Effects of irradiation on sludge organic carbon and nitrogen mineralization

Irradiation used to eliminate pathogens in sewage sludge may deleteriously affect properties of sludge organic matter. Three anaerobically digested, dewatered sewage sludges were irradiated with gamma-irradiation at absorption doses of 0, 6 and 12 kGy, respectively, then mixed with a soil and incubated for 16 wk. Evolved CO2 and concentrations of NH4+N and NO3−N in the sludge-soil mixture were measured. The effects of irradiation were not consistent in the three sludges tested. Irradiation did not significantly affect sludge organic C nor N mineralization in Guelph sludge (GS). However, in sludges from Ottawa (OS) and Toronto (TS), C mineralized in irradiated treatments was less than from the non-irradiated treatments. In response to slower organic matter decomposition caused by irradiation in the OS and TS, the inorganic N concentrations were higher with irradiated than with non-irradiated in the OS, but the converse was true for the TS. This was probably related to the difference in C-to-N ratio of the organic matter (19.5 in OS and 9.5 in TS). Increasing the irradiation doses did not significantly affect inorganic N concentrations with the OS, but with the TS, the high dose (12 kGy) treatment resulted in higher C and N mineralization than did the low dose (6 kGy).