Hydraulic Considerations in Declining-Rate Filtration

Flow control through granular filters is an important factors in filter performance in both water and wastewater treament. Declining-rate filtration plants operate without effluent rate control devices on each filter. All the filters in the plant operate at the same water level and the same head loss. Therefore, the cleanest filter operates at the highest filtration rate and the dirtiest filter operates at the lowest rate. The filters decline in rate as each filter is backwashed in succession. Declining-rate filtration plants produce better water quality and require less head loss than constant-rate or constant water level filters. The design of declining-rate plants has been hampered by the absence of rational approaches to several hydraulic questions which are answered in the paper as follows: What total head loss is required? How can excessive filtration rates be prevented without the use of rate controllers? How can the water level variations in the filter be predicted and how are they affected by the number of filters in the plant? How does the plant respond to imposed load variations