Schmutzdecke Characterization of ClinoptiloliteAmended Slow Sand Filtration Daniel R. MeNair, Ronald C. Sims, Darwin L. Sorensen, and Matthew Hulbert Studies were conducted to characterize the growth and composition of the schmutzdecke and the associated particle removal efficiency for a field-scale experimental slow sand filtration (SSF) system used for the biological treatment of Logan River water. A naturally occurring ammonium-selective zeolite, clinoptilolite, was added as a surface amendment to the sand bed of the SSF system as a reservoir of nitrogen for algae comprising the schmutzdecke. A predominance of filamentous algae at the surface of the filter was observed throughout the entire experimental period. Single-cell algae were observed at depths in the filter from 0.5 to 1 in. (15 to 30 mm). Particle removal was superior in clinoptilolite-amended SSF, even at filtration rates two to four times greater than conventional SSF rates. The effect of schmutzdecke maturation on the removal of Giavdia lunz6liu cyst-size particles was also investigated. The growth of algae following the initiation of a filtration cycle, i.e., the maturation period of the schmutzdecke, was associated with an increase in the ability of the amended filter to remove Giardiu lumbliu cyst-size particles. The zeolite-amended SSF system treated drinking water for longer periods of time at higher filtration rates than achievable with conventional SSF without the zeolite surface amendment. Slow sand filtration (SSF) is an appli- cation of filtration technology for the treatment of drinking water and is characterized by the flow of raw water at a rate of 0.04-0.08gpm/sqft (0.10-0.20 m/h) through a bed of sand approx- imately 3 ft (1 m) deep.’ The primary objective of filtration is to remove par- ticles from suspension and produce an effluent with a turbidity of less than 1 ntu.2 The ability of the SSF processto 74 RESEARCH AND TECHNOLOGY remove and inactivate particles assmall as viruses has beenreported.” Although mostconventional water treatment tech- niques use physicochemical mecha- nisms, SSF combines physicochemical processes with biological processes for improving water quality.’ The US Envi- ronmental Protection Agency has taken an active role in evaluating the applica- tion and effectivenessof SSFfor treating community surface water supplies.” The relationship between biological activity in the filter and SSFperformance is well established.The schmutzdecke. or surface biological mat, is composed of algae, bacteria, and other microorgan- isms, including protozoa and rotifera.’ The principal mass of the schmutzdecke is located on the surface of the filter, with lesseramounts of active biomass continuing throughout the filter bed.“-12 The level of bacteria presentin treated water has been shown to be related to the maturity of the schmutzdecke.l:j,lJ Bacterial removal by SSF is also a function of the growth and the popula- tions of organisms living within the schmutzdecke, I.!i.il.lS-ii The type of algae present within the schmutzdecke has specifically been shown to be critical to filter perfor- mance.“Jj If filamentous algae are pre- dominant in a filter, a biological mat with high tensile strength is formed. The result is a decrease in the buoyancy of the biological mat, an increase in the filtration rate, and a decrease in the resistance to flow. Filamentous algae JOURNAL AWWA Copyright (C) 1987 American Water Works Association