Cellulose – which comes fom toilet paper – is sieved from the raw wastewater (influent) by means of a fine sieve. This reduces the load of solid matter in the subsequent biological wastewater treatment process, thus reducing the energy consumed for biological treatment.
The CADoS research project is seeking to enhance significantly both the water line and the sludge line. The process reduces operational costs in the water line while it also greatly simplifies the processing of sludge. At the same time it creates possibilities for using the separated sieved material as a raw material for biogas and for the production of bioplastics.
Wastewater is cleaned with bacteria in the biological treatment process of the Ulrum wastewater treatment plant. The dirt in the wastewater is food for the bacteria. This causes the bacteria mass (sewage sludge) to grow. The surplus sewage sludge is thickened on-site and then taken by tanker trucks to a central processing plant at Garmerwolde in the province of Groningen. This is where the sewage sludge, after bio-fermentation with additives like polymer and iron chloride, is dewatered by a pressure installation. Subsequently, third parties thermally dry the dewatered sludge, after which it is taken by truck to cement furnaces in the south of the Netherlands. The furnaces incinerate the sludge as waste. The residual ash becomes part of the cement. The cement is a raw material for products like paving such as kerbstones and cement bricks.
Cellulose Assisted Dewatering of Sludge (CADoS)
A trial installation will be used at the Ulrum wastewater treatment plant mid-2014, which will reduce the operational costs of the existing treatment process in the water line. Even more important, it will significantly simplify sludge processing:
- existing processes such as the thickening of sludge will be eliminated;
- doses of chemicals used for sludge processing will be substantially reduced. The produced sieved material (containing about 40% dry matter) is a raw material that will be transported by road to a raw material customer in the region;
- the released waste will be reduced in volume closer to source and reused regionally as a raw material. This means fewer transport movements.
The fine sieve being used in the CADoS project is of the ‘rotating sieve belt’ type and is already in service for the mechanical pre-treatment of domestic wastewater. The fine sieve allows removal of the suspended matter present in the wastewater by means of filtration.
Organic material like food waste and kitchen and garden waste (KGW) is processed in various ways to create products or raw materials. This is done by composting or fermenting. Composting is an aerobic process with active bacteria that need oxygen to live. They decompose organic material and convert it to an earth-like mass, i.e. compost. If the decomposition of organic material occurs under anoxic conditions, it is called an anaerobic process. Bacteria are then active that convert the organic material into (for example) methane, water, carbon dioxide and nitrogen. Methane is particularly attractive, because it is also the principal element of natural gas and biogas. Methane is excellently usable as a fuel for the production of sustainable energy. The sieved material released in the CADoS process is an excellent raw material for fermentation.
Bio-fermentation involves filling steel reactor tanks of a few thousand cubic metres with organic material. By keeping the material in motion and ensuring ideal conditions like moisture content and temperature, the bacteria convert the material into products including methane. The released biogas is captured. Several fermentation techniques exist. Some systems use a lot of water and keep the material moving by means of agitators. A dry digester uses less water and pumps keep the fermenting material in motion. A third system is a horizontal digester, whereby a horizontally rotating agitator keeps the material moving slowly. All systems produce biogas that is usable as an energy source.
The biogas obtained from fermentation contains between 55 and 60% methane; the rest is mainly carbon dioxide. The gas is usable to good effect in special motors that provide electricity or heat. Upgrading biogas to green gas optimises the yield because no energy is lost in the form of heat, as in the case of electricity production. What’s more, every cubic metre of green gas supplied to the gas grid saves 1.8 kg of CO2.
Bioplastics from fermentation
Attero has a special two-stage fermentation installation at Venlo. Kitchen and garden waste is sprinkled with water in composting tunnels. This starts a natural process in the material, i.e. hydrolysis. Bacteria present in the waste convert organic compounds – like fats, proteins and carbohydrates – into fatty acids, amino acids and alcohol. These are the building blocks of organic compounds. One of their properties is that they are soluble in water. The tunnel is sealed off from the outside air. This enables staff to influence the climate in the tunnel in a way that optimises conditions for the bacteria and speeds up the process. These volatile fatty acids are the source of nutrition for the bacteria for the production of bioplastics. Attero thus has a unique link between KGW and bioplastics.
Production of bio-based raw materials for industry (including the chemical industry) is important with a view to sustaining the Dutch economy. The CADoS project is facilitating this sustainability process.
With CADoS we plan to achieve both economic and technological advantages.
Economic advantages at treatment plant:
- lower total energy consumption;
- less consumption of chemicals;
- lower sludge volume, less sludge transport;
- smaller treatment plants in future.
Economic advantages with sludge valorisation:
- biogas in the fermentationsieved matter;
- opportunities for further valorisation of examinations of higher-value commodities (say: board or bioplastics)
- collecting data to substantiate the technology in order to bring them further on in the market;
- increase biogas production, reducing CO2;
- optimize operation of water treatment plants;
- footprint in wastewater treatment.