The physicochemical conditions of high-rate algal ponds were responsible for a more than 97% reduction in the infectivity of Cryptosporidium parvum oocysts in neonatal mice. The use of semipermeable bags of cellulose showed that pH, ammonia, and/or light seems to be a major factor for the inactivation of oocysts in wastewater, supporting the importance of alga-based systems for safer reuse of treated wastewater.Present conventional wastewater treatment is very expensive for application in rural areas and, moreover, does not guarantee the inactivation of Cryptosporidium parvum oocysts from sewage (3,12,16,18). The high-rate algal pond (HRAP) is a low-cost wastewater treatment system designed to achieve two goals: secondary wastewater treatment and algal biomass production. The HRAP is a combination of intensified oxidation ponds and an algal reactor. Algae supply the oxygen demand for bacterial degradation of organic matter, and bacteria excrete mineral compounds that provide the algae with nutrition. HRAPs have proved effective in removing organic matter (13) and in reducing bacterial contamination (8) and the number of nematode eggs (1), but no data are available on their role in removing Cryptosporidium oocysts, a subject of special interest when dealing with rural wastewater. We will focus this study on the effect of the HRAP physicochemical conditions on the viability of Cryptosporidium oocysts as measured with a neonatal mouse infectivity model. HRAP pilot plants. Two identical pilot plants fed with urban wastewater were used for this study (ponds A and B) (Fig. 1). The average physicochemical characteristics of the ponds during the study period are shown in Table 1.C. parvum oocysts. Oocysts were obtained from the feces of an experimentally infected lamb, purified according to the procedures of Arrowood and Sterling (2), and stored at 4°C in a 2.5% (wt/vol) aqueous potassium dichromate solution until use.Treatment of oocysts in HRAP. Regenerated cellulose semipermeable bags with 14,000-Da porosity were used for the experiment. These bags allowed oocysts to be in contact with the small ions present in the water while reducing the effect of other mortality factors such as bacterial and/or fungal contamination or predation. The bags were filled with 50 ml of sterile water and 10 8 oocysts. Two bags were placed in each pond for 3 (pond A) and 10 (pond B) days. Two other bags without oocysts were also placed in the ponds to test the osmotic interchange through the semipermeable membrane, and two bags with oocysts were stored at 4°C in sterile water and used as a control. At the end of the hydraulic retention time (HRT), the oocysts were washed and centrifuged with distilled water. The sediment was resuspended and adjusted to obtain 5 ϫ 10 5 oocysts/25 l.Bioassay for viability. Two hundred fifty-seven suckling mice from 21 individual litters (7 to 16 mice/litter) of NMRI mice were divided in four groups of 3-to 4-day-old mice, day 0 postinfection (p.i.). Mice in groups A (98 mice) and B (79 mice) were inoculated...

The viability of Parascaris equorum eggs was studied in two experimental pilot-scale high-rate algal ponds (HRAPs) working in parallel with 4 and 10 days hydraulic retention time respectively. Semi-permeable bags of cellulose (15000 daltons pore size) were used to study the effect of physico-chemical conditions on the survival of these helminth eggs. Three thousand eggs were used in each bag. Replicates of these bags were submerged for 4 and 10 days in the HRAPs and egg viability was compared with that in control bags submerged in sterile water. After 4 days exposure, 60% reduction in viability was achieved, reaching 90% after 10 days, much higher than the 16% and 25% found in the control bags for 4 and 10 days respectively. Ionic conditions of the HRAP may have been responsible for up to 50–60% of the egg mortality, suggesting that mortality due to the ionic environment could be more important than physical retention and other potential removal factors.