Department Urban Water Management

Transition to decentralised wastewater treatment systems – its effects on the operation of sewers

Effects of drastic changes in wastewater quality and quantity on sewer systems

Increasing the efficiency of urban wastewater systems is a critical challenge for global sustainability. In most affluent cities, centralized wastewater infrastructures are widely established. However, in many cases, these investment intensive centralized systems may no longer provide the best solution. These cases may include situations in which expensive rehabilitation is needed or when flexibility of the systems is desired due to anticipated changes in climate, in land use and in demography. Attractive technical alternatives to replace or complement existing centralized systems are being widely developed. These include novel systems for drastic reduction in domestic water consumption and for decentralized wastewater reuse. Decentralized systems can provide desired flexibility and localization of required services and performances. Implementation of such systems changes the quality and quantity of the wastewater discharged to the sewer system. Such changes can negatively affect the operation of existing centralized systems. These effects may include phenomena such as the accumulation of solids in sewer pipes, increasing rates of blockages and the formation of unpleasant or toxic gases (such as volatile organic acids, hydrogen sulphide or methane).

We aim at examining the effects various transition scenarios of wastewater infrastructures from centralized to hybrid systems, combining centralized and decentralized options, may have on the operation of existing centralised systems. These effects shall be examined by simulating an integrated model of the sewer system, integrating treatment and transport processes. Domestic wastewater flows (quality and quantity) will be generated stochastically and serve as input to a municipal sewer network model. The sewer model will include hydrodynamic simulation of flows, a gross solids and pollutant transport module and a model of the interaction of deposited solids with the biochemical processes occurring in the sewer pipes. Pipes will be modelled as a series of continuously stirred tank reactors (CSTRs) in which the complex quality processes are modelled using the Petrsen Matrix. The simulation system that will serve as a basis for the research is SIMBA# (developed by ifak, Magdeburg).

Stochastic inference of the simulation results together with sensitivity analysis of the model will provide the means to develop a simplified model for assessing the expected effects of incorporating decentralized wastewater treatment systems on any existing sewer system. This will spare the high computational efforts needed to simulate the very detailed and complicated model, enabling relatively easy and fast assessments to be achieved.