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Dealing with rising sewage problem

Friday September 29, 2017 Published in Letters to the Editor

Regarding the article headlined “The new economy of excrement” in the Wednesday, September 20 edition of your newspaper.


Having past experience of sewage disposal management, I have retained an interest in the subject. This is my second visit to Rarotonga, the first being some five years ago. I have noticed a huge increase in the number of fellow tourists visiting, as well as the building of new homes and accommodation. I am told that at any one time there are around 5000 tourists on the island alongside around 10,000 residents.

From my enquiries, it seems that septic tanks and drain-aways are the norm for sewage disposal. This got me wondering as to what was happening to the liquid content as well as the solid waste component.

A quick reckoning of the quantities involved makes startling reading:

Based on the tourist/resident numbers above, I calculated monthly productions of 225 tons of faeces (0.5 kg per person per day), 450,000 litres of urine (1 L), 4.5 million litres of toilet-flushed water (1 flush), 13.5 million litres of shower water (1 30-L shower), and 600,000 litres of laundry water from hotels (one daily wash from 40 hotels) and 3.2 million litres from homes (one weekly wash from 4000 homes).

Evidenced by the current service provided to pump out septic tanks and dispose of the excess, it’s also clear that many septic systems are not functioning as well as they should. This excess ends up in settling ponds at the waste disposal site. The dirty water is then pumped up the mountain where it enters the ground and surface-water catchments, eventually reaching the lagoon.

In the context of this already creaking sewerage infrastructure, industry goals to increase tourist numbers by anything close to 20 per cent a year will push the current system beyond capacity. It’s worth noting that 20 per cent growth per year is a doubling of tourist numbers (and tourist faeces) every three and a half years. It’s hard to comprehend how this is sustainable.

My proposed solution is not new and has been tried and tested in many countries. It is environmentally friendly, cost-effective and economically viable system. The accompanying diagram shows how the process works.

Sewage is pumped out of septic tanks and transported to settling tanks located at the highest point of the sewage disposal farm. Solids settle to the bottom of the tank and are gravity-fed to evaporation ponds where, once dry, they can be collected and used as fuel or fertilizer.

The liquid from the top of the settling tank is gravity-fed to biological filters – multiple concrete tanks filled with crushed stone.

The liquid is distributed onto the stone surface, draining through the aggregate and past the billions of bacteria which feed on the organic content. This leaves only the inorganic materials such as nitrates and phosphates. Helpfully, these form the basis of man-made fertilizers and, less helpfully, contribute to the algae bloom and seaweed growth in the lagoon.

The liquid then flows into a series of filtration ponds - concrete dams with established reed beds and vegetation. The reeds take up the nitrates and phosphates and turn them into plant material. Since the water is free of organic pollutants and sewage, fish can be bred in these ponds for human consumption or, together with the reeds themselves, processed for animal feed.

The water coming out the final pond is free from faecal bacteria, organic and inorganic compounds and can be used for crop irrigation, e.g. taro, or allowed to join the nearest water course.

The costs of installing such a project are not inconsiderable, but must be regarded as a public investment. It is also an investment in the future of the tourist industry, since tourists will not want to visit an island left fouled by the failure of today’s infrastructure to cope with tomorrow’s visitors.

A report in the September 21 edition of CINews (“Developed nation status a challenge”), states that the $70 million from overseas development assistance support will disappear next year.

This means tourism is going to have to do some very heavy lifting to make up the shortfall, because it’s not as if you can sell your tuna stocks twice. Lose the golden goose that is tourism and the CI economy will be on the rocks.

I would be interested to know what the CI government has in mind to address the problem of current and future sewage disposal.

It must take the bull by the horns and purchase suitable land in the higher areas of the island in order to build the infrastructure necessary for the coming decades.

Yes, this will mean some landowners making sacrifices, but they will be for the greater good of the island, its environment, people and tourism-dependent economy.

You have a beautiful island and a very big decision ahead: Either make the necessary investment in public sewerage infrastructure at a cost now, and with benefits to come.

Or, don’t do that, and have to make the investment later anyway, after the natural environment upon which the whole economy depends has been degraded beyond recognition.

            David Hawkins