The only sewage digestion process producing a guaranteed pathogen-free sludge is batch thermophilic digestion in which all of the sludge is maintained at 50oC (122oF) for 13 days. Other sewage digestion processes will allow the survival of worm eggs and possibly pathogenic bacteria. Typical sewage treatment plants instead use a continuous process where wastewater is added daily or more frequently, thereby guaranteeing the survival of pathogens (see Figure 7.2).
I took an interest in my local wastewater treatment plant when I discovered that the water in our local creek below the waste-water discharge point had ten times the level of nitrates that unpolluted water has, and three times the level of nitrates acceptable for drinking water.33 In other words, the water being discharged from the water treatment plant was polluted. We had tested the water for
nitrates, but we didn't test for pathogens or chlorine levels. Despite the pollution, the nitrate levels were within legal limits for wastewater discharges.
Waste stabilization ponds, or lagoons, large shallow ponds widely used in North America, Latin America, Africa and Asia, involve the use of both beneficial bacteria and algae in the decomposition of organic waste materials. Although they can breed mosquitoes, they can be designed and managed well enough to yield pathogen-free waste water. However, they typically yield water with low concentrations of both pathogenic viruses and bacteria (see Figure 7.4).
Most mouldering and commercial composting toilets are relatively anaerobic and compost at a low temperature. According to Feachem et al., a minimum retention time of three months produces a compost free of all pathogens except possibly some intestinal worm eggs. The compost obtained from these types of toilets can theoretically be composted again in a thermophilic pile and rendered suitable for food gardens (see Figure 7.5 and Table 7.14). Otherwise, the compost can be moved to an outdoor compost bin, layered and covered with straw (or other bulky organic material such as weeds or leaf mould), moistened, and left to age for an additional year or two in order to destroy any possible lingering pathogens. Microbial activity and earthworms will aid in the sanitation of the compost over time.
Complete pathogen destruction is guaranteed by arriving at a temperature of 62oC (143.6oF) for one hour, 50oC (1220F) for one day, 460C (114.80F) for one week or 430C (109.40F) for one month. It appears that no excreted pathogen can survive a temperature of 650C (1490F) for more than a few minutes. A compost pile containing entrapped oxygen may rapidly rise to a temperature of 550C (1310F) or above, or will maintain a temperature hot enough for a long enough period of time to destroy human pathogens beyond a detectable level (see Figure 7.6). As pathogen destruction is aided by microbial diversity, as discussed in Chapter 3, excessively heating a compost pile, such as by forcing air through it, can be counter-productive.
Table 7.14 indicates survival times of pathogens in a) soil, b) anaerobic decomposition conditions, c) composting toilets and d) thermophilic compost piles.
This is a good subject to discuss in greater detail as it is rarely a topic of conversation in social circles, yet it is important to those who are concerned about potential pathogens in compost. Therefore, let's look at the most common of human worm parasites: pinworms, hookworms, whipworms and roundworms.
A couple of my kids had pinworms at one time during their childhood. I know exactly who they got them from (another kid), and getting rid of them was a simple matter. However, the rumor was cir-
Was this article helpful?