Abstract
The simple operational and maintenance requirements of slow sand filtration (SSF), coupled with its high biological treatment efficiency makes it an attractive technology. The main limitation of SSF is its vulnerability to high suspended solids loadings. Pre-treatment in such instances can be achieved by gravel filtration. Cases of gravel pre-filtration underperformance have been attributed to significant fractions of influent suspended particles in the colloidal range. Maximum limits of turbidity for the application of gravel pre-filters have also been suggested. A simple intervention to overcome such water quality constraints can be through the dosing of a coagulant (aluminium sulfate) upstream of the pre-filtration stage; enhancing the pre-treatment efficiency, in what could be defined as direct (gravel) filtration. Previous studies have investigated its use as a pre-treatment for SSF; however, the results emphasise pre-filtration treatment efficiency and do not consider the effectiveness of the pre-treatment in protecting the slow sand filters. Also, because of the potential toxic effects of A1 residuals, its upstream use in relation to a biological SSF treatment has never been properly evaluated. The objectives of this study were: assess the efficiency as well as the effectiveness of chemically- enhanced up-flow gravel filters in series (UGFS) as pre-treatment for SSF and study the impact of aluminium residuals on the treatment performance and potential effects on biological activity. Preliminary experiments and first set of runs with chemically-enhanced pre-filter showed evidence of wall-effects due to the media size/column diameter ratio. This experimental design shortcoming was thought not to have affected the overall trends of results and was addressed for the set of pilot-experiments on which most conclusions are based. The results from the experimental work have shown that chemically-enhanced gravel pre-filtration can be effective only if coagulant dose is carefully controlled. Contrary to previous research, it was found that when resorting to chemically-enhanced pre-treatment (with alum) turbidities of less than 2 NTU (nephelometric turbidity units) should be targeted for in order to ensure an efficient and effective SSF pre-treatment. Such control will minimise A1 residuals that can otherwise cause a premature blockage of the slow sand filter by A1 hydroxide precipitates even with influent turbidities below 10 NTU. This was speculated to occur possibly due to size and mechanical properties of deposits retained on the uppermost layers of the SSF beds. A1 speciation analyses revealed that A1 residuals from chemically-enhanced pre-filtration were found to be mostly of inorganically- bound Al. This fraction consisted mainly of A1 in its form which is considered to be potentially more bioavailable (and possibly toxic) to aquatic (micro)organisms. However, slow sand filtration column trials found that filters dosed with the potentially more labile form of A1 did not show any significant difference in terms of treatment performance parameters and biological activity indicators. It has been demonstrated that chemically-enhanced pre-filtration (with alum) may not be an effective pre-treatment on the basis that it may cause an early blockage of the slow sand filters. There was no evidence of effect of A1 on biological treatment of the slow sand filtration process.