Anerobik çürütücülerde karışım şartlarının çürütücü verimine etkisinin araştırılması
Başlık çevirisi mevcut değil.
- Tez No: 75176
- Danışmanlar: DOÇ. DR. LÜTFİ AKÇA
- Tez Türü: Yüksek Lisans
- Konular: Çevre Mühendisliği, Environmental Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Çevre Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 85
Özet
ÖZET Bu çalışmada 10 İt hacimli labaratuvar ölçekli tam karışımlı anaerobik bir reaktöre eşit oranlarda karıştırılmış damlatmalı filtre ile ön çökeltme çamuru beslenerek, 10 gün sabit hidrolik bekletme süresinde, mezofilik şartlarda kesikli karıştırmanın çürüme performansına etkisi araştırılmıştır. Reaktör her gün İl45 (+30dk) 'te beslenmiştir. Sıcaklık reaktörün etrafındaki su ceketinde 35°C sıcaklığındaki suyun sürekli devir daim ettirilmesiyle sabit tutulmuştur. Reaktörün üst kısmına çürüme gazının kaçmasına mani olacak şekilde sıcaklık ve pH probu ve düşey milli dönme hızı ayarlanabilen elektrik motoru monte edilmiştir. Karıştırma sisteminin çürüme performansına etkisini araştırmak üzere karıştırma motoruna devreye girip çıkma süresi minumum 15 dk olan bir zaman rölesi ile karıştırıcı, 45 dk çalışma 15 durma, 15 dk çalışma 15 dk durma ve 15 dk çalışma 30 dk durma hallerine ayarlanmıştır. Besleme 250 ml/dk debi ile peristaltik pompa kullanılarak manuel olarak yapılmıştır. Çürümüş çamurun reaktörden çıkışı besleme esnasında savaklanarak gerçekleştirilmiştir. Gaz debisini ölçmek için 10 ml'ye duyarlı bir gaz ölçer kullanılmış, gaz analizi için 2 İt civarında gaz, hacim deplasmanı yöntemiyle pet bir kapta depolanmıştır. Reaktör ham çamur ile beslendikten hemen sonra pH'nın 7.15 civarından 6.9'a kadar düştüğü, 3-4 saat sonra tekrar yükseldiği görülmüştür. Reaktörün sıcaklığını 20°C'ye düşürüp 2-3 gün besleme yapılmadığı takdirde çürüme gazının CO2 bileşeninin %15'in altına kadar düştüğü, besleme tekrar başladığında sistemin 1 gün içinde eski performansına ulaştığı tesbit edilmiştir. Karışımın olmadığı periyot 30 dk'nın üzerine çıktığında üst kısımda yüzücü madde ve köpüğün arttığı gözlenmiştir. Çökelme özelliği kötü olan biyolojik çamurların anaerobik çürütülmesinde, çürütücünün 30 dk'dan fazla olmayan aralıklarla kısa süreli karıştırılmasının sistemin performansına olumsuz bir etkisinin olmadığı tesbit edilmiştir. IX
Özet (Çeviri)
THE EFFECT OF MIXTURE CONDITIONS FOR ANAEROBIC DIGESTERS ON DIGESTION PERFORMANCE SUMMARY Anaerobic digestion processes is videry used for the treatment of municipal wastewater sludge. The purposes of the treatment are stabilization and volume reduction of the sludge. Anaerobic digestion is capable of achieving both these purposes, by converting soluble and particulate organic matter into carbondioxide and methane gases. The advantage of anaerobic sludge digestion and relatively cheap anergy coupled with poor operations of anaerobic sludge digesters further added to lack of interest in anaerobic digestion of sludges. Since production of methane makes anaerobic digestion attractive from an energy standpoint, interest in this process has increased in recent years. Although anaerobic digestion reduces the volume of sludge solids, separation of the solids from the liquid in digested sludge is required to reduce cost and the handling and final disposal of the sludge. At most wastewater treatment facilitates, the dewatering step is one of the most expensive and important processes. Anaerobic digestion is one of the oldest processes which have been used extensively and effectively by many municipal facilities where the systems are heated and mixed and operated like biological reactors for the stabilization of sludges. Anaerobic digestion involves the decomposition of organic matter by bacteria in the absence of oxygen. The major applications have been in the stabilization and volume reduction of sludges produced from treatment of wastewater and in the treatment of some industrial wastes. The advantages of anaerobic digestion over other processes used for sludge treatment used for sludge treatment are;. The rate is not limited by oxygen transfer.. Thereisalowyieldofbioroass.. Pathogens are inactivated due to long detention time.. Methane is useful end product. The production of methane gas, which is useable source of energy. The process is a net enenrgy producer at most treatment facilities in which anaerobic sludge digestion is used. The energy produced is in excess to that required to maintain the temperature of the digesting sludge and to meet the energy requirements for mixing.. Reduction in the mass and volume of sludge through the conversion of organic matter in the volatile solids to methane, carbon dioxide, and water.Solids destruction usually is aproximately 25-40% of the feed sludge solids can result in reduction in the cost of sludge disposal.. Production of a solids residue that may be used as a soil conditioner. The anaerobically digested sludge contains nitrogen and phosphorus and other nutrients as well as organic material that can improve the fertility and texture of soils.. Pathogens associated with the feed sludge are inactivated during the anaerobic digestion processes. The principal disadvantages of anaerobic sludge digestion are:. The capital costs are high. Large, covered tanks along with pumps for feeding and circulating sludge, heat exchangers and compressor for gas mixing are required.. Long hydraulic detention times, in excess often days, are required to develop and maintain a population of methane producing bacteria.. The Quality charactristic of the supernatant from anaerobic sludge digestion are poor. The supernatants contain suspended solids, dissolved and particulate organic materials, nitrogen, and phosphorus. Mechanism Of Anaerobic Digestion Effective anaerobic degradation of complex organic matters to methane is a result of the combined coordinated metabolic activity of the digester microbial population. Most of the constituents of wastewarter sludge are insoluble and hence are not avaliable for assimilation by bacteria. At present, anaerobic digestion is divided conceptually into three steps to describe the microbiology and chemistry of anaerobic digestion. In the first step, complex organics are hydrolysed by the fermentative bacteria to free sugars, alcohols, volatile acids, hydrogen, carbon dioxide. Essentially no organic waste stabilization occurs during hydrolysis; the organic matter is simply converted into a soluble form that can be utilized by the bacteria. Subsequently, the alcohols and volatile acids longer than two carbons are oxidised to acetate and hydrogen by the obligate H2 producing acetogenic bacteria in the second step. In the final step, the methanogens readily utilise the hydrogen produced to reduce, CO2 to CH4 and convert acetate drectiy into C02 and CH4 This conversion model is shown in figure I When an anaerobic digester is working properly, these three steps of degradation are dynamic equilibrium; that is, the volatile organic acids are converted to methane at same rate that they are formed from the more complex organic molecules. If the pH drops below approximately 6.6 because of an unbalance of these steps, significant inhibition of the methanogenic bacteria occurs. At a pH of approximately 6.2, the acid conditions cause acute toxicity to these bacteria. Moreover, this pH does not stop acid production and the fermentative bacteria continue to produce acids until the pH drops 4.5 or 5.0 XICelluloze and Hemicelluloze H,0- Fermentative Bacteria Soluble Organics H 02+ C02 Acetogenic Bacteria Acetic Ac it Methanogenic Bacteria CH4 C02+CH4 Figure I Anaerobic digestion of organic solids. The anaerobic processes is essentially controlled by the methane bacteria because of their slow growth rate and sensitivity to environmental change.It is suggested that the bacteria utilising acetic acid were the most important, and that this step was considered as the rate limiting step in the overall process. Although methane fermentation may be the rate limiting step for sewage sludge digestion at 35 °C and short detention time (less than 10 day), hydrolysis of the organic solids is the rate limiting step at longer detention time. The other rate limiting step is that cellulose hydrolysis which is converted into methane. Process Description Of Anaerobic Digestion Anaerobic digesters that process a municipal sewage sludge can be either low-rate or high- rate, depending on mixing and heating provided. Two stage digesters, in which a high rate digester is followed in series by a low-rate digester, also have been designed and operated. In low-rate digesters, the contents of the tank are usually unheated and unmixed. Bubbles of gas, generated during the sludge digestion, provide the only mixing. As a result the contents of the tank stratify, forming the distinct zones: a floating layer of scum, a middle level of supernatant, lower zone of sludge. Most decomposition occurs in the lower zone. The loading rate to these digesters is usually less than 0.8 kg VS /m3 day and typical detention time are in excess of 30 days. In high-rate digesters, the contents of the digester are heated and mixed to provide the best conditions for the biological process. The reasons for mixing domestic sludge digesters are to provide efBient utilization of the entire digester volume, to prevent stratification and temperature gradients, to disperse metabolic end products and any toxic materials contained in the influent sludge, and to maintain intimate contact between the bacteria bacterial enzymes, and their substrates. The effect of inefficient mixing on process kinetics is manifested in the decrease in effective system volume and hence decrease in solids retention time. Additional concerns associated with inefficient mixing are foaming and scum formation, and excessive solids deposition. Inefficient mixing results in a decrease in effective system volume and may reduce the effective volume of digester by as much as xn70%, leaving anaerobic actual volume utilization of only 30%. Mixing causes better contact of bacteria with the raw sludge, destroys the scum layer, and evenly distributes toxins and metabolic waste products. Typical loading rates are from 1.6 to 6.4 kg VS/m3day and the required detention time can be less than 1 5 days. According to the experiment, following results are obtained. For every kg of COD destroyed, regardless of substrate source (0.35 ±0.04) n m of methane is produced. (0.35 n m3 /kg or 0.35 n It /gr) On this basis, it can be easety defined whether anaerobic systems are economic and feasible or not.. Daily monitoring of gas flowrate, methane production and percent CO2, along with daily measurement of pH, and total alkalinit, allows for the earliest possible dedections of stress. H2 also plays a key role in anaerobic digesters. By controlling gas flowrate and organic load it is rare to come across to operation problems unless the system is fed properly.. Intermittent mixing can put into practiced according to the setüeability properties of digesting sludge. This also slightly increases the performance of the reactor. The effect of Intermittent mixing on reactor performance is shovn in Figure n and Figure HI. Intermitance period between two mixing turns should not exceed 30 minutes to prevent the foam formation and accumulation of flooding materials at the surface. j 1145 min. mixer I I on and 15 min. j I °ff 1 I B 15 min. mixer j 1 on and 15 min. ! \ off. i i l D 15 min. mixer ; on and 30 min. I off Figure II The Relationship between COD,^. and intermittent mixing. As it can be seen in figure 11 ana uu, can be removed with 50 % efficiency. in Figure II and in, volatile solids, in other wods organic wth 50 % efficiency. matter xm. After feeding, a reduction in the pH can be observed due to the excess production of vollatile acids. At the beginning of a new feeding, volatile acids production rate is observed to be greater than the consumption rate until a time period of 4 hours.. Gas production is 0.85-1. 1 per m3 per 1 kg VSJw Figure Di The Relationship between VSSmb. and intermittent mixing. CO2 content of gas may reduce to below 15 % when reactor is not fed for a long period. The reduction of carbondioxide content of digestier gas is shown in figure IV. This is due to complete consumption volatile fatty acids and conversion of CO2 to liquid phase as a result of a slight increase in pH. 14 12 ST10 (0 I 8 10 CO O 7İ İLU4^tî44i \.A 1 1 » i i i i H i i I M 1 M ! j_j! j ? î ? i i 111 ITmTT" i !. -%C02 \ İ i j j j M j- JLX-L,,,,,, s i- 30 27 24 21 18 « 15 O 12 ^ 9 6 3 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 Time, day Fİ Jîgure IV Average Daily Gas Production - Feeding Frequency and Gas Content XIV
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