|Title:||Hydraulic characteristics and plant-microorganism micro-ecosystem of sand-sludge soil (Eco-Soil) for anti-desertification|
|Advisors:||Wai, Wing Hong Onyx (CEE)|
|Subject:||Sewage sludge -- Recycling.|
Sewage sludge -- Characterization.
Desertification -- Control.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Civil and Environmental Engineering|
|Pages:||xxx, 284 pages : color illustrations|
|Abstract:||Desertification has been one of the severe worldwide environmental problems due to the loss of land productivity, including water retention capacity and nutrients. However, nutrients are rich in activated sludge from wastewater treatment processes. As a win-win strategy, this study aims to develop and investigate a novel growing medium, which is the mixture of arid sand and thickened activated sludge, namely the Eco-Soil, to rebuild the land productivity from the margin of the arid area. This study carried out comprehensive investigations of the hydraulic properties and plant growing capabilities of the Eco-Soils with different Sand-Sludge Volumetric Ratios (SSVR), 100:0, 80:20, 60:40, 40:60 and 80:20. The results were compared with the properties of commercial potting soil. The Soil Water Characteristic Curve (SWCC) of the Eco-Soils were determined by experiments and numerical simulations using the van Genuchten model in the RETC software. The seed germination index experiment was carried out to study the phyto-toxicity of Eco-Soils. The growth of Abelmoschus esculentus (Okra) in Eco-Soil was also investigated to reveal the productivity improvement. In addition, the runoff patterns of the soil under rainfall intensities from 10 to 100 mm/hr were investigated to elucidate the maximum rainfall retention capacity. The results reveal the relation between SSVR and Sand Sludge Weight Ratio (SSWR) and indicate the hydraulic properties, which include bulking density, particle density, porosity, particle size distribution, field capacity and hydraulic conductivity, affected by activated sludge. Seed germination percentage, root length ratio and seed germination index further illustrate the plant affinity for Eco-Soil leachates. The optimal SSVR of Eco-Soil for anti-desertification purposes is suggested to be 60:40 based on the performance of hydraulic properties and phyto-toxicity results.|
The SWCC of Eco-Soils changes when sludge content increases indicating that the water retention abilities of Eco-Soils can be greatly improved by sludge introduction and the performance is comparable to potting soil. The numerical simulation results show that the van Genuchten model fits well to the Eco-Soil in the suction head range of 0~-1.19×10⁶ cmH₂O. The regression relation between the sludge weight content and the two essential fitting factors, α and n, offers a way to introduce the sludge weight ratio into the van Genuchten model to develop numerical SWCC equation for Eco-Soil. Observations and findings of the plant vertical development, the dried weights of organs, the root shoot ratio (RSR), the pod and seed productions show that the plant growing in Eco-Soils of SSVR 60:40 and 40:60 perform comparably and stably. From the consideration of maximizing the utilization of activated sludge and achieving sustainable development of vegetations, the SSVR 60:40 of Eco-Soil is recommended. The rainfall runoff results show that the maximum rainfall retention under one hour rainfall on arid sand is improved from 3.62 to 16.08 L and is linearly related to the sludge weight ratio. Finally, based on the present comprehensive analysis, Eco-Soil of SSVR 60:40 has suitable physical properties and plant affinity for anti-desertification.
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