With the impending ban on disposal of sewage sludge to sea, a reliable method 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 particular, it is essential that odour and ammonia emissions and damage to the growing crop are minimized. The most promising way forward appears to be placement 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 effectively at depths of between 50 and 80 mm, was compared with the Greentrac slipper foot shallow injector tine fitted with a disc cutter. Trials were conducted both in the soil dynamics laboratory, to observe force and disturbance characteristics, and in the field to investigate odour and ammonia emissions following the application of anaerobically digested sewage sludge in November into winter wheat, and pig slurry in May into spring wheat. Investigations in the soil dynamics laboratory showed that draught forces were comparable between tines, (approximately 0.5 kN at 50 mm depth); however a 30% reduction in the vertical force required to enable the improved injector tine to penetrate and maintain the desired working depth was observed compared with the slipper foot tine. This characteristic is critical in the 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 operation. In addition, the wings on the improved injector tine were found to promote greater soil mixing, resulting in lower surface exposure of bio-solids following application. Results suggest that odour emissions, following a typical application of anaerobically digested sewage sludge to arable land in November, are sufficiently low [80-120 odour units (ou)/m(2)] as to present no difficulties for the proposed disposal system regardless of the method of application, and reductions in ammonia emissions of up to 80%, (300 mgNH(3) N/m(2)), may be possible 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% level of confidence, can be achieved through the adoption of the improved injection system. Similarly, ammonia volatilization following the same application was significantly reduced ( > 1000 mg NH3 N/m(2)) by the use of a shallow injection method (at the 95% level of confidence) compared with surface application. The results from the May application indicate a significant trend towards further reductions (up to 200 mgNH(3) N/m(2)) in ammonia emissions using the improved injector tine. (C) 1998 Silsoe Research Institute.
