Drivers of changing urban flood risk: A framework for action

This study focuses on drivers for changing urban flood risk. We suggest a framework for guiding climate change adaptation action concerning flood risk and manageability in cities. The identified key drivers of changing flood hazard and vulnerability are used to provide an overview of each driver's impact on flood risk and manageability at the city level. We find that identified drivers for urban flood risk can be grouped in three different priority areas with different time horizon. The first group has high impact but is manageable at city level. Typical drivers in this group are related to the physical environment such as decreasing permeability and unresponsive engineering. The second group of drivers is represented by public awareness and individual willingness to participate and urbanization and urban sprawl. These drivers may be important and are manageable for the cities and they involve both short-term and long-term measures. The third group of drivers is related to policy and long-term changes. This group is represented by economic growth and increasing values at risk, climate change, and increasing complexity of society. They have all high impact but low manageability. Managing these drivers needs to be done in a longer time perspective, e.g., by developing long-term policies and exchange of ideas.

Co-digestion of grease trap sludge and sewage sludge

Redirection of organic waste, from landfilling or incineration, to biological treatment such as anaerobic digestion is of current interest in the Malmö-Copenhagen region. One type of waste that is expected to be suitable for anaerobic digestion is sludge from grease traps. Separate anaerobic digestion of this waste type and co-digestion with sewage sludge were evaluated. The methane potential was measured in batch laboratory tests, and the methane yield was determined in continuous pilot-scale digestion. Co-digestion of sludge from grease traps and sewage sludge was successfully performed both in laboratory batch and continuous pilot-scale digestion tests. The addition of grease trap sludge to sewage sludge digesters was seen to increase the methane yield of 9-27% when 10-30% of sludge from grease traps (on VS-basis) was added. It was also seen that the grease trap sludge increases the methane yield without increasing the sludge production. Single-substrate digestion of grease trap sludge gave high methane potentials in batch tests, but could not reach stable methane production in continuous digestion.

Reducing the total discharge from a large WWTP by separate treatment of primary effluent overflow

At many large wastewater treatment plants (WWTPs) the increased hydraulic load, caused by combined sewer systems during storm events, results in primary effluent overflow when the capacity of further treatment is exceeded. Due to stringent effluent standards, regulating the total discharge from the WWTPs, the Rya WWTP in Göteborg and the Sjölunda WWTP in Malmö will have to reduce the impact of primary effluent overflow. Separate, high rate, precipitation processes operated only during high flow conditions have been investigated in pilot units at the two WWTPs. Precipitation in existing primary settlers operated at a surface loading of 3.75 m/h removed phosphorus to 0.35 mg/l. The Actiflo process was also shown to remove suspended solids and phosphorus well. BOD was reduced by 50-60%. With such processes the overall effluent concentrations from the plants can be reduced significantly. Key upgrading features are small footprints, short start up time and high efficiency.

Digestion of sludge and organic waste in the sustainability concept for Malmö, Sweden

Anaerobic digestion of sludge has been part of the treatment plant in Malmö for many years and several projects on optimisation of the digestion process have been undertaken in full scale as well as in pilot scale. In order to facilitate a more sustainable solution in the future for waste management, solid waste organic waste is sorted out from households for anaerobic treatment in a newly built city district. The system for treatment of the waste is integrated in a centralised solution located at the existing wastewater treatment plant. A new extension of the digester capacity enables separate as well as co-digestion of sludge together with urban organic waste from households, industry, restaurants, big kitchens, food stores, supermarkets, green markets etc. for biogas production and production of fertiliser. Collection and pre-treatment of different types of waste are in progress together with examination of biogas potential for different types of organic waste. Collection of household waste as well as anaerobic digestion in laboratory and pilot scale has been performed during the last year. It is demonstrated that organic household waste can be digested separately or in combination with sludge. In the latter case a higher biogas yield is found than should be expected from digestion of the two materials separately. Household waste from a system based on collection of organic waste from grinders could be digested at mesophilic conditions whereas digestion failed at thermophilic conditions.

Upgrading the Sjölunda WWTP according to a novel process concept

The Sjölunda wastewater treatment plant in Malmö, Sweden, was upgraded for extended nutrient removal in 1998-1999. The design was based on future effluent standards of 10 mg BOD7/l, 0.3 mg total-P/l and 8 mg total-N/l. The upgrading concept took into consideration existing processes and structures, resulting in a cost-effective and compact upgrading. To introduce nitrification, the existing trickling filters for BOD-removal were converted to a nitrifying mode. A sequencing batch reactor for nitrification of supernatant was necessary to control the ammonia load. Denitrification was accomplished in a moving bed biofilm reactor with addition of external carbon source. The future effluent standards could be met by the upgraded plant. The trickling filters were stable despite varying loading conditions. High rates and low effluent ammonia concentrations were achieved. Essential features for stable post denitrification were control strategies for carbon source dosage and avoiding phosphorus limitation.

Sustainability concept for a newly built urban area in Malmö, Sweden

National goals regarding sustainable development from the Swedish government were decisive in the planning process when the newly built city district Western Harbour was established in the city of Malmö, Sweden. A systems analysis was used as a tool for evaluating different collection and treatment systems. This type of analysis does not include more subjective factors such as acceptance of organic fertilizers based on human waste and user acceptance of collection schemes. These aspects, however, will often determine the success of a technical solution. When the system for collection and treatment of old organic waste and wastewater was designed, both subjective and objective factors were considered. This meant that a centralised solution for wastewater treatment was selected. In order to facilitate a more sustainable solution for sludge management a treatment process with recovery of phosphorus will have to be introduced. Organic waste is sorted out and treated in an anaerobic digestion process. Source sorting of solid organic waste has been difficult to implement in Sweden due to inadequate sorting discipline. As a consequence two relatively new systems are tested in the area. A comprehensive evaluation will be carried out during a period of two years.

Use of phosphorus release batch tests for modelling an EBPR pilot plant

The aim of this study was to evaluate how routinely performed phosphorus release tests could be used when modelling enhanced biological phosphorus removal (EBPR) using activated sludge models such as ASM2d. A pilot plant with an extensive analysis programme was used as basis for the simulations. Without any calibration the prediction of phosphorus removal was poor and the initial release rates from the simulations were not similar to those found from the laboratory tests. A period with low organic loading was chosen as a calibration period. In this period averages of daily influent measurements were used as influent parameters. First, calibration was performed in order to fit effluent COD and MLVSS in the sludge. Next, the phosphorus content in the sludge was decreased to the measured level by decreasing the fermentation rate. Finally, the initial phosphorus release rate was calculated from a simulated batch test and the PHA uptake rate was increased to fit this release rate with the average initial rates from laboratory batch tests performed during the period. The calibrated model was verified with data from the subsequent period where acetate was dosed.

Population dynamics in wastewater treatment plants with enhanced biological phosphorus removal operated with and without nitrogen removal

The population dynamics of activated sludge in a pilot plant with two activated sludge systems, both designed for enhanced biological phosphorus removal (EBPR), but one of them with (BNP) and the other without (BP) nitrogen removal, was monitored during a period of 2.5 years. The influent water to the pilot plant was periodically manipulated by external addition of phosphorus (P), acetate and glucose, respectively. The population dynamics and the in situ physiology were monitored by quantitative fluorescence in situ hybridization (FISH) and microautoradiography. Significant P removal was observed in both systems throughout the whole period, with significant increases of the P removal when substrates were dosed. The activated sludge in both systems contained large amounts of dense clusters of gram-negative, methylene-blue staining coccoid rods during the whole period. A large part of the clusters belonged to the beta Proteobacteria, whereas the rest of the clusters belonged either to the Actinobacteria or to the alpha Proteobacteria. The relative abundance of Rhodocyclus-related bacteria in the activated sludge varied significantly in both systems during the whole period (from 6 to 18% in BNP, and from 4 to 28% in BP). However, no statistically significant correlation of the Rhodocyclus-related nor any of the other investigated bacterial groups to the P content of the activated sludge (correlation for all groups investigated was always < 0.5) was observed. A significant 33Pi uptake was observed by the beta Proteobacteria (part of them Rhodocyclus-related, the identity of the rest unknown) and the Actinobacteria. However, not all of the Rhodocyclus-related bacteria showed 33Pi uptake. The P removal in the investigated plants is thus believed to be mediated by a mixed population consisting of a part of the Rhodocyclus-related bacteria, the Actinobacteria and other, yet unidentified bacteria.

Effect of solids retention time and wastewater characteristics on biological phosphorus removal

The paper deals with the effect of wastewater, plant design and operation in relation to biological nitrogen and phosphorus removal and the possibilities to model the processes. Two Bio-P pilot plants were operated for 2.5 years in parallel receiving identical wastewater. The plants had SRT of 4 and 21 days, the latter had nitrification and denitrification. The plant with 4 days SRT had much more variable biomass characteristics, than the one with the high SRT. The internal storage compounds, PHA, were affected significantly by the concentration of fatty acids or other easily degradable organics in the wastewater, and less by the plant lay-out. The phosphorus removal is mainly dependent on availability in the wastewater of fatty acids but also by the suspended solids in the effluent, which is higher in the plant with nitrification-denitrification, probably due to a higher SVI or denitrification in the settler. The addition of glucose to the influent seems to have an effect on the performance of the plants similar to that of acetic acid. In spite of great load variations over time to the pilot plants and the different operational modes, the study of population dynamics showed less significant variations with time which has importance in relation to modelling. The overall conclusion of the comparison between the two plants is that the biological phosphorus removal efficiency under practical operating conditions is affected by the SRT in the plant and the wastewater composition. Thus great care should be taken when extrapolating results from one type of plant to another. Indirectly the experiments confirm that results from lab-experiments with artificial wastewater are difficult to extrapolate through modelling to real life wastewater and conditions. The 2.5 years time series can be valuable in verification of models for Nitrogen and Enhanced Biological Phosphorus Removal.

Storage and degradation of poly-beta-hydroxybutyrate in activated sludge under aerobic conditions

This research analyses the accumulation and degradation of poly-beta-hydroxybutyrate (PHB) in experiments with pulse addition of acetate to samples of activated sludge from pilot-plant and full-scale wastewater treatment plants. The experiments are divided into two periods: a feast period defined as the time when acetate is consumed and a famine period when the added acetate has been exhausted. In the feast period the significant process occurring is the production of PHB from acetate. The produced PHB is utilised in the famine period for production of glycogen and biomass. According to modelling results approximately 90% of the total potential growth occurs in the famine period utilising the stored PHB. The degradation rate for PHB in the famine period is found to be dependent on the level of PHB obtained at the end of the feast period. It was found that multiple order kinetics gives a good description of the rate of PHB degradation. The examined sludge of low SRT origin is found to degrade PHB faster than long SRT sludge at high fractions of PHB. The observed yield of glycogen on PHB in the famine period is in the range of 0.22-0.33 g COD/g COD depending on the SRT. The storage pool of glycogen in the examined sludge is more slowly degraded than PHB (COD/COD/h).

Radar based rainfall forecast for sewage systems control

There has been an increasing demand for accurate rainfall forecast in urban areas from the water industry. Current forecasting systems provided mainly by meteorological offices are based on large-scale prediction and are not well suited for this application. In order to devise a system especially designed for the dynamic management of a sewerage system the "RADAR" project was launched. The idea of this project was to provide a short-term small-scale prediction of rain based on radar images. The prediction methodology combines two methods. An extrapolation method based on a sophisticated cross correlation of images is optimised by a neural network technique. Three different application sites in Europe have been used to validate the system.