Laboratory based, unsaturated drainage experiments were undertaken using worked Bikenibeu beach coral sand from South Tarawa in Kiribati, to examine its drainage and effective microbial removal properties. Bacterial and viral indicators (E. coli J6-2, E. faecalis and MS2 phage) along with viral pathogens (adenovirus; echovirus; norovirus; rotavirus) were drained under gravity through coral sand-packed columns, serving as physical models of a domestic effluent drainage field. Experiments using clean coral sand and coral sand which were conditioned with Dissolved Organic Carbon (DOC), in the form of domestic effluent, were investigated. The results show that coral sand has a higher affinity for attenuating viruses than bacteria. All organisms examined showed removal efficiencies over 4-Log Removal Values (LRVs) which is equivalent to a 99.99 % reduction in the target microorganisms.

Attenuation mechanisms such as absorption and to a lesser extent physical straining likely play a major role in the ability of coral sand to attenuate the microbial tracers used in this study. The application of DOC in the form of domestic effluent may have provided additional binding sites on the surface of the coral sand and increased the adsorption rate of viruses but the bacterial indicators (E. coli J6-2, E. faecalis) did not appear to be influenced by DOC. It was also found that changes in ionic strength, such as rainfall, increased the mobilisation of microorgansims within coral sand. Long term field scale studies are required to verify the laboratory results as well as incorporating the effects of kind tide events, fluctuations in groundwater, effects of prolonged rainfall and examining microbial transport within coral sand under saturated conditions.

These findings could have important implications for the use of locally available materials, such as coral sand, to improve household onsite wastewater treatment in Kiribati and offer enhanced protection of groundwater resources and reduce diarrheal disease.