Technical Report

Sustainable biogas production in municipal wastewater treatment plants

November 2015

This report deals with anaerobic digestion (AD) of sewage sludge, an energy- and nutrient-rich by-product of wastewater treatment plants (WWTP). The objective is to promote sustainable practices and technology, focussing on energy efficiency of biogas production and utilisation. An overview of the AD process in WWTP is given, along with standard energy performances, nutrient recycling and different process options and their impacts. It is not intended as a detailed technical guideline for project managment. The report is aimed at energy policy and decision makers as well as WWTP operators and was produced by IEA Bioenergy Task 37, an expert working group that addresses challenges related to the economic and environmental sustainability of biogas production and utilisation.

Anaerobic digestion (AD) is a proven technology for sewage sludge treatment and which allows generation of renewable energy from the same process. During AD, microorganisms break down the organic matter contained in the sludge and convert it into biogas, a mixture of mainly methane and carbon dioxide, which can be used for electricity, heat and biofuel production. At the same time, the sludge is stabilised and its dry matter content is reduced. The benefits of AD of sewage sludge are widely recognised and the technology is well established in many countries. Today, a high proportion of biogas produced in AD plants is from those on municipal wastewater treatment sites (see Table 1) and there is still an enormous potential to exploit worldwide.

Sewage sludge is produced in wastewater treatment plants (WWTPs) as part of the water cleaning process (Figure 1). The sludge contains the particles removed from the wastewater, which are rich in nutrients and organic matter, leaving the water clean for its release into nature. Growing population centres and expanding industry, which are increasingly well served by wastewater treatment facilities, result in rapid growth of sewage sludge production.

As important consumers and generators of energy, WWTPs are one of the numerous players influencing developments towards energy sustainability. The present brochure aims to encourage sustainable and efficient production, conversion and utilisation of biogas in municipal WWTPs, including the closing of nutrient cycles, whenever this is legally and technically possible. It is addressed to energy policy- and decision makers as well as WWTP operators. It allows understanding context and interactions involved in the treatment processes, but it is not meant as a technical guideline for project management. Specialists must also be involved for detailed studies and implementation of new treatment concepts and technologies.

The three main objectives of the brochure are:

• To acknowledgement the importance of AD in WWTPs
• To provide information about benefits and challenges
• To encourage optimisation of biogas production and utilisation