Sand has been the main filter media used in rapid gravity filtration since their emergence in the 19th century. This dominance is due to its low cost, availability and extensive experience which has led to dependable and predictable performance. Over recent years multi-media filters have become the typical filter arrangement. Sand still remains the preferred filter medium in the lower layer with typically anthracite used in the upper layer. A limitation to match previous work has been the emphasis on overall performance but mechanistic analysis as to the reasons for the variations compared to sand has been rare. The fundamental effects of particle size and consolidation on filtration performance and headloss are known but were not often accounted for in the reported research. This has limited the academic contribution of previous work and made it more difficult to compare with the data for this thesis. At an average treatment works the highest costs are associated with the use of chemicals (30 %) and power (60 %) required mainly for pumping. Rapid gravity filters are one of the least energy demanding stages in this system, only requiring pumping for backwashing and air scour, assuming gravity feed was incorporated into the design. Energy efficiency of water treatment has become more important and the research was conducted to determine if the use of novel new media could be used to improve the performance of the filters with regards to turbidity and headloss. For example, the result presented within this thesis demonstrates through the use of angular media improved performance to benefit both turbidity and headloss performance. This was obtained from slate having a sphericity of 0.49 compared to sand at 0.88. In addition the use of novel materials with different physical properties has allowed an extension to analysis of performance using fundamental filtration mechanisms. The greater range of properties available from the novel media used in this thesis compared to sand has suggested additions to this theory. The use of surface reactive materials, including limestone, has shown the removal of additional contaminants such as phosphorus, iron, aluminium and manganese not typically associated with rapid gravity filtration. An assessment of the impact these reactions had on typical filter performance criteria, for example turbidity, headloss and life expectancy. The results showed an 97 % removal of Fe in the limestone compared to 13 % for sand. This was brought about by the precipitation of hydroxide, coagulation, a pH change and consequent co-precipitation. In the case of iron and aluminium removal this pH induced change was theorized as the most likely cause of coagulation within the filter bed itself leading to improved turbidity removal performance. Filter media chosen for laboratory and pilot study in this work was firstly assessed using British Standards tests, but additional tests were added that could provide additional characterisation data. The media were selected based on an individual fundamental property that differed from the other media selected whilst retaining the standard RGF size. Filtralite for example offered a high surface area, limestone a more active surface and slate a plate-like particle shape. Glass had a very smooth surface texture and as a recycled material better sustainability. Four of these filter media (Sand (control), Glass, Filtralite and Slate) were then selected for further on-site pilot plant studies, based on results from the laboratory work. Both the laboratory and pilot study suggested that turbidity and headloss performance could be improved by changes in media specification. The results showed that after particle size, angularity of the media was the most important factor affecting turbidity and headloss performance. A greater angularity led to improvements in filter run time with for example a doubling of filter run time with the slate compared to sand for the same turbidity removal in the pilot plant. Previous literature had suggested an improvement in turbidity performance but that head loss would deteriorate but this was not seen in the data from this research, with slate (sphericity of 0.49) offering improved headloss performance. This improvement was attributed to the varied packing of the filter bed and associated porosity variations throughout the filter. The objectives of the pilot study were to provide understanding of scale-up factors and adjust these theories with real variable clarified water. Real water chemistry is too complex to model and enabled experiments more typical of the variation that a rapid gravity filter would encounter. The pilot plant is 0.07 % the plan area of a full scale filter compared to the 0.01 % of the laboratory columns. Results corroborated the laboratory work on the effect of extreme particle shapes on filter performance. The pilot study also highlighted problems from floc carry over with the use of clarified water and quantified the impact it had on filtration performance. In this case floc carryover changed the performance of the pilot plant results significantly. Thus an overall conclusion from the work was that an integrated design approach to filters, to account for the clarifier type the likelihood of floc carryover and raw water anticipated could be further researched. There were also limitations to the current monitoring equipment that could not quantitatively measure the floc carryover because of large particle size.

Biological filtration (biofiltration) of drinking-water is gaining popularity due the potential for biodegradation of an array of contaminants not removed by traditional drinking-water processes. However, previous research has suggested that biomass growth on biofilter media may lead to increased headloss, and thus, greater energy and water requirements for backwashing. Research has suggested that the main cause of headloss might be due to extracellular polymeric substances (EPS) rather than the bacterial cells themselves. As EPS production has been shown to increase under nitrogen- and phosphorus-limited or -depleted conditions, the goal of this research was to add to the body of knowledge regarding biofiltration by studying the relationship between EPS production and nutrient limitations in drinking-water. Batch experiments with a synthetic groundwater were run with a mixed community of drinking-water bacteria under nutrient-balanced (a molar carbon to nitrogen to phosphorus ratio [C:N:P] of 100:10:1), nutrient-limited (e.g., C:N:P of 100:10:0.1), and nutrient-depleted conditions (C:N:P of 100:0:1 or 100:10:0). After 5 days, growth was measured as the optical density at 600 nanometers (OD600), and the concentrations of free and bound carbohydrates and proteins, the main components of EPS, were measured. In batch experiments with 2.0 and 0.2 g/L as carbon (mixture of acetic acid, mannitol and sucrose) increases in EPS production per OD600 and decreases in growth were noted under nutrient-depleted conditions. When the same experiments were conducted with a pure culture of Bacillus cereus, bound polysaccharides normalized to OD600 increased under nitrogen- and phosphorus-depleted conditions. Since previous research suggested that Bradyrhizobium would be an important player in EPS production in drinking-water biofilters, similar batch experiments were conducted with Bradyrhizobium. However, due to experimental challenges with Bradyrhizobium japonicum USDA 110, differences in EPS production under nutrient limitations could not be reliably assessed. Additional work is required with Bradyrhizobium. Recommendations for future work include the replication of these batch conditions in steady-state chemostats containing biofilm attachment media and in bench-scale columns. Additionally, future work should include experiments at carbon concentrations as low as 2 mg/L to match typical carbon concentrations in drinking-water biofilters. / text