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Escherichia coli was found in drying pan samples at low mean concentrations of 160 and 580 CFU g−1 DS for Boneo and Somers WwTPs, respectively (Table 2). Escherichia coli is typically found in raw wastewater in concentrations of 106–108 CFU 100 mL−1 and wastewater treatment processes typically provide a 1 to 2 log10 reduction in the indicator bacteria (Koivunen et al. 2003). Solar drying processes (such as the drying pan and solar drying shed used at Boneo, and drying pans at Somers WwTPs) are also effective at reducing numbers of faecal pathogens (Bennamoun et al. 2013). Rouch et al. (2011b) reported a >5 log10 reduction in E. coli during drying pan treatment for 8–15 months compared to the sludge entering the pan. The relatively low numbers of E. coli detected following drying pan treatment were consistent with these previous findings. The results demonstrated that the harvested drying pan sludge may typically comply with the T2 standard for E. coli of <103CFU g−1 DS (EPA Victoria 2004). However, the Somers drying pan 2 sample exceeded this limit at 1,100 CFU g−1 DS (replicate data not shown).

Table 2

Bacterial pathogens in drying pans and one- and two-year biosolids stockpiles at Boneo and Somers WwTPs

Sample dateStockpile core depth (m)E. coli (CFU g−1 DS)a
Salmonella spp. MLE
MeanS.D.a
Boneo WwTP 
 Drying panb 
  30-Apr-15/  7-Dec-15 n/a 160 200 <0.50 
 Drying pan route – 1-year stockpilesc 
  13-Mar-15 2.8 <14 n/a 0.20 
  11-Nov-15 2.0–2.5 <16 n/a <0.20 
 Drying pan route – 2-year stockpilesc 
  13-Mar-15 2.3 <14 n/a <0.20 
  19-Mar-15 2.0 <14 n/a 0.20 
 Solar dryer route – 1-year stockpilec 
  11-Nov-15 1.7–2.3 <25 n/a <0.20 
 Solar dryer route – 2-year stockpilec 
  13-Mar-15 2.7 <22 n/a <0.20 
Somers WwTP 
 Drying pand 
  30-Apr-15 n/a 580 320 <0.30 
 1-year stockpilesc 
  18-Mar-15 1.5 <15 n/a <0.20 
  12-Nov-15 1.5 15 3.0 <0.20 
  12-Nov-15 1.5 <15 n/a <0.20 
 2-year stockpilec 
  18-Mar-15 2.8 <17 n/a <0.20 
Sample dateStockpile core depth (m)E. coli (CFU g−1 DS)a
Salmonella spp. MLE
MeanS.D.a
Boneo WwTP 
 Drying panb 
  30-Apr-15/  7-Dec-15 n/a 160 200 <0.50 
 Drying pan route – 1-year stockpilesc 
  13-Mar-15 2.8 <14 n/a 0.20 
  11-Nov-15 2.0–2.5 <16 n/a <0.20 
 Drying pan route – 2-year stockpilesc 
  13-Mar-15 2.3 <14 n/a <0.20 
  19-Mar-15 2.0 <14 n/a 0.20 
 Solar dryer route – 1-year stockpilec 
  11-Nov-15 1.7–2.3 <25 n/a <0.20 
 Solar dryer route – 2-year stockpilec 
  13-Mar-15 2.7 <22 n/a <0.20 
Somers WwTP 
 Drying pand 
  30-Apr-15 n/a 580 320 <0.30 
 1-year stockpilesc 
  18-Mar-15 1.5 <15 n/a <0.20 
  12-Nov-15 1.5 15 3.0 <0.20 
  12-Nov-15 1.5 <15 n/a <0.20 
 2-year stockpilec 
  18-Mar-15 2.8 <17 n/a <0.20 

DS = dry solids; S.D. = standard deviation; MLE = maximum likelihood estimate of the mean; n/a = not applicable.

aIf no E. coli were detected the concentration was assumed to be equivalent to the LoD to calculate the mean and standard deviation where applicable.

bMeans/MLEs of two composite samples of the same drying pan.

cMeans/MLEs of five composite samples.

dMeans/MLEs of four composite samples of four drying pans.

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