All life on Earth is based on the global cycle of water. Often, however, there is too much or too little water, or the rain comes too early or too late, or the quality of the water is so bad that it endangers life. In the 1970s, Finland was faced with water resources of rapidly decreasing quality. The ecologically vulnerable lakes were being exposed to high levels of industrial, agricultural and household pollutants, making it necessary to search for new and innovative ways of purifying the country’s water resources. The dissolved air flotation (DAF) process is one of these solutions, and has been established at over 200 plants across Finland.
‘We are in the midst of a water crisis that has many faces … the overriding problem is one of water quality and management.’
(UN World Water Development Report, 2003)
Water resources must be understood in terms of the water cycle. Water resources are renewable only to a limited extent, as water usually flows through catchments that are self-contained. Water resources are also variable over time and space, with huge differences in availability and quality for different parts of the world and during different seasons. The variability of water quality is one of the essential characteristics of water resource management. Most efforts are intended to overcome this variability and to reduce the unpredictability of water resource flows.
Water quality depends on a number of interrelated factors, including geology, climate, topography, biological processes and land use, combined with the length of time the water has been present. Over the past 200 years, human activities have infringed upon the water cycle to such an extent that there are now few examples of natural water bodies. Industrial development and the intensification of agricultural practices have produced waste products that are transported by surface water, groundwater and atmospheric processes into water storage systems.
In 2003, the UN World Water Development Report stated that 6000 people, mostly children under the age of 5, were dying every year from water-related diseases. About 1.4 billion people lack safe drinking water. In the industrialised world, water-related risks are not usually due to insufficient hygiene, but arise from products of the water treatment process. The harmfulness of these products is based on long-term consumption. They are often created by reactions between organic matter and chlorine. Organic material from domestic sewerage, municipal waste and agro-industrial effluent is the most widespread pollutant globally. Organic matter can directly contaminate water, but can also contain high concentrations of nutrients, particularly nitrogen and phosphorus, and this results in abnormal plant growth which depletes oxygen, so killing animal life (a process known as eutrophication).
Water Sources in Finland
Almost 200,000 lakes cover 10 per cent of Finland’s land area of 338,000 km2. Although the water quality in most of these lakes is good enough for drinking, it has not always been this way. In the 1970s, Scandinavian lakes were suffering from the effects of intensified agricultural production and European industrial processes. The environmental outlook was poor. Finland’s groundwater resources are small and shallow, and without rivers to carry pollutants to open sea, they are particularly vulnerable to pollution. Thus, Finland has had to cope with its own discharges, which eventually end up in the sensitive Baltic Sea, as well as suffering the acidification of waters by airborne pollutants from Western Europe. However, large investments in advanced environmental technology, which began in the 1960s, have restored Finland’s clean and reliable water sources. Finland is now one of the world’s leading countries in environmental technology. The United Nations World Water Development Report ranked Finland first out of 122 countries in terms of the quality of water and the ability and willingness to improve it. Finland was also first of 147 in the overall Water Poverty Index, calculated according to water use.
The concentration of natural organic matter in Finnish waters is 2.5-fold compared to the world average. In Finland, organic matter is removed by biological means, while phosphorus is removed by chemical means. The biological removal of nitrogen is also becoming more common. Biological processes have been adopted by the Finnish Water and Waste Water Works Association (FIWA). However, the northern location of Finland means that the application of biological solutions is a specific challenge, since low temperatures slow down the rate of the biological process. The biological removal of phosphorus is not well understood (certain cells of biomass store far more phosphorus than normal), in spite of operating full-scale treatment plants.
Finnish Water and Waste Water Works Association (FIWA)
The FIWA is a nationwide joint organisation of water and wastewater works. The members of FIWA cover about 85 per cent of the volume of the Finnish water services. The main duties of FIWA are:
- To promote the common interests of its members;
- To prepare technical and administrative guidelines for its members’ use;
- To promote research activities;
- To provide information;
- To advise and to help its members in technical, administrative and juridical matters;
- To provide supplementary education and training courses for water services personnel;
- Management of international affairs.
The representatives of the member utilities participate in FIWA’s activities through several Working Groups. FIWA cooperates with national and local authorities and research institutes. FIWA is the representative of Finland in the IWA (International Water Association)and in the EUREAU (European Union of National Associations of Water Suppliers and Waste Water Services).
Finland’s Water Services Technology Programme
‘We know the problem: it is one of management.’
(UN World Water Development Report, 2003)
Finland has tackled the problem of water management directly through its Water Services Technology Programme. Funded by the National Technology Agency of Finland (Tekes), the programme was first implemented from 1997 to 2001. It targeted the development of the Finnish water services sector. The primary objectives of the programme were:
- Improvement of the technological competitiveness of businesses working in the field of water services;
- Introducing new technologies at water and sewage works;
- Development and introduction of new products that are designed to satisfy the water service needs in rural areas;
- Promotion of research and development activities in Finnish water services.
These objectives were achieved through the development of innovative equipment and new processes, thereby testing new technical solutions to water treatment. This was facilitated by the co-operation between research units, water and sewerage treatment works, private companies, and regulatory authorities. The programme targeted the technical development of water services for communities and rural areas; issues regarding industrial water services were not included unless they had a direct impact on the community water services. The key issues of the programme were:
- Water treatment technology that ensures good quality drinking water;
- Maintenance and rehabilitation of water supplies and sewerage systems;
- Wastewater treatment and sludge disposal;
- Water supply and sanitation in rural areas;
- Pre-treatment technologies for treating local pollution sources of the community sewerage networks.
The programme has resulted in improvements to the quality and productivity of the services provided by the water and sewage treatment sector in Finland. Significant new technical innovations have been implemented. Functional solutions have been discovered to resolve water service-related problems faced by households in rural areas. A particularly successful example of such innovations is the dissolved air flotation (DAF) process, which has become a Finnish speciality in water treatment.
Dissolved Air Flotation
Traditionally, sand filtering and chlorination have been used to purify the water from inland sources. Prior to the 1970s, energy was relatively cheap and so obtaining fresh water and releasing wastewater presented no real problems. At the time, there was little incentive for looking into alternative water treatment techniques. However, the situation changed as energy and chemicals became more expensive and an increased concern about environmental pollution produced an increase in taxes on water and wastewater dumping. This marked the end of cheap and easy water management. It was clear that there was not much room for improvement with chemical water treatment. With chlorination, regular replacements of large amounts of water are necessary to prevent the build-up of potentially hazardous amounts of chlorinated hydrocarbons and organic carbon.
The dissolved air flotation (DAF) process is a very efficient, biological clarification process. In Finland, the Rictor company has undertaken research into flotation processes since 1965 and there are now over 200 flotation plants in the country. In the DAF process, the introduction of small bubbles separates suspended solid matter in the water. These bubbles float the material to the surface of the tank, a process known as ‘flocculation’.
Pumping the material-containing water at a high pressure and then mixing it with compressed air in the ‘contact zone’ tank achieves this flocculation. When the pressure is released, the dissolved air precipitates as a cloud of micro bubbles, which attach to the particulate matter and so cause it to rise to the surface. The attraction between the air bubbles and particles is a result of adsorption forces, which are due to the characteristics of the particle surface. This attachment of bubbles to the particle reduces the density of the particle, resulting in increased buoyancy. The rising flocks (gathered filaments of material) form a sludge blanket, which is removed into a sludge channel. The purified water is led through a separate channel at the bottom of the basin, known as the separation-zone.
Dissolved air flotation characteristics
- The volume of air released into the contact zone (where air meets the water and material) must be sufficient to attain positive buoyancy – i.e. to make the flocks float. The more flocks, the more air is required.
- The ‘air-adhesion efficiency’ indicates how well the air bubbles attach themselves to the flock. This determines how much air is required.
- Different plants have very different air flotation efficiencies.
- Not all sludge responds in the same way to DAF processes.
- The higher the sludge volume, the lower the level of flotation that can be acquired. This has produced two categories of sludge: ‘poorly-settling sludge’ and ‘normal sludge’.
- The contact zone of a DAF reactor is the zone where sludge particles and air mix for the first time to form flocks.
- The intensity of mixing and time of mixing are of importance, but there are no generally accepted specifications for the contact zone size or input speeds and timing.
- Motionless conditions are required for the flocks to float to the surface.
- The depth of the flotation tank is not considered to have a direct influence on the sludge-thickening performance, unless the tank becomes excessively shallow.
- The float-layer is the layer of sludge resting on the water in the separation zone that is to be removed.
- The most important thickening mechanism is the draining of excess water from the part of the float layer above the water line.
- An excessively deep build-up of sludge below the water level could increase the flow of water under the float layer. This may cause the float layer to be eroded from below.
- The scraper blade cuts cleanly into the float layer and only slices off the top part of the layer, without disturbing the rest of the float layer.
- Problems can include ‘knockdown’ (parts of the float layer break loose when the scraper moves over the sludge), ‘depression’ (the float layer is pushed downward by the scraper and pushes up again after the scraper has passed), and ‘rolling’ (the float layer breaks into strips which roll and partially decompose as the scraper passes over).
- If these problems occur, the float layer is mixed with the underflow of water. The thinner the float layer, the more severe this mixing becomes. The depth of the float layer should be at least 150 mm above the water level to prevent mixing.
The DAF process is particularly well suited for the removal of flock formed in the treatment of low alkaline, coloured water. This type of flock tends to be very fragile and voluminous, making traditional gravity sedimentation ineffective. Flotation processes do not require large, heavy flock in order to achieve efficient solids removal. This results in lower chemical dosages and reduced time required for flocculation. The compressive forces applied to the sludge by the flocks rising from below result in greatly reduced volumes of wastewater. This enhances the efficiency of chemical use. An equally important benefit of the technology is the efficiency of the clarification process. Since the performance of the filtration process is directly affected by the amount of solids in the clarified water, the high degree of solids removal in the DAF process results in an overall increase in system performance.
The Finnish Product
Heavy winter snow and thick ice-cover on nearly 200,000 shallow lakes is a key element of Finland’s idyllic Nordic landscape. The ecologically sensitive lakes have stimulated Finland’s technological innovators to develop new methods and equipment to protect and benefit the natural environment. During 30 years of research, Rictor has produced many specialised technologies for purifying natural waters and wastewaters. The compact, efficient and economic DAF process is one of these solutions. A concerted effort by Finland’s authorities (such as FIWA and Tekes) and research institutions (such as Helsinki University of Technology) means that these solutions are continuously being adapted to Finland’s changing conditions. While DAF processes offer an environmentally sound and efficient technique, emphasis on the future means that new solutions and improvements to established technologies must always be the target.
Dudok van Heel, W. H. and van der Toorn, J. D. (1998). ‘A biological approach to water purification: a practical application: the Delfinaario in Tampere, Finland’. Aquatic Mammals. 14(3): 92-106.
Haarhoff, J. and Bezuidenhout, E. (1999). ‘Full-scale evaluation of activiated sludge thickening by dissolved air flotation’. Water SA. 25(2):153-166.
Kuronen, R., Ødegaard, H. and Brooker, N. (2002). Water Services 1997-2002: Final Report. Helsinki: National Technology Agency.
United Nations and the World Water Assessment Programme (2003).World Water Development Report: Water for People, Water for Life. Oxford: UNESCO.
Finnish Water and Waste Water Works Association (FIWA) www.vvy.fi
Helsinki University of Technology www.aalto.fi
National Technology Agency of Finland (Tekes) www.tekes.fi
Donor and Supporting Organisations
Department for International Development (DFID) www.dfid.gov.uk
World Bank www.worldbank.org
European Union of National Associations of Water Services (EUREAU) www.eureau.org
ITDG Technical Briefs answers.practicalaction.org
International Water Association (IWA) www.iwa-network.org