A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 1997

The prime objective of this research was to investigate the suitability of Bangkok domestic sewage sludge and nightsoil sludge in agricultural application with respect to Potentially Toxic Elements (PTEs). Pot studies as well as Field experiments with Sunflower (Pacific-33) and Grass (Zoysia matrella) were conducted at Agricultural Research Field and CIM building Lawn, AIT. Analyses of sludges showed that both sewage sludge from Huay Kwang and nightsoil sludge from Nong Khaem exceeded the limit set-up by Australian Guidelines for unrestricted use with respect to Zn, Cu and Ni. However, the sludges were considered safe for landscaping/agriculture/forestry/land rehabilitation. As per German Standards, all the heavy metals except Zn in both the sludges were well below the limit for safe agricultural application. Pot studies with pure sewage sludge and nightsoil sludge showed that seeded sunflower plants and volunteer plants (tomato plants) accumulated appreciably high concentration of Zn, Mn and Cu compared to Cr, Pb, Ni and Cd. Nothing could come-up voluntarily in pure sewage sludge pots. Pure nightsoil sludge showed highly promising as lots of plants (tomato, chili, egg plants etc.) grew voluntarily . The sunflower plants as well as volunteer plants were quite healthy until the end of the experiment. Recreational grass/Turf application of sewage sludge and nightsoil sludge gave a significantly good yield due to higher application rate of sludge in terms of nitrogen (100 to 200 kgN/hac). But in respect of PTEs, no significant differences were found with 0.50 and 0.25 cm sludge application and with control plot. Field experiment with sunflower depicted that for most of the treatments, the plant available and total heavy metal concentration were higher in plots with higher loading rates of sludge. The concentration of heavy metals decreased with time. It was also found that sewage sludge contributed higher concentration of heavy metals in the soil compared to nightsoil sludge and artificial fertilizer. However, the concentration in the soil was well below the UK Standards, German Standards and the US EPA Regulation. High concentration of Zn, Mn and Cu was detected in the sunflower plant leaves. The upper concentration of these metals was: Zn= 123.4 mg/kg dry matter; Mn = 734.1 mg/kg dry matter; Cu = 43 .9 mg/kg dry matter. None of these metal concentrations exceeded the tolerable limits of plant. Appreciable amount of heavy metals was accumulated in the seed. But these metals are not significant from dietary intake point of view. The application of sludge to agricultural land exhibited positive response to the plant growth. The effect of sludge was found statistically not significant with respect to PTEs accumulation in plant tissues. Bangkok domestic sewage sludge and nightsoil sludge is therefore regarded as safe and beneficial to landscaping development and agricultural application in this region.