Media surface properties and the development of nitrifying biofilms in mixed cultures for wastewater treatment
Highlights
► Eight plastics supported nitrifying biofilms in wastewater. ► Nitrification did not correlate with surface roughness nor biomass concentration. ► Nitrification was inversely proportional to adhesion force with heterotrophs present.
Introduction
Biofilm processes are frequently used for aerobic biological wastewater treatment. The conventional trickling filter has been a mainstay and recent developments include rotating biological contactors (RBCs) and biological aerated filters (BAF) (Boller et al., 1994, Mendoza-Espinosa and Stephenson, 1999). These processes are often designed to remove ammonia as well as organic carbon, as measured by biochemical oxygen demand (BOD). As heterotrophic bacteria responsible for BOD removal have faster growth rates and higher yield coefficients compared to autotrophic nitrifying bacteria, in order for combined BOD removal and nitrification to occur, the reactor is designed to operate at a lower BOD loading (Grady et al., 1999). Common to all the processes is an inert support media on which a biofilm grows. Selection of appropriate materials has been the aim of much research in the past but has almost entirely focussed on mesoscale, i.e. reactor-scale, investigations such as using laboratory scale reactors or pilot plants e.g. Moore et al. (1999). Most attempts to manipulate bacterial population to improve treatment processes are undertaken with suspended growth systems (Stephenson and Stephenson, 1992); for example, the use of immobilisation in gels to enhance performance of activated sludge (Bouchez et al., 2009). However, for biofilm systems, research has concentrated on the selection of the media, and has usually been for a single species under laboratory conditions, e.g. Robledo-Ortíz et al. (2010). The experiments described in this paper were designed to select biofilm support media to treat wastewaters that would enhance growth of nitrifiers in the presence of heterotrophs. Atomic force microscopy (AFM) was used to determine the important media properties for the enhancement of nitrification.
Section snippets
Materials and methods
A 175 l tank was set up as a fed-batch aerobic reactor treating settled primary sewage in order to grow biofilm on 8 different plastic media. In one half of the tank 9 aquarium diffuser stones (Aquatics-online Ltd., Bridgend, UK) were used to aerate the tank at a rate of 2–4 l/min; the plastic media samples were submerged in the other half to reduce the scouring effect on biofilm that would attach to the plastic. Each sample was end on to the flow in order to aid the even distribution of liquid
Results and discussion
Plastics ranked in order of roughness (roughest to smoothest) based on AFM measurement of Ra were PE > PTFE > Ny > PVC > Tu > ABS > PP > PC (Table 1). Average roughness (Ra) values ranged from 6 to 603 nm (PC and PE respectively). The Rmax and SA measurements were also greatest for PE at 6.58 nm and 6322 nm2 respectively and least for PC at 0.28 nm and 5369 nm2 respectively. Plastics could generally be ranked in a similar order based on Rmax and SA values, with the exception of PTFE and ABS. Irregular, rough
Conclusions
The results may reflect species-related differences in adsorption capacity i.e. that nitrifiers are better adapted to adhere to low-energy surfaces, or possibly the inability of low-energy surfaces to support greater biomass associated with the rapid growth of heterotrophic biofilm. Experimental work on activated sludge flocs has shown that nitrifiers bind together very strongly, with extracellular polymers probably responsible (Larsen et al., 2008), which would aid their attachment to low
Acknowledgements
This work was supported by a UK Engineering and Physical Science Research Council (EPSRC) Platform Grant (Grant No. GR/S 64523/01). The authors wish to acknowledge the inspiration of Derek Rodman now of Clearfleu Ltd.
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Present address: Alpheus Environmental Ltd., 49a Bromham Road, Bedford MK40 2AA, UK.
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Present address: James Hutton Institute, Craigiebuckler, Aberdeen AB15 8QH, UK.