In recent years there has been some concern over the standard of sewage treatment in the Cook Islands due to problems with marine algae blooms in the coastal lagoon of Rarotonga and Aitutaki. While water sampling for bacteria in 2006 has shown that the lagoon is clean and safe for swimming throughout the year, during the warmer months algae has been observed to bloom for extended periods, smothering coral and reducing oxygen levels in the water. The reduction in oxygen levels has been suggested as the cause of small numbers of fish washing up dead on the beach. Reef fish have also become poisonous to eat through consuming microscopic algae called "dinoflagellates" that cause ciguatera fish poisoning. The abundance of dinoflagellates has been linked to high nutrient levels in research elsewhere. Though there have been no outbreaks of water borne disease, the health of the coastal lagoon in Rarotonga and Aitutaki is of concern with respect to public health.
Most homes in the Cook Islands treat their waste in septic tanks and soak holes. Only one village on Rarotonga has sewerage and a central treatment plant. While septic tanks have been a good form of sewage treatment in the past, the increasing density of development in some areas has made them too crude a treatment method for the natural environment to receive. The Sewage Treatment Improvement Program is for the purpose of reducing nutrient enrichment of our lagoons and bacterial contamination of our streams for the better health of our people. It aims to achieve this by improving on-site sewage treatment and facilitating the development of sewage infrastructure for areas with high density development.
