This must be the most frequently asked question, and unfortunately there is no simple answer or solution. Firstly, it must be emphasised that this problem commenced with the introduction of rain water tanks, and is by no means a new problem.
The consumption of rain water tainted with “wrigglers” presents no health hazard, but can be easily prevented with the installation of a 20 micron filter after the pressure pump.
Ensuring that the tank remains free of mosquitoes, wrigglers and larvae is proving to be more difficult. The assumption that the mozzies somehow get through the screen is wrong, “around it” would be the more appropriate term. Most inlet strainers do not make a perfect seal on the tank. This is true not only on steel water tanks, but also on plastic tanks particularly when the sunlight starts to buckle the plastic.
A 2-3mm gap between the strainer and the tank seem insignificant, but is plenty big enough to let female mozzies enter the tank and lay eggs. We developed a screwed inlet strainer which seals perfectly and allows no gap between the tank and the strainer. Unfortunately this strainers are custom made and therefore rather expensive. The second way for mosquitoes to enter the tank is much more difficult to seal.
Roof guttering will always retain small paddles of water after a rain event. This is the perfect place for female mozzies to lay their eggs, which of course are washed into the tank with the next rain. The 0.98mm aperture of the inlet strainer is far to coarse to catch microscopic eggs, which pass without difficulties into the tank to hatch.