Ecological sanitation

Introduction

Ecological sanitation can be viewed as a three-step process dealing with human excreta: (1) Containment, (2) Sanitisation, (3) Recycling. The objective is to protect human health and the environment while limiting the use of water in sanitation systems for hand (and anal) washing only and recycling nutrients to help reduce the need for artificial fertilisers in agriculture. An essential step in the process of sanitation is the containment of pathogens that can cause disease. Without containment and sanitisation, a vicious circle develops where the pathogens in excreta are released back into the environment, re-infect people through consumption of contaminated water or food, and are then excreted again, only to begin the cycle over. Ecological sanitation systems are designed around true containment and provide two ways to render human excreta innocuous: dehydration and decomposition. The Ecosan concept is based on following principles:

• Prevent diseases (must be capable of destroying or isolating faecal pathogens)
• Protect the environment (must prevent pollution and conserve valuable water resources)
• Return nutrients (must return plant nutrients to the soil)
• Culturally acceptable (must be aesthetically inoffensive and consistent with cultural and social values)
• Reliable (must be easy to construct and robust enough to be easily maintained in a local context)
• Convenient (must meet the needs of all household members considering gender, age and social status)
• Affordable (must be affordable and accessible)

Ecological sanitation is an alternative to the linear approaches to carry waste (excreta, soapy water, etc.) to water bodies. It is based on an ecosystem approach. The nutrients and organic matter contained in human excreta must be considered as a resource and properly treated for its contribution to food production systems. Systems typically work with urine-diverting dehydration toilets (often with soil-'flush'), flush-urine-diversion toilets and black water systems for example based on the vacuum toilets connected to a biogas plant.

Figure 1 illustrates a possible scenario for closing the nutrients cycles and simultaneously preserving fresh water from pollution. This scenario can be achieved with the application of ecological sanitation, base on ecological principal.

Figure 1 Material flows in ecological sanitation (Source: Otterpohl et al., 1997)

Material flows in domestic wastewater

Different particular wastewater streams are forming the domestic wastewater (Figure 2). The wastewater originating from toilets is called black water and can be further divided into yellow water (Urine with or without flush water) and brown water (Toilet wastewater without urine). Additionally, grey water is that part of domestic wastewater which originates from kitchen, shower, wash basin and laundry.

The typical characteristics of the streams of domestic wastewater, shown in table 1 clearly show that yellow and brown water contain most of the nutrients discharged to sewers in the conventional sanitation. This means that they are generally wasted instead of being used as fertilizers (except the small portion of nutrients being contained in sludge which is used sometimes as fertiliser after sanitisation).

Figure 2 Domestic wastewater originates from different sources

Table 1 The typical characteristics of the streams of domestic wastewater
(Compiled from: Geigy, Wissenschaftliche Tabellen, Basel 1981, Vol.1, Larsen and Gujer, 1996 and Fittschen and Hahn, 1998)

Due to pathogens, brown water poses high health risk, but it represents a very small volume flow in domestic wastewaters (only 50 litres are excreted per person per year). In conventional systems, this small volume is mixed with other streams of domestic wastewater with higher volume flows: yellow water (tenfold volume flow compared to faeces) and grey water. Grey water volume flows depend on habits. That is why a wide range is given for grey water volume flow: 25,000 to 100,000 litres per person per year. This figure is related to Central European patterns. Of course, also extremely smaller grey water volume flows per person can be found, especially in regions with water scarcity. Additionally toilet flush water has to be taken into consideration (which might be up to 10 litres per toilet use).

It is very impressive that the large volume flow of grey water is accompanied by comparably small nutrient mass flows (about 3 % of the total nitrogen mass flow and 10 % of the total phosphorus mass flow (phosphorus concentration can still be lowered by using phosphorus free detergents) discharged with domestic wastewater). However, about one third of the potassium (which is also important for plant growth and a limited fossil fertiliser component) mass flow of domestic wastewater is contained in grey water. Because of the large volume flow of grey water (compared to yellow and brown water), its potassium concentration is quite low (commonly below 10 mg/l), however. Because of its low contribution to mass flow of the nitrogen and phosphorus in domestic wastewater and its high volume flow grey water turns out to belong to the water cycle and represents a splendid source for wastewater reuse. As grey water contains nearly half of the organic load of domestic wastewater, this is the main group of pollutants to be removed from grey water before its eventual reuse. Therefore, treatment of grey water is far cheaper than treatment of total domestic wastewater as there is no need of costly nitrification and denitrification processes mostly practiced in modern municipal wastewater treatment plants.

The scheme clearly demonstrates that greatest part of the nutrients nitrogen, phosphorus and potassium of domestic wastewater are contained in the comparably small volume flow of yellow water. Moreover, urine contains trace metals required for plant growth. Only about 10 % of the organics of domestic wastewater are urine borne. From these reasons, yellow water has to be taken into consideration as fertilizer, and is thus related to the food cycle rather than to the water cycle.

Brown water contributes greatly to the phosphorus load of domestic wastewater and can thus also be considered as fertiliser. Moreover, the organic solids make brown water a splendid candidate as a soil conditioner after suitable treatment. Therefore, also brown water is belonging to the food cycle.

Yellow water as fertiliser

Separate collection of yellow water is possible with urine-diversion (UD) toilet, a suitable technology to separate the urine and faeces at source (See figure 3 and 4). Usually, the toilet has two bowls, the front one for urine and the rear one for faeces. Each bowl has its own outlet from where the respective flow is piped out. The flush for the urine bowl needs little water (0.2 l per flush) or no water at all whereas flushing water for faeces bowl can be adjusted to the required amount (about 4 to 6 l per flush) or no water in case of Urine-diverting dehydration/composting toilets.

Among the flows of wastewater, yellow water contains most of the nutrients. One person produces on average 3.92 kg of nitrogen, 0.38 kg of phosphorous and 0.97 kg of potassium per year. These nutrients, such as nitrogen in the form of urea, phosphorus as super phosphate and potassium as an ion, are in a form which are ideal for uptake by plants. Beneficially, urine contains very low levels of heavy metals and pathogens. These heavy metal concentrations are much lower than those of most chemical fertiliser. In Sweden, for instance, urine contains less than 3.2 mg cadmium per kg of phosphorus compared to 26 mg Cd/kg of phosphorus in commercial fertiliser and 55 mg Cd/kg of phosphorous in sludge.

Figure 3 Urine Diversion (UD) Toilet(Source: Ruediger)

Figure 4 Double-vault UD toilet(Source: Esrey et al., 1998)

Because of the following reasons wasting of yellow water is not sustainable: 1st: Production of fertiliser is energy intensive, draws on very limited fossil resources and causes environmental problems. Generating nitrogen (N) from air requires a considerable amount of energy. Mining and refining the raw materials for phosphate production generates a huge amount of hazardous wastes. Reserves of phosphate (P) and potassium (K) are definitely limited on a time scale of only a couple of human generations especially with regard to economic constraints. Moreover, also sulphur is a limited fossil resource and is required by plants. Also sulphur is contained in yellow water. Therefore, we should use yellow water as fertiliser which contains reasonable amounts of these nutrients.

2nd: If yellow water is wasted (i.e. it is added to municipal wastewater), it will either contribute to eutrophication of surface waters because of its nutrient content or it will even lead to groundwater contamination (high nitrate concentrations can be generated by transformation of ammonia, and sometimes high nitrate concentrations are detected in ground waters contaminated with domestic wastewater - they can lead to lethal methemo¬globinemia of babies drinking water from such contaminated sources).

3rd: Even if domestic wastewater is treated in modern treatment plants, adding yellow water to wastewater is disadvantageous. Residual nitrogen compounds (mainly nitrate) escape with the effluents even of very efficient wastewater treatment plants and contribute to surface water eutrophication. But the main disadvantage of adding yellow water to municipal wastewater is that a great deal of energy consumption for operating activated sludge tanks is required for ammonia removal (aeration for nitrification). Removal of phosphate from municipal wastewater requires an additional biological stage and/or precipitation stages (addition of iron or aluminium salts). It is assumed that nutrient removal requires about 50 % of energy consumption in German wastewater treatment plants. It is clear that production of electric power is contributing to the greenhouse effect and leads to emission of air pollutants.

4th: Because of its content of nutrients, yellow water can substitute a reasonable amount of synthetic fertilisers. It is assumed that about 50 % of crops needed by human beings can be fertilised with human excreta. When yellow water is wasted, this means additional energy consumption for fertilizer production.

Brown water as soil conditioner

Soil degradation caused by human activities is alarming worldwide. The main causes of soil degradation are: erosion, fertility decline, overcropping and use of synthetic fertiliser. Since synthetic fertiliser does not contain organic matter which prevents soil erosion, reuse of brown water as soil conditioner plays important role to reduce the soil degradation as brown water contains most of the organic solids in domestic wastewater.

Like yellow water, separate collection of brown water is possible with the Urine-diversion toilet (See figure 3 and 4). As faeces are the predominant source of pathogens of all streams of domestic wastewater, they have to be sanitised prior to their use as soil conditioner. However, killing of pathogens is facilitated when faeces are collected separately. Possible treatment techniques are dehydration (utilization of solar energy) or composting/vermicomposting. Together with organic solids helping to prevent soils from desertification, also nutrients are transferred to agricultural fields with processed brown water.

Source separation of different particular domestic wastewater flows (related to the nutrient as well as to the water cycle) facilitates sanitisation of human excreta as well as nutrient recovery from excreta – and also purification of the grey water stream.

Composting and dehydration toilet systems

In composting toilet systems either a toilet with urine diversion or no urine diversion is used. In case of no urine diversion toilet, faeces and urine with or without toilet paper depending on the user’s habit drop into the composter located just below the toilet. In urine diversion toilet, urine is collected separately and kept in a storage tank until it is ready for use in agriculture or just treated in a soak pit or evapotranspiration bed. However, leaching from soak pit can cause heavy groundwater pollution. Faecal materials are composted or dehydrated for a long time and reused as fertiliser and soil conditioner in agriculture.

Urine diversion is crucial for dehydration toilets. The non-diluted faecal materials (faeces and toilet paper if used) drop into the dehydrating vault located just below the toilet and are dehydrated with the help of heat (Solar), ventilation and the addition of dry materials.

Double-Vault dehydrating toilets have been widely used in many parts of the world. In this toilet, urine is diverted to a collection tank or soak pit under the toilet vault or outside the toilet and faeces drop into one of the two vaults located below the toilet’s seat. When one vault is full, it is sealed and another vault is used. Dry materials like ash or soil or a mixture of sawdust/lime or soil/lime are added after the defecation. The added dry material assists the desiccation process and raises the pH, which aids in pathogen reduction.