Sewage sludge: turning an environmental pollutant into an agricultural resource

Agro-industrial wastewater treatment plants generate large quantities of sewage sludge daily. In Metro Manila [Philippines] alone, as much as 21,000 tons of sewage sludge could be accumulated annually. The disposal of sewage sludge by either incineration or dumping into bodies of water causes a serious threat to the integrity of the environment. A pollutant such as sewage sludge may be turned into an agricultural resource by using it as organic fertilizer. However, the issue of heavy metal accumulation and pathogenic contamination of the soil must be addressed. Sewage sludges from Maya Farms and Century Canning Corporation [Philippines] were found to pass the US and German standards for the critical levels of heavy metals. For pathogen control, irradiation with gamma radiation coming from 60 effectively eliminated the indicator organisms such as coliforms by as much as 95 percent. Biological composting and alkaline stabilization using a combination of heat, high pH (12), and drying are alternative procedures which may be used. Fertilization studies using 15N isotope dilution technique under field conditions revealed that for short-duration crops such as pechay, immobilization of sewage sludge N occurred during the main crop but it was subsequently mineralized during the residual crop. This indicates the need for incorporating the sewage sludge into the soil way ahead of seeding time if short-duration crops (6-7 weeks) are to be grown. The residual effect of sewage sludge on pechay is much better than that of ammonium sulfate; however, CCC sewage sludge had much more residual effect than Maya sewage sludge. In the case of upland rice, the regression of grain yield with N rate follows a curvi-linear function which was described by a second degree polynomial equation. Based on the equation, the equivalent N rates for the application of 90 kg N/ha and 180 kg N/ha as CCC sewage sludge were 33 and 55 kg N/ha, respectively. Therefore, the application of 90 kg N/ha and 180 kg N/ha as CCC sewage sludge was as effective as 3.3 and 5.5 bags of ammonium sulfate, respectively. With proper management, an environmentally hazardous waste in the form of sewage sludge could be utilized effectively as an organic fertilizer for sustainable agricultural production