Sewage Sludge as a Replacement for Chemical Fertilizers


In view of increasing world population and depleting land and water resources, the yield of agricultural crops per unit area has to be increased. This may be achieved through use of costly chemical fertilizers, their hazardous effect like contaminating underground water notwithstanding. Sustainable agriculture under the situation demands management of soil organic matter and rational use of organic inputs such as sewage sludge, crop residue, green manure and food industry wastes. The high nutrients and organic matter contents of sewage sludge make it a useful supplement to enhance soil fertility. Hence, land application of sewage sludge offers a potential means for using this waste material in agriculture, instead of the current disposal practices of land filling and incineration. This will be not only economically viable but also environmentally acceptable. However, presence of heavy metals causes a major problem for such a utilization of sludge, especially in situations where waste treatment facilities are inadequate or missing as is common in most developing and under developed countries. In Pakistan for instance, so far very little attention has been focused on the treatment of urban and industrial wastes. Here, the untreated sludge is mixed with agricultural soil as a supplemental fertilizer especially near big towns to produce vegetables which go to the market without any assessment of hazardous effects on the health of consumers rising from presence of heavy metals in the produce.
Studies have confirmed that sewage sludge enhances crops yield and may replace chemical fertilizers. In an independent study in Pakistan for instance, sewage sludge application to wheat at the rate of 40 tons ha-1 produced almost similar grain weight as those obtained with nitrogen:phosphorus:potassium (120:60:60 kg ha-1) applied as chemical fertilizer. Sludge improved biological as well as physical properties of soil. The experiments were conducted for three successive years from where it is evident that sewage sludge applications to wheat field did not affect grain quality in the short term but over time, accumulation of Zn, Cu, Pb and Cd tends to start in soil. This indicates that metals accumulation in soil will be a problem if sewage sludge will be used on long term basis. To avoid this problem, it is recommended that sewage sludge application should be discontinued after three years and repeated after two years break. However monitoring of soil and grain for heavy metal accumulation is necessary. More continuous long- term experiments are needed to improve our understanding of the effects of sewage sludge on soil fertility and crop yield to contribute to the development of sustainable agricultural practices.
The returning of sewage sludge into crop fields is considered to be one of the most effective methods in reducing the use of industrially produced fertilizer, which consumes high cost oil energy. Research has proved the beneficial effects of this practice in agriculture leading to improved physical, chemical and biological properties of soil following sewage sludge application. Consequently, significant increase in crops yield over control has been observed. However, repeated application of sewage sludge may gradually increase heavy metal concentration in the soil. This warrants a cautious approach to adopt the practice.