Recycled and Virgin Plastic Carriers In Hybrid Reactors for Wastewater Treatment Etienne Paul, Delmira Beatriz Wolff, Juan Carlos Ochoa, Rejane Helena Ribeiro da Costa ABSTRACT: The reduction of organic and nitrogen pollution of waste- water was investigated in two hybrid reactors and compared with the re- duction obtained by using a conventional activated sludge reactor (ASR) run as a control. Both HR-1 and HR-2 were activated sludge systems where a low-density carrier, P1 (polyethylene) for HR-1 and P2 (recycled plastics) for HR-2, was added. Firstly, the three reactors were operated at 10 days Suspended Solid Retention Time (SRT SS ), leading to a complete nitrification. Secondly, the SRT SS for each reactor was lowered to 3 days. Nitrification was lost for the ASR but remained complete for HR’s. Respirometric techniques were used to measure fixed or suspended biomass activities for heterotrophic and autotrophic biomass. More than 90 % of the autotrophic activity was found on the supports whatever the SRT SS used. The results may underline the role of the carrier geometry or surface characteristics on the autotrophic/heterotrophic microorganism distribution. Water Environ. Res., 79, 765 (2007). KEYWORDS: Activated sludge, biofilm, floating carriers, hybrid reactor, wastewater treatment doi:10.2175/106143006X123139 Introduction Numerous studies are currently being developed to investigate the combining of procedures using suspended cultures with fixed biomass systems called Hybrid Systems (HR) for domestic waste- water treatment (Al-Sharekh and Hamoda, 2001; Ødegaard et al., 2000) and chemical wastewater treatment (Wessman et al., 2004). This type of system can be constituted either by a separate unit (combined system), as for example a trickling filter preceding a conventional activated sludge reactor, or by the combination of two types of biomass (suspended 1 fixed) in the same reactor. The HR can be used for the upgrading of activated sludge systems through the addition of material supports to the aeration tank (to absorb an increase in applied organic volumetric load and/or to improve their performance with regard to nutrient removal without additional tank construction), or in new treatment plant installations. Different carrier materials are available on the market for use as biofilm supports, which can be firmly fixed in the aeration tank (such as rigid plastic structures or flexible ropes which are stretched in a framework) (Mu ¨ller, 1998) or move freely in the activated sludge. This type of process has given excellent results in relation to the elimination of carbon and nitrogen, a reduction in the sludge production and an improvement in the settling of biological sludge (Gebara, 1999). A new technology for HR with a Moving Bed Biofilm Reactor (MBBR) associated with activated sludge is being utilized in more than 100 treatment stations around the world, for the removal of organic material, with nitrification and denitrification occurring in a single reactor (Ødegaard et al., 2000). The aim is the growth of a biofilm on a floating plastic carrier acting as a support, which moves freely in the biological reactor (Andreottola et al., 2000). In the scope of nitrogen removal, competition for space in the biofilm fixed on the support has to be studied. Among major restrictions that limit the industrial application of moving beds in the wastewater treatment chain are the cost of these supports and the lack of performance description that would help the system design. It is therefore useful to test the cheapest supports such as those based on recycled plastic. Therefore, the objectives of this work are firstly the character- ization of the carbon and nitrogen removal efficiencies of two hybrid systems using two types of biofilm support. These hybrid systems are fed with a real domestic wastewater, run at rather low temperature (16 6 1 8C), and successively operated under two Sludge Retention Times (SRT SS ) of 10 and then 3 days. Secondly, the determination of the partitioning of the autotrophic and heterotrophic biomass between the Suspended Solids (SS) and the biofilm developed on the supports. Materials and Methods Pilot plant description. Hybrid reactor (HR). Two-bench scale HR pilot plants were used in parallel to compare process performances with respect to the type of carrier used. A schematic representation of one HR chain of the pilot plant used is described in figure 1a. The HR and settler volumes were 22 and 2 liters respectively. Temperature inside the reactors was controlled at 16 6 0.5 8C by means of a cryogenic unit and monitored together with the dissolved oxygen concentration and the pH by means of two Consort units (R321 and R301 respectively). In both HR’s, aeration was supplied by diffusion through small holes made in a plastic tube, at an air flow rate of 0.2 vvm (Volume of air/(Volume of liquid  min 21 )). This aeration rate was sufficient to keep a dissolved oxygen concentration of at least 2 mgO 2  L 21 . Mixing of liquid and supports in the biological reactors was ensured by two propellers run at 150 rpm. This stirring velocity is enough to have a rather good homogeneity in the support spatial distribution in the reactor without the generation of a too high abrasion. Supports were retained inside the reactor by means of a separation device based on the density difference between the liquid and the supports at the outlet of the reactors; a down-flow recovery of the liquid is performed at a low liquid velocity that allowed the support/liquid separation. Control reactor (CR). A conventional activated sludge system without carrier additions was used as control reactor. This reactor was a reproduction of the other reactors used as HR’s. Supports for biofilm development. Two plastic supports were utilized to permit the development of a biofilm over its surface. Photographs of the noncolonized supports were given in figure 2a for carrier 1 (named P1) and figure 2b for carrier 2 (named P2). P1 July 2007 765