Relationships between clogging in irrigation systems and plankton community structure and distribution in wastewater reservoirs

The development of large populations of cladocerans and copepods in reservoirs that store wastewater for crop irrigation causes severe clogging in irrigation systems. In most reservoirs in Israel water is filtered at the banks of the reservoirs before being sent to irrigation. This avoids/reduces clogging in the irrigation systems transferring the problem to the storage reservoirs. To cope with this problem we studied relationships between clogging and particle size distribution in a range of secondarily treated wastewater reservoirs with different characteristics and water management. Since the reservoirs are deep and thermal stratification develops, measurements of the time required to clog 100 μm pore net filters and samples for physico-chemical and microscopic analyses were collected in the surface, deep epilimnion and hypolimnion. Factor analysis allowed identifying two independent sets of environmental conditions and planktonic species that lead to clogging of irrigation filters. The first set is related to thermal stratification and the development of a planktonic community with a complex web of feeding interactions in which the organisms capable of clogging filters are mainly copepods. The second set occurs in spring and fall, and includes planktonic organisms forming a short food chain in which the organisms capable of clogging filters are mainly cladocerans. A third factor was organic loading, mostly related to the entrance of fresh wastewater into the reservoirs during the irrigation season, which had a negative effect on nitrification and promoted blue green algae development and copepod reproduction. The analysis of potential management procedures to avoid filter clogging indicates that in these reservoirs manipulations of food web interactions in the planktonic community structure towards smaller organisms that do not clog filters would not be effective against copepods. A better option to decrease clogging events is to avoid pumping water out from the deep epilimnion, preferably pumping from the hypolimnion.