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United Nations Environment Programme
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

5. Chemicals and Nutrients

The health risks associated with chemicals found in wastewater and sludge may need to be given more attention, particularly as the pace of industrialization increases in developing countries. In many areas, municipal wastewater and industrial wastes are combined, creating a potentially dangerous mixture of toxic substances that must be handled carefully (see table 2).

Municipal sludge may contain high concentrations of heavy metals such as cadmium, lead, nickel and chromium. When industrial wastewater is also present concentrations of these metals can be 10 to 20 times higher (Chang, Page and Asano, 1995). In 1983, it was estimated that incineration of sewage sludge worldwide introduced between 3 and 36 tonnes/year of cadmium into the atmosphere (Nriagu and Pacyna, 1988). This cadmium eventually is deposited to terrestrial and aquatic environments. Heavy metals such as cadmium and mercury concentrate in the tissues of many filter-feeding shellfish, fish, and in some cases terrestrial plants (WHO, 1992; WHO, 1989) and thus can pose significant health threats to consumers of these products. For example, Grössman (1988 in WHO, 1991) found elevated levels of nickel in green vegetables, cabbage, onions, beans and peas grown on soils contaminated with nickel from sewage sludge application.

Organic chemicals found in wastewater and sludge may also cause adverse health effects, but direct evidence is limited. However, this does not mean that there are no serious health effects associated with use of water contaminated with industrial wastes. Rather, it is a reflection of the difficulty of associating chronic exposure of chemicals and chemical mixtures to diseases with long latency periods. In some parts of China where irrigation with wastewater heavily polluted with industrial wastes has occurred for many years, health effects have been observed. For example, wastewater irrigation was associated with a 36% increase in hepatomegaly (enlarged liver), and a 100% increase in both cancer and congenital malformation rates compared to control areas (Yuan, 1993).

To minimize adverse health and environmental effects from these components, industrial wastes should be adequately pre-treated to remove these chemicals or should be treated separately from municipal wastewater and excreta.

Poor irrigation practices with untreated or partially treated wastewater also impact the quality and safety of groundwater in shallow aquifers and surface waters that may supply drinking water. Wastewater related nitrate contamination of aquifers has been extensively documented in both developed and developing countries. High concentrations of nitrates in drinking water can cause methaemoglobinaemia (“blue babyE syndrome). Numerous cases of methaemoglobinaemia due to nitrate exposure in bottle-fed infants have been reported in Eastern Europe and the United States over the last ten years Eincluding several infant deaths (Knobeloch et al., 2000). It has been suggested that chronic exposure to nitrates in drinking water may also be linked to cancer, thyroid disease and diabetes (Knobeloch et al., 2000).

Excessive nutrients (primarily nitrogen and phosphorus), in wastewater, sludge and excreta, may contaminate surface waters and can cause eutrophication. Eutrophication of freshwater sources may create environmental conditions that favor the growth of toxin-producing cyanobacteria. Toxins produced by cyanobacteria can cause gastroenteritis, liver damage, nervous system impairment, and skin irritation. Chronic exposure to cyanobacteria toxins has been associated with liver cancer in animals and may cause similar effects in humans (Chorus and Bartram, 1999). Health problems associated with cyanotoxins have been documented in several countries including Australia, Brazil, Canada, China, England, USA, and Zimbabwe. In China, Ling (2000) found that liver cancer mortality was nearly six times higher in people who obtained their drinking water from eutrophic ponds where high concentrations of cyanobacterial toxins (microcystins) were present than in people who obtained their drinking water from deep wells. Exposure to these toxins has usually been through contaminated drinking water or recreational contact with the water. However, it is also likely that consumers of contaminated freshwater fish or shellfish might be exposed to these toxins (Chorus and Bartram, 1999).

Table 2: Chemicals of Health Concern Found in Untreated Municipal Wastewater and Excreta

Chemical Health Effect
Halogenated Compounds
Chloroform
DDT
Di and tri-chlorobenzenes
Tetrachloroethane
 
Skin irritation, nausea, embryo/fetotoxic
Nervous system damage, cancer (?)
Liver, kidney, and blood damage
Liver damage, nausea
Heavy Metals
Arsenic
Cadmium
Chromium
Lead
Mercury
Nickel
 
Gastrointestinal, skin, and nerve damage, cancer
Gastrointestinal, kidney and lung damage
Lung and skin damage, cancer
Nervous and immune system, and kidney damage, embryo/fetotoxic
Brain and kidney damage, embryo/fetotoxic
Lung, brain, kidney, liver, spleen and skin damage, cancer
Inorganic Chemicals
Cyanide
Flouride
Hydrogen Sulfide
Nitrate
 
Brain and heart damage, shortness of breath, death
Dental and skeletal fluorosis
Nausea, vomiting, mucous membrane irritation
Methaemoglobinaemia
Nutrients
Nitrogen
Phosphorus
 
Cause eutrophication which facilitates the growth of toxin-producing cyanobacteria and other harmful algae
Organic Chemicals
Benzene
Phenol
Toluene
Xylene
 
Anemia, dizziness, leukemia
Irritation of skin, eyes, and gastrointestinal tract, systemic toxicant
Brain and kidney damage
Confusion, dizziness, memory loss, embryo/fetotoxic
Other Chemicals
Endocrine disruptors
 
Pharmaceuticals
 
 
Reproductive/developmental effects in wildlife, various potential effects in humans

Reproductive/developmental effects in wildlife, various potential effects in humans
Source: Chang, Page, and Asano, 1995; National Research Council, 1998; WHO, 1999c; WHO, 1992; WHO, 1991; WHO, 1989; ATSDR, 2000.

There is also some evidence to indicate that nutrient enrichment of marine waters is partially responsible for other (i.e. non-cyanobacterial) toxic algal blooms (Epstein, et al. 1993). Toxins produced by these algae accumulate in the flesh of filter-feeding shellfish. When the toxins in the shellfish are consumed a variety of illnesses may result, including; paralytic shellfish poisoning, diarrhoeic shellfish poisoning, amnesic shellfish poisoning, ciguatera poisoning and neurotoxic shellfish poisoning (Hallegraeff, 1993). Several of these toxins can cause serious health effects - even death in certain circumstances (WHO, 1986).

In the last several years chemicals that mimic hormones or have anti-hormone activity, and interfere with the functioning of endocrine systems in various species have been identified in municipal wastewater. Endocrine disruptors as they are known, derive from many sources including pesticides, persistent organic pollutants, nonionic detergents and human pharmaceutical residues. Many of these substances are resistant to conventional wastewater treatment and may persist in the environment for some time (National Research Council, 1998). Human health effects potentially linked to exposure to these chemicals include: breast, prostate and testicular cancer; diminished semen quantity and quality; and impaired behavioural/mental, immune and thyroid function in children. Although direct evidence of adverse health effects in humans is lacking, reproductive abnormalties, altered immune function, and population disruption potentially linked to exposure to these substances has been observed in amphibians, birds, fish, invertebrates, mammals, and reptiles (WHO, 1999b).

6. Treatment and Risk Management

There are many ways in which adverse health effects to humans from the release of untreated/inadequately treated municipal wastewater and excreta into the environment can be minimized. Some of the most important methods include proper wastewater and excreta treatment and disposal, waste minimization (e.g., reducing the volume of the wastes or the use of toxic chemicals), providing separate treatment or diversion of wastewater and sludge with high concentrations of toxic chemicals (industrial wastes), protection of water sources, restriction of shellfish harvesting and bathing in polluted waters, safe water handling and storage, and improving personal hygiene including hand washing with soap and safe disposal of faeces.

This document will focus on the proper treatment and disposal of wastes (i.e., the removal or significant reduction of harmful components from wastes and disposal of wastes so that humans will not be exposed to harmful substances) and improving personal hygiene.

6.1 Treatment options (see table 3)

Human pathogens in municipal wastewater and excreta generally pose the greatest threat to human health. Wastewater and excreta should be effectively treated to reduce threats to human health.

Most of the world's population does not have access to adequate sanitation. Conventional sanitation relies on centralized sewerage systems and requires large amounts of water to carry the wastes away. This type of approach may work well in developed countries and large cities but is impractical for many other locations. The cost of a sewerage system (which is usually more than four times that of on-site alternatives) and its requirement of a piped water supply preclude its adoption in the many communities in developing countries that lack adequate sanitation (Franceys, et al. 1992). On-site disposal, can effectively protect public health, facilitates the reuse of valuable waste components (nutrients, organic matter, and water), keeps the waste where it is produced, reduces contamination with toxic chemicals, and generally is the least expensive alternative. On-site disposal alternatives range from simple composting latrines to complex bioreactor systems. Refer to Source Book.

Where land is available at low cost and sewerage is conveyed to centralized locations, well-designed waste stabilization pond (WSP) systems have proven to be very effective in treating wastes. The advantages of WSP systems are that they cost little to run, are simple to maintain, produce small volumes of sludge, facilitate waste reuse and are good at removing pathogens - especially helminths which are not adequately removed in most primary and secondary treatments.

Conventional wastewater and excreta treatment strategies (primary and secondary levels) in developed countries have traditionally focused on the removal of suspended solids and pollutants that used oxygen in the receiving waters to decompose (Biological Oxygen Demanding substances or BOD) and less on the reduction of pathogens and nutrients. These processes usually are expensive to operate due to the large energy, skilled labor, infrastructure, and maintenance requirements. Tertiary treatments must be added to the process to effectively reduce pathogen and nutrient levels. Frequently different tertiary treatments may be used in combination to reduce pathogen levels to very low or undetectable levels. For example, a filtration step (i.e., microfiltration, ultrafiltration, multimedia filtration) or reverse osmosis may be combined with disinfection. Addition of further treatment steps however, significantly increases the cost and complexity of the process.

Table 3: Removal Efficiencies of Excreted Microbes

Treatment Process Removal (log10 Units)
Bacteria Helminths Protozoa Viruses
Primary 0-1 0-2 0-1 0-1
Primary + chemical coagulation 1-2 1-3 0-1 0-1
Secondary Treatments
Activated sludge + 2B sedimentation 0-2 0-2 0-1 0-1
Biofiltration 0-2 0-2 0-1 0-1
Aerated lagoon + settling pond 1-2 1-3 0-1 1-2
Oxidation ditch 1-2 0-2 0-1 1-2
Waste stabilisation ponds 1-6 1-3 1-4 1-4
Effluent storage reservoirs 1-6 1-3 1-4 1-4
Tertiary Treatments
Chlorination (free) 2-6 0-1 0-3 0-4
Combined chlorine 1-2 ND 1-2 1-2
Ozone disinfection 2-6 0-1 1-2 0-4
UV disinfection 2-6 ND 1-3 2-4
ND= No Data
Sources: Feachem et al., 1983; Rose et al., 1996; Rose et al., 1997; National Research Council, 1998; Karimi, et al., 1999; Clancy et al., 1998; Lazarova et al., 2000; Sobsey, 1989

6.2 Treatment of excreta and sludge

Sludge that is produced from municipal wastewater treatment processes also must be treated carefully before it can be safely disposed of or used as a fertiliser. Many of the organic and inorganic chemicals as well as the pathogens are concentrated in the sludge. When industrial wastewater is mixed with municipal wastewater high concentrations of toxic chemicals may be found in the sludge turning it into a hazardous waste that requires special disposal to limit adverse health and environmental effects. Additionally, toxic chemicals can interfere with the biological processes that are used to treat the wastes thus reducing the effectiveness of the treatment. Therefore, it is imperative to separate toxic industrial wastes from municipal wastes before treatment to preserve sludge reuse options and protect public health.

Helminth eggs are extremely durable and can remain viable for long periods of time. Therefore, excreta and sludge treatments usually require storage periods for many months to ensure helminth egg inactivation. Storage times can be reduced when the excreta and sludge are allowed to digest at higher temperatures. WHO recommends each of the following procedures for the treatment of sludge, excreta, and nightsoil to minimise adverse health effects prior to sludge disposal or use of sludge or excreta as a fertiliser (Mara and Cairncross 1989, Helmer and Hespanhol, 1997):

  • Storage for 12 months at ambient temperatures (warm climates), or
  • Batch thermophilic anaerobic digestion at 50 - 55°C for 13 days, or
  • Forced-aeration co-composting of excreta with domestic refuse for one month (temperature should rise to 55 - 60°C), followed by 2 - 4 months of maturation, or
  • Raw sludge and excreta can be safely applied to the land by subsurface injection, or placed in trenches and covered with at least 25cm of soil before crops are grown (where soil conditions and the depth to the water table permit), or
  • Liquid nightsoil can be stored for one week and applied to land but the settled sludge which may contain high concentrations of helminth eggs should be stored for at least a year or digested as above.

6.3 Hygiene

Improving personal hygiene generally requires major behavioural changes (and an adequate supply of water), but is one of the most important steps in minimising adverse health effects from municipal wastewater and excreta. Numerous epidemiological studies have documented the importance of washing hands with soap and water. This act alone, can cut the transmission of diarrhoeal disease by a third (WHO, 1993b; Esrey et al., 1985; Esrey and Habicht, 1986; Huttly, 1990; Esrey et al., 1991). Additionally, it is critically important to dispose of faeces, particularly children's faeces, in a safe manner. Children frequently are the victims of diarrhoeal disease and other faecally/orally transmitted illnesses, and thus may act as sources of pathogens. Getting children to use sanitation facilities (or designing child-friendly toilets), and implementing school sanitation programmes, are important interventions for reducing the spread of disease associated with waste and excreta (WHO, 1993b).

7. Conclusion

Poor wastewater and excreta disposal continues to be responsible for a significant proportion of the world's infectious disease burden. This burden is not distributed equally, waterborne illnesses predominantly affect the poor and the young. However, when basic water, sanitation, and hygiene interventions are applied, waterborne illnesses can be effectively reduced. Moreover, low cost interventions can be implemented to reduce the transmission of many diseases. For example, the safety of drinking water can be improved by protecting sources from faecal contamination and disinfection. Wastewater and excreta can be treated in waste stabilization ponds to reduce pathogen concentrations and facilitate the reuse of nutrients. Hygiene education and the promotion of hand-washing is also extremely effective in reducing faecal-oral disease transmission. As poor countries industrialize, it is important to minimize the production of toxic chemicals and to treat industrial wastes separately when high concentrations of dangerous chemicals are present.

Reducing the adverse health effects associated with wastewater, sludge and excreta is possible but takes sustained effort at the individual, community and national levels. Additionally more emphasis must be found on finding sustainable approaches for reducing health hazards associated with wastewater, sludge and excreta, and at the same time, closing the nutrient cycle and protecting limited fresh water sources and the environment.

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