3 Method and case studies
4.3 Sensitivities, parameter variations and system modifications
4.3.3 Modifications of the systems
Figure 4.31: Impact of varying prices of phosphorus on benefits from nutrient recovery (initial parameter marked in the figure)
4.3.3 Modifications of the systems
The selected systems represent only an extract of possible system variations. Processes included in the systems can be modified in order to check the possibilities for improving the systems’ performances. This goes beyond the variation of specific parameters as discussed in the previous section, and refers to changes in the general system setup and in specific processes. In what follows, some possible modifications for each system are suggested (see Table 4.12); the impact of such modifications on the results is also assessed.
Table 4.12: Measures included in the system modifications System Modification
1 CurS Reduction of toilet flush water 2 NurS AshDec as P recovery process
3 NuRU Improvements in nutrient recovery processes (stripping and MAP precipitation)
4 CoDig Replacement of vacuum system Thickening of digested slurry Omission of N recovery (stripping) 5 BlaD Phasing out of bottled water 6 CompU Heat recovery
1 CurS
Current developments show a trend towards a decrease in demand for toilet flush water. The volume of cistern flush can be reduced by do‐it‐yourself kits or new toilet models can be installed. For example, a low flush toilet with dual flush (2 l/4 l) uses about 14 l p‐1 d‐1 (af Petersen et al., 2001). Therefore, the modelling of System 1 CurS is carried out with this reduced flush water volume instead of the previous calculation with 30 l p‐1 d‐1. As a result, the required drinking water and the wastewater inflow to the WWTP are reduced by about 15% to 16%. This also cuts energy requirements and costs for water supply (minus 15% and minus 2% respectively). Also those processes in wastewater disposal and treatment that are flow‐dependent, such as pumping, show decreased energy demand and costs (minus 10% and minus 1% respectively). The effort required for nutrient elimination in the WWTP increases due to the fact that the effluent needs to comply with standards for concentrations, which in turn are negatively affected by a decreased water volume and constant nutrient loads. For example, energy and cost requirements for nitrogen elimination increase by about 2%, but the overall cost and energy balances of the WWTP decrease slightly by about 1‐3%.
For the overall cost calculation of this modification, a toilet cost of about 360 € is assumed opposed to 275 € for a conventional toilet. This outweighs any potential savings in water supply and wastewater treatment and results in a total cost increase of 1% for the whole system.
2 NuRS
The technology underlying the nutrient recovery process from sewage sludge in the model is the Seaborne® process. This process is selected due to data availability and the existing pilot plant. Other technologies are currently under development (see also Section 2.4.3) and are also tested in large‐scale. Their practicability and success will need to be assessed in the future. One of the more promising technologies is the so‐
called AshDec process, developed within the scope of the EU 6th Framework Project SUSAN (see also http://www.susan.bam.de/). Sewage sludge ash from mono‐
incineration is subjected to several thermo‐chemical processes resulting in a licensed fertiliser product called PhosKraft® (Hermann, 2008). For detailed modelling not enough data is available yet. However, some approximate calculations can be carried out using the basic information given in Annex A.8.
Table 4.13: Comparison of AshDec and Seaborne results
Value Unit
a) Potassium is added to the process as potassium chloride or potassium sulphate and therefore the recycled potassium is not waste- or wastewater-borne
b) Net energy consumption (i.e. minus savings in WWTP) - not applicable
3 NuRU
As mentioned in Section 4.2.5 treatment of urine is only more energy‐efficient than the transport of untreated urine if the distance between intermediate storage and agriculture exceeds 110 km. However, treatment (i.e. MAP precipitation and stripping) has additional advantages such as the reduction of micropollutants, easier application and last but not least the acceptance of the products by farmers. The overall efficiency of
the system could be improved by measures to reduce the energy consumption of the recovery processes. For example, heat recovery from the existing sludge treatment at the wastewater treatment plant or from thermal waste treatment, could reduce the heat consumption of the stripping process. Energy savings for the stripping process of up to 40% seem realistic with improved process setups (Tettenborn et al., 2007). Regarding the overall energy consumption of the system this would decrease the specific energy demand by about 2% to 544 kWh p‐1 y‐1. Regarding MAP precipitation, the cost of magnesium is one of the main contributors, making up about 65% of the total costs (Esemen and Dockhorn, 2009). Esemen and Dockhorn (2009) show that the use of seawater containing high levels of magnesium, reduces the cost by about 75%
compared to conventional operational supplements. Other substrates rich in magnesium, such as the wastewater from potassium mining, could also possibly be used for precipitation. Considering the overall costs of the system, this cost reduction seems negligible (less than 0.5%). For the detailed design of the processes however, such saving potentials should be considered.
4 CoDig
Besides the reduction of flush water and the addition of more organic waste (see also Figure 4.28 and Figure 4.29), other measures are feasible to reduce the energy consumption of System 4 CoDig. For example, the vacuum system (toilets and sewerage) for the collection of blackwater could be replaced by extreme‐low‐flush toilets that use as little water for flushing as vacuum toilets (e.g. 0.6‐1 l per flush) but have a lower energy consumption (af Petersen et al., 2001). If the energy consumption for the vacuum sewerage is disregarded, the total energy consumption is reduced by about 16%. However, the flushing ability and other practical experiences need to be considered in more detail to come to a decision as to which of the two approaches is more favourable. Another innovation worthwhile to be mentioned is a patent hold by Hamburg Wasser (Li, 2007). This patent is for a vacuum blackwater collection system connected to a centralised vacuum source that theoretically reduces the energy consumption when compared to conventional vacuum systems. However, no practical experiences have been reported yet, and thus no data on possible energy savings are available.
Furthermore, thickening of the collected blackwater before transport to the centralised treatment is an option that could possibly cut down on energy demand. Assuming the installation of decentralised thickening devices, such as gravity thickeners, the total solids content of the blackwater could be increased about threefold. This would reduce the volume of the blackwater to about one third of the original volume. The excess water (sludge liquor) can be discharged into the centralised sewer system, together with greywater. Assuming an energy demand for thickening of about 10 Wh m‐3blackwater and
an annual cost of approximately 0.74 € y‐1 m‐3blackwater (all data based on Puchajda and Oleszkiewicz, 2008), this measure would require less than 1 kWh p‐1 y‐1 and 2 € p‐1 y‐1. Potential benefits outweigh these requirements due to reduced transport requirements.
Energy savings could amount to about 13 kWh p‐1 y‐1 and cost savings could be in the range of about 18 € p‐1 y‐1. Therefore, the total energy demand could be decreased by about 2% and total costs could be decreased by about 6%. Thickening of blackwater could possibly also increase the biogas yield due to longer retention times. Puchajda and Oleszkiewicz (2008) report that about 27% more energy can be produced by digestion of thickened sludge with a TS of 6% instead of 3%.
The nitrogen recovery in System 4 CoDig, which is based on stripping of nitrogen from sludge liquor, eliminates the need for nitrogen removal in the activated sludge reactor.
Also energy from fertiliser production is saved. However, an assessment of a modified system, that does not include any nitrogen recovery, reveals that the overall energy balance and the overall cost balance could both be decreased by about 3% if the stripping process is omitted.
5 BlaD
System 5 BlaD achieves the highest recovery ratio, yet it is the most unfavourable system with regard to energy demand and costs. This is also partly due to the assumption that bottled water is used for drinking and cooking purposes. A better choice would therefore be to treat the recycled greywater to a level where the water is fit for human consumption. However, social perception and acceptance would need to be ensured100. Another option for this system is a centralised water supply as in Systems 1 to 4. The locally treated greywater could then be infiltrated, discharged into the nearest watercourse or used for purposes such as irrigation. The resulting energy savings amount to about 20% of the total energy demand, decreasing the specific demand to about 709 kWh p‐1 y‐1. However, this value is still greater than the energy demand of the other systems due to the relatively high energy consumption of the vacuum system and the anaerobic digestion under current assumptions.
Only a combination of measures discussed in Section 4.3.2 (i.e. addition of more organic waste, reduction of flush water) and an alternative provision of drinking water instead of bottled water, could bring down the energy consumption to values comparable to the other systems. For example, a centralised drinking water supply, the addition of 80%
100 In future, additional sources of drinking water might be developed including decentralised water
supply options. For example, Hristovski et al. (2009) suggest that water generation from household energy production by hydrogen fuel cells would be sufficient for potable water supply for human consumption.
organic waste to digestion, and a reduction in flush water consumption down to 4 l p‐1 d‐1 would reduce the energy consumption significantly to 597 kWh p‐1 y‐1; a reduction of about 33%. This value is even lower than the current energy consumption, highlighting the potential for possible energy savings. Also the replacement of the vacuum system by a low‐flush toilet system as discussed above, could be a measure to reduce the energy demand. Replacing bottled water by a centralised water supply alone, could decrease costs by about 22% to 267 € p‐1 y‐1. This is still greater than the costs of the other systems, but in a comparable range.
6 CompU
System 6 CompU could be adapted by the introduction of urine treatment or the replacement of recycled water by a centralised water supply (as discussed for Systems 3 NuRU and 5 BlaD). The expected improvements in terms of energy and costs are considered to be rather marginal, so that these measures are not discussed here in detail.
Another measure that could be an appropriate improvement, not only for System 6, but also for the other systems, is heat recovery from wastewater or greywater. This measure is worthwhile to look at, although energy requirements for household purposes such as heating of water for showering, etc., are not included within the system boundaries of this study101. Heat recovery from wastewater (i.e. connected to the sewerage system), which could be an addition to Systems 1 to 4, is currently being tested by Hamburg Wasser (Werner and Augustin, 2009). Heat exchangers can be used to utilise the heat in the wastewater for heating or cooling purposes. For more general information on this topic please refer also to Section 2.5.2 and BFE (2009). Also, the decentralised heat recovery from greywater102 directly by local energy recovery, seems to be a promising alternative, e.g. by recovering the heat from spent shower water for heating water.
Cooling 1m3 of greywater by 1°C can theoretically provide 1.16 kWh, but there will be losses in heat exchanger and heat pump devices. The coefficient of performance of a heat pump fed with shower water can be up to about 10 (Menerga, 2009). According to Forstner (2009) about 15 kWh can be recovered from 1000 l greywater. Taking the average water consumption for showering into consideration (about 30 l p‐1d‐1),
101 Another possibility to save energy, that shall be briefly mentioned here, are appliances to reduce the
consumption of warm water, for example, water‐saving shower heads (anti‐legionella devices should be preferred).
102 Greywater has on average a higher temperature (about 28°C to 40°C) than mixed wastewater (about
12°C to 15°C) since it originates from activities such as showering and washing with warmer water than, for example, toilet flushing.
164 kWh p‐1 y‐1 could be recovered. This is about 26% of the required energy demand in System 6, which, however, doesn’t include any energy for heating or cooling. Assuming a cost103 of about 400 € p‐1, a lifetime of 12.5 years and an interest rate of 3%, the annuity of this measure amounts to about 39 € p‐1 y‐1, which is an extra charge of about 18% of the total specific costs of System 6.