3 Method and case studies
4.1 Process descriptions
4.2.7 Discussion of the results
In this section a summary of the criteria nutrient recovery, water consumption, energy use and costs of the different systems is given (see also Figure 4.23). The main objective of the development of the five alternative systems (systems 2 to 6) is the recovery of nutrients. This is reflected by replacement rates93 for mineral nitrogen or phosphorus fertiliser between 4% and 29%. In general, the replacement of phosphorus fertiliser is comparatively higher than the replacement of nitrogen fertiliser. Particularly System 2 NurS recovers a good deal more phosphorus than nitrogen. The recovery of one nutrient in surplus means that the other nutrients still need to be added in the form of balanced mineral fertiliser.
Figure 4.23: Potential change of nitrogen and phosphorus fertiliser use, groundwater
extraction, energy demand and annualised costs compared to the current situation [%]
93 In this context it needs to be noted that the model uses average fertiliser application rates. In farming
practice, however, nutrient application depends on specific crop and soil requirements.
The highest replacement of mineral fertiliser is achieved by System 5 BlaD with replacement rates of nitrogen and phosphorus fertiliser of about 15% and 29%
respectively. However, the transport of digested slurry and particularly the use of bottled water, counteract these benefits and make this system the most unfavourable in terms of energy use (40% higher than currently) and costs (114% higher). In addition, biogas production from the current system setup seems to be too low to make up for increased energy demand for digestion and vacuum sewers. In this regard please refer to Section 4.3.3 for a discussion of significant parameters, which can have a positive effect on the efficiency of the system.
Water consumption can be reduced by source separation of urine or blackwater.
System 3 NuRU and 4 CoDig reduce water consumption by about 20 to 25%, whereas the decentralised systems (5 BlaD and 6 CompU) require almost no external water sources due to the recycling of process water and the use of rainwater.
Overall energy demand is reduced by the introduction of source separation of urine (3 NuRU) or by nutrient recovery from sludge (2 NuRS). All other systems show higher energy needs. The relatively poor performance of systems 4 and 5 in particular is rather unexpected; these systems are characterised by the anaerobic digestion of waste flows and the related energy generation from biogas. The reason for this is the current system setups, including for example, the relatively high flush volumes of vacuum toilets and the relatively low amount of organic waste added to the digestion process. A parameter variation as carried out in Section 4.3.2 is therefore essential.
Costs for Systems 3 to 6 are significantly higher than costs for the current situation (see also Section 4.2.6). This picture changes in the case of greenfield development, i.e. if the use of already existing infrastructure is neglected. Then, overall costs for Systems 3 NuRU and 6 CompU are even less than for System 1 CurS, and also the relative costs for Systems 4 CoDig and 5 BlaD are greatly reduced. Considering the detailed cost breakdown, a reallocation of costs to the source (i.e. the households) can be observed by the introduction of source separation. This does not mean though, that the costs need to be actually borne by the households themselves. Cross‐subsidies could for example be introduced.
The substitution of water from decentralised water supply systems by bottled water (as modelled in System 5 BlaD) is considered to be a very negative factor with regard to energy and cost balances94. If social acceptance allows, treatment levels for recycled
94 Bundanoon in Australia was the first community world‐wide to ban bottled water from its shops to
protest against the use of resources related to bottled water. On request, re‐usable bottles are filled with tap water by local businesses (Wälterlin, 2009).
water should be high enough to ensure the provision of safe drinking water (as included in System 6 CompU). If the consumption of recycled water is not socially acceptable, the provision of drinking water can also be ensured by a centralised system.
However, a dual system (centralised provision of drinking water and decentralised facilities for process water) seems to be economically inferior compared to one single system.
Although System 3 NuRU has one of the lowest overall fertiliser replacement rates (about 8% for both nitrogen and phosphorus), it shows the lowest overall energy demand; current energy use is reduced by 12%. Also, water extraction is reduced by more than 25%. It is therefore one of the more promising systems, particularly in the case of new developments (i.e. greenfield) where total costs are even less than those of System 1 CurS.
All in all, the analysis shows that none of the systems is superior in all criteria.
Therefore, an aggregation of the criteria would be needed to come to a general ranking of the systems. However, such an aggregation requires the weighting of the criteria, which is beyond the scope of this study because stakeholders need to be involved in the weighting process. The depiction of the discrete results furthermore keeps the analysis transparent. Using multi‐criteria decision support, such as a decision‐matrix where the performance regarding a range of different criteria is assessed, would furthermore require that the criteria be independent95.
Therefore, it is up to the planner or decision‐maker to evaluate the results according to the specific needs. Trade‐offs need to be taken into account as benefits related to some criteria are connected with drawbacks in other criteria. For decision making a transparent and well‐coordinated process would be required.
It is important to note that modelling results always depend on the assumptions used for the model approach. Therefore, the results shown here should always be considered in the context of the system setup and the selected parameters. The next section (Section 4.3) details the analysis by varying system parameters and system setups, in order to assess the sensitivity and variability of the modelling results and to look for measures to improve the resource efficiency of the systems.
95 This means that, for example, the replacement of nitrogen fertiliser, which is energy consuming process, as a target, must be separated from the energy balance to avoid any double counting.