Modelling the bioenergy potential of municipal wastewater treatment plants.

A municipal wastewater treatment plant accounts for a large portion of the total energy consumption of a municipality. Besides their high energy demand, the plants also display a significant bioenergy potential. This is due to the utilisation of the energy content of digester gas and sewage sludge if there exist suitable units. To maximise the energy recovery efficiency of wastewater treatment systems (WWTS), it is important to analyse the amount of digester gas and sludge produced in different types of plants. Therefore, the present paper deals with designing a tool to answer the following research questions: Which bioenergy potentials occur in different plant types? Which mass and energy flows are related to the specific potentials? Which utilisation processes for the potentials can lead to a high energy recovery efficiency of WWTS? Preliminary analyses with the designed tool were focused on estimating the level of electric and thermal energy self-sufficiency of different plant configuration scenarios including or excluding digester gas and/or sludge utilisation units. First results based on the level of self-sufficiency and associated energy and disposal costs show that a digester gas and sewage sludge utilisation should be considered when designing future WWTS.

Case study: Is the 'catch-all-plastics bin' useful in unlocking the hidden resource potential in the residual waste collection system?

Austria's performance in the collection of separated waste is adequate. However, the residual waste still contains substantial amounts of recyclable materials - for example, plastics, paper and board, glass and composite packaging. Plastics (lightweight packaging and similar non-packaging materials) are detected at an average mass content of 13% in residual waste. Despite this huge potential, only 3% of the total amount of residual waste (1,687,000 t y-1) is recycled. This implies that most of the recyclable materials contained in the residual waste are destined for thermal recovery and are lost for recycling. This pilot project, commissioned by the Land of Lower Austria, applied a holistic approach, unique in Europe, to the Lower Austrian waste management system. It aims to transfer excess quantities of plastic packaging and non-packaging recyclables from the residual waste system to the separately collected waste system by introducing a so-called 'catch-all-plastics bin'. A quantity flow model was constructed and the results showed a realistic increase in the amount of plastics collected of 33.9 wt%. This equals a calculated excess quantity of 19,638 t y-1. The increased plastics collection resulted in a positive impact on the climate footprint (CO2 equivalent) in line with the targets of EU Directive 94/62/EG (Circular Economy Package) and its Amendments. The new collection system involves only moderate additional costs.