Active heterotrophic and autotrophic biomass distribution between fixed and suspended systems in a hybrid biological reactor J.C. Ochoa*, J. Colprim** , ***, B. Palacios***, E. Paul* and P. Chatellier*** * Laboratoire d’ingénierie des Procédés de l’Environnement (LIPE-EA833), Institut National des Sciences Appliquées de Toulouse, Complexe Scientifique de Rangueil, 31077 Toulouse, Cedex 4, France ** Laboratori d’Enginyeria Quimica i Ambiental (LEQUIA), departament de EQATA, Universitat de Girona, Campus montilivi s/n, E-17071 Girona, Spain.(E-mail: J.Colprim@lequia.udg.es) *** Centre of International Research for Water and Environment (CIRSEE), Lyonnaise des Eaux, 38, rue du Président Wilson, F-78230 Le Pecq, France Abstract This paper presents the results obtained when modifying sludge retention time (SRT 8, 5, and 3.7 days during phases A, B and C respectively) for a hybrid biological reactor (HR) compared with a classical activated sludge process. The study was conducted by following active biomass evolution and distribution for two lab-scale pilots plants operating with the same conditions, one acting as HR and the other as the control reactor (CR) without support material. At the end of phase C, support material was split into two fractions between both reactors to study the effect of support to reactor volume ratio (Fr). Active biomasses in suspended and fixed systems were calculated using respirometric techniques. Evolutions of active autotrophic and heterotrophic biomasses for both reactors are presented during all the operational periods and it is observed that in the HR biomass concentrations are up to double that in the CR, mainly due to the presence of support material. When studying biomass distribution in HR, autotrophic biomass is mainly located over the support material (from 95% to 99% during periods A and C respectively) while only about 60% of heterotrophic biomass is located over the support. Keywords Autotrophs; biomass distribution; heterotrophs; hybrid reactor; OUR; oxygen uptake rate Introduction Combining suspended and fixed biomass systems has arisen as an alternative to classical acti- vated sludge systems to improve plant performance. Such systems, known as hybrid systems, are able to improve nitrification potential by increasing cellular retention time independent of purge flows. An important aspect to enable high performance in these configurations is the carrier support characteristics (Odegaard, 2000). Using support material with high specific surface area, total active biomass inside the reactor is highly increased and thus higher volu- metric substrate conversion rates are possible, in addition, some effects on sludge quality, such as reducing bulking phenomenon (Andreottola et al, 2000), have been observed. Distribution of microorganisms in fixed systems (biofilm) could be studied by means of microbiological techniques or activity measurements. Some authors studied the effect of C:N ratio in the feed medium on spatial distribution of nitrifiers and heterotrophs using microslicer and FISH techniques (Okabe et al., 1995; Lazarova et al., 1998; Satoh et al., 2000, Gieseke et al., 2001), or the utilization of oxygen microelectrodes for the measure- ment of activity in biofilm on small suspended particles (Van Loosdrecht et al., 1995). The use of activity measurement techniques are based on the measurement of oxygen uptake rate (OUR) and have been proved to be a reliable method for determination of viable bio- mass in wastewater treatment, (Jorgensen et al., 1992; Riefler et al., 1998). In this paper the biomass distribution between the liquid (suspended biomass) and the support (fixed biomass) as well as their composition (heterotrophic and autotrophic Water Science and Technology Vol 46 No 1–2 pp 397–404 © IWA Publishing 2002 397 Downloaded from https://iwaponline.com/wst/article-pdf/46/1-2/397/476862/397.pdf by guest on 01 November 2018