Biomass leachate treatment by reverse osmosis

Leaching of alkali metals and chloride from biomass reduces fouling and slagging in furnaces and boilers, especially for grasses and other herbaceous crops used as fuel. Leaching also produces large volumes of leachate for disposal or treatment. Leachates obtained by water-washing rice straw were treated by reverse osmosis (RO) for water recovery and to evaluate ion and chemical oxygen demand (COD) rejection by the membrane as a function of feed concentration. Recovery above 90% was achieved with rejection of COD and ions other than carbonate also above 90%. Similar results would be expected to apply to other biomass leachates. Flux approaching a recommended 40 1 M h was maintained at 90% recovery at a pressure half the maximum recommended pressure of the membrane tested. Flux declined above 90% recovery due to increasing osmotic pressure and probable fouling as salt concentrations likely exceeded solubility limits at the membrane surface. Cost of RO processing for a leaching facility handling 50,000 Mg year(-1) of biomass and generating 1.9 Ml day(-1) of leachate is estimated at $671 Ml(-1) of leachate exclusive of brine disposal. The cost increases to $719 Ml(-1) leachate for a facility of approximately half this capacity. RO costs add $7-8 Mg-1 to the cost of fuel processed. RO treatment reduces the volume of fresh water required for leaching and the volume of brine disposed. Where sufficient municipal wastewater treatment capacity exists the addition of RO to a leaching system may not be economically justified if costs assessed on the basis of strength are higher than costs assessed for flow. For treatment by evaporation ponds, RO offers a clear advantage over direct leachate disposal, reducing costs by 50% to 70%. Land application of both leachate and concentrate can take advantage of nutrients extracted from the biomass and could be used in the production of additional biomass. Application over an area one-tenth or more the original source area may be required to manage salts and to ensure that sustainable application rates are not exceeded. (C) 2003 Elsevier Science B.V. All rights reserved.