Assessment of the Quality of Liquid Digestate from Small-scale Anaerobic Biodigesters Used for Crop Irrigation in Urban and Peri-urban Maseru, Lesotho

Abstract

Objective: The aim of the study was to determine the quality of Liquid Digestate (LD) from selected small scale anaerobic digesters for biogas production and assess the suitability for crop irrigation. Methodology: The selection of the parameters was guided by national standards and international guidelines for the agricultural use of wastewater and wastewater treatment products. The analysis was carried out using standard methods. Results: The results showed that most of the parameters determined of the LD from the anaerobic reactors were within the ranges of the national standards and the FAO recommended guideline limits for crop irrigation with wastewater; pH (6.75-8.50), alkalinity 12.5-45.7 mg/L), EC 0.39-1.30dS/m), COD (82.3-158.0 mg.O2/L, SS (1.35-6.17 mg/L) and TDS (249.6-832.0 mg/L). The LD from the reactors contain some considerable amounts of plant nutrients; total nitrogen (11.5- 33.1 mg/L), ammonium nitrogen (2.3-22.0 mg/L), total phosphorous (1.5-121.6 mg/L) calcium (37.68-438 mg/L), magnesium (15.25-127-36 mg/L), sodium (4.67-32.47 mg/L), chloride (9.30-19.5 mg/L) and potassium (12.07-39.50 mg/L). The mean concentrations of the micronutrients cobalt (0.67-0.94 mg/L), copper (0.78-1.08 mg/L), iron (0.851.93 mg/L), manganese (0.09-0.20 mg/L), nickel (0.82-1.48 mg/L) and zinc (0.31-2.24 mg/L) were greater than the FAO guideline limits for wastewater used in crop irrigation, which suggests that the LD are potential low-cost biofertilizer. The level of toxic metals arsenic (0.65-0.87 mg/L), cadmium (0.70-0.97 mg/L), chromium (0.71-0.98 mg/L) and lead (0.55-1.46 mg/L), were higher than the recommended levels for use of treated wastewater in crop irrigation. The numbers of the common pathogenic microorganisms determined were much lower than the FAO and WHO recommended limits indicating that the liquid digestate will not pose any major health risk a biofertilizer. Conclusion: The nutrients and other parameters indicated that the quality of the LD is good enough to be used for crop irrigation without any restrictions.However, the LD should be used with caution, because of the levels of the toxic metals, which may accumulate in the soil after prolonged application. Also, extension services on best practices for the agricultural application of the LD should be made available to potential users to mitigate any potential negative environmental and health impacts.