Biofilm monitoring by photoacoustic spectroscopy T. Schmid, U. Panne, C. Haisch and R. Niessner Institute of Hydrochemistry, Technical University of Munich, Marchioninistr. 17, 81377 Munich, Germany Abstract The use of photoacoustic spectroscopy (PAS) as a new biofilm monitoring technique is presented. Growth and detachment of biofilms at three different positions inside a flow channel were monitored by photoacoustic measurements in the visible spectral range (λ = 532 nm). The experimental approach allows the investigation of the influence of various process parameters (e.g. pH or flow conditions) on growth and detachment of biofilms. In addition, the distribution of the attached biomass can be monitored by depth-resolved photoacoustic measurements. Keywords Biofilm; monitoring; photoacoustic spectroscopy Introduction Biofilms are aggregates of microorganisms, which occur at the interfaces of aqueous systems. Biofilms consist of water (85–95% wet weight), microbial cells, and extracellular polymer substances (EPS), such as polysaccharides, proteins, and other biopolymers. Biofilms which are attached to solid surfaces can be found in natural and engineered water systems. The unwanted growth of biofilms in technical processes reduces the water quality, increases the pressure differentials in membrane processes, and reduces the efficiency of heat exchangers. Despite these negative effects of biofilms, attached microorganisms are used also in beneficial applications, i.e. removal of organic pollutants in wastewater treat- ment plants (Wilderer and Characklis, 1989). In the context of biofilm formation in technical processes, parameters such as growth, detachment, thickness, and chemical composition of the biofilm have to be monitored on-line by a nondestructive analytical technique. In this paper, the use of photoacoustic spectroscopy (PAS) for on-line monitoring of biofilms is presented. Photoacoustic spectroscopy is based on the absorption of electromagnetic radiation inside a sample where non-radiative relaxation processes convert the absorbed energy into heat. Due to the thermal expansion of the medium, a pressure wave is generated which can be detected by microphones or piezoelectric transducers (Rosencwaig, 1980). The ampli- tude p of a photoacoustic signal generated by a laser pulse inside solid or liquid samples can be generally described by , (1) where C p is the heat capacity, β is the thermal expansion coefficient, c is the speed of sound in the medium under study, E 0 is the laser pulse energy, and μ a is the absorption coefficient of the sample (Tam, 1986). If a short laser pulse is used for excitation, a time-resolved recording of the photo- acoustic signal allows a depth-resolved investigation of the light absorption inside the irra- diated part of the sample (Karabutov et al., 1995). The distance between an absorbing object inside the sample and the sample surface can be calculated as (2) Water Science and Technology Vol 47 No 5 pp 25–29 © 2003 IWA Publishing and the authors 25 p c C E ∝ β μ 2 p 0 a z ct = ⋅ , Downloaded from https://iwaponline.com/wst/article-pdf/47/5/25/422486/25.pdf by guest on 08 November 2018