Fraunhofer-Institute for Environmental, Safety and Energy Technology (UMSICHT),
Osterfelder Strasse 3, D-46045 Oberhausen, Germany.

Abstract: This article presents possible renewable energy technologies (RET) for energy recovery from waste water worked out on the example of two model regions of Vietnam.


Climate change and increasing energy consumption have to lead to preventive use of resources. In particular in the waste water treatment and drinking water branch, energy recovery will have to be integrated into the different process steps in order not only to minimise the waste of energy but also to cover the rapidly increasing energy demand, as a result of population growth. Over 70 percent of the increase in the worldwide primary energy demand from 2004 to 2030 will have to be met in the developing countries [5]. The average amount energy spent for waste water treatment varies between 30 and 60 kWh per capita and year. Currently, about 44 million people are being added to Asia’s urban population every year, equivalent to 120,000 people a day. This trend is expected to continue. The United Nations forecasts Asia’s urban population to reach almost 2 billion people by 2015 or nearly half the regional population. In Japan, about 6 billion kWh have been used in 1,900 installations for wastewater treatment in 2001 [2]. In many of these countries, public and private wastewater disposal systems are often at a low-level mechanisation or even still missing at all. One decentralised approach may be an appropriate solution for collection, treatment and disposal near to the place of generation, especially for regions far off from the urban agglomerations. Centralised solutions will be preferred in regions with population and industry agglomeration. The choice of the appropriate technology presently depends on several factors such as the location of water use, composition of the wastewater, expertise and money. In future, energy recovery in/from wastewater treatment plants to cut operation costs and to recover energy for other purposes will be another important decision factor for the choice of an appropriate technology. The availability of clean drinking water and the treatment of wastewater is a development issue in “non-developed” countries, just as the supply of energy for citizens (Millennium Development Goals) [4], thus the integration of energy recovery into waste water treatment process is backed politically.

In Vietnam, 78 % of the population are living in rural areas. At the end of 2006, over 91 % of all rural households or 97.8% of all rural municipalities respectively have had access to electricity. The energy supply, however, in rural areas is even worse, because the energy grid is not stable and energy supply breakdowns occur often. The recent plan for energy supply in rural areas in Vietnam stated that by 2010 95 % of all households will be supplied with energy. By 2020, the energy supply is designed to rise to 100 percent coverage of all households. Some households in rural areas can only be supplied through decentralised energy sources due to cost efficiency. In Vietnam approx. 1,100 municipalities with around 750,000 households and 3 million inhabitants will not receive centralised national energy supply in middle or short term future. These households have to rely on decentralised solutions [3].

Within the IWRM - Nam Dinh region, investigation focuses on two regions, one industrial zone (My Trung) and one rural region (Tong Xa).


According to information issued by WHO, 95 % of the urban and about 74 % of the rural population (in Vietnam) theoretically have access to secure water sources. Due to insufficient treatment capacities in Vietnam, large amounts of wastewater flow untreated directly into rivers and lakes because of missing financial resources or maintenance. The largest producers of wastewater are industrial enterprises of different branches as paper, textile or metal industries.

The situation in Nam Dinh regarding the health of the citizens and the pollution prevention of the environment is alarming. For the rural population in Tong Xa there are still less possibilities to get clean drinking water and vice versa to get their wastewater clean. Tong Xa is a handicraft village with small enterprises and with a high level of pollution in the neighbourhood of these small enterprises. There are only few widespread discharge systems to clean the water by buffering trough the soil, no technical facility exists to clean the wastewater. The situation of the energy supply is similar. In developing countries, energy supply often relies on various own or easily importable fossil sources. In the case of Tong Xa, most citizens have their own small biogas plant; and this biogas is used directly for cooking. Additionally, citizens often use rice straw pellets for cooking and heating. The supply of electricity is often realised via illegal connections and in small enterprises such as foundries energy is covered by different sources depending on the available source, for instance coal or gas. In Nam Dinh city, the energy also comes from fossil resources. With regard to climate change, efforts have to be made to produce energy also from renewable sources.

1. Objectives of the IWRM project

The aim of the research project in the province Nam Dinh is the development and model-based set up of wastewater treatment systems, pertaining to industrial and municipal wastewaters, each for rural and urban areas. Technological standards are lacking especially in these rural areas, which leads to significant air and water pollution. Cleaner Production (CP) measures are necessary, aerobic and anaerobic treatment processes for industrial and municipal wastewater will be planned and first steps for the installation of a pilot plant will be developed.


The dissemination of renewable energy technologies (RET) in developing countries is crucial for the energy efficiency and also for the active support of climate protection, being the most important goal of the United Nations’ Framework Convention on Climate Change. The use of renewable energy technologies has further benefits such as the reduction of local pollutants, allowing the electrification of rural areas without having costly investments and reducing fuel import dependency [1].

One of the ways to reduce operation costs and generate energy for other applications is energy recovery in/from wastewater treatment plants.

There are two approaches to include RET in wastewater treatment plants, independently of centralised or decentralised disposal structures:

§     In existing plants, optimisation of plants

§     In planning process, new design of plant

Figure 1 shows the source to produce energy in different process steps within an existing e.g. centralised wastewater treatment plant.


Figure 1. Sources of renewable energies from wastewater (acc. to [8])

1. Biogas: Anaerobic digestion

Anaerobic digestion is a biochemical reaction performed in the absence of oxygen by microorganisms. The process works by feeding sewage sludge to a closed reaction tank with controlled temperature in the mesophilic (30-40 °C) and the thermophilic (50-70 °C) range. The end product of the microorganism reaction is biogas and stabilized sludge. The biogas mainly composed of methane (CH4) and carbon dioxide (CO2) can be converted to both electricity and heat. The amount of gas produced depends on the amount of organic waste fed to the tank and on the temperature.

Currently, energy is generally recovered only in state-of-the-art wastewater treatment facilities at the sludge digestion stage (heat at about 15 kWh per capita and year and electricity at about 10 kWh per capita and year). More consequent and more efficient usage of the heat and electricity production could double the output. In Vietnam, which in the public wastewater treatment sector is often considered a “donor driven market” by various experts, processes and technologies follow the demands of the Worldbank, ADB and other [6].

Additional capacities could be available by using different sorts of waste, in co-fermentation processes. At present, about 25 kWh per capita and year are recovered in the wastewater treatment sector by anaerobic digestion of sludge [9]. A surplus of about 30 kWh per capita and year from biogas out of organic matter could be generated.

2. Heating/Cooling: Waste heat from sewage sludge

The utilization of unused energy such as industrial waste heat is one important measure to save energy consumption for global warming mitigation and to reduce domestic and industrial heat waste. The waste heat from sewage sludge incineration and melting plants can be used for heating facilities and buildings. A high, however presently unused energy potential is owned by heat from wastewater (ref. to heat pumps). Energy recovery rates of about 110 kWh per capita and year can be realized by a temperature drop after exchanger modules of almost 2°C. Cooling of larger buildings is another option for this renewable energy resource.

3. Heat Pumps

Wastewater temperatures range between 10 °C and 20 °C in Europe in the course of the year. In tropical regions, wastewater temperatures can reach up to 25 °C and in some places even more. Hence wastewater seems to be an ideal heat source for the efficient operation of heat pumps. The technology for energy recovery from wastewater is simple and already tested in different examples. Core elements of the technology are a heat exchanger to remove energy from wastewater and a heat pump to utilize the energy for heating or cooling of buildings.

Flow rates of minimum 15 l/s are required to remove energy from sewers. Every litre per second of wastewater can result in heat outputs of approximately 8 kW of the heat pump. Heat recovery from larger sewers has the advantages of continuous and adequate flow rates. This concept might have the bigger potential compared to heat recovery direct from WWTPs.

4. Bio-Oil/Syngas: Transformation to oil and/or gas

Under carefully controlled conditions and extreme temperatures of 450 to 1,000 °C sludge can be converted to fuel in a chemical reaction. Other processes are gasification that produces syngas which is similar to natural gas, and pyrolysis, that produces biological oil which is similar to diesel fuel. These methods are interesting as it is a possible alternative to sludge incineration. However, operation costs are high: Above all, high temperatures must be guaranteed and the conditions have to be controlled carefully, to prevent the creation of harmful by-products such as hydrogen cyanide. Several pilot projects in Europe and the US for sewage treatment plants are struggling with such problems.

5. H2: Hydrogen

Hydrogen can be produced from a various range of materials and it provides energy with minimal air pollution. Within the waste water treatment process the organic waste as well as the high carbohydrate wastewater from breweries could be served as a source for the hydrogen production. The conversion process / the fermentation uses bacteria which need organic parts to produce hydrogen. So far the yields have been very poor and are usually about far beyond the theoretical maximum.

6. Electricity: Microbial Fuel Cells (MFC)

Microbial fuel cells are devices using bacteria as catalysts to oxidize organic and inorganic matter and generate current. In MFCs, bacteria are separated from terminal electron acceptors at the cathode so that the only means for respiration is to transfer electrons to the anode. Electrons flow to the cathode as a result of the electrochemical potential between the respiratory enzyme and the electron acceptor at the cathode. Electron transfer from the anode to the cathode is matched by an equal number of protons moving between the electrodes so that electroneutrality is maintained [7].

So far, microbial fuel cells have been developed only under lab conditions which are able to run small devices such as pocket-sized ventilators.


Based on present IWRM related approaches different concepts and adapted measures for modern wastewater treatment concepts in Tong Xa village have been developed. As shown above various options exist to implement renewable energy sources in existing WWTPs. The target of new concepts should be the implementation from the very beginning. Adapted solutions should secure on the one side a reliable and save wastewater treatment and on the other side optimised energy concepts. Besides an enhanced and sustainable use of natural energy sources these concepts can guarantee an improvement in refinancing modern the increasing costs for modern wastewater treatment.

Modern decentralised approaches do consist of interdisciplinary methods and should not only focus on the single water problem. One example of modern approaches is shown in Figure 2.

Figure 2. Wastewater treatment concept in Tong Xa by use of different energy resources [10]

Optimisation of biogas plant operation by novel DSS (decision support structures) systems (ref. to Fig. 3) will result in cost effective wastewater treatment. Especially industrial areas to be developed will benefit from these decisions related tool. Enterprise of certain industrial sectors could be using this tool to optimise a specific substrate load in their process chain. With these DSS suggestions including an optimised mode of operation can be made but also accounts for the necessity of future extensions and investments can be planned.


Figure 3. Tool for optimizing the operation of anaerobic treatment plants
 in mixed industrial areas


As has become evident in this article, there are different options for future wastewater treatment facilities in order to cope with the demands arising from climate change and increasing commodity prices. Depending on location, legal regulations, sewer systems and specific demands, solutions range from biogas utilization to heat pumps. Before designing an energy installation, the identification of barriers is mandatory for a sustainable planning and appropriate measures at different levels (local- regional- national).

Recently, major parts of applications for renewable energies require high investment costs, while the market energy prices for consumers of RE are - compared to those of energy from fossil carriers- still too low in order to create good market entry conditions and to push the use of RE. In Vietnam, the use of RET is politically backed up thus the basis for using RE is created. Especially in the two regarded regions, the industrial zone (My Trung) and the rural region (Tong Xa) the identification of barriers are important to choose the appropriate technology to recover energy and thus also to save costs. Biogas recovery and the use of the heat from sewage sludge could be alternatives in future industrial regions. In the rural regions the use of sludge from the small household facilities for energy purposes is very typically, but often does not meet the increasing demand, one alternative could be to plan larger decentralised systems for whole residential quarters based on shared waste water treatment and finally biogas recovery.


1. Environmental research of the Federal Ministry of the Environment, Nature Conservation and Nuclear Safety, 2007. Promoting Renewable Energy Technologies in Developing Countries through the Clean Development Mechanism

2.   GEC Sanitation Programme,, 10.05.2008

3. GTZ/Abteilung Umwelt und Infrastruktur Sektorvorhaben TERNA Windenergie, 2007. Energiepolitische Rahmenbedingungen für Strommärkte und erneuerbare Energien. Eschborn.


5. International Energy Agency (IEA), 2006. World Energy Outlook 2006. Paris.

6.  Länderbericht Vietnam, 2005.  Private Universität Witten/Herdecke gGmbH.

7. Logan, B.E.; Hamelers, B. et al. 2006. Microbial Fuel Cells: Methodology and Technology ENVIRON. SCI. & TECHNOL. Published on Web 07/14/2006.

8. Parliamentary Office of Science and Technology,, 31.07.2007.

9. Rosenwinkel K.H., 2006. Energieverbrauch und -erzeugung in der Wasser-, Abwasser- und Abfallwirtschaft Nutzung erneuerbarer Energien. Wasserwirtschaft im Wandel, oral presentation, 29.11.2006, Berlin.

10. iaks, 2008. Tong Xa concept, internal project information.