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.

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