrient potential of the spentwash. The contamination of surface and ground waters, destruction of aquatic life and excessive accumulation of salts in soils pose serious threat to sustenance of soil and environmental health. Technologies based on scientific experimentation are needed for effectively utilizing this valuable resource in agriculture without any environmental hazards.

Acknowledgement

We gratefully acknowledge M/s Rajshree Sugars and Chemicals Ltd, Coimbatore, South India for funding this research project. Thanks are also due to Mr. R. Murugaragavan, Ms. M.P. Malathi and Dr. B.B. Saliha for providing data.

References

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