LIV. J. B. HENSON. 



In practice, however, the beds are not always allowed 

 to run continuously. At certain times of the year each bed 

 is shut off in rotation at short intervals. The water is 

 lowered to a point a few inches below the surface of the 

 sand, which is thus exposed to the direct action of the air 

 and sunlight for a day or two, if practicable. The bed is 

 then recharged and filtering resumed. This practice was 

 adopted to check the growth of vegetable and animal life 

 in the water on the beds, but it requires careful control. 



The clear water tank was originally uncovered, and under 

 the iulluence of sunlight vegetable growths developed 

 rapidly and caused much trouble. The filaments were 

 drawn into the pump suction, found their way into the 

 water reticulation pipes, and choked the strainers of water 

 meters. At certain seasons of the year, the growth was 

 so abundant as to necessitate the cleaning out of the tank 

 every three or four weeks. This, besides being expensive, 

 caused in convenient stoppages. A roof was constructed 

 over the tank, and light being excluded the growth of 

 aquatic plants roused. No further trouble from this source 

 lias since been experienced. 



Occasionally the finer growths of weeds on the filter 

 beds collect the minute bubbles of gas which are evolved at 

 the surface of the sand. The bubbles become entangled and 

 accumulate until a buoyancy is gained sufficient to lift the 

 weed from its anchorage, and in doing so a patch of the 

 skin of the bed is torn off, leaving the bare sand exposed. 

 Through this bare sand, no doubt, the water percolates 

 faster than through the protected areas round it until a 

 fresh skiu is formed. 



No bacteriological examinations of the water are made. 

 Samples of the water before and after filtration are taken 

 once a month and forwarded to the Board of Health, 

 Sydney, for chemical examination. These examinations 



