Human faeces contain most of the pathogens with a potential of causing diseases. Therefore, source control of faeces from household wastewater prevents these disease-causing pathogens gaining access to water bodies where they survive longer than on land and pose a long-term threat to human health. The most beneficial is when it is kept separated at source which avoids dilution of faeces. The separated solid fractions, which are easily biodegradable, can be treated biologically. When organic matter is decomposed oxidatively, heat is produced due to self heating capacity. This self produced heat will create self-hygienization of the matter.

The mostly applied methods for the sanitation of separated faecal waste are composting and dehydration. Treatment methods based on dehydration can reduce pathogens effectively, because there is a rapid pathogen destruction at moisture content below 25 %. Composting of a sufficient amount of fresh and easily degradable organic materials can produce heat, which raises the temperature of the materials. At the temperature of 60 °C and above, most of the pathogens are destroyed. Low temperature composting takes long time to kill the pathogens. The rate of reduction of pathogens is significantly dependant on time and temperature. The higher the temperature of the materials, the shorter the time for destroying the pathogens and vice versa. The factors such as a high pH, competition for food, antibiotic action and the toxic by-products of decomposing organism play a significant role in eliminating or reducing pathogen.

Feachem et al. (1983) stated that three months retention time will kill all of the pathogens in a low-temperature composting toilet except for worm eggs (Table 5).

Table 5: pathogens survival by composting (Feachem et al., 1983)

There is a synergistic correlation between time and temperature (see figure 5). The hatched areas refers to safety zone, where due to the combination of time and temperature all pathogens will be killed. Also the factors such as competition for food, predator-prey relationships and antagonism help to reduce or eliminate pathogens.

Desiccation by drying and adding high-alkaline additives is the best way to kill pathogens. Addition of ash helps in raising pH and decreasing moisture of faecal material. Both of them shorten the surviving time of pathogens. There are also other additives such as saw dusk, dry soil etc. Plant ash is the most effective additive to eliminate pathogens.

Figure 5: Combination of time and temperature of pathogens elimination. Hatch area represents complete pathogens elimination due to the combined effect of time and temperature (Feachem et al., 1983)

In summary, the die-off rate of the pathogens depends on the environmental condition of the place where they reside. The following factors are lethal to most of the pathogens:

• high pH (> 9)

• Low moisture contain (< 25%)

• High temperature (> 55 ^OC) over more than 10 hours

• Long retention time (> 6 months)

• Ammonia and high salt content

• Limited nutrients (competition for food)

• predator-prey relationships

• antagonism

High pH can be obtained by adding alkaline material such as ash or lime (but lime is not preferable) that reduces the moisture additionally. Moisture can be lowered by drying. Solar dryer can be used for this purposes, also high temperatures can be achieved at least part of the year in hot climate regions. High ammonia and salt can be obtained from urine. Long retention time, ammonia and high salt content, limited nutrients availability, predator-prey relationships and antagonism can be obtained in multi-chamber batch composting process.

The hygiene risk associated with urine is quite small compared to that with faeces. The fate of the pathogens entering into urine collection tank due to faecal contamination in urine diversion toilets is of vital importance for the hygiene risks related to the handling and reuse of the urine. For urine mainly temperature and the elevated pH (~9) in combination with ammonia has been concluded to affect the inactivation of micro-organisms. Bacteria like Salmonella (i.e. Gram-negative bacteria) were inactivated rapidly, whereas viruses were hardly reduced at all at low temperatures (4-5°C), (see table 6).

Table 6: Inactivation of microorganisms in urine, given as T90-values (time for 90% reduction) in days (Höglund,2001)

(a) Survival experiments performed at 5°C.

In practice, complete elimination of pathogens may not be possible in any kind of sanitation. Therefore, secondary barrier such as personal, food and domestic hygiene must be included to destroy the pathogens completely. Therefore, hygiene awareness and proper education are the crucial points for on-site faecal waste management.