Tier 1: Assessment Using Emission Factors

In more heterogeneous and larger catchments or in situations with enhanced wastewater disposal, the main sources of nutrients in a catchment might not be obvious, nor may be the pathways by which they enter the river system. In such a setting, the tier 1 approach based on emission factors (also called “export coefficient method”) is helpful to obtain a first semiquantita- tive overview of the contribution of main sources of nutrient emissions to a waterbody - namely, municipalities (including households and industries) and agriculture.

Municipalities

As discussed in section 7.3.3, phosphorous is an essential nutrient in human nutrition and human excreta, which therefore are the dominating source of P in municipal wastewater, and other significant sources may be P from detergents or commercial and industrial wastewater, particularly from food processing enterprises or fertiliser industry. An estimation of P emissions from municipalities (which includes wastewater from households, commerce and industries) is possible on the basis of the following information:

• • the number of people in the catchment, with 1.3-1.7g P emitted per person and day (Zessner & Lindtner, 2005);
• • whether or not detergents used locally are P free (i.e., which type is typically sold on local markets), with emissions per person amounting to 2-3 g if detergents contain P;
• • the share of the population connected to sewer systems;
• • the estimated amount of commerce and industry likely to emit P;
• • the level of wastewater treatment (no treatment, biological treatment with removal only of organic substance [“C removal”], or treatment with P removal/P precipitation).

In municipalities without any significant commercial and industrial activity, P in wastewater predominantly originates form households. In case of high commercial and industrial activities, experience shows that those activities may increase P loads in wastewater by up to 1.5g per person and day (Zessner & Lindtner, 2005), and this may serve as first rough estimate. Any large-scale fertiliser or food processing industries are not included in this number and would have to be accounted separately.

The impact of populations not connected to sewer systems on P emissions is highly dependent on their sanitation system. Where this is through subsurface or soil treatment, the impact will usually be small and can be neglected for a first estimate. Where household wastewater is directly discharged into a river or its tributary, the total load from the population would have to be accounted as emission into the surface waters.

Wastewater collected in sewers is usually discharged to surface waters, with P emissions depending on the level of treatment. Emissions are:

• • 100% of P in raw wastewater in case of no treatment;
• • 60-70% of the P concentration in raw wastewater in case of biological treatment without P removal;
• • 10-20% of the P concentration in raw wastewater in case of biological treatment with P precipitation or biological P removal (with further reduction by a factor of up to 10 if a combination of postprecipitation and filtration step is added after conventional treatment) (Heinzmann & Chorus, 1994).

Example: There are four municipalities in a catchment. The first is a town and has 50 000 inhabitants (inh), all connected to the public sewer system, an average amount of commercial and industrial activity and a wastewater treatment plant, including biological C removal without P removal. The markets in town sell P-free detergents for washing laundry and dishes. The other three municipalities are small settlements with all together 1500 inhabitants not connected to sewer systems and buying in the markets of the town.

P emissions from the town to surface waters=(1.5g P/(inhxd) [from households] + 0.75 g P/(inhxd) [from commercial activ- ity])x50 000 inhx0.65 (P in effluent after treatment) = 73kg P/d=2.7t P/yr.

P in wastewater from settlements=1.5g P/(inhxd)x 1500 inh=2.3kg P/d=0.8t P/yr. The fate of this P load is not known. Catchment inspection would serve to collect evidence whether wastewater disposal from these settlements could directly seep into surface waters of the catchment.

Agriculture

The main external pathway by which nutrients reach a farm is the input of external (mineral) fertiliser and the input of feedstuff for livestock. Farm animals process their feed, excreting faeces and urine which is then spread as manure and slurry on fields. Nutrient exports from the farm as loads to a waterbody can ideally be avoided if feed and manure are kept in an internal on-farm cycle, with the phosphorus content of the agricultural products (as output of P from the agricultural production process) approximately balancing the P input through externally imported fertiliser and feedstuff. While this situation appears idealistic, agricultural nutrient budgets (see Box 7.3) have indeed proven to be a highly effective approach to controlling nutrient loads (see section 7.5.2).

Soil is the essential medium in this production process as it provides the nutrients to the plants for their growth. As discussed above, during the production process, nutrients not taken up by the plants may be transported from agricultural areas to the waterbody, and while nitrogen in form of nitrate is very soluble, P is usually adsorbed to a high extent to soil particles, if supplied to soil in higher amount as needed by the plants. This may lead to increased concentrations of P in agricultural soils, and if erosion occurs, this can transport P together with soil particles to surface waters. Depending on soil properties and soil saturation with P, it might also be transported in soluble form and reach surface waters with surface run-off, tile drainages, interflow or groundwater, but in most settings, particulate transport with erosion dominates. Losses of P from agricultural soil are impacted by many factors, and the load emitted into surface waters is determined by the concentrations of P in soils and the amount of soil mobilised from the field that reaches the waterbody. Because of the high numbers of factors that determine this process, even rough estimates of P emissions from agricultural soils to waterbodies are less straightforward than for wastewater from municipalities. Nonetheless, a first rough tier 1 estimate based on emission factors is possible for this source as well. Where more precise quantification is needed, more elaborated quantifications (tier 3) are recommended in cooperation with modelling specialists. For this tier 1 approach, the following information needs to be known:

• • arable area in the catchment;
• • basic information on slopes of land used for agriculture;
• • connectivity of arable land to the waterbody or its tributaries (ranging from “well connected” to “poorly connected”, e.g., due to the interception of erosion through natural vegetation or buffer strips);
• • plant cover of crops during rainy seasons (missing to high);
• • soil properties (clay, silt, loam, sand);
• • density of livestock, particularly cattle, and fertilisation level (high to none);
• • erosion abatement in place (high to none).

Inputs of P from the catchment to the waterbody range from about 0.1-0.2kg P/(haxyr) from areas with dense perennial vegetation cover up to 5.0 kg P/(haxyr) from arable land (Franke et al., 2013). The highest values can be expected if high P concentrations in soils occur in situations with pronounced soil erosion and the eroded soil is easily transported to the surface waters. High P concentrations can be expected if agricultural management is characterised by high livestock densities and/or fertilisation levels exceeding plant requirements resulting in high P surpluses. P surpluses in soils can be estimated via soil nutrient budgets from the amount of nutrient in fertiliser, manure and slurry spread on the fields and pastures in relation to the amount in the harvests leaving the farm (for further information and data, see EUROSTAT (2019) and FAOSTAT (2019)). With clay/silty soils, the concentrations in eroded soil material may be further enriched as fine particles usually have the highest concentrations and are predominately transported.

Several local factors determine the levels of soil erosion. Firstly, soil erosion is impacted by the energy with which raindrops mobilise soil particles when they reach the surface. This is especially high if the crops grown have a low plant cover during rainy season (which is often the case for, e.g., maize, soya bean) and in regions with high rainfall intensity (volume per area and time). Secondly, the slope of a field and its length determine the transport capacity of water during surface run-off. Therefore, soil erosion increases at fields with steep and long slopes. Clayey/silty soils are especially vulnerable against soil erosion as small particles are more readily mobilised and transported as larger particles from sandy soils. Further, high organic (humus) content of the soil and improved soil structure reduce credibility. If specific erosion abatement measures are in place, erosion is reduced.

Winter crops and mulch seed, for instance, increase the coverage of soils and thus hinder rain to mobilise and transport soil particles. High transport to surface waters of eroded material can be expected if the fields where erosion takes place are well connected to surface waters (high connectivity): run-off from such fields may directly enter surface water because there are no other types of land uses (e.g., buffer stripes) between them and the river is not able to hinder the transport of soil particles.

Example: A catchment of 50 km² has a share of 30% arable land. The amount of fertiliser applied is in a medium range; the region is hilly with a significant share of steep slopes and pronounced connections between arable land and creeks of the catchment. Silty soils prevail; no specific erosion abatement measures are in place. About 35 km² in the catchment is covered with natural vegetation or grassland. Input into surface waters can be assumed to be at the low end of the range given above, that is, 0.1-0.2 kg P/(haxyr) from these areas as soil loss from these areas and P content of soil material are usually low:

• • P emissions form naturally covered land and grassland =
• 0.1-0.2kg P/(haxyr)x3500 ha=0.35-0.701 P/yr.

About 15 km² are covered with arable land. We assume relatively high levels of P emissions due to unfavourable conditions with respect to erosion (high soil loss) but only average fertilisation levels (moderate P concentrations in soils) of 1.5-3.0kg P/(haxyr):

• P emissions form arable land = 1.5-3.0 kg P/(haxyr) x 1500 ha = 2.25-4.501 P/yr.

Franke et al. (2013) present a more elaborate tier 1 approach for nutrient emissions from agricultural fields in the context of grey water footprint calculations. This can be applied if fertilisation levels are known. If requirements for the quantitative assessment of nutrient emissions are higher, the higher-level tiers discussed below should be applied. These tiers require including experts in the field of nutrient monitoring and nutrient emission modelling in the planning team.