Samples of sewage sludges, taken over a 12-month period from 9 Scottish sewage works, contained on average 0.24 cysts of Globodera spp. (potato cyst-nematodes) of which 11% were viable. The incidence was not significantly related to season or to the presence of vegetable-processing effluent. Exposure of cysts in sludge to mesophilic anaerobic digestion (35 °C, 30 min) cold anaerobic digestion (9 weeks), pasteurisation (70 °C, 30 min) and aerobic thermophilic digestion (60 °C, ld) reduced viability of eggs within the cysts by almost 100%. Sludges so treated can therefore be considered to be free from infection risk to potato crops, although the non-infective cysts may still be recovered. Treatment with lime at pH 11.5 (20 °C, 24 h), by aerobic stabilisation in an oxidation ditch (7 weeks) and by activated-sludge treatment (5d) did not reduced viability acceptably. Accelerated cold digestion did not reduce viability sufficiently after the usual 15 weeks but rendered eggs completely non-viable after 21 weeks. The results show that even sludge treated to destroy viable cysts should not be applied to land used for growing seed potatoes and subject to testing for freedom from infestation. Treatment destroying viability should increase the acceptability of sludge for ware potato growers, although the numbers of cysts applied in untreated sludge would be unlikely to increase significantly levels of cysts in soils already infested.

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