Upflow sand-roughing filtration

Slow sand filtration requires low turbidity water for effective operation. In most cases this means pretreating the raw water to reduce its turbidity. Lately horizontal gravel filters have become very popular as a means of pretreatment. They are simple to operate, have a large mass storage capacity, and can cope with high turbidity shock loads. However, cleaning them can be a problem. An alternative pretreatment, which this study is concerned with, is upflow sand roughing filtration. It has the potential advantages of simple design, ease of operation and cleaning, and low area requirements. A study of upflow versus downflow was done using two small filter columns. The results showed that although downflow exhibited a better turbidity removal than upflow, upflow allowed longer filter runs and better utilisation of bed depth. This was in agreement with earlier findings. The effect of backwash water quality was also investigated. Both filters were run in upflow direction; one was washed with tap water, the other with polluted raw water. The results suggested, unexpectedly, that a filter washed repeatedly with polluted water produced slightly better turbidity removal than one washed with tap water. This indicated some form of maturation, which could be biological in nature, taking place. Experiments with horizontal gravel filters clearly showed the importance of biological mechanisms in maturation. Four new filter columns, 50mm in diameter, were constructed. These allowed further investigation of the effect of bed depth. Backwash rate and duration were investigated to find an optimum. Very little of the wash water was required to clean the filters, the majority being required to flush the turbidity out of the system. The long-term effect of backwash on filters was investigated, as was the effect of wash water quality. Tap water, raw water and raw water polluted with settled sewage were used to wash the filters. The results indicated that the filters washed with raw water and polluted water proved to be as good as regards turbidity removal as those washed with tap water. This suggested that raw water backwash could be used effectively for washing upflow roughing filters.