2 Resource efficiency in sanitation
2.2 Criteria for resource efficiency assessments of sanitation systems
2.4.2 Nutrients in wastewater flows
Nutrient uptake is essential for human living, yet, almost all the nutrients that are taken up are excreted in urine or faeces22. In addition, detergents, personal care products, foodstuff, etc. are sources of nutrient discharges into wastewater. Nutrients in
20 Income growth, urbanisation and shift in consumer preferences result in increased meat demand, which is expected to rise by more than 50% by 2030 (FAO, 2008).
21 Estimations of anticipated fertiliser consumption for bio‐fuel production are variable but go as high as
28% in 2011 (Cassman et al., 2006 cited in FAO, 2008).
22 Only children accumulate a few percent of the nutrients consumed in their body (Jönsson and Vinneras, 2004).
wastewater are considered in this study from two perspectives. First, nitrogen and phosphorus can lead to pollution of water sources. Therefore, emissions e.g. from combined sewer overflows or from wastewater treatment plant outlets, need to be avoided. In this context, it needs to be kept in mind that nutrient elimination is one of the most energy‐consuming steps in wastewater treatment. Second, nutrients in wastewater represent a potential source of nutrient supply for agricultural purposes and can therefore contribute to mineral fertiliser replacement.
The nutrient content of wastewater flows is rather variable. For example, the nutrient load in excreta and the allocation to the respective flows, i.e. faeces and urine, depend on the kind of nutrition and the digestibility of the food. Furthermore, most data on nutrients in wastewater are only available for mixed wastewater and not for source separated flows. Literature reviews on nutrient composition of source separated flows were done by e.g. Henze (1997), Herrmann and Klaus (1997), Londong and Hartmann (2006) as well as Niederste‐Hollenberg and Otterpohl (2000). Each of these reviews however, are based on a maximum number of ten values per parameter and all authors conclude that the database needs to be increased in order to eliminate the effect of dissimilarities. Therefore, this study uses average values derived by a review done by the author for the task group on new sanitation concepts of the German Water Association (DWA) (Meinzinger and Oldenburg, 2009; see also Oldenburg et al., 2008b).
The review included more than 200 European references on specific nutrient contents of wastewater and organic waste flows. The results of this review are shown in Figure 2.4 and Annex B.
The highest ratio of the nutrients N, P and K can be found in urine. Up to 80% of the daily load of nitrogen and about 50% of phosphorus and potassium are present in urine (see Figure 2.4). This suggests various benefits regarding the separation of this flow. On the one hand, urine separation can be useful in terms of reduced nutrient loads to the wastewater treatment plant or for selective inflow to the treatment plant to reduce nutrient peak flows. On the other hand, separated urine represents a flow particularly suitable for the provision of plant nutrients. Other than phosphorus, faeces contain relatively low nutrient loads. Nevertheless, this flow and its separation from the total wastewater flow can play an important role in terms of energy provision due to its high content of organic matter (see Section 2.5) and can be considered for provision of plant nutrients in combination with urine. Greywater contains relatively high loads of sulphur, but also nitrogen, phosphorus and potassium are present. Greywater characteristics are highly dependent on the habits of the users, appliances in the household (e.g. dishwasher) and the detergents that are being used. This impact can be observed, for example, by looking at phosphorus contents in greywater. The last decades have seen a general decrease in average P contents in domestic wastewater in Europe due to the ban on phosphates in washing agents in many countries. Nowadays
phosphates in detergents are replaced by organic phosphorus ingredients. However, in dishwashing detergents phosphates cannot be replaced yet. The increased use of dishwashing agents is offsetting the reduction due to detergents to the point where phosphorus loads in greywater are again increasing. Londong and Hartmann (2006) showed that the dishwashing agents in a household with three members can contribute to a daily load of about 0.4 g P per person.
Figure 2.4: Nutrient distribution and specific loads [g p-1 d-1] in urine, faeces and greywater (Source: compiled by the author)
The characteristic values derived above are based only on European references and reflect the nutritional habits and water‐use patterns applicable to this region. Due to this dependency the context is important and for regions other than Europe representative values for such regions should be used. For regions like most of Africa, data relating to household nutrition and water usage is scarce. Jönsson et al. (2004) developed a method to calculate the expected nutrient content in excreta based on national nutritional data (i.e. food protein content), which is published by the Food and Agriculture Organization (FAO)23. This method is used to estimate the nutrient content in faeces and urine in the case of Arba Minch according to the following formulae (Jönsson et al., 2004)24:
N = 0.13 * total food protein
23 National food balance sheets can be found on http://faostat.fao.org/site/368/default.aspx#ancor (last access on 9 September 2009).
24 Comparing the calculated nutrient contents derived from FAO data for Germany with average data in
Figure 2.4 results in differences of 16%, 3% and 39% for N, P and K respectively.
P = 0.011 * (total food protein + vegetal food protein) K = 0.015 * (total food protein + 4.6 * vegetal food protein)
Based on data from FAO (2009b) on food protein content in Ethiopia25, nitrogen, phosphorus and potassium loads are calculated to be 6.9 gN p‐1 d‐1, 1.1 gP p‐1 d‐1 and 4.1 gK p‐1 d‐1 in urine and faeces together. The digestibility of the food affects the distribution of nutrients between urine and faeces. More processed and therefore easier digestible food such as consumed in the European diet, generally results in more nutrients being excreted via urine (Jönsson and Vinneras, 2004). Therefore, it can be expected that the ratio of nutrients excreted via faeces is higher in Ethiopia than in Europe. But for the purpose of this study the same distribution as derived from the European references are used. This means that 85% of N, 63% of P and 77% of K are found in urine. Also greywater characteristic values differ significantly in the African context compared to European data, since water scarcity often leads to multiple uses and therefore higher pollutant concentrations. As stated previously, for many regions in Africa, which includes Ethiopia, little data is available. In general, very high and usually very variable organic matter concentrations can be observed, e.g. with COD values reported to be as high as 8000 mg l‐1 (Raude et al., 2009). The same study, which was carried out in a location in Kenya similar to Arba Minch in Ethiopia, reports about variable nitrogen and phosphorus concentrations ranging from 2 to 340 mg l‐1 and 1 to 13 mg l‐1 respectively. With a reported per capita water consumption of 10 l p‐1 d‐1 in this study, this translates to 0.02‐3.4 gN p‐1 d‐1 and 0.01‐0.1 gP p‐1 d‐1.
Theoretically, the nutrients in excreta of the world population would be sufficient to replace roughly about a quarter of the global N fertiliser consumption, a fifth of the P fertiliser consumption, and a third of the K fertiliser consumption. Of course, this calculation is rather hypothetical, since the population distribution is not equal to the distribution of fertiliser use and it does not seem realistic to recover 100% of all nutrients from urine and faeces. These considerations also show that excreta‐based nutrient content and particularly the nutrient content of single flows such as urine do not show the same proportion of nutrients as used in mineral fertiliser. Therefore, site‐
specific supplemental application of particular nutrients could be required in addition to the use of nutrients recovered from human waste flows.
In the next sections possibilities for nutrient recovery from mixed and source separated wastewater are introduced.
25 In 2003, the total food protein consumption in Ethiopia was 53.41 g p‐1 d‐1 and the vegetal protein consumption was 47.14 g p‐1 d‐1 (FAO, 2009b).