July 22, 2002
By Lucille Davie
FOR years there've been grumbles that Zoo Lake is polluted by animal droppings from the zoo across Jan Smuts Avenue. No longer. The zoo's increasingly sophisticated purification system is not only filtering the animal droppings - it's also cleaning out the muck floating in from Hillbrow's stormwater drains.
Johannesburg Zoo is upstream from the Zoo Lake. For years, polluted water from its hippo, seal and polar bear cages passed on to neighbouring Zoo Lake, where adults and children row boats. But today, thanks to a water purification system that has been six years in development, the ecoli count in water leaving the Zoo is zero.
The spruit that flows through the zoo rises near Valley Road in Parktown. It enters the zoo already polluted, largely from the stormwater run-off from the streets in overcrowded Hillbrow.
The Department of Water Affairs stipulates a maximum ecoli count of less than 130. The ecoli count of the water entering the Zoo varies from month to month, but in March this year, for example, it was 19 000. The discharge from the animal enclosures, however, pushes the ecoli count even higher, to as much as 75 000.
But the zoo has now perfected a water purification system which cuts the ecoli count - right down to zero. Construction of the purification system began six years ago, and it has been operational for the past two years, with steady improvements in performance.
"Since we got this purification system up and running over the past two years, no one can accuse us of sending polluted water into Zoo Lake," says Dominic Moss, Zoo representative of fauna.
The purification system is simple but effective. The water from the main storm water channel through the zoo is diverted into a catchment tank, passing through a metal sieve where all the debris is removed.
From this tank the water moves into a large 6-8-metre deep settlement tank. Here the solids settle to the bottom of the V-shaped tank, which is then channelled into the city's main sewer system. The water from this tank then overflows into a series of three reed dams.
The first dam has a base of up to two metres of gravel stones, covered with up to half a metre of water. As the water moves across the first dam, it trickles through reeds planted in a herringbone shape. This acts to slow the flow of the water, and, together with the stones, filter the water. The water in this dam is murky and muddy.
The reeds in the second dam have been planted in half-moon shapes, again with the aim of slowing down the flow of the water. Ducks are happy to swim in this dam, where the water is clear.
The reeds in the third dam are randomly planted. They thrive on bacteria, and cleanse the water in the process. More reeds are being planted here, with improved results. At the end of May this year, the ecoli count on water passing through the system was down to zero - earlier that month it had been 110.
The zoo has asked the city's water utility, Johannesburg Water, to test the water on a monthly basis. "The Johannesburg Zoo remains committed to a clean environment and will take any measures necessary to ensure that all its activities are in line with this commitment," says Moss.
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