Water is variously claimed to be completely miscible at up to 30% or 55% by weight of total liquid, above which an aqueous layer separates. Any water that does separate must be carefully managed as it will be heavily contaminated with dissolved organics and require extensive treatment before disposal. Utilisation and consideration of oil on a "wet" basis, therefore, seems to be more sensible. An alternative approach is to condense the liquids at a temperature above the dew point of water i. e. above about 110°C. This has been used successfully by Bio — Alternative in their continuous carbonisation unit (128) but produces a very dirty gas which has to be burned or incinerated immediately. Roy has also used this approach to preferentially separate fractions from his vacuum multiple hearth pyrolyser (129), and early work at Waterloo also employed hot and cold water condensers (18). Indirect cooling has the disadvantage of increasing the residence of the vapour at temperatures where further reaction can still occur, thus impairing the product quality and yield. As processes increase in scale, this effect would become more pronounced. The current approach is for rapid quenching of the total product stream for maximum recovery of the liquid fraction

Water content is important as it has several effects: it reduces the heating value, affects the pH, reduces the viscosity, influences both chemical and physical stability, reduces potential pollution problems from waste water disposal and could affect subsequent upgrading processes (130). The interactions are poorly understood. The water is difficult to measure and remove, since evaporation or distillation at normal temperatures of around 100°C can cause significant and potentially deleterious physical and chemical changes in the liquid. Lower temperature drying is not successful due to the nature of the relationship between

water and the organic component in which the water seems to be chemically combined, analogous to water of hydration.

A key feature of flash pyrolysis processes for liquids is that no discrete aqueous phase is produced as all the water of reaction and feed water is incorporated or dissolved in the product bio-oil. This water is thus incinerated when the oil is combusted, and there are no environmental or pollution implications.

From slow pyrolysis processes, however, water is produced in significant quantities of typically between 20 and 40% wt on the feed, depending on feed moisture content. If a liquid product is collected from slow or conventional pyrolysis units, then the maximum water load of the liquid product is around 20 % wt. Above this level a discrete aqueous phase separates. This water phase is highly contaminated with dissolved and suspended organics, with a COD of typically 150000 (125, 126). This therefore represents a major problem of disposal or utilisation. In the selection of the primary pyrolysis products this waste water must be considered. If biological treatment is not appropriate or too expensive, part of the heat of combustion of the products will be required for incineration of this heavily contaminated water fraction. A potentially more attractive alternative route than incineration is oil condensation above the dew point of water, i. e. about 110- 120°C. The water then stays in the vapour phase and can be burned with the product gas (128). The pyrolysis gas should primarily be used for this purpose but this may not be enough in cases where the primary feedstock has a high water content and the gas is required for feed drying. Since slow and conventional pyrolysis is now only considered for charcoal production or for disposal of difficult wastes, this waste water problem is not significant in the context of bio-energy.