Dewatering methods are available for most high-water-content virgin and waste biomass. This suggests that the moisture content of such feedstocks can be readily adjusted before conversion. This is not the case, however, because it is often difficult to reduce moisture content to the level desired at reasonable cost.

The equipment used for dewatering includes filters and screening devices of various types, centrifuges, hydrocyclones, extrusion and expression presses, water extractors, and thickening, clarifying, and flotation hardware. The pro­cessing methods encompass a broad range of water-removal techniques. They
can also incorporate the use of chemical flocculants and surfactants and high- and low-temperature treatments. The drawbacks to the dewatering of high — water-content biomass by most of these methods are numerous. Direct physical separation of the occluded moisture in aquatic species by dewatering is nor­mally not feasible unless the biomass is subjected to physical processes that disrupt the cell walls. Solar drying in open air is a low-cost option for moisture reduction as already pointed out, but most high-water-content biomass species begin to decompose, some quite rapidly and often with a relatively large loss in carbon and energy content, when dried under these conditions. In contrast, municipal biosolids are often dewatered to 5 to 20 wt % solids content, and some of the advanced dewatering methods are capable of increasing the solids content to as high as 50 wt % or more. The drying methods used commercially in wastewater treatment plants facilitate final disposal, but they are costly and afford products that are still far from the preferred moisture content range of feedstocks for thermochemical conversion.

Strict physical processing of high-water-content biomass for partial removal of moisture can sometimes be accomplished by combined use of shearing or cutting devices and mechanical pressing. Some of the dewatered products produced by these techniques can sustain their own combustion, can be com­bined with low-moisture feedstock for thermochemical conversion, or can be fabricated into briquettes or pellets for use as fuels. Overall consideration of the difficulties of dewatering high-water-content biomass suggests that microbial conversion processes should be used so the feedstock does not have to be dewatered or dried and can be used as such.

Although it is relatively costly, one drying method deserves special mention because it is used commercially for several high-water-content waste biomass streams such as brewery and fermentation industry wastes, food and dairy industry wastes, and primary and secondary municipal biosolids. The technique is based on the equivalent of multiple-effect evaporation and vapor recompres­sion so that most of the water exits the process as liquid, except in the last effect, to avoid losing the latent heat of vaporization. Several advanced processes have been developed to separate water from solids at lower energy inputs than conventional, single-effect drying systems. With the advent of large centrifugal compressors in the 1960s, it became possible to mechanically recompress the water vapor from an evaporative stage to drive that same stage rather than another as in multiple-effect evaporation. Using standard technology, the dewa­tering of high-water-content biomass can utilize mechanical vapor recompres­sion to raise the solids concentration to near 30 wt %, followed by multiple — effect evaporation to raise the solids concentration to near 50 wt %, followed by a rotary dryer if desired for further moisture reduction (с/. Crumm and Crumm, 1984). The efficiency of the segment of evaporation that mechanical vapor recompression accomplishes is very high. At an energy equivalent of 10.5 MJ/kWh (10,400 Btu/kWh), mechanical vapor recompression can vapor­ize 1 kg of water for less than 0.46 MJ (1.0 lb for less than 200 Btu). The Carver-Greenfield process is based on combining mechanical vapor recompres­sion with multiple-effect evaporation to dry high-water-content biomass and other solid suspensions. Many full-scale units have been placed in operation since the first facility was installed in 1961. One unit was used at the Hyperion wastewater treatment plant in Los Angeles from 1987 to early 1995 to dry 40 t/day of biosolids wetcake to 99+% total solids content (Haug, Moore, and Harrison, 1995). The process has since been replaced by rotary steam dryers because it was not possible to reach the design capacity of the unit.