PhD project title: Untapping the energetic potential of grey water: microbiological safety and... (# of pos: 14)

Topic background
mWater circularity has become a requirement for new urban developments, as preserving usable water represents a basic, but an important first step for a sustainable economy. Separation of wastewater streams at the source is a valuable approach for the effective recovery of resources such as nutrients, water and organics. For instance, concentrated toilet water can be treated via anaerobic digestion to produce energy in the form of biogas, and phosphorus can be recovered as calcium phosphate in the same anaerobic reactor. High quality reuse water and heat can be extracted from greywater (discharges from laundry, showers and sinks) after treatment with nanofiltration membranes, among others. Nanofiltration reduces the organic matter content and microorganisms, which otherwise would promote the growth of biofilms and pathogens downstream. In this framework, biofilm formation is an event naturally occurring in many water distribution environments which can be detrimental, posing a serious threat for the safety of water re-utilization. Thus, a step further into circularity can be achieved by tapping out the energetic potential of grey water in a microbiologically safe manner.

Research challenges 
The energy potential of grey water lays in the relatively large volumes produced, which have high temperatures. The idea is to use residual greywater for district heating, by applying heat pumps in a decentralized approach, closing the energy loop in a household. The system will be applied at a neighbourhood level, where grey water is first treated aerobically to remove organic matter and nutrients. Thereafter advanced treatment is applied by means of a nanofiltration unit. In this stage, bacteria and micropollutants are removed. Warm effluent is then stored (at a temperature around 20°C) and with the use of a heat pump, the energy is extracted and stored in a high temperature storage tank for reuse in households.

Dependency by fossil fuels and natural gas, and an abundance of greenhouse gas emissions are just some of the negative side-effects of our current heating systems. Clearly, there’s an urgent need to transform this obsolete system into a sustainable one. This project idea satisfies three main targets set by the European Green Deal, namely investing in environmentally-friendly technologies, decarbonising the energy sector and ensuring buildings are more energy efficient.

Objectives and methodology
The focus of this project will be in recovering energy and produce reusable, high quality water in a decentralized manner. A multidisciplinary approach will be used to draw all the possible scenarios, from initial water purification to its final re-utilization.

In the first phase a lab-scale setup will be realized, with particular attention to the (polymeric) materials utilized because of their strict relationship in promoting bacterial/biofilms growth (including pathogens). Within the setup, the microbiological safety of the water during the whole cycle will be assessed via several analytical and microbiological techniques.

Lab-scale investigations will focus on:

• Quality of greywater after the nanofiltration treatment, assessing analytical and microbiological parameters.
• Characterization of the bacterial population growing inside the system setup.
• Assessment of energy efficiency of the whole process via mathematical modelling.
• Effects of storage of grey water after nanofiltration and the greywater effluent reuse after storage at high temperature.
• Possible disinfection/cleaning strategies to be applied at different levels of the system.