Pioneering technology to reduce nitrogen emissions

Elmo ceo, Nalle Johansson is proud of his company's achievements in reducing the environmental impact of emissions to air and water. He told Leather International that the company had been pursuing dynamic environmental policies for many years and the environmental management system has independent certification. He explained that 'by changing processes and chemicals and using traditional techniques, we had come as far as was possible in tackling nitrogen pollution in our wastewater. To make further progress and achieve further improvements, we needed to introduce a completely new treatment process for the tanning industry.' This goal has been realised with an impressive investment of approximately €5.45 million, of which around €900,000 was contributed by the EU's LIFE environmental fund. The investment underlines the company's commitment to production in Sweden. At the inauguration of the new plant, Johansson stated he shared the concern about the increasing trend towards moving industrial production and, therefore, jobs out of Sweden. He emphasised that Elmo had done the opposite in recent years, concentrating production in Svenljunga by shifting manufacturing back to Sweden from Denmark and the US. However, Johansson stated: 'That does not mean that we will exclude the possibility of locating certain parts of production, such as die-cutting and sewing the finished leather, closer to our customers or in places where it is cheaper to perform them. Our core activity, however, the actual manufacturing of leather, will remain in Svenljunga.' New technology/biological treatment of wastewater The technology behind the new plant is based on nitrification/ denitrification, an advanced form of biological treatment. The technique was originally used by municipal treatment plants but had never previously been employed in the tanning industry as the properties of tanning industry wastewater were considered to be technically unsuitable. The process involves pollutants in the wastewater being broken down by micro-organisms in a series of tanks. Oxygen is added to the wastewater to increase the number of micro-organisms. In the next stage, the oxygen is cut off and the micro-organisms are forced to eat the pollutants in order to survive. As a result, nitrogen emissions are reduced by 80% as most of the nitrogen-containing pollutants are converted into nitrogen gas which appears naturally in the atmosphere. This is a major advance compared with traditional tanning industry wastewater treatment which cuts nitrogen emissions by around 30%. The process Wastewater will be pumped from Elmo's tannery via two pump stations to the new treatment plant. The water will pass through a 2mm grid before being pumped from the first station (T21 in Figure 1). This is an existing pump station that Elmo will take over from the municipality. The second pump station (T01) will pump the wastewater up to the first treatment stage. Stage 1 - Biological treatment The wastewater is first pumped into a 2,000m³ tank (T02.1). The tank has two functions: acting both as an equalisation tank for flow and pollution content, as well as an oxygen-infusion tank. Micro-organisms in the water oxidise a large amount of the sulfides and other organic material in the water. Oxygen is introduced via robust mixers to ensure stability in the process. Oxygen supply is regulated by a redox-meter located in the tank. Phosphoric acid is added to the water to provide sufficient phosphor to ensure the optimum nutritional balance necessary for the micro-organisms that will consume the pollutants in the wastewater. The pH value can be adjusted by adding either calcium hydroxide or sulfuric acid. If necessary, a special agent can be added to reduce excess foam. The wastewater then travels from the first aeration tank to a sedimentation tank (T04) via a deaeration tank (T03) where iron chloride (FeCl₃) and a polymer can be added to improve sedimentation. In the sedimentation tank, a large amount of the suspended substances is eliminated. Part of the sludge formed in the sediment is pumped via the sludge pump station (T05) back to the first treatment stage (T02.1). This step recovers activated sludge and nutrient and thus improves the overall effect of the treatment process. The remaining sludge is pumped to the sludge holding tank (B01) where dewatering takes place. The treated water is pumped onward via an intermediate pump station (T06) to the second tank (T02.2) where further oxygen infusion can take place if necessary. The water is pumped in stages, depending on which treatment process is used in T02.2. Stage 2 - Biological treatment Final processing of the wastewater takes place in the second tank (T02.2). The tank has a volume of 5,100m³ which allows space for nitrogen reduction. At this stage, the nitrogen mainly consists of ammonia. Nitrogen reduction takes place in two stages: Nitrification - ammonia (NH₃) is reduced to nitrogen gas which is then taken up by the surrounding atmosphere. This process requires anaerobic conditions and is dependent on access to organic material in the wastewater. Denitrification - nitrate (NO₃) is reduced to nitrogen gas which is then taken up by the surrounding atmosphere. This process requires anaerobic conditions and is dependent on access to organic material in the wastewater. During the nitrification process, oxygen is supplied through an ejector-aerator at the bottom of the tank. The aeration tank is equipped with an oxygen meter that ensures the required amount of oxygen is introduced, minimising energy consumption. In order to ensure optimum efficiency of the nitrification process, treated water from the municipal treatment plant can be added to the tank to dilute the water. To maintain the temperature required for optimum nitrification, the municipal water can be heated with a heat exchanger (T13) that utilises the heat in the water leaving the new plant. If the temperature gets too high, municipal water is added to cool the tank down. The switch between aerobic and anaerobic conditions is made by turning off the aeration process during nitrification. Anaerobic conditions occur relatively quickly because the wastewater is still highly oxygen-consuming. The tank is equipped with a mixer to optimise energy consumption and ensure the best mix conditions during nitrification periods. Phosphoric acid can be added to the water here, too, if there is not enough phosphor present after stage 1. The pH value can be adjusted by adding either calcium hydroxide or sulfuric acid. The wastewater is then gravitated from the second aeration tank to a sedimentation tank (T08) via a deoxygenation tank (T07) where iron chloride (FeCl₃) and a polymer may be added to improve flocculation and sedimentation. The suspended substances are eliminated in the sedimentation tank. The sludge formed in the sediment is pumped via the sludge pump station (T09), in part, back to the second treatment stage (T02.2). This step is performed to recover activated sludge and nutrient and thus improve the overall treatment effect. The remaining sludge is pumped to a dewatering step, the sludge holding tank (B01). The treated water is pumped onward to a disc filter (T18), which has a maximum size of 10m to ensure that levels of remaining suspended substances are low. Final testing A flow-controlled sampler is used in the well after the disc filter to check the pollution levels of the treated water. Sludge handling The resulting sludge is pumped to the first sludge treatment plant and then collected in a tank (B01), where the pH level can be adjusted by adding lime. A polymer is added and the sludge is dewatered in centrifuges. After dewatering, the sludge is stabilised using lime and collected in containers for transportation to the municipality's waste management station at Moga. The plant has been designed to meet the wastewater standards shown in table 1: