Research on the effect of sludge fertilizer on farmland and the safety of heavy metals in a karst area

Guilin is a typical karst area where the hydraulic proximity between surface water and ground water is close. From the point of environmental conservation, karst aquifers are more vulnerable than non-karst aquifers because their underground conduits provide no filtration. Therefore, it is necessary to pay close attention to contamination from heavy metals in the groundwater if a sludge fertilizer, which usually contains relatively large amount of heavy metals, is applied to the land surface in this area. To determine the effect of sludge fertilizer and the possible toxicity of heavy metals when the sludge of municipal wastewater is used in agriculture, a series of experiments was undertaken. Firstly, sludge was extracted from the sewage treatment plant of Guilin and the plant nutrient and heavy metal was then analyzed. Secondly, a complex organic fertilizer of sludge (OCFS) was produced. This OCFS was applied to rice land to determine its effect as a fertilizer. Lastly, the heavy metal contents of agricultural products after application of OCFS were studied. It was shown that (1) there were large amounts of plant nutrients in the sludge: on average it contained 39.6% organic matter. The content of total nitrogen, phosphorus, and potassium was 48.3, 21.1, and 8.5 g/kg on average, respectively. The heavy metal contents of the sludge are in accordance with the National Standard and, basically, the sludge is suitable for agricultural application. (2) The fertilizer of sludge was effective. The output of rice was increased by 18-19% when OCFS was used. The effect of sludge fertilizer was better than that of a complex fertilizer sold on the market. When sludge fertilizer was tested against others on the market, no difference could be found in the heavy metal content of the rice and rice stems. Therefore, sludge fertilizer can be assumed to be safe and effective for agriculture. (3) The extraction rate of heavy metals ranged from 23 to 60% when the sludge was extracted by 0.1 mol of EDTA, i.e., 23 to 60% of metals may be absorbed by plants.