Analaysis of volatile odour compounds in digested sewage sludge and aged sewage sludge cake

Abstract

A number of malodorous compounds were identified in digested sludge and the corresponding sludge cakes. The major components included a number of sulfides. The levels of these compounds were monitored in order to investigate the influence of the sludge treatment process and length of cake storage on the malodour of treated sewage sludge. Two sewage treatment works (STWs) were surveyed. One of the works (Site B) produced very malodorous sludge cake, whilst the other produced a much less odorous cake (Site A). The analysis consisted of purge and trap sampling of liquefied samples followed by gas chromatography-mass spectrometry (GC-MS). Afteran initial screening of samples, by full mass scan GC-MS analysis, targetcompounds were identified for quantification. Sulfide levels inthe majority of the liquid digested sludges were significantly lower than that in sludge cakes. Large differences in the level of sulfides were found in stockpiled sludge cakes. The majority of malodorous sludge cakes had higher sulfide levels than less odorous sludge cakes. The most abundant compounds were dimethylsulfide, dimethyldisulfide and dimethyltrisulfide. Thehighest sulfide levels were found during the first month of stockpiling. Whenthe odour threshold concentration of these compounds was taken into account and odour unit values were calculated, dimethyltrisulfide was found to have a proportionally greater impact on odour than any of the other compounds studied. The use of sulfide levels as indicators of malodour formation and possible strategies for reducing odour in stockpiled sludge cake are discussed.