Urine Storage

On average, a human being discharges about 50 L of faeces, 500 L of urine and produces up to 100 000 L of greywater every year depending on diet and personal water availability and water use.

The actual amount of human excreta and the nutrient content depends on the diet and varies between countries as well as between individuals.

Excreta contain all essential micronutrients and an average amount of plant available macronutrients of 4.5 kg/person/year (kg/p/a) for nitrogen, 0.6 kg/p/a for phosphorus and 1.2 kg/p/a for potassium (SUSANA 2009). Faeces are by far the fraction that contains most of the pathogens (TETTEY-LOWOR n.y.). Urine is virtually sterile. It makes up less than 0.5 % of the household wastewater (TETTEY-LOWOR n.y.), but contains most of the essential plant nutrients, accounting for about 80 % of the nitrogen (N), 50% of the phosphorous (P) and 60% of the potassium (K) from all nutrients excreted per year per person (JOHANSSON 2000). P and K from urine are generally found as inorganic ions, which are directly plant available. N form urine is transformed into ammonium during storage. Hence, urine has properties similar to commercially available chemical fertilisers.

Fertiliser value of urine and faeces excreted per person per year in different countries (using FAO data). Source: JOENSSEN et al. 2004)

In conventional toilets, urine is often flushed down the pan together with the faeces and flushing water leading up to 20 000 L per person per year of severely polluted wastewater. This is based on a European average for a toilet that uses about 8 L of water per flush. However there are toilets (mainly in North America) that use up to 23 L (5 gallons) just for one flush what leads to almost 50 000 L polluted fresh water per person and year.
Treatment of wastewater, in order to prevent pollution of the environment, requires large infrastructures and investments. Phosphorus, nitrogen and other nutrients can be recovered during the treatment process, but operation requires a lot of energy and therefore remains very expensive. Most often nutrients removed during the treatment process are lost to the air (in the case of nitrogen), or end up in the excess sludge and are dumped in sanitary landfills, incinerated, etc. but usually not recycled to agriculture.

To increase sustainability (recycle nutrients, decrease wastewater effluents etc.), alternatives to conventional wastewater treatment have been suggested and the aim is often to reuse the plant nutrients from excreta (urine and faeces) as a fertiliser (SCHOENNING n.y.). One concept is source-separating sanitation systems which include blackwater systems, where the wastewater from toilets is treated separately from the greywater produced in the kitchen, bathroom, etc.), urine-diverting systems with separate handling of urine and different types of dry systems where human excreta is handled without the use of flush water (SCHOENNING n.y.). The aim of urine diversion is to collect urine for use as a fertiliser in agriculture and eliminate the eutrophicating discharge of nutrients into surface waters (WHO 2006 Vol. IV). Urine diversion may be practised using both composting and dehydration toilets. When flush water is used, the remaining brownwater can be mixed with organic household waste and treated by liquid composting or digestion (TETTEY-LOWOR n.y.). In dry systems, faeces can be composted or dehydrated (see also anaerobic digestion, biogas digester, biogas settler, composting small and large scale, storage and dehydration of faeces).

Virtually every toilet system can be adapted for urine diversion using a urine diversion toilet pan (see also urine diversion components) and separate collection and handling (see also urine diversion dehydration toilets, urine diversion flush toilet, terra preta toilet, etc). An easy way to collect the urine is also to install urinals (see also urinals). Urine can be collected “dry” (without flushing water), but there are also water-reliant systems (using flushing water for the urine). Flushing urine results in a dilution of the urine with water leading to enhanced precipitation and clogging of collection pipes due to reaction of the minerals contained in the water with the urine (see below). Furthermore, the treatment is more difficult: Not only are nutrients more diluted, but also pathogen die-off is reduced due to a lower rise in pH and ammonium formation (see below).

Once collected, the urine can either be used directly in the garden or on the field, infiltrated into an evapotranspiration bed, stored on-site or off-site for later use as a liquid fertiliser or further processed into a dry powder fertiliser (WINBLAD & SIMPSON-HERBERT 2004) (see also urine desiccation, struvite precipitation). See use of urine (small-scale and large-scale) to learn more about the application to crops.

For single households, the urine can be used without storage for all types of crops, provided that the crop is intended for the household’s own consumption and that one month passes between fertilising and harvesting, i.e. time between last urine application and consumption (SCHOENNING & STENSTROEM, 2004). Direct use or short storage periods are also applicable for small domestic systems in developing countries, where in addition higher ambient temperature will also increase inactivation rates and safety (SCHOENNING & STENSTROEM, 2004). If urine is contaminated (i.e. by cross contamination), it needs to be treated. For urine coming from larger systems (community level), cross contamination with faeces cannot be ruled out and treatment is thus obligatory (see also DAGERSKOG (2010) and KASSA (2010) in ECOSAN CLUB (2010)).

Human urine does not generally contain pathogens that will be transmitted through the environment (HOEGLUND 2001). The hygiene risks associated with diverted urine are mainly a result of contamination by faeces (JOENSSON et al. 2004). The simplest, cheapest and most common method to treat urine with the aim of killing pathogens and preserving nutrients is extended storage in storage tanks (GTZ 2009).

The pathogen removal is based on a combination of increased pH, ammonia concentrations, temperature and time.

During storage, pH raises form around 6 to 9 due to the decomposition of urea into ammonia/ammonium (NH4+/NH3) and hydrocarbonate. This is facilitated by the natural enzyme urease present in the urine. At this high pH (which means that the solution is alkaline) bacteria, protozoa, viruses and intestinal helminths die off over time (GTZ 2009). An environment with high temperature enhances this effect. Time itself also leads to pathogen kill (GTZ 2009). And further inactivation of pathogens is expected in the soil after application (e.g. by predation, solar radiation) and the risk for infection by ingestion of crop will be reduced during the time between fertilisation and consumption (HOEGLUND 2001).

Average chemical composition of fresh urine (literature values) and stored urine (simulation). Source: GTZ (2009)

The high pH of the stored urine not only influences the pathogen die-off, but also induces precipitation (the formation of a bottom sludge). The main precipitate from urine with high pH is struvite (MgNH4PO4) and calcium phosphate (Ca10(PO4)6(OH)2) crystals (GTZ 2009). They are formed because at high pH, the initial concentrations of phosphate, magnesium, calcium and ammonium are no longer soluble (JOENSSEN et al. 2004). In additions, skin cells, hair and excreted organic complexes will also settle (GTZ 2009). The end result may be hard precipitates or soft, viscous, past-like precipitates. Incrustations tend to occur on the inner walls of pipes and pipe bends. Soft deposits occur in storage tanks (where they form a sludge at the bottom of the tank) and in near horizontal urine pipes. Precipitation in urine pipes and storage tanks occurs in both water-flushed and waterless systems. In waterless systems, more soft deposits tend to occur (GTZ 2009), while in flushed systems, more hard deposits tend to occur due to the mineral (magnesium) contained in the water. In waterless systems, up to 30% of the phosphorus contained in urine can be transformed into sludge, which forms as a bottom layer in the collection tank provided the collection pipes have a slope of at least 1 % and are wide enough (> 75 mm) (JOENSSON et al. 2004). The sludge can be used for crops with high P demands or handled with the rest of the urine (JOENSSON et al. 2004). In the latter case, the sludge should be mixed before spreading to get an even dosage (JOESSON et al. 2004).

The high pH of the urine coupled with its high ammonium concentration also means that there is a risk of losing N in the form of ammonia with the ventilated air (JOENSSEN et al. 2004), because at high pH ammonium (NH₄⁺) transforms into ammonia (NH₃), which is highly volatile. However, these losses are easily eliminated by designing the system in such a way that the tank and pipes are not ventilated, but just pressure equalised with the air (JOENSSEN et al. 2004). This also eliminates the risk of bad odours from the urine system (JOENSSEN et al. 2004).

Safety recommendations for urine storage are mainly aimed at reducing the risks from consuming urine-fertilised crops. It will also reduce the risk for the persons handling and applying urine (SCHOENING & STENSTROEM 2004).

Urine from community level and larger (urban) systems always needs to be sanitised (stored) because cross-contamination can never be ruled out. A larger system means that crops are not consumed by the households from which urine has been collected (WHO 2006 Vol. IV), i.e., there is a risk that they are exposed to pathogens to which they have not been exposed before. For these systems, the recommended storage time at 4 to 20°C is between one and six months, depending on the type of crop to be fertilised (SCHOENNING & STENSTROEM 2004). Generally, the WHO guidelines (table below) can be adopted for larger (urban) systems in developing countries if the withholding time of one month between fertilisation and harvest is adhered to (SCHOENNING & STENSTROEM 2004). For urine that is significantly contaminated, a longer storage time and/or a higher temperature is recommended and specific recommendations for large-scale systems may need to be adapted based on local conditions, accounting for behavioural factors and the technical systems selected (e.g. mismanagement) (WHO 2006 Vol. IV).

If a family uses its own urine, the risk of disease transmission via fertilisation and crops is very low — the risk that diseases are transmitted directly, e.g. by handshaking, coughing or by improper hygiene behaviours is much higher.

Recommenced storage times for urine mixture (urine and water, a pH of at least 8.8 and a nitrogen concentration of at least 1 g/L is assumed) based on estimated pathogen content (gram-positive bacteria and spore-forming bacteria are not included in the underlying risk assessments, but are not normally recognised as causing any of the infections of concern) and recommended crop for larger systems (where crops will be consumed by individuals other than members of the household from which the urine was collected). Source: WHO (2006 Vol. IV)

During storage the urine should be contained in a sealed tank or container. This prevents humans and animals coming into contact with the urine and hinders evaporation of ammonia, thus decreasing the risk of odour and loss of plant-available nitrogen (SCHOENNING & STENSTROEM 2004).

To avoid faecal contamination, special precautions must be taken during instances of diarrhoea and when children or unaccustomed adults use the toilet (HOEGLUND 2001).

Protection (e.g. wearing gloves) and awareness of risks is important, especially for those handling un-stored urine at the household level (HOEGLUND 2001). Using suitable fertilising techniques and working the urine into the soil (see also use of hygienised urine in agriculture, small scale and large scale), as well as letting some time (at best one month) pass between fertilisation and harvesting will decrease the exposure of humans and animals to potential pathogens (HOEGLUND 2001).

For storage, urine should preferably not be diluted. Concentrated urine provides a harsher environment for microorganisms, increases the die-off rate of pathogens and prevents breeding of mosquitoes (SCHOENNING & STENSTROEM 2004). Thus, the less water that dilutes the urine the better (SCHOENNING & STENSTROEM 2004).

Urine is very corrosive and therefore tanks should be made of resistant material, e.g. plastic or high quality concrete, while metals should be avoided (JOENSSEN et al. 2004).

Urine storage is the cheapest option of urine treatment. However, the costs for land and storage tanks can be significant.