The effect of injector tine design on odour and ammonia emissions following injection of bio-solids into arable cropping

With the impending ban on disposal of sewage sludge to sea, a reliable meth od of incorporating sewage sludge into cropped arable land would provide an alternative outlet for water companies. However, certain criteria must be met before widespread acceptance of such a system could be achieved. In par ticular, it is essential that odour and ammonia emissions and damage to the growing crop are minimized. The most promising way forward appears to be p lacement of the bio-solids below the soil surface using tine injectors. The performance of a shallow winged injector tine with an integral cutting edge, developed at Silsoe College, which is capable of operating effectivel y at depths of between 50 and 80 mm, was compared with the Greentrac slippe r foot shallow injector tine fitted with a disc cutter. Trials were conduct ed both in the soil dynamics laboratory, to observe force and disturbance c haracteristics, and in the field to investigate odour and ammonia emissions following the application of anaerobically digested sewage sludge in Novem ber into winter wheat, and pig slurry in May into spring wheat. Investigations in the soil dynamics laboratory showed that draught forces w ere comparable between tines, (approximately 0.5 kN at 50 mm depth); howeve r a 30% reduction in the vertical force required to enable the improved inj ector tine to penetrate and maintain the desired working depth was observed compared with the slipper foot tine. This characteristic is critical in th e design of a full-sized machine where a large operating width is desirable in order to minimize the area of land trafficked during the injection oper ation. In addition, the wings on the improved injector tine were found to p romote greater soil mixing, resulting in lower surface exposure of bio-soli ds following application. Results suggest that odour emissions, following a typical application of an aerobically digested sewage sludge to arable land in November, are sufficie ntly low [80-120 odour units (ou)/m(2)] as to present no difficulties for t he proposed disposal system regardless of the method of application, and re ductions in ammonia emissions of up to 80%, (300 mgNH(3) N/m(2)), may be po ssible using the improved injection system over the Greentrac slipper foot injector. Results from an application in May, using a stronger source (pig slurry), indicate that a 60% reduction in odour, significant at the 85% lev el of confidence, can be achieved through the adoption of the improved inje ction system. Similarly, ammonia volatilization following the same applicat ion was significantly reduced ( > 1000 mg NH3 N/m(2)) by the use of a shall ow injection method (at the 95% level of confidence) compared with surface application. The results from the May application indicate a significant tr end towards further reductions (up to 200 mgNH(3) N/m(2)) in ammonia emissi ons using the improved injector tine. (C) 1998 Silsoe Research Institute.