| dc.description.abstract | Globally, water demand increases due to the population growth, urbanisation, industrialisation and improving living standards. Water consumption varies from household to household, region to region and country to country depending on several factors. The largest proportion of household water use generates wastewater which is transferred through centralised sewer systems (separate or combined systems) or discharged in decentralised sewerage systems such as septic tanks (STs). The performance of centralised sewer systems faces many challenges as a result of climate change and urbanisation uncertainties. These two are expected to have serious effects on urban drainage systems resulting in localised flooding due to increased frequency of wet weather events.
To effectively manage the water demand, improve the performance of both centralised and decentralised systems and control flooding, the understanding of the household water use variations, the performance of sustainable urban drainage systems and STs is critical.
The primary objective of this study is to investigate the influence of decentralised water management strategies on the conventional centralised wastewater and stormwater management infrastructure in a developing country and propose alternative sustainable designs. The study is based on Sirte city in Libya. Sirte has experienced rapid population growth, considerable urbanisation, changes in land-use patterns, frequent flooding and water scarcity problems. The city has both conventional STs and a centralised sewer system.
The study used a questionnaire survey involving 380 households in the city to collect data on household characteristics (demographic and socio-economic) and water consumption in both winter and summer seasons. The survey also collected data on STs characteristics and their extent of use.The collected data was analysed using the IBM SPSS Statistics package (v. 25), cluster analysis, and ANOVA test. The data for all water end uses and household characteristics were used to develop time patterns (i.e. temporal water consumption and wastewater generation profiles). The data was also used to develop statistical regression models, using the stepwise-multiple-linear regression technique, to estimate future demand as a function of household characteristics. In order to assess the effectiveness of alternative drainage networks, simulations were developed in Stormwater Management Model (SWMM) using the existing sewer network data from Sirte city. From the survey data, the wastewater generated was estimated to be 80% of the average seasonal water consumption and diurnal patterns were developed. These were used as an input data to the SWMM simulations. STs were incorporated in the simulations to investigate their impact on both quality (in terms of biochemical oxygen demand and total suspended solids) and quantity of wastewater entering the network in winter and summer seasons. In addition, simulations were done for a range of design storm events for several return periods and different rainfall durations to investigate the performance of the existing sewer system in terms of volumes of flooding, number of nodes flooding, and surcharged.
Additionally, the SWMM simulations were tested for four water demand management (WDM) strategies: greywater recycling, efficient fixtures, use of both greywater recycling and efficient fixtures and reuse of STs as rainwater harvesting (RWH) system storage tanks. The simulations were done for several stormwater events, different durations and time interval of rainfall. Three scenarios were used to test the performance of RWH storage tanks according to their initial rainwater content. Moreover, the influence of different capacities of RWH storage tanks were tested in terms of the reduction of flooding volumes, number of nodes flooding and surcharged.
The results suggest that water consumption in the city is 255 l/p/d in winter and much higher (380 l/p/d) in summer. The seasonal diurnal water consumption patterns for all end uses have multiple peaks, which are significantly different from typical profiles for the developed countries. With respect to the STs survey, the results indicated that around 229 of 380 surveyed households use STs and most of the STs in Sirte have holes in the walls and sand floors. The majority of the households in the city prefer to use STs with one compartment rather than two compartments.
The SWMM simulation results suggest that the efficiency of STs increases with its size (i.e. plan area). This is because the increased plan area does help with 4 improved sedimentation and therefore solids removal. Moreover, the temperatures of wastewater in STs have a mariginal positive influence for pollutants removal. With respect to the WDM strategies, the results suggest that using the first three strategies marginally reduce flooding volume. The results also show that the reuse of the existing STs as RWH storage tanks (after closing the holes and sealing the floor) appears as the most efficient strategy for reducing localised flooding for all storm events caused by the existing sewer system and can reduce the demand of water for non-potable applications. | en_GB |
