Interception of small particles by flocculent structures, sessile ciliates, and the basic layer of a wastewater biofilm

We investigated attachment processes of hydrophobic and hydrophilic particl es (diameter = I mum) to mature biofilms grown on clay marbles in a sequenc ing batch biofilm reactor. During a treatment cycle with filtered wastewate r containing different fluorescent beads, the progression of particle densi ty in various biofilm compartments (carrier biofilm, basic biofilm layer, b iofilm flocs, and sessile ciliates) was determined by How cytometry, confoc al laser scanning microscopy and automated image analysis. Particles were a lmost completely removed from wastewater by typical processes of particle r etention: up to 58% of particles attached to clay marbles, up to 15% were a ssociated with suspended flocs, and up to 10% were ingested by sessile cili ates. Ingestion of particles by ciliates was exceptionally high immediately after wastewater addition (1,200 particles grazer(-1) h(-1)) and continued until approximately 14% of the water had been cleared by ciliate filter fe eding. Most probably, ciliate bioturbation increases particle sorption to t he basic biofilm. Backwashing of the reactor detached pieces of biofilm and thus released approximately 50% of the particles into rinsing water. Clay marbles in the upper part of the reactor were more efficiently abraded than in the lower part. No indications for selective attachment of the applied hydrophobic and hydrophilic beads were found. As a consequence of intercept ion patterns, organisms at elevated biofilm structures are probably major p rofiteers of wastewater particles; among them, ciliates may be of major imp ortance because of their highly active digestive food vacuoles.