Zanzibar's rudimentary waste management and sanitation facilities are nowhere near keeping pace with the increase in the Tanzanian island's population, according to officials.
"Our population is growing enormously. Complete villages are springing up within very small spaces. There is also a high immigrant population from the mainland," Hamza Juber Rijal, a senior ecologist and head of environment education at Zanzibar's department of energy, told IRIN.
Some 1.1 million people live in Zanzibar, compared to 300,000 in 1964,
Rijal said, and the island’s stunning beaches attract tens of thousands of tourists every year.
In the capital, Zanzibar city, only a minority of residents, those living in the historical centre - Stone Town - are connected to the sewerage network, which consists of a mere 25km of pipes, according to the municipal council (ZMC).
At the last census in 2002, the city's population was 206,000 with annual growth estimated at 4.5 percent.
"What we mainly have are septic tanks and soak pits. Within the periphery of the suburbs, there are places with no soak pits or septic tanks; some people even have no toilets," Rijal said.
A 2006 government directive requires hotels to treat their own sewage, but this rule is widely flouted.
Considerable amounts of sewage, including from septic tanks where only minimal treatment takes place, are discharged directly into the sea: The island has no sewage treatment plant.
"Primary liquid waste treatment is only done at the septic tank where there is only [a] 30 percent reduction of the BOD [biological oxygen demand - a measure of water quality] before being discharged into the ocean along any of the 27 sea outfalls," Mzee Juma, a ZMC sanitary engineer said.
Sludge from septic pits and latrines also seeps into the sea from the mangrove stands where it is dumped. Contamination has already been noted in the Maruhubi area, north of Zanzibar city, according to a government report prepared for the WIO-LaB project, coordinated by the UN Environment Programme and the Global Environment Facility.
Maruhubi is used by private sludge emptiers and is prone to flooding during high tides. Mangroves growing there help absorb some of the organic waste.
"Some scientists say it is not safe to swim close to the beach because of a rise in pathogens. There is an indication of sea water pollution with e-coli forms detected along the coast," said Juma.
Studies have shown that nutrient levels in near-shore waters are higher than normal for tropical seawaters, indicating anthropogenic inputs. Faecal and total coliform levels of up to 70/100ml and numerous thousands per ml of seawater, respectively, have been reported in the waters fronting the Zanzibar Municipality, said the WIO-LaB report.
About 9,000 to 12,000 cubic metres of liquid waste is discharged into the sea daily.
According to the WIO-LaB report, the use of contaminated water and seafood is a concern. "Contamination of biota [plant and animal life], including those harvested for food such as bivalves [kind of mollusc], has been reported, as have water-borne diseases such as dysentery, diarrhoea, cholera and typhoid, among others," it said.
Fishing and tourism are the main economic activities in Zanzibar. Pollution and coral reef mining for lime are, however, among activities threatening the fragile marine ecosystems there, according to experts.
"Untreated municipal and industrial wastes are currently the main threats to the quality of water, sediment and biota in Tanzania's coastal waters, specifically in the urban areas of Dar es Salaam, Zanzibar, Tanga and Mtwara," it added.
Poor maintenance and over-utilization of onsite disposal systems such as pit latrines causes them to overflow, with the high water table in many areas compounding the pollution problem.
According to ZMC's Juma, there is an urgent need for the establishment of a sewage treatment facility. "ZMC is also looking into the construction of intercepting sewers along the coastline for more waste treatment before sea discharge. Other options include the construction of longer sea outfalls to the deep sea where there is a higher dilution factor."
Solid waste management is also inadequate. Of about 200 tons generated daily, only 45 percent is moved to dumping sites, with the remainder left in open spaces. About 0.5kg of solid waste is generated per capita per day - 80 percent of it organic - according to estimates.
"At the dumpsite there is no controlled tipping; people just dump; the waste just stays there and decomposes," he said. Complaints have been raised over rodent infestation, bad smells and smoke from the site about 12km from Zanzibar city.
Garbage collection capacity in the municipality is low with only six trucks. "Public awareness is very low; people dump garbage anywhere," Juma added.