The Effect of Drying and Size Reduction Pretreatments on Recovery of Inorganic Crop Nutrients From Inedible Wheat Residues

Abstract

Inorganic nutrients can be easily recovered from ALS crop residue solid wastes by aqueous leaching. However, oven-drying and milling pretreatment of these residues has been frequently required to accommodate crop scientists and facility storage limitations. As part of a research study that will compare three different bioreactor technologies for processing these wastes, we realized that different drying and size reduction pretreatments had been utilized for each technology. This article compares the effects of residue pretreatment on recovery of nutrients by leaching. Pretreatments included three drying methods [fresh, oven-dried (70°C overnight), and freeze-dried)] and two size reduction methods [chopped (2 cm length) and milled (2 mm diameter)]. Determination of mass balances (dry weight and ash content of solids) before and after leaching indicated solublization was least for fresh residues (23% dry weight loss and 50% for ash loss) and most for freeze-dried residues (41-47% dry weight loss and nearly 100% for ash loss). Mineral recovery of major elements (NO 3 , PO 4 , K, Ca, and Mg) in leachates was poorest for fresh residues. P and K recovery in leachates was best for oven-dried residues, and Ca, Mg, and N recovery best for freeze-dried residues. The differences in recovery for N, P, and K in leachates were minimal between chopping and milling and slightly better for Ca and Mg from milled residues. conversion of crop residue (15). However, oven-drying and milling pretreatment of these residues (inedible roots, stems, leaves, and chaff) had been frequently required to accommodate crop scientists and facility storage limitations. As a first step, wheat residues were extracted with water at room temperature to remove soluble compounds that might later interfere with cellulose hydrolysis. Upon analysis, this aqueous extract (leachate) was found to contain significant quantities of many plant nutrients (nitrate, K, Ca, Mg, phosphate, etc.). This leachate could be used to replenish wheat hydroponic solutions, but soluble organic compounds were also extracted (7,8). These compounds led to problems in the nutrient delivery system if not first removed from the leachate