Use of rice hull ash in water treatment

Abstract

Rice hulls, the largest milling byproduct of rice, constitute one fifth of the paddy by weight. The hulls which can be obtained at relatively low cost, are in abundant supply in most developing countries, particularly in the Asian region. Hull contains approximately 20% silica by weight and, on combustion, yields a porous ash having a silica content of approximately 90 percent. This thesis investigates the following two possible applications of rice hull ash in water treatment processes: (i) the use of rice hull ash as a filter medium; (ii) the use of rice hull ash as a coagulant aid. Rice hull ash comprising amorphous silica was produced from locally available rice hulls, using a specially constructed incinerator. Scanning electron microscopic studies were conducted to evaluate the microstructure of this ash as well as diatomaceous earth and filter sand. The laboratory filtration experiments were conducted at slow to semi-rapid filtration rates in order to investigate the effectiveness of rice hull ash medium in removing turbidity, bacteria and colour from water, and to compare its performance with a conventional sand filter. Synthetic water was made by adding suitable amounts of kaolin clay, Escherichia coli suspension, coffee/leaf extract, to laboratory tap water. Filtrate quality and head loss were considered as the major parameters in assessing the performance of these filters. A number of thin layer filter experiments were conducted to obtain the variation of turbidity with depth in rice hull

ash filters, under selected operating conditions. Solubility studies of rice hull ash at various strengths of sodium hydroxide and for various steeping periods were made to evaluate the optimum conditions for silica solubilisation. A dilution procedure including partial neutralization and Subsequent aging prior to further dilution was adopted to convert alkali soluble rice hull ash into activated silica. Coagulation of dilute clay suspensions (£ 40 m g / L ) , using alum, activated silica and selected polyelectrolytes were conducted with the aid of a jar test apparatus. The filtrate turbidity for approximately 750 mm depth of rice hull ash medium was equal to or less than that of a sand medium, at rates of filtration 0.25 to 2.0 m 3 / m 2h and a turbidity range of 20 to 60 FTU. The rate of head loss in the sand medium was 1.5 to 5 times more than in an ash filter. The optimum rate of filtration for the ash filter occurred at 1.0 m 3 / m 2 h , with a rate of head loss of 52 mm/d. At filtration rates of 0.5 and 1.0 m 3 / m 2 h , for an influent Escherichia coli concentration of 100 - 2000 no/mL, approximately 70% to 90% reductions in bacterial numbers were achieved by 750 mm depths of rice hull ash media. Colour removal of at least 30% was achieved by shallow depths (< 320 mm) of ash media, at slow rates of filtration (< 0.25 m 3 / m 2 h ). The results obtained from thin layer filtration experiments were analysed using a statistical filtration model known as the chi-square distribution analogy. This technique was successful in predicting the performance of rice hull ash filters at specific operating conditions. The optimum removal of silica from rice hull ash occurred when ash was steeped in 5% NaOH solution for a period of 24 h. A procedure for the preparation of activated silica from rice hull ash was developed. The addition of 5 mg/L of activated silica as a coagulant aid during the coagulation of turbid water (4 0 mg/L of kaolin clay) with 50 mg/L of alum at pH value of 6, was sufficient to achieve a residual turbidity of 1.2 FTU. The coagulation of the same water with alum or alum-polyelectrolyte at similar conditions led to higher residual turbidity.