e> Pergamon Waf. SCI. Vo\. 33, No.8, pp. 7-15,1996. Copyright © 1996 IA WQ. Published by Elsevier SCIence Ltd Pnnted In Great Bntaln. All rights reserved 0273-1223/96 $15'00 + 0·00 PH: S0273-1223(96)00255-7 IMMOBILIZATION IN FIXED FILM REACTORS: AN ULTRASTRUCTURAL APPROACH C. S. Harendranath*, K. Anuja**, Anju Singh**, A. Gunaseelan**, K. Satish** and Krishna La1a** * Regional Sophisticated Instrumentation Centre, Indian Institute o/Technology, Powai, Bombay 400076, India ** Bombay Zonal Laboratory, NEERI, Worli, Bombay 400018, India ABSTRACf An ultrastructural approach has been attempted to study the surface features of 29 packing media reported in lilerature and a few more potential candidates for immobilizing micro-organisms. The electron micrographs reveal tmportant features like degree of smoothness/roughness, microcrystals and fibres, ridges, macro and micro pores < 196> their dimensions, depth and distribution and thereby the biomass accumulation capacity of the media. Based on microscopic observations the packing media have been classified as (i) smooth - polypropylene bead, glass bead, peristaltic tube, porcelam, powdered activated carbon, perspe)(, polyvinyl chloride and glass (ii) uneven - straw, paddy stem, nylon, sand, gravel and stone (iii) porous - jute, gravel, soil, granulated clay, limestone, ceramic, shell, refractory brick, diatomaceous earth, casuarina seed, granular activated carbon thermocol. sponge, pumice stone and polyurethane foam. The results clearly show that ultrastructural e)(amination and image analysis can be a quick, effective and direct visual technique for selecting support media for bioreactors. An e)(ample of application of quantitative image analysis for providing quantitative geometric description of surface features is also presented. Copyright © 1996 IA WQ. Published by Elsevier Science Ltd. KEYWORDS Fixed film reactor; ultrastructure; packing media; porosity; surface area; scanning electron microscopy; wastewater treatment; immobilization potential. INTRODUCTION Anaerobic processes have proven to be effective in treating various wastewaters owing to the inherent advantages like less consumption of energy for operation and minimum sludge production. However. a major drawback in reactors based on the anaerobic processes is the slow generation rate of methanogens. This problem is sought to be overcome in present day advanced reactors by effective biomass retention (Young and Yang. 1989; Lettinga et al.. 1980; Van den Berg and Lentz. 1979). thereby increasing the contact period between the biomass and the waste, The biomass retention is initiated in fixed film reactors by immobilization on support media, Thus the characteristics of the support material used determine the efficiency of the treatment process.