Using Phages for Characterization of Effluent Quality in a Stabilization Pond
417
3 Results and Discussion
The field results show the efficiency of the treatment in the removal of pathogen
indicators (Figure 2). It is clear the significant reduction of the three indicators
monitored throughout the treatment process.
The results are presented separately for the summer and the winter because
there is a significant difference in wastewater temperature during these two
seasons (Figure 3). It leads to a low efficiency in the removal of the
microorganisms when the temperature is less during winter.
During the summer season there is a total reduction in the rocky pond of 2.5 log
for fecal coliforms, 3 log for somatic coJiphages and 4.5 log for F+ phages. The
removal rate decreases during the winter being 1.1 log, 1.3 log and 2.1 logs
respectively, for each type of indicators. The reduced efficiencies show that the
removal of F+ phages is greater than fecal coliforms and somatic coliphages
removal during both seasons. This result is in contradiction with the statementthat
viruses are in general more resistant to wastewater treatment processes than
bacteria (Feachem et al. 1983; Gerba and Yates, 1998).Tumer and Lewis (1995)
noted similar results in their study of pathogen indicators behavior in oxidation
ponds. This finding suggest that either there is a secondary source of phages that is
not detected by our methods (e.g. unanalysed host cells) or that some other
mechanisms of microbial removal is dominant (e.g. sedimentation or predation) in
this treatment system.
There is an additional microbial removal after the stabilization reservoir and the
storage of about 2-3 log units for the three indicators. This is due to their higher
retention time, that in this case is about one month or even more in the
stabilization reservoir and about 150 days in the storage, depending on the
irrigation necessities. Ponds with mean retention time between 15 and 22 days
reduce coliform content by two to three orders of magnitude (Liran et aI., 1994).
Also Rangeby et al. (1996) concluded that retention time was the most important
factor to affect bacterial die-off since it allows other changes in the pond
environment that affect the survival of the microorganisms.
417
3 Results and Discussion
The field results show the efficiency of the treatment in the removal of pathogen
indicators (Figure 2). It is clear the significant reduction of the three indicators
monitored throughout the treatment process.
The results are presented separately for the summer and the winter because
there is a significant difference in wastewater temperature during these two
seasons (Figure 3). It leads to a low efficiency in the removal of the
microorganisms when the temperature is less during winter.
During the summer season there is a total reduction in the rocky pond of 2.5 log
for fecal coliforms, 3 log for somatic coJiphages and 4.5 log for F+ phages. The
removal rate decreases during the winter being 1.1 log, 1.3 log and 2.1 logs
respectively, for each type of indicators. The reduced efficiencies show that the
removal of F+ phages is greater than fecal coliforms and somatic coliphages
removal during both seasons. This result is in contradiction with the statementthat
viruses are in general more resistant to wastewater treatment processes than
bacteria (Feachem et al. 1983; Gerba and Yates, 1998).Tumer and Lewis (1995)
noted similar results in their study of pathogen indicators behavior in oxidation
ponds. This finding suggest that either there is a secondary source of phages that is
not detected by our methods (e.g. unanalysed host cells) or that some other
mechanisms of microbial removal is dominant (e.g. sedimentation or predation) in
this treatment system.
There is an additional microbial removal after the stabilization reservoir and the
storage of about 2-3 log units for the three indicators. This is due to their higher
retention time, that in this case is about one month or even more in the
stabilization reservoir and about 150 days in the storage, depending on the
irrigation necessities. Ponds with mean retention time between 15 and 22 days
reduce coliform content by two to three orders of magnitude (Liran et aI., 1994).
Also Rangeby et al. (1996) concluded that retention time was the most important
factor to affect bacterial die-off since it allows other changes in the pond
environment that affect the survival of the microorganisms.