Evsel atıksu arıtma tesislerinin optimum tasarımı /
Yazar: ERCAN ÇİTİL
Danışman: YRD. DOÇ. DR. DURSUN ATİLLA ALTAY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Yüksek Lisans, 1995
Konu: Çevre Mühendisliği = Environmental Engineering
Dizin: Atık su arıtma tesisleri = Waste water treatment plants ; Evsel atık su = Domestic sewage

Özet:
ÖZET Bu tez çalışmasında, temel olarak evsel atıksu arıtma tesislerinin optimum tasarımı incelenmiştir, özellikle sistem verimini önemli ölçüde etkilediği bilinen aktif çamur ünitesi son çökeltme havuzuna ilişkin hesaplar akı modeline göre yapılmış ve bunun sistem seçimine olan etkisi araştırılmıştır. Bu amaçla çeşitli maliyet kriterlerinin minimum yapılmasını sağlayacak sistemin seçimi, çeşitli debilerde incelenmiş sonuçlar tablo ve grafik olarak verilmiştir. Tezin birinci bölümünde, bu tez çalışmasının amaç ve kapsamı açıklanmıştır. İkinci Bölümde, modellerin çözümünde kullanılan optimizasyon tekniklerinden genel olarak bahsedilmiş, tez çalışmasında kullanılan tamsayı programlama tekniği daha detaylı bir şekilde incelenmiş, ayrıca arıtma sistemlerinin optimizasyonu konusunda daha önce yapılmış çalışmalar sunulmuştur. Ayrıca çalışma sırasında kullanılan ve EPA(Environmental Protection Agency) tarafından geliştirilen EXEC-OP bilgisayar programı tanıtılmıştır. Üçüncü Bölümde, Atıksu Tasfiye Metodları hakkında kısa bilgi verilmiş, çökeltme prosesinin atıksu arıtımındaki yeri ve önemi açıklanmış ve akı modeli hakkında bilgi verilmiştir. Dördüncü Bölümde, su kalite standartları ele alınmış, Türkiye'de kullanılan standartlar incelenmiştir. Tezin beşinci bölümünde, tasarımı düşünülen evsel atıksu arıtma tesisi ile ilgili veriler sunulmuş, çeşitli debilerde ve farklı çökelme özelliğine sahip çamurlarla, standartlara uygun çıkış suyu kalitesini sağlayan en iyi sistemler belirlenmiş, akı modelinin optimum sistem seçimi üzerine etkisi incelenmiştir. Çamur çökelme özelliğinin modele dahil edilmesinin hem sistem seçimini(özellikle biyolojik arıtma ünitesini) hem de maliyetleri önemli ölçüde etkilediği belirlenmiştir. Altıncı bölümde ise bu tez çalışmasında elde edilen sonuçlar özetlenmiştir. XI

Abstract:
THE OPTIMUM DESIGN OF MUNICIPAL WASTEWATER TREATMENT PLANTS SUMMARY In the present thesis, basically, the optimum design of municipal wastewater treatment plants is investigated. Especially, in view of the fact that the design of the secondary sedimentation tank as a part of the activated sludge system has a significant impact on the overall system performance, the calculations pertaining to the design of such tanks are carried out by means of a flux model and, the influence of such a modelling on the choice of the system design is examined. To this end, the selection of the system design which will minimize a number of cost criteria is considered with respect to varying flow rates, and the results obtained from this research are presented in a number of tables and graphs. The graphs show the relationships between flow rate-construction cost, flow rate-total annual cost, flow rate-annual operation and maintenance cost. Also, flow rate- land requirement are drawn. The goal and the scope of the present thesis are presented at the first chapter. Due to the fact that the treatment systems require a high amount of investment, the reliability of the economic analysis in the design of these systems becomes a crucial problem. The correspondence of the economic analysis to the real systems wholly depends on the employed mathematical model. The problem of the sludge precipitation that one faces in the biological treatment plants might have a negative impact on the overall performance of the system. In the previous studies, it has been observed that the secondary sedimentation tank is one of the process units which have a significant impact on the optimum design. It has also known that the performance of this process unit mainly depends on the precipitation properties of the sludge which comes from aeration tank. At the second chapter, various optimization techniques are widely used in the solution of the mathematical models are briefly considered. Then, integer programming method which is used in my thesis, is presented in more detail. Furthermore, some previous studies on the optimization of the design of the wastewater treatment plants are mentioned in this chapter. XIIThe studies on the optimization of the treatment plants which have been carried out in the field of environmental engineering, were mainly those which aim at an optimum utilization of the natural absorption capacity of the receiving water. Later, together with these studies, some new models have been developed in order to determine the optimum design which would minimize the costs of the treatment plant. In the present study, mainly the biological treatment units, especially the activated sludge system and its modifications are taken into consideration. The reason for such a consideration is the proliferation of the current studies on the activated sludge systems.Out of these previous studies, special attention is paid to the EXEC- OP program which has been developed by EPA. The third chapter focuses on the wastewater treatment methods. After a discussion of the role and significance of the precipitation process in wastewater treatment, some basic information about "Flux Theory" is presented. The flux can be defined as the matter which passes through a unit area in unit time. The flux can be generated either 1.by gravitation.or, 2.by the sludge disposal from the bottom of the tank. The secondary sedimentation tank are dimensioned by taking two main functions into consideration: clarification and thickening. The thickening function is expressed by the flux concept. In current days, the aeration and sedimentation tanks are dimensioned independently of each other. Nevertheless, such an approach has a negative influence on the plant performance and administration. For this reason, in the present study, the secondary sedimentation tank is designed according to the flux model. Since Water Quality Standards play a crucial role in the optimum design of the wastewater treatment plants, the fourth chapter is devoted to the presentation of the receiving water and drainage standards(especially those of Turkey). in the fifth chapter, first, the data pertaining to the municipal wastewater treatment plant to be designed are presented. Then the best system designs which provide the required effluent water quality standards are determined. Furthermore, the influence of the flux model on the choice of the system design is investigated. The optimum plant design is determined to satisfy the effluent water quality standards of Turkey by using various combinations of the initial construction cost, total annual cost, annual operation and maintenance cost, total energy produced. Indeed, EXEC-OP programme contains eight criteria. They are as follows: XIII1.Initial construction cost. 2.Annual operation and maintenance costs, including all energy costs. 3.Total equivalent annual cost consisting of amortized capital costs plus annual operation and maintenance costs. 4.Gross energy consumption consisting of the direct electrical energy needed to operate equipment, the kwh equivalent of all fuel consumed, and the kwh equivalent of the energy used in chemical production. 5. Gross energy production as the kwh equivalent of the usable energy contained in treatment by-products such as digester gas. 6.Net energy consumption which is difference between criteria 4 and 5. 7.Total land utilization, excluding those process units with small land requirements. 8.Subjective system undesirability rating. In this optimization process, the various flow rates ranging from 5000-500000 m3/day are taken into consideration. The selected alternative treatment systems are calculated for 5000, 15000, 40000, 75000,115000,150000,200000 and 500000 m3/day flow rates. It is observed that the inclusion of the sludge properties into the model has a significant impact both on the choice of the system design(especially,on the choice of biological treatment unit) and also on the ultimate cost values. The results that are obtained in the present thesis are summarized at the sixth chapter. For a municipal wastewater, the wastewater treatment plant is composed of sludge pumping, screening, grit chamber, primary sedimentation tank, activated sludge system(or trickling filter) secondary sedimentation tank and chlorination units. The sludge treatment units of the system are chosen in general as thickening, lime stabilization and land spreading. As the flow rate increases (Q>1 00000 m3/day) according to the İSKİ(İstanbul Water and Sewerage Administration) Regulations(BOD5= 20 mg/l and TSS=50 mg/l) anaerobic digestion unit replaces with lime stabilization. For "Water Pollution Control Act" (BOD5= 45 mg/l and TSS=30 mg/l) such a replacement is required only after the flow rate level of 200000 m3/day. According to the criterion of the minimization of the total annual cost, the biological treatment unit of the chosen system comes out to be trickling filter. Whenever construction cost and total annual are considered together, and the secondary sedimentation tank is dimensioned according to the flux model, the mechanical aeration activated sludge system is chosen as the biological treatment unit for those wastewater with a precipitation property of a high quality. Such a choice is valid until SVI(Sludge Volume lndex)= 150 ml/g. However, for an SVI value higher than 200 ml/g, trickling filter becomes preferable. The reason for XIVsuch a change is an increase in the need of sludge treatment together with an increase in the surface area of the secondary sedimentation tank in order to keep the same effluent water quality. Necessary surface area of the secondary sedimentation tank of the activated sludge system is given in Table 1. The inclusion of the sludge precipitation properties into the model results in a consideration of the operation of the plant at the design level. Hence, the optimum design of the plant is carried out both by minimizing costs and by enabling the proper operation of the plant. Even if an effluent sludge quality of SVI = 50 ml/g cannot always be reached easily, a system designed according to an SVI value of 150 ml/g despite an increase in costs, does not prevent the choice of activated sludge system. The cost of electricity is an important factor in the choice of the system design. If the cost of the electricity is increased from 0.1 $/kwh to 4 $/kwh, due to an increase in the operation and maintenance costs, the trickling filter is chosen instead of the activated sludge system. This shows that, in the system design, the energy policies of the governments and the energy costs in the future should be taken into account. Table 1 Necessary Surface Area of The Secondary Sedimentation Tank of The Activated Sludge System XVThe water quality standards and the wastewater discharge standards are the determining factors in the choice of the system design. If the standards are too restrictive, the plant costs significantly increase and the attainment of the desired effluent water quality gets more difficult. Also, there shouldn't be big differences between various standards that are used in a country. Especially, for the developing countries like Turkey, the standards should be determined so as not to pollute the receiving waters, and furthermore, they should be arranged so as to allow the use of best available technologies. XVI