OVERVIEW: ADVANTAGES OF USING DEMINERALIZED AND DEAERATED MAKE-UP WATER IN DISTRICT HEATING SYSTEMS • The risk of corrosion is reduced because the corrosion process requires salts and oxygen to react. • By reducing the salt content in the water, the risk of microbiological growth in the district heating system is reduced, which again reduces the risk of microbiological corrosion. • The need for chemicals is significantly reduced (typically more than 90%) because the “buffer” in the water is reduced by reducing the salt content. The “buffer” defines how much NaOH is needed to change the pH. Water with a low buffer gets effected by NaOH more easily than water with high buffer. Therefore, less chemicals are needed for demineralized water than just softened water. • Using demineralized water in the district heating system makes detection of raw water break-in much more efficient
Demineralization and deaereation plant at Aars District Heating. The complete make-up water treatment plant consists of a water softener plant, a reverse osmosis unit (RO), a membrane degassing unit (MDU) and a dosing station for dosing NaOH. The RO and MDU units are shown here.
than in systems with just softened water. This is because raw water break-in into demineralized water creates bigger fluctuations in the parameters of the system. • By using demineralized water with low conductivity instead of softened water, the corrosion which will occur when raw water breaks into the system will be the relatively unharmful general surface corrosion instead of aggressive local corrosion. • An MDU-plant removes oxygen and carbon dioxide in the demineralized water by means of membrane contactors, a vacuum pump and nitrogen as a sweep gas. This method is very efficient and reduces the oxygen content in the demineralized make-up water from 5-10 mg O2/liter to below 0.02 mg O2/liter.
GRAPH 1: CORRELATION BETWEEN PH, CONDUCTIVITY AND SALTS
CASE STUDY: PRACTICAL EXPERIENCE WITH IMPROVING THE WATER QUALITY IN A DISTRICT HEATING SYSTEM. Aars District Heating Company is a CHP plant that produces and distributes electricity and heat for about 5500 households. Heat is transmitted through one transmission line to 6 distribution networks. The make-up water was previously produced in a water softener and deaerated in a traditional vacuum deaerator. In July 2018, the water treatment strategy was changed. Today the make-up water is demineralized. The make-up water is produced in a central water treatment plant, and all make-up water is added to the transmission line. All distribution networks get their make-up water from the transmission net. Since then the use for oxygen scavenging chemicals has been eliminated completely. In addition, the use for chemicals for pH adjustment has been reduced 95%.
Graph 1 shows the development in the water quality in the transmission line in the period July 2018 to October 2019. In this period, the water quality has changed from softened water to demineralized water. Thus it follows the “Recommendations for Water Treatment and Corrosion Prevention”, authored by the Danish District Heating organization. In the beginning of January 2019, a system leak occurred and as an emergency, soft water with higher conductivity was used as additional make-up water. This can be seen as the increased content of chloride, sulfate and conductivity. A slight increase in the content of sulfate can be seen October 2019. A possible explanation for this is that sulphate reducing bacteria had been reduced due to the improved water quality. The reason is that harmful bacteria simply do not get any nutrients (salts) to support life. Therefore, the bacteria do not any longer “eat” the sulphate. This explanation is supported by measurements in the content of sulphide (not shown on the graph). In the beginning of the period the level of sulphide was 0.5 mg/l but in October 2019 it was reduced to below the detection limit.
D I S T R I CT ENERGY - SUS TA I NAB L E C I T Y T RANS FORMAT I ON
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