that remove particles, such as sedimentation, filtration or coagulation-flocculation are employed. therefore, waste stabilization ponds (lagoons), storage basins, constructed wetlands, rapid filtration or upflow anaerobic sludge blanket (uasb) reactors can be used. helminth ova cannot be inactivated with chlorine, uv light or ozone (in the latter case at least not with economical doses because >36 mg/l ozone are needed with 1 hour contact time). inactivation of helminth ova can be achieved in sewage sludge treatment where the temperature is increased over 40 °c or moisture is reduced to less than 5%. best results can be obtained when both of these conditions are combined for an extended period of time. details about the contact time under these conditions and other related environmental factors are generally not well-defined for every type of helminth egg species. helminth eggs are considered highly resistant biological structures. indicator organism identification and quantification of helminth eggs at unam university in mexico city, mexico this section is an excerpt from indicator organism § indicator helminth eggs. the eggs from helminths (parasitic worms) are a commonly used indicator organism to assess the safety of sanitation and wastewater reuse systems (such schemes are also called reuse of human excreta).: 55 this is because they are the most resistant pathogens of all types of pathogens (pathogens can be viruses, bacteria, protozoa and helminths). it means they are relatively hard to destroy through conventional treatment methods. they can survive for 10–12 months in tropical climates. these eggs are also called ova in the literature. helminth eggs that are found in wastewater and sludge stem from soil-transmitted helminths (sths) which include ascaris lumbricoides (ascaris), anclostoma duodenale and necator americanus (hookworm), and trichuris trichiura (whipworm). ascaris and whipworm that are identified in reusable wastewater systems can cause certain diseases and complications if ingested by humans and pigs. hookworms will plant and hatch their larvae into the soil where they grow until maturity. once the hookworm eggs are fully developed, they infect organisms by crawling through the organism’s skin. the presence or absence of viable helminth eggs ("viable" meaning that a larva would be able to hatch from the egg) in a sample of dried fecal matter, compost or fecal sludge is often used to assess the efficiency of diverse wastewater and sludge treatment processes in terms of pathogen removal.: 55 in particular, the number of viable ascaris eggs is often taken as an indicator for all helminth eggs in treatment processes as they are very common in many parts of the world and relatively easy to identify under the microscope. however, the exact inactivation characteristics may vary for different types of helminth eggs. the technique used for testing depends on the type of sample. when the helminth ova are in sludge, processes such as alkaline-post stabilization, acid treatment, and anaerobic digestion are used to reduce the amount of helminth ova in areas where there is a large amount. these methods make it possible for helminth ova to be within the healthy requirements of ≤1 helminth ova per liter. dehydration is used to inactivate helminth ova in fecal sludge. this type of inactivation occurs when feces is stored between 1-2 years, a high total solids content (>50-60%) is present, items such as leaves, lime, earth, etc. are added, and at a temperature of 30°c or higher. environmental samples for the purpose of setting treatment standards and reuse legislation, it is important to be able to determine the amount of helminth eggs in an environmental sample with some accuracy. the detection of viable helminth eggs in samples of wastewater, sludge or fresh feces (as a diagnostic tool for the infection helminthiasis) is not straight forward. in fact, many laboratories in developing countries lack the right equipment or skilled staff required to do so. an important step in the analytical methods is usually the concentration of the eggs in the sample, especially in the case of wastewater samples. a concentration step may not be required in samples of dried feces, e.g. samples collected from urine-diverting dry toilets.