A light and electron microscopic study of decayed CCA-treated radiata pine (Pinus radiata) wood from a cooling tower

IRG/WP 94-10056

A P Singh, R N Wakeling, D R Page

An inspection of an industrial cooling tower in New Zealand showed surface decay of 12 year old Pinus radiata wood panels treated with CCA preservative to a retention of around 15 kg/m³ of salt. Wood decay micromorphology typical of that caused by soft rot fungi, white rot fungi, 'stripy' and 'v-shaped' erosion bacteria and cavitation bacteria were all commonly seen using a light microscope (LM). Some evidence of the presence of tunnelling bacteria was also seen but was not as common. Soft rot was largely absent from the wettest regions sampled such as spray-line supports and side panels in close proximity to the spray lines, and erosion bacteria attack was the predominant type in these areas. Transmission electron microscopy (TEM) showed that unusual patterns largely consisting of troughs, depression and granulations in wood cell walls, were in most instances, almost certainly caused by erosion bacteria, but in others, tunnelling bacteria were also present. Several decay patterns seen under light microscope as matrices of fine troughs parallel and perpendicular to the cellulose microfibres were difficult to characterise in terms of previous classification but were also thought to have been caused by erosion bacteria. The distinction made by previous classification between patternms formed by erosion and cavitation bacteria needed to be questioned on the basis of observations made. Whilst the TEM showed that erosion and tunnelling bacteria were often present in close association within the wood cell walls, light microscopy suggested that, in the majority of section examined, all the types seen were clearly seperated by regions of undegraded cell wall. The observations underscore the importance of erosion bacteria in wood decay under the conditions of a cooling tower where in-service timbers are kept constantly wet by the spray from water sprinklers. Also of significance is the great diversity of decay types seen, in particular the presence of cavitation bacteria and white rot fungi has not previously been recorded for high retention CCA treated cooling tower timbers.


Conference: 94-05-29...06-03, Nusa Dua, Bali, Indonesia

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