Sewerage plants substandard

Two thirds of the sewage treatment plants on private property tested in Cayman fall below legal standards.

The Water Authority tested wastewater at 186 private sewerage treatment plants between April last year and February this year and found that 126 of them fell below the legal limit of the amount of oxygen needed to break down bacteria in septic tanks.

According to the Water Authority Law, biochemical oxygen demand, or BOD, levels in treated sewage in both public and private sewerage treatment plants should be 30 parts per million.

In one private plant tested by the Water Authority, the treated sewage was greater than 3,144 parts per million of BOD – more than 100 times under the legal standard. The two next worst plants were measured at 1,410 and 1,254 parts per million.

Mike Smith, a specialist in wastewater treatment at the Water Engineering and Development Centre at Loughborough University in the UK, explained the impact of wastewater with high levels of biochemical oxygen demand.

‘An increase in BOD, caused by readily degradable organic material contained in human and animal waste, can result in depletion of the dissolved oxygen in water. Bacteria use oxygen in water to break down the BOD, so fish and other aquatic organisms have to compete with bacteria for what oxygen is dissolved in water.

‘If oxygen in the water is used up, fish may die, the water may become black and unsightly, and the water may smell foul. In some cases, however, fish may feed on the waste materials that increase BOD, so the environment may be affected if fish are attracted to discharges.’

BOD is the amount of oxygen needed by bacteria in septic tanks to break down large organic molecules into smaller molecules.

The Water Authority took the samples between 17 April, 2008, and 26 February, 2009. There are 524 private sewerage treatment plants on Grand Cayman.

The Authority also tested for total suspended solids, or TSS. The legal standard for this in Cayman is also 30 parts per million, and the Authority found that 100 of them – more than half – were below the legal limit.

One site was found to have an alarming 8,240 parts per million of suspended solids, another had 1,670 parts per million of TSS, while the third worst was found to have 1,305 parts per million.

Director of the Water Authority Gelia Frederick-van Genderen said; ‘The results over 1,000 are results that do need further examination on our part, as these are higher than could be explained by waste being concentrated. With the ongoing monitoring programme, these sites will be revisited.’

She said testing was ongoing and by this week, about half the plants on the Island had been sampled.

On the effects of suspended solids on the water and sea-life, Mr. Smith said: ‘Suspended solids appear as cloudiness in the water. High concentrations of TSS may be aesthetically unpleasant and may suffocate aquatic life if they form a blanket that reduces light transfer.

‘TSS can also destroy bottom flora when it settles, and may result in fish kills. Inert insoluble materials can affect the gills of fish causing death.’

The Water Authority launched island-wide testing of onsite sewerage plants. Originally, it tested plants on their designs, at the planning stage, rather than on the plants’ performances once they were operational.

The Authority, which can test seven samples a week, said plants that had failed to meet legal standards of BOD and TSS had done so due to most systems not being properly operated and maintained once they were installed.

The results of the tests done so far have been provided to the relevant stratas.

The Authority can prosecute owners that do not maintain their septic tanks properly, but it admits this is only done as a last resort and it works with owners to meet the standards.

Mrs. Frederick-van Genderen said high BOD levels were primarily a concern in surface waters, while high TSS results in fouling of wells, resulting in overflows.

Mrs. Frederick-van Genderen said the vast majority, approximately 95 per cent, of the testing had been initiated by the Water Authority as an overall approach to review the plants, but others had been following complaints.

The information on the tests carried out at the 186 sites was released to the Caymanian Compass under a Freedom of Information request, which was initially turned down. Following an internal review by the chief officer of the authority, Mrs. Frederick-van Genderen, it released data on the results of tests, but with the names of the locations redacted.

The Compass has appealed to the Freedom of Information Commission against that decision and is asking for the names of the estates and sites where the tests were taken to be provided to the public.

Mrs. Frederick-van Genderen

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