Sewage sludge as a potential fertilizer

The increasing volume of sewage resulting from the ever-increasing population is becoming an environmental risk, especially to the marine ecosystem where the sewage sludge is usually thrown. Knowing that sewage sludge is a rich source of nutrients for crop production, the use of sewage sludge as fertilizer shows to be a potential solution to the sewage disposal problem. However, sewage sludge fertilizer is not very well appreciated in our country, unlike in China and some European countries. Hence, Estanislao, et al. (SRA) verified the value of sewage sludge as fertilizer through a study “Productivity improvement of soils planted to sugarcane with sewage sludge”.

The study was done under greenhouse and field conditions, using Phil 8583 sugarcane variety grown on Angeles loamy sand. The greenhouse experiment determined the effects of nitrogen (N) fertilizer at 90 kg/ha and 180 kg/ha; sewage sludge, at 10 t/ha, 20 t/ha, 30 t/ha, and 40 t/ha; and different proportions of combined sewage sludge and nitrogen fertilizer, on crop growth (in terms of plant height, tillering, and biomass yield) for a four-month period.

The field experiment was conducted at the SRA-LAREC, Paguiruan, Floridablanca, Pampanga during the period April 2000-May 2001. It measured the growth and yields of sugarcane (in terms of germination count, tillering, plant height, millable stalks juice analysis, can tonnage, and sugar yields) with sewage sludge at 40 t/ha, 80 t/ha and 120 t/ha; with N fertilizer at 90 kg/ha, and 180 kg/ha; and different proportions of combined sewage sludge and N fertilizer.

Findings

• The sewage sludge used in the study had a pH of 7.1 (neutral), and contained the following nutrients: nitrogen (0.15% N), phosphorus (0.06% P2O5), potassium (0.01% K2O), calcium (0.15% CaO), magnesium (0.09% MgO), sodium (0.01% Na), zinc (67.75 ppm Zn), copper (11.99 ppm Cu), manganese (28.18 ppm Mn), iron (2,375.00 ppm Fe) and organic carbon (1.75% C).
• The greenhouse experiment showed that except for biomass yields, plant height and number of tillers were not affected by sewage sludge and N fertilizer application. The performance of 40 t/ha sewage sludge alone (without N fertilizer), wit a biomass yield of 2.22 kg/bag, was similar to that of the 90 kg/ha N fertilizer combined with sewage sludge at 10-40 t/ha, and 180 kg/ha N fertilizer alone, or a mixture of 180 kg kg/ha N and sewage sludge at 10-40 t/ha. The optimum combination of N fertilizer and sewage sludge was 90 kg/ha N and 20 t/ha sewage sludge, obtaining a biomass yield of 2.67 kg/bag.
• The field experiment showed that sewage sludge at the rate of 40-120 t/ha could give almost the same performance as that of 90 kg/ha N fertilizer in terms of the number of millable stalks, and cane tonnage. The number of millable stalks in 40-120 t/ha sewage sludge (125-137.75) was not significantly different from that in 90 kg/ha N fertilizer (130). Likewise, cane tonnage in 40-120 t/ha sewage sludge (36.20-43.17 tc/ha) was not significantly different from that of 90 kg/ha N fertilizer (43.39 tc/ha).

  To a certain level, the combination of sewage sludge and N fertilizer further increased the number of millable stalks and cane tonnage. With 180 kg/ha N fertilizer alone, the number of millable stalks was 165.50, which was not significantly different from that of the combined 180 kg/ha N fertilizer and 40-120 t/ha sewage sludge (154.50-165.00). Similarly, cane tonnage from the combined 90 kg/ha N fertilizer and 80-120 t/ha sewage sludge was higher (58.18-58.76 tc/ha), but did not differ significantly from that of 180 kg/ha N (58.29 tc/ha) and the combined higher rates of N fertilizer and sewage sludge (56.88-64.07 tc/ha).
  

• Germination rate, tiller counts, and sucrose content were neither affected by sewage sludge nor by N fertilizer.
• Plant height increased significantly with application of either sewage sludge or N fertilizer, i.e., from 104.35 m (without sewage sludge or nitrogen fertilizer) to 122.51-137.09 m with 40-120 t/ha sewage sludge, and 141.33-152.29 m with 90-180 kg/ha N fertilizer. However, no further increase in plant height was observed from increasing the rates of combined sewage sludge and N fertilizer. The performance of 120 t/ha sewage sludge (137.09 m) was at par with that of 90 kg/ha N (141.33 m); and the combined 10 t/ha sewage sludge and 90 kg/ha N fertilizer (157.27 m) had the same performance as the combined 180 kg/ha N fertilizer and 40-120 t/ha sewage sludge (145.47-156.99 m).
• Sugar yields were observed to increase with an increase in the rate of fertilizer application. Sugar yields (89.71-103.83 Lkg/ha) obtained from 40-120 t/ha sewage sludge were relatively lower than that obtained from the 90-80 kg/ha N fertilizer and from the combined N fertilizer and sewage sludge, with 107.08-157.07 Lkg/ha.
• The sewage sludge contained the following heavy metals: Arsenic, Cadmium, Chromium, Lead, Mercury, and Selenium. Lead content was highest. Although within the acceptable standard limits set by the DENR (Table 1), these metals would accumulate in the soil over time through continued application, and this poses risks to the environment and health of the people.

Implications of the Findings

• The findings indicate the great potential of sewage sludge as a substitute or supplement for N fertilizer. However, precautionary steps must be taken to address the risks of heavy metal accumulation and pollution of water bodies from runoff due to application of large volume of sewage sludge fertilizer. Nevertheless, converting the sewage sludge into more potent, compact, or concentrated forms of fertilizer materials may alleviate its possible adverse environmental effects.

Table 1. Heavy metal content of the soil after harvest at varying levels of liquid sewage sludge application.

* Per Department of Environment and Natural Resources Department Administrative Order No. 29.

Source: PCARRD, 2002. Highlights 2001, Los Banos, Laguna.

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