Sewage Sludge as Fertilizer in Soybean Production
by
Tawadchai Suppadit
The Graduate Program in Environmental Management, School of Social Development and Environment, National Institute of Development
Administration, Bangkapi, Bangkok 10240, Thailand
This research sought to study the growth, yield, yield components, seed quality, including nutrient and heavy metal content of soybean cultivar Chiang Mai 60 (CM. 60) by using sewage sludge from domestic wastewater treatment as fertilizer. The experiment used a completely randomized design, divided in 6 treatments with 4 replications and was conducted from February to June, 2004. Sewage sludge was mixed with soil at the rate of 5, 10, 15
and 20% by weight, and chemical fertilizer (12-24-12) was applied at the rate of 10 grams/basin.
The results showed that soybean growth, yield, yield components, seed quality, protein and lipid were significant (P<0.05), showing the best potential productivity at 5% by weight and being better than chemical fertilizer. The residues of heavy metals (lead, cadmium and mercury) accumulated in leaves and seeds, including in soil before and after the study were also significant (P<0.05) related to the quantity of sewage sludge used. Soil nutrients of all treatments were also significant (P<0.05). The data varied similarly to the residues of heavy metals. The replacement of sewage sludge for chemical fertilizer in plant production including soybean could be as a nutrient source. However, it must used in an appropriate rate. Moreover that, it should not be used in plants for human and animal consumption because heavy metals may accumulate in plant products.
Key words : nutrient, potential productivity, heavy metal
Introduction
The extensive communities and industrial developments continue to cause environmental problems from amounts of water waste and pollutions in Thailand (Suppadit, 2003). Domestic wastewater is one of these problems which is related to community growth and population increase. Water pollution is caused by wastewater discharge or leakage, or discharge without control and treatment. In the future, clean water for consumption may not be available in Thailand (Suppadit, 2004). Therefore, the Thailand government tries to improve water quality with the proper operation of wastewater treatment systems. The major aim of wastewater treatment is to remove as much as possible suspended solids before the remaining water is discharged back to the environment. After treatment, sludge content is about 60.0 grams/person/day (Tunnukit, 1999) which differs in type, characteristics, and composition depending on water utilization activities of these communities (Chawsithiwong, 2000). Sludge is composed of organic compounds, nitrogen, phosphorus and potassium at levels of 50.0-80.0, 2.50-5.00, 1.50-2.00, and 0.020-0.500 percent/dry weight, respectively (Suntornvongsagul, 1994). At present management sewage sludge involves discharging it to public lands (Suppadit, 2004) which still has many problems for environment. A new concept for sewage sludge management involves its use as a fertilizer for crop production, including field and vegetable crops (Sermviriyakul, 2004). This study sought to apply sewage sludge to replace chemical fertilizers. This method may decrease the costs of soybean production, improve the environmental health and safety in long-term period and provide evidence for sewage sludge management in a community.
Materials and Methods
Experimental Place
Trials were conducted at Kleang District, in Rayong Province from February to June of 2004. The trialswere in an artificial greenhouse that measured 6.00 m wide ื8.00 m long ื2.00 m high (96.0 m3), and useda plastic roof. Corrugate iron and blue net were used as a border around the greenhouse.
Experimental Design
This study was a separate, completely randomized design with 4 replications. The treatments evaluated are as follows:
T0: Control (without sewage sludge or chemical fertilizer)
T1: Soil mixed with sewage sludge, 5%/(w/w)
T2: Soil mixed with sewage sludge, 10%/(w/w)
T3: Soil mixed with sewage sludge, 15%/(w/w)
T4: Soil mixed with sewage sludge, 20%/(w/w)
T5: Chemical fertilizer formula 12-24-12, 10.0 g/basin, applied after 20 days after emergence (DAE)
Soybean cultivar Chiang Mai 60 (CM. 60) was evaluated. Soil was mixed with dried sewage sludge into total weight 10 kg/basin (T1 - T4). Besides, soil weight of T0 and T5 was 10 kg/basin, which did not mix with sewage sludge. After two weeks, soybean seeds were planted in each plastic basin, thinned one week after, leaving four seedlings/basin. These were watered until the R7 stage (beginning maturity). The entire plots area was weeded by hand. Tobacco was used for insect control. Data recorded were planting date, stage of emergence, number of nodes, height, leaf area, dry matter, number of pods/plant, number of seeds/pod, 100 seeds dry weight, and yield/basin. Seed germination and vigor were recorded based on ISTA Rule (1985). Proteins and lipid were measured by the Kjeldahl method and by extraction, respectively. Heavy metals were measured by using the methods of atomic - direct aspiration for lead and cadmium and atomic absorption - cold vapor for mercury.
SAS program version 6.12 (SAS Institute, 1996) was used for analysis of variance and DNMRT (Duncans New Multiple Range Test) to compare the experimental treatments.
Results and Discussion
Potential Growth and Yield
The number of soybean nodes was significant for all treatments (P<0.05) (Fig. 1). The highest node/ plant of 15.0 nodes was obtained from the 5% mixture which was not different from the chemical fertilizer (14.5 nodes). Similar results were obtained by Srisomboon (1999), who reported 15.0 soybean nodes. The 5% mixture was the optimum for soybean production. Increasing the quantity of sewage sludge caused a decrease in the number of nodes which could be due to the increasing level of heavy metals related to the quantity of sewage sludge used.This may be a factor affecting soybean growth and development. Cationic metals which are more soluble in acid soil appeared in this experiment. These have been found to inhibit plant photosynthesis, decrease plant dry matter accumulation and soil microorganism activities and increase NO-3 accumulation (Panichasakpatana, 1996). Although, the 20% mixture showed the highest value of nutrients, this level showed the lowest number of nodes and had the highest value of soil salinity, which is not appropriate for soybean production.
Conclusions
The growth and yield of soybeans responded to sewage sludge incorporation in soil. Incorporation of sewage sludge at 5% by weight showed the best soybean growth and potential productivity. The use of sewage sludge as replacement for chemical fertilizer in soybean production could be as a nutrient source. However, sewage sludge should not be used at more than 5% by weight.
Suggestions
Application of sewage sludge may cause problems from soil acidity, soil salinity and heavy metals. Thus, soybean growers should check pH and Na - content in sewage sludge and adjust soil pH to 6.5 or higher before actual planting. For heavy metals, further study can be made to monitor bioavailability of heavy metals. Careful plant selection is important for the use of metal - contaminated soils. Plants translocate larger quantities of metals to their leaves than to their fruits or seeds. The greatest risk of food chain contamination is in leafy vegetables like lettuce or spinach. Another hazard is forage eaten by livestock. Flowering plants and ornamental plants are new sectors that can use sewage sludge as fertilizer substitute.
Acknowledgement
Authors would like to express their appreciation for the laboratory support of this work by Mr. Nakarin Pripwai, Chiang Mai Rajabhat University and researchers of Chiang Mai Field Crops Research Center, Thailand.
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