2. Chemical Precipitation in Fellingsdams
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2008:104 PB MASTER’S THESIS Fellingsdams An investigation of wastewater treatment by chemical precipitation in ponds in northern Sweden Wen Zhang M.Sc. in Environmental Engineering CONTINUATION COURSES Department of Civil and Environmental Engineering Division of Sanitary Engineering Universitetstryckeriet, Luleå 2008:104 PB • ISSN: 1653 - 0187 • ISRN: LTU - PB - EX - - 08/104 - - SE Luleå University of Technology MASTER THESIS Fellingsdams - An investigation of wastewater treatment by chemical precipitation in ponds in northern Sweden WEN ZHANG Master of environmental engineering Department of Civil, Mining and Environmental Engineering Luleå University of Technology Abstract Fellingsdams are widely used in northern Sweden as an economical and easy operating method to treat the wastewater. They are divided into in-pond and pre-pond systems by different locations of dosage. Aluminium salts, iron salts and slaked lime are the most common coagulants that have been used, since they are cheap and easy to react with water. Turbidity and detention time are two important factors in fellingsdams. Turbidity is mainly influenced by precipitation and algae. From the perspective of actual detention time, the pond geometry should be long and narrow. When using slaked lime as coagulant, there is usually a large amount of sludge accumulated around the influent pipe because of fast precipitation. In the fellingsdam at Nyliden, 90% of the total sludge was found accumulated in about 20 meters. Aluminium salts and iron salts react slower with water, so the formed sludge curves are modest. To remove the sludge, excavation and pumping are the choices but differs due to different conditions. Dewatering is the main way to treat the sludge. The main task for a fellingsdam is to remove the organic carbon, nitrogen and phosphorus. The effluent values from yearly environmental reports 2001-2007 were analyzed, most values were below the demands. The coliform bacteria concentration is a problem in the effluent water. A high pH environment guarantees low bacteria concentration in the effluent. Laboratory analyses were made on samples from a few aluminium fellingsdams and iron fellingsdams, only the pond in Hede reduced the coliform bacteria concentration below 10 cfu/100ml. Mellansel fellingsdam, an iron pond, had a high value of 6600 cfu/100ml in the effluent. Future utilization of the effluent water is also discussed; irrigation is one of the uses. The energy forest in Vika Strand, Falun is a good example. Luleå University of Technology Acknowledgements The investigation includes lots of field work and data analysis, thus the field cooperation and data collection are very important. I would like to express my gratitude to all the managers and operators at Örnsköldsvik, Härjedalen, Östersund, Ljusdal and Bollnäs municipalities; they provided a detailed background information of each fellingsdam during the field investigation, and also presented the environmental reports. I appreciate my supervisor Professor Jörgen Hanaeus for offering me this opportunity to work with him. I improved a lot through this study, the theoretical knowledge I learned at the university became practical. We visited a lot of fellingsdams to get water and sludge samples. It was hard to take samples from the frozen ponds in cold winter, and sometimes we need to row a rubber boat to get samples in the ponds. But I realised that I have never learned that much, the theoretical knowledge is important but not enough, whereas practical experience can be harder and challengeable. And also I am grateful to his patient guidance and unconditional support. Finally, I would like to give my special thanks to my parents whose patient love enabled me to complete this work. Wen Zhang Nov. 2008 Luleå Luleå University of Technology TABLE OF CONTENT 1. BACKGROUND INFORMATION………………………………………………1 1.1 Fellingsdams..…………………………………………………………………..1 1.2 Fellingsdams in Sweden………………………………………………………..1 1.3 Aim and method of this investigation………………………………………….1 2. CHEMICAL PRECIPITATION IN FELLINGSDAMS………………………….3 2.1 Pre-pond and in-pond systems…………………………………………………3 2.2 Transporting process…………………………………………………………...4 2.3 Chemical precipitation…………………………………………………………6 2.3.1 Aluminum salts……………………………………………………………6 2.3.2 Iron salts………………………………………………………………....7 2.3.3 Slaked lime …………………………………………………………..….8 2.4 Turbidity………………………………………………………………………..9 2.5 Detention time………………………………………………………………...14 2.6 Investigated fellingsdams……………………………………………………..15 2.6.1 Lime fellingsdams………………………………………………………..15 2.6.2 Aluminum fellingsdams……………………………………….…………20 2.7 Suggestions……………………………………………………………………24 3. SLUDGE ACCUMULATION…………………………………………………….26 3.1 Sludge quantities……………………………………………………………...26 3.1.1 Sludge depth in fellingsdams…………………………………………….26 3.1.2 TS and organic content of the sludge…………………………………….32 3.2 Sludge removal and treatment………………………………………..……….38 4. REMOVAL OF ORGANIC MATTER, PHOSPHORUS, NITROGEN AND BACTERIA…………………………………………………………………….…….43 4.1 Organic matter………………………………………………………...………43 4.2 Phosphorus…………………………...……………………………………….46 4.3 Nitrogen……………………………………………………………………….47 4.4 Bacteria…………………………………………………….………………….49 5. FUTURE DEVELOPMENT……………………………………………………....50 CONCLUSIONS…………………………………………………………………....51 REFERENCE……………………………………...…………………………………52 APPENDIX Luleå University of Technology 1. Background information 1.1 Fellingsdams A fellingsdam also called “a pond or a lagoon using chemical precipitation”, is a pond system to separate nutrient and organic matter by adding coagulant. The main process of this wastewater treatment method is chemical precipitation. Compared with most other methods of treating wastewater, a pond system provides several advantages. It is a simple process which is easy to operate and inexpensive. Meanwhile the drawback of fellingsdam is that to achieve certain discharging water quality, a reasonably large area is necessary. So the method is not suitable to apply in cites of high population density. The treatment of sludge which is generated during the chemical precipitation should also be considered. The difference between fellingsdam and common wastewater stabilisation ponds system or lagoons is the chemical dosage. Sunshine is an important driving factor for a pond system, so during winter time, when the pond is covered by snow and ice, it will not work. Since the operation of a fellingsdam mainly relies on a chemical reaction, it can work quite well during a winter period. 1.2 Fellingsdams in Sweden The use of ponds or lagoons for wastewater treatment can be traced back to centuries ago. For instance, ponds have been used in China for more than 2000 years (Baozhen, 1987). A use of the fellingsdam method to treat the wastewater is rather common in northern Sweden, because the population in the north is small and the area is huge. In cold region, lagoons do not work well in winter; add chemical coagulant can help to precipitate organics, nitrogen and phosphorus. The population density in these areas is small and most of these regions are covered by forest which makes fellingsdam a better solution to treat wastewater compared with a compact wastewater treatment plant. Besides, it is also more economical. A number of fellingsdams were built several decades ago in Sweden. Most of them are in good condition and still in operation. 1.3 Aim and method of this investigation To investigate the operation conditions of the existing fellingsdams and to find out any latent problem are the aims of this task. Some suggestions and ideas for both existing fellingsdams and future constructions. Fellingsdams from 5 municipalities were investigated namely Örnsköldsvik, Härjedalen, Östersund, Ljusdal and Bollnäs. In a total of 16 fellingsdams turbidity and - 1 - Luleå University of Technology sludge samples were taken for laboratory analysis. 6 of them were lime ponds, and 10 of them were aluminium ponds. One iron pond located at Mellansel, was also investigated regarding the effluent quality. More of fellingsdams information was collected from yearly reports of each municipality. - 2 - Luleå University of Technology 2. Chemical precipitation in fellingsdams 2.1 Pre-pond system and in-pond system According to the location of dosage, fellingsdams can be divided into two different groups: pre-pond systems and in-pond systems. These two different fellingsdam systems are shown in figure 1. Influent Influent Dosage Dosage Effluent Effluent a) Pre-pond system b) In-pond system Figure1. Location of dosage at fellingsdams Pre-pond system is to add the chemical close to the inlet of the first pond. There is sometimes no pre-sedimentation tank, all the precipitation will occur in the ponds. In-pond system is to add the chemicals within the ponds. There can be one or more pre-settling ponds before dosing. Use of the pre-settling pond is to remove big particles from the wastewater and achieve a better treatment result. There are advantages and drawbacks for both of these two different fellingsdams. For the pre-pond system it is easier to mix the chemicals into the wastewater without any installation of specific mixing facilities by using the kinetic energy of the water at a pumping station or in a sloping inlet pipe. It is also easier to include organic matter in the precipitation since it may be less dissolved at the inlet end of the pond. Furthermore, when lime is used as the coagulant, a high pH can be kept throughout the pond system which prevents the formation of hydrogen sulfide (H2S). H2S create an unpleasant