Feasibility of wastewater treatment in collection networks

Dissertation for Master's degree

In the field of civil engineering, environmental engineering

Abstract

In this research, the effect of biological reactions on the removal of organic substances in wastewater during transfer from production sources to the treatment plant in conventional networks and modern small diameter networks was investigated. For this purpose, a part of these sewage collection networks was designed and built. The pipes used are made of PVC and the length of the pipes reaches 15 meters in total. Also, the diameter of the pipes used was 0.1 m, which was designed by gravity. In order to create a suitable surface for the attachment of microorganisms, a plastic net was attached to the inner surfaces of the tube as a mediator. The contact surface of the sewage with the nets, when the pipes were full, was 3.77 square meters. In order to carry out tests related to the removal of organic substances, synthetic wastewater with the same pollution intensity as urban wastewater was used. The built model was operated under aerobic conditions and the temperature changes of the wastewater were in the range of 3-20. After three weeks have passed since the start-up, the tests related to the removal of organic matter including BOD, COD, TN, NH3-N and NO3-N were performed. The surface of the created biofilm was irregular and its average thickness was between 3-4.7 mm. The surface density of the formed biofilm was between 22.3-33.1 grams of biomass per square meter (dry weight). After 3 weeks, the oxygen consumption rate reached almost 0.21 mg/L/min. The highest reduction was in COD concentration with a 77% reduction. Also, the highest BOD5 removal rate was 73%. Due to the high concentration of COD and the short residence time, the nitrification process did not take place and the changes in the concentration of nitrogen compounds were insignificant. The results indicate that if the conditions are ready, the rate of biological reactions in the collection networks is relatively high and with proper planning, this potential can be used optimally. In remote and mountainous areas where the construction of a treatment plant faces problems due to implementation problems, and also in areas where it is not possible to build a treatment plant due to lack of capital, this approach can largely prevent pollution from entering the environment. Biofilm

                                                        Introduction In this chapter, we have a brief look at the importance of research in the first place and a reference is made to the role of collection networks in sustainable development. The general physical, chemical and biological processes that occur in wastewater collection networks and their importance are discussed. Finally, the necessity of the research, hypotheses and objectives of the research have been stated.

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Today, sewage collection networks are considered one of the important infrastructures of human societies and play an important role in the development of cities. The main role of these structures can be considered as collecting sewage from the cities, preventing the spread of epidemic diseases and providing public health conditions. The operating cost of sewage collection networks is very high and includes almost 75% of the costs related to the overall process of sewage treatment. Therefore, the maintenance of these facilities is very important.

The optimal and efficient design of wastewater collection networks plays an important role in sustainable development plans. These facilities are very expensive and in case of problems such as corrosion in these networks, the executive management will bear heavy costs. The release of hydrogen sulfide gas in wastewater collection networks causes problems such as the release of dangerous gases into the atmosphere and the corrosion of sewage pipes and transmission facilities [1]. The role of today's wastewater collection networks, which were used to collect wastewater since the middle of the 19th century, has only been to transfer wastewater from production sources to the treatment plant. Research has shown that the composition of wastewater is constantly changing during transportation [2]. These changes in the quality of wastewater are caused by physical, chemical, or biological processes that occur in the network, but nowadays the impact of these processes is not taken into account when designing and operating networks.

In rainy conditions, hydraulic phenomena and the transfer of solid waste materials are very important, while in these conditions, biological and chemical processes are usually less important.However, in non-rainfall conditions, which prevail almost 95% of the time in many countries, biological and chemical processes may have an impact on the performance of the sewer and the interaction between the sewer and the subsequent treatment processes in the treatment plant.

Probably because the activities of researchers and executive agents are more dedicated to the rainfall conditions, the biological and chemical performance of the collection network has been less attention. However, it is clear that the biological and chemical processes of wastewater during transport cannot be ignored. These processes may initially have effects on the performance of the collection network itself and then on the treatment plant facilities, the environment and people who are in direct or indirect contact with the wastewater. Most of the existing researches in the field of the collection network are dedicated to the planning, design, operation and maintenance of these networks, and less attention has been paid to the aforementioned reactions in scientific activities.

The processes that occur in the collection network have different phases that generally have a complex system. These processes may occur in different phases, including the fluid phase, the formed biofilm phase, the sewage sediments phase, the air in the network and finally the sewer wall phase [3]. These processes have a significant impact on the urban atmosphere, for example, aromatic compounds may be dispersed in the urban atmosphere. Also, wastewater treatment plants and local wastewater receiving systems are affected by physical, chemical, and biological reactions in networks. In addition to receiving materials discharged into the network, these facilities also receive products resulting from network processes such as sludge and purified water.

There are several examples that show the importance of these processes, for example, the effect of sulfide under anaerobic conditions is known. Sulfide is a serious danger to humans because it is smelly and toxic and may also cause corrosion problems in the network [4]. In addition, anaerobic conditions may cause the production of easily degradable substrates that disrupt phosphorus removal and denitrification in the treatment plant and increase the need for treatment plant facilities. If the collection network is under aerobic conditions, these easily degradable organic materials are removed and particles are produced that are easily degradable [8]. Therefore, with correct and efficient design, the conditions governing wastewater during transportation in the collection network may be improved, and the potential of collection networks can be used to remove organic matter from wastewater, and on the other hand, a positive interaction with the subsequent treatment processes in the treatment plant can be created. The executive management assumes that the wastewater treatment is done completely in the treatment plant and the role of the collection networks is only to collect and transfer the wastewater from the production sources to the treatment plant [5]. The growth of the world population has created a competitive environment between designers and engineers in the field of implementing and maintaining the wastewater collection network and managing these types of systems, because with the growth of the population and the increase in per capita water consumption, the wastewater produced in cities also increases. and designers are always looking for new methods of designing new networks. But from the technical and economic point of view, the issue of wastewater treatment is still considered as a complex and expensive issue. Although the technology in the field of equipment for large treatment plants has made significant progress, there is still a problem of wastewater treatment in some small cities. Among the researches carried out to reach an applicable technology, the use of the collection network as a biological reactor is a suitable solution.

The optimization of the collection networks and the old sewage treatment plants in cities whose population has grown excessively can be considered as a suitable solution in urban planning.

Using the collection network as a pre-treatment facility with Reducing the organic load of wastewater reduces the need for treatment plant equipment