Study on dewatering effect of sewage sludge by adding rice husk flour

At present, finding valuable application of rice husk should be targeted for further research, and how to deal with sewage sludge has become an issue that requires urgent attention. In this study, rice husk flour, an agricultural waste material, was used to condition sludge and enhance sludge dewaterability. The effect of rice husk flour dosage on sludge dewaterability was investigated, and the surface element content and main components of rice husk flour and the microstructure of sludge cakes was investigated to analyze conditioning mechanisms. In addition, the effect of adding rice husk flour on turbidity and soluble chemical oxygen demand (SCOD) of sludge filtrate were also investigated, the advantages of adding rice husk flour for future filtrate treatment were evaluated. Results showed that the dewaterability of sludge conditioned with the rice husk flour and FeCl3 (138.09 g/kg dry solids, DS) was much superior to the sludge conditioned with rice husk flour, and the optimal dosage of rice husk flour was 70%. With the optimal rice husk dosage, the sludge specific resistance to filtration (SRF) decreased by 59.73%, the net sludge solids yield (YN) increased by 45.27%, and the solid content of sludge cake increased from 13.99% to 23.97%, compared with the sludge conditioned with FeCl3 (138.09 g/kg) alone. When the rice husk flour was used to condition sewage sludge combined with FeCl3 (138.09 g/kg), the large cracks were found in the sludge cake easily by an environmental scanning electron microscope (ESEM) and the coefficient of compressibility of sludge cake was decreased to 0.92 (the coefficient of compressibility of raw sludge cake and sludge cake conditioned with FeCl3 alone were 1.21 and 1.08, respectively). So the sludge cake became more incompressible and permeable and the sludge moisture could easily pass through. Therefore, the rice husk flour was proved to play a supportive role as a skeleton builder in the sludge cake for sludge conditioning and dewatering. Surface element contents and zeta potential of rice husk flour tested by an energy-dispersive spectrometry (EDS) and Zetasizer Nano analyzer showed that the stability of sludge colloidal system was not destroyed by charge neutralization, and the sludge particles could not congregate with each other under the treatment with rice husk flour alone, it might be the main reason why adding rice husk flour and FeCl3 was superior to adding rice husk flour alone for sludge conditioning and dewatering. Moreover, adding rice husk flour could reduce turbidity and SCOD of filtrate (the turbidity decreased from 11.89 NTU to 2.91 NTU and SCOD decreased from 664.87 to 79.93 mg/L, respectively) because the rice husk flour addition might adsorb the pollutants in sludge supernatant. As important indices of water quality, the reduction of turbidity and SCOD was benefited to further filtrate treatment because of the possible significant reduction of difficulty and cost, that is to say that adding rice husk flour as a conditioner aid can also improve the quality of filtrate for further filtrate treatment. Therefore, using rice husk flour to condition sludge is economically feasible and promising. © 2019, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.