Sludge Watch ==> Ottawa Research - No significant reduction in Crypto during sludge digestion

[Maureen Reilly]

Sludgewatch Admin:

As we look at the startling rise of documented cryptosporidium infections in 
the USA, we should note that sewage digesters do little to reduce the number 
of cyptosporidium eggs in digested dewatered sludge.

I have been called to many rural homes who have Cryptosporidium infections 
adjacent to sludge spreading sites.


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Fate of Cryptosporidium oocysts, Giardia cysts, and microbial indicators 
during wastewater treatment and anaerobic sludge digestion


Christian Chauret, Susan Springthorpe, and Syed Sattar


Abstract: The extent of reduction in selected microorganisms was tested 
during both aerobic wastewater treatment and anaerobic digestion of sludge 
at the wastewater treatment plant in Ottawa to compare the removal of two 
encysted pathogenic protozoa with that of microbial indicators. Samples 
collected included the raw wastewater, the primary effluent, the treated 
wastewater, the mixed sludge, the decanted liquor, and the cake. All of the 
raw sewage samples were positive for Cryptosporidium oocysts and Giardia 
cysts, as well as for the other microorganisms tested.

During aerobic wastewater treatment (excluding the anaerobic sludge 
digestion), Cryptosporidium and Giardia were reduced by 2.96 log10 and 1.40 
log10, respectively.

Clostridium perfringens spores, Clostridium perfringens total counts, 
somatic coliphages, and heterotrophic bacteria were reduced by approximately 
0.89 log10, 0.96 log10, 1.58 log10, and 2.02 log10, respectively.

All of the other microorganisms were reduced by at least 3.53 log10. Sludge 
samples from the plant were found to contain variable densities of 
microorganisms. Variability in microbial concentrations was sometimes great 
between samples, stressing the importance of collecting a large number of 
samples over a long period of time.

In all cases, the bacterial concentrations in the cake (dewatered biosolids) 
samples were high even if reductions in numbers were observed with some 
bacteria. During anaerobic sludge digestion, no statistically significant 
reduction was observed for Clostridium perfringens, Enterococcus sp., 
Cryptosporidium oocysts, and Giardia cysts.


A 1-2 log10 reduction was observed with fecal coliforms and heterotrophic 
bacteria. However, the method utilized to detect the protozoan parasites 
does not differentiate between viable and nonviable organisms. On the other 
hand, total coliforms and somatic coliphages were reduced by 0.35 log10 and 
0.09 log10, respectively.

These results demonstrate the relative persistence of the protozoa in sewage 
sludge during wastewater treatment.

Key words: Cryptosporidium, Giardia, indicators, wastewater, sludge.