3 Method and case studies
4.1 Process descriptions
4.1.4 Surface areas and sewerage
Mass and nutrient flows
The Process Sewerage is only used in the Systems 1 to 4 (CurS, NuRS, NuRu, CoDig), whereas the Process Surface Areas is also part of the systems without centralised wastewater treatment.
The sewerage system of the City of Hamburg consists of mixed sewerage, i.e. domestic wastewater and rainwater is discharged in one common sewer covering an area of about 9,500 ha, and separate sewerage with separate sewers for rainwater and domestic wastewater covering 20,000 ha (Hamburger Stadtentwässerung (HSE), 2000). This means, that part of the rainwater enters the WWTP together with the domestic wastewater, whereas another part is directly discharged into surface waters or infiltrated. Excess rainwater in the mixed sewer system as a result of intense rain can cause so‐called combined sewer overflow (CSO), leading to the release of polluted wastewater into surface waters. The different flows of the rainwater and sewer systems are reflected in the ceMFA model by using respective transfer coefficients for the output flows of the process Sewerage (see also Figure 4.3 and Annex A.2). The main input to
67 It should be noted that the centralised water supply in Germany is not for profit and cost‐covering, which means that all costs are reflected in the water price (bdew, 2008). Therefore, the approach of using water prices as costs for water supply is appropriate.
the sewerage is the domestic wastewater flow from the households, which is calculated based on the volume of wastewater generated in the households.
Figure 4.3: Processes Surface Areas and Sewerage
The sewer system, which for most part consists of conventional gravity sewers, together with some pressure sewers, is subjected to infiltration of groundwater to the sewer and infiltration of wastewater from the sewer into the ground. Groundwater infiltration into sewerage is difficult to determine. Values in literature vary and refer to inconsistent units, such as infiltration per sewered area, per length of sewer or per person (Franz, 2007)68. For this study, an infiltration rate per unit of sewered area of 700 m3 ha‐1 y‐1 is calculated, based on data from Hamburger Stadtentwässerung (2000). Infiltration from the sewer into the ground is taken into account in the model as a percentage of the total flow, i.e. 2.3% of the total wastewater flow in the sewer is assumed to infiltrate into the ground (Hamburger Stadtentwässerung (HSE), 2000). Another transfer coefficient of 3%, again based on existing data (Hamburger Stadtentwässerung (HSE), 2000), is used for determining the combined sewer overflow. Since no accumulation or degradation is assumed in the sewerage, a mass balance is used to calculate the total inflow of sewerage to the WWTP. The energy demand for the process sewerage due to pumping
68 Franz (2007) reports about average infiltration ratios of, for example, 0.1 l s‐1 hared‐1, 0.1 l sec‐1 kmsewer‐1 or 55‐250 l cap‐1 d‐1. The calculated value used in this study of 700 m3 ha‐1 y‐1 translates to 0.02 l s‐1 hared‐1 showing that infiltration rates in Hamburg are comparatively low.
is calculated using a black box approach. The flow entering the WWTP is multiplied by a specific electricity demand of 0.12 kWh m‐3 (Balkema, 2003).
The Process Surface Areas, which includes neither agricultural areas nor surface waters, is introduced in the ceMFA model to allocate rainwater flows to the respective processes. Part of the rainwater evaporates, whereas another part directly infiltrates into the soil. Transfer coefficients of 0.5 (evaporation/rainfall) and 0.3 (infiltration/rainfall) are used for calculating these mass flows. The part of the runoff that enters the sewer system is calculated using a transfer coefficient of 0.216, which applies only to the sewered areas, whereas infiltration and evaporation apply to the total area. This transfer coefficient that determines the inflow to the sewerage is based on current data of rainwater in the sewer system (Hamburger Stadtentwässerung (HSE), 2000). The direct runoff to surface waters is then derived, based on a mass balance. In Systems 5 and 6 the flow Runoff to Sewerage is not included since no centralised system exists, but an additional flow Water Supply to Decentralised Treatment is introduced. This flow represents the rainwater that is used to replenish any water losses occurring in the decentralised small‐scale water cycles, where greywater is treated for use as process water. In Systems 5 and 6 excess rainwater is assumed to runoff or infiltrate without any sewer system or additional treatment. However, rainwater treatment might be required in particular areas such as car parks before rainwater can be infiltrated into the soil. Yet, this process is neglected within the scope of this study.
A literature review is used for estimating the respective nutrient concentrations in the different flows. The selected values can be found in Annex A2. These concentrations are multiplied by the mass flows in order to determine total nutrient flows.
Energy
For the conventional sewerage in Systems 1 to 469 an electricity consumption of 0.12 kWh m‐3 is assumed (based on Balkema, 2003). In Systems 4 CoDig and 5 BlaD a vacuum sewer system is used for the transport of blackwater. Vacuum sewers have the advantage that no infiltration into and from the sewers occurs and that also small volumes of wastewater can be transported. However, energy is required for the creation of the vacuum. Remy and Ruhland (2006) compiled the electricity demand for vacuum systems from several references. According to them the specific annual electrical energy demand varies between 7 and 51 kWh cap‐1 y‐1. Considering the amount of wastewater
69 In system 4 CoDig only greywater is discharged via the conventional sewer system, whereas blackwater is transported in vacuum sewers.
transported in the different cases this can be converted to an electricity demand70 ranging from 3 to 28 kWhel m‐3.The lower values are cited as being possible but such values have not been achieved in practice, whereas the higher values are a result of systems not working at full capacity. For this study an electricity demand of 15±5 kWhel m‐3 is assumed.
Costs
Costs for the Process Surface Areas and Sewerage relate to conventional sewerage in Systems 1 to 4 and vacuum sewerage in System 4 and 5. In addition, rainwater infiltration that must be taken into account in Systems 5 and 6 is covered in this section.
Investment costs for sewers are greatly influenced by various factors such as diameter, depth, soil type, material, etc. In the calculation model costs per metre for conventional sewers are approximated to average about 300±50 € m‐1 (Guenthert and Reicherter, 2001; Oldenburg and Dlabacs, 2007). The total wastewater sewer length in Hamburg is 3,764 km (Statistikamt Nord, 2009). No differentiation is made between combined or separate sewers. Since no detailed design has been prepared, costs for pump stations are approximated by specific costs derived from a pump station design prepared by Oldenburg and Dlabacs (2007). According to their calculations pump stations require about 2 € p‐1 for civil works (lifespan 50 years), 1.6 € p‐1 for machinery (lifespan 12.5 years) and 0.4 € p‐1 for electrical components (lifespan 12.5 years). A 20% error margin is assumed for these values.
Vacuum sewers (Systems 4 CoDig and 5 BlaD) have an investment cost per unit length of sewer of about 40‐110 € m‐1 (Balkema, 2003; Herbst, 2008; Oldenburg and Dlabacs, 2007). An average cost of 60±15 € m‐1 is assumed. By approximation the length of the vacuum sewers is assumed to be about 50% of the current conventional sewers, since the buildings are connected in clusters and connecting mains can be avoided. This results in a specific vacuum sewer length of 1.1±0.2 m p‐1. The cost for vacuum stations are stated by Oldenburg and Dlabacs (2007) to be about 30 € p‐1 for civil works and 30 € p‐1 for machinery and electrical equipment. Herbst (2008) specifies the costs for vacuum stations using Equation 4‐5:
is cited to be as low as 0.7 kWh m‐3 (ATV 1995 quoted in Remy and Ruhland, 2006).
According to this formula the specific costs for vacuum stations servicing clusters of 3,000 to 5,000 persons ranges respectively between 67 and 55 € p‐1. Therefore, 30±3 € p‐1 for civil works and 30±3 € p‐1 for machinery and electrical equipment are used as cost parameters with a lifespan of 50 and 12.5 years respectively. Blackwater storage vessels are assumed to cost 125±12 € m‐3, of which 80% is for civil works and 20% for machinery and equipment (Oldenburg and Dlabacs, 2007).
For sewer maintenance it is assumed that the cost amount to 21 € p‐1 y‐1 and 19 € p‐1 y‐1 respectively (Wagner,2004 and Reicherter, 2003 cited in Herbst, 2008). Regarding maintenance of the vacuum sewer system another approach is followed compared to the conventional sewer system since the vacuum sewer system is smaller. For the vacuum sewer system maintenance a requirement of 1.3±0.1 € m‐1 is assumed (Oldenburg and Dlabacs, 2007). In addition, costs for electricity used for operation of both sewer systems (as calculated in the ceMFA model) are taken into account.
In Systems 5 BlaD and 6 CompU rainwater infiltration is required in some areas, since the combined sewerage network is not in use anymore. Currently, about 9,500 ha is connected to the combined sewer. It is assumed that on 80% of these areas rainwater infiltration processes with a cost of 3±0.3 € m‐2 and a lifespan of 20 years is applicable71. The remaining areas are assumed to be drained naturally or by separate rainwater pipes, which are not included in the analysis since they are the same in all systems.
Operation and maintenance costs of the rainwater infiltration system are set at 3% of the investment.