Clogging of groundwater recharge basins by cyanobacterial mats

Abstract

Cyanobacterial mats, which develop on the surface of groundwater recharge basins in the Dan Region Wastewater Reclamation Project in Israel, tend to reduce the rate of effluent infiltration into the ground. The purpose of this work was to study the cyanobacterial populations found in the basins and to elucidate their contribution to the clogging process. Three cyanobacterial types were isolated and their clogging capacity was determined in a simulation system. Simulation experiments were also used to study the effects of various flooding regimes on mat development and permeability. In the recharge basins, clogging events have occurred in the autumn, winter or spring. During these seasons, Phormidium autumnale was found to be one of the dominant cyanobacterial types in the mat. This organism is capable of rapid gliding, forms raft-like structures, produces an extracellular sheath, and secretes copious amounts of mucus. The remarkable clogging capacity of P. autumnale is thought to be related to these features. Another cyanobacterium, which predominates in the mats during the summer, does not express this combination of features and its clogging capacity is much lower. The recharge basins are normally flooded with effluent for 24 h and dried for 48 h. In a simulation experiment clogging was enhanced when cyanobacterial mats were not allowed to dry thoroughly between floodings. Alteration of the recharge regime to 10 h of flooding during the night only, followed by 38 h of drying, enabled temporal separation of light availability and water availability. Under this regime the development of cyanobacterial mats was significantly retarded. It is concluded that night-flooding can alleviate the operational problems associated with clogging of the recharge basins by cyanobacterial mats.