3 Method and case studies
4.1 Process descriptions
4.2.1 Nutrient emissions to the environment
Nutrient emissions to the environment are included in the model as emissions to soil/groundwater, and to surface water, and as gaseous emissions in the case of nitrogen and sulphur. Groundwater contamination can result from infiltration of wastewater from sewers into the ground, infiltration of rainwater runoff from surface areas and infiltration from agricultural areas. Nutrient flows to surface water are from various sources, namely rainwater from separate sewer systems, wastewater from combined sewer overflow, effluent of the wastewater treatment plant85, rainwater runoff from surface areas, erosion and runoff from agricultural areas. Gaseous emissions can occur
85 According to UBA (2003) nitrogen emissions from German wastewater treatment plants and sewers contribute 17% and 4% respectively of the total emissions into surface waters. Regarding phosphorus these emissions are about 35% and 10% respectively.
at the wastewater treatment plant and the composting facilities. Emissions into air are however, beyond the scope of this study and are only briefly discussed here.
Emissions can be used as an indication of potential environmental pollution. However, they can only give an indication of the risk. For example, infiltration of nitrogen into the soil may result in serious groundwater pollution. If however, the distance to the aquifer is sufficient and intervening soil properties are favourable, this risk might be minimal.
Although considerations relating to risk assessment have not been included in this study, total loads are used as indication.
Nitrogen
Figure 4.9 illustrates the total nitrogen emissions to groundwater and surface water from the water‐system related processes of the six investigated systems. The total emissions to groundwater and to surface water are summarised in Table 4.5. The effluent of the wastewater treatment plant, although adhering to current standards, contributes the largest share of emissions (about 84‐86% of the total discharges to surface waters)86. These emissions can be reduced by 17% (3 NuRU) and 42% (4 CoDig) through the introduction of source separation of urine and blackwater, respectively.
Source separation can in addition reduce pollution from infiltration of wastewater from sewers into the ground, as well as from the combined sewer overflow. If, for example, blackwater is transported in separate vacuum sewers (4 CoDig), nitrogen emissions from both sources can be reduced by 90%. The reduction of emissions as a result of combined sewer overflow has in the past been tackled through an extensive programme implemented by Hamburg Wasser. This reduces the overall emissions into the surface waters; the current increase of intense rain however, is a factor that can in future result in again increased sewer overflows (Kopp, 2007).
Systems 5 BlaD and 6 CompU, which are based on the assumption that the current centralised wastewater system is replaced by decentralised systems, show greatly reduced nitrogen emissions to the environment. Sewer related emissions (infiltration from sewers and sewer overflow) and discharges from the WWTP do not exist in these two systems. Differences in rainwater emissions in 5 BlaD and 6 CompU are also due to other changes in rainwater management like the use of rainwater to replenish process water.
86 The results of the model for discharge parameters N, P and CSB of the WWTP show a general
correlation to real data (e.g. Hamburg Wasser, 2008a). Discrepancies observed are usually due to variations in assumptions, such as not taking industry into account, incorrect nitrogen discharge assumed (13 mgNtot l‐1 instead of the actual 15.4 mgNtot l‐1) or variations in rainwater volume in different years.
Figure 4.9: Total nitrogen emissions to soil/groundwater and surface water from
water-system related sources [tN y-1]
Comparing the pollution from the urban wastewater system to that from agriculture, the large contribution of agricultural nitrogen discharges (e.g. from runoff or erosion) to the overall nitrogen emissions becomes apparent. According to the results of the ceMFA model, the nitrogen infiltration into soil from the total agricultural area amounts to 6200±2300 tN y‐1, and the nitrogen discharge into surface waters from runoff and erosion of farmland is about 4300±1600 tN y‐1. When considering only the administrative borders of Hamburg (i.e. without the hinterland area), agricultural emissions from infiltration and erosion/runoff contribute 235±87 tN y‐1 and 163±61 tN y‐1 respectively.
Table 4.5: Total nitrogen emissions to soil/groundwater and surface water from processes related to the urban water system [tN y-1]
1: CurS 2: NuRS 3: NuRU 4: CoDig 5: BlaD 6: CompU Emissions to soil/
groundwater Emissions to
surface water
1418 fertilisation or from wastewater flows, incineration of sewage sludge, and denitrification of nitrates in the wastewater treatment plant. The latter results in emissions of nitrogen gas (N2), which enters again the nitrogen cycle and does not have any negative impact on the environment. Incineration and composting processes can however, result in the emission of nitrogen in the form of nitrogen oxides, which do
have an environmental impact in terms of greenhouse gas emissions and acid rain.
Nitrogen oxide emissions can be reduced by technical measures. These considerations however, go beyond the scope of this thesis, since no differentiation regarding the form of nitrogen is made, but total nitrogen is referred to.
The error margins of the results are relatively variable; this is particularly so for the nitrogen discharge from the wastewater treatment plant in System 4 CoDig, which seems rather variable (62% relative error). A sensitivity analysis reveals that a variety of parameters have an impact on this value. Among them are parameters such as nitrogen incorporation into biomass in the WWTP, the degree of nitrogen recovery in the stripping process of the WWTP (nutrient recovery process), the nitrogen content in greywater, and also the nitrogen content in organic waste. The latter is particular to this System 4, since organic waste is added to the digestion process, thereby increasing the nitrogen load in the activated sludge process because of recirculation of sludge liquor.
Phosphorus
The distribution of phosphorus emissions for the different systems (Figure 4.10) is similar to that of nitrogen emissions. As for nitrogen, discharges of phosphorus from the effluent of the wastewater treatment plant contribute the largest fraction of emissions for the Systems 1 to 4 (about 76‐81% of the total discharges to surface waters).
Source separation of urine or blackwater can reduce the WWTP discharges by about 15‐
17%. Discharges from combined sewer overflow can be reduced by about 32% (3 NuRU) or 75% (4 CoDig) by source separation. The total phosphorus emissions, which are shown in Table 4.6, are in the range of 33 to 70 tP y‐1 and 25 to 122 tP y‐1 for soil/groundwater and surface waters, respectively.
Figure 4.10: Phosphorus emissions to soil/groundwater and surface water from the different sources [tP y-1]
Erosion and runoff from farmland contributes about 68±25 tP y‐1 to diffuse phosphorus discharges to surface waters. Referring only to the agricultural area within Hamburg’s city boundaries, this value is 3±1 tP y‐1. Despite phosphorus being less mobile than nitrogen, there is some phosphorus infiltration, which amounts to 56±21 tP y‐1 for the total agricultural area and 2±1 tP y‐1 for the agricultural areas within Hamburg.
Table 4.6: Total phosphorus emissions to soil/groundwater and surface water from processes related to the urban water system [tP y-1]
1: CurS 2: NuRS 3: NuRU 4: CoDig 5: BlaD 6: CompU Emissions to soil/
groundwater Emissions to
surface water
122