Atık Aktif Çamurun Aerobik Stabilizasyonuna Ön Çökeltmenin Etkisi

Abstract

Atıksu arıtılması sonucu oluşan arıtma çamurları ham halde iken yüksek miktarda organik kirletici içermekte olup, bozunma eğilimindedir. Bu sebeple arıtma çamurları nihai bertaraf edilmeden önce mutlaka kararlı (stabil) hale getirilmelidirler. Ancak çamur arıtma, atıksu arıtma işleminin en karmaşık ve maliyetli aşamasını oluşturmaktadır. Bu yüzden çamur karakterine en uygun çamur stabilizasyon ve arıtma metodunun seçilmesi büyük önem taşımaktadır. Aerobik (havalı) çürütme ile çamur stabilizasyonu, küçük kapasiteli (≤20.000 m3/gün) biyolojik arıtma üniteleri için en uygun ve ekonomik çözümü sunmaktadır. Diğer çamur stabilizasyonu yöntemleri gibi aerobik stabilizasyon sonunda üretilen ürünün kalitesi uygulanan atıksu arıtma proseslerinden doğrudan etkilenmektedir. Bu çalışmada atıksu arıtımı sırasında ön çökeltme aşamasının mevcudiyetinin aktif çamur sisteminden kaynaklanan atık biyolojik çamurun aerobik stabilizasyonuna etkileri incelenmiştir. Bu kapsamda çamur yaşları 8 gün olan ve biri çökeltmeye tabi tutulmuş, diğeri ise tutulmamış atıksu ile beslenen iki aktif çamur reaktörü aynı koşullarda işletilip ve daha sonra bu reaktörlerden elde edilen farklı karakterdeki iki çamur 61 gün süre ile aerobik (havalı) şartlarda çürütülmüştür. Elde edilen sonuçlar karşılaştırılarak ön çökeltmenin aerobik stabilizasyona etkisi ortaya konulmaya çalışılmıştır. Çalışmadan elde edilen başlıca sonuçlar şunlardır: (1) her iki aktif çamur sisteminde çıkış suyunun KOİ konsantrasyonu ve sistemlerin KOİ giderim verimleri birbirine oldukça yakındır; (2) iki saatlik ön çökeltmenin uygulanmaması, aerobik çürütme safhasına giren atık aktif çamurun AKM konsantrasyonunda %55, UAKM konsantrasyonunda %56 oranlarında artışa sebep olmuştur; (3) katı madde oranındaki büyük farka rağmen iki çamurun UAKM:AKM oranı stabilizasyona başlandığında ~%78, 49 günlük stabilizasyon sonunda ise ~%58‘dir; (4) OTH ölçümlerine göre ön-çökeltmesiz reaktörde üretilen çamurda biyolojik aktivite stabilizasyon başlangıcında %34 daha fazladır. Ancak 28. günde her iki reaktördeki aktivite benzer seviyelere indiği görülmüştür; (5) ön-çökeltme uygulanmamış reaktörde üretilen çamurun aktivitesindeki ve çamur yoğunluğundaki göreceli fazlalık ön-çökeltmede ayrıştırılabilecek zengin biyobozunur organik maddelerin sisteme beslenmesi ile açıklanabilir; (6) aktivitedeki değişimlere paralel olarak ön-çökeltmesiz atıksu ile üretilen çamurdaki UAKM ve TOK giderim verimi diğer reaktöre kıyasla ilk 28 günde daha yüksektir; (7) stabilizasyonun 49. gününün sonunda her iki reaktörde UAKM giderimi %65-68 ve stabilizasyonun 61. gününün sonunda TOK giderimi %53’tür; (8) Ön çöktürmenin işlemi uygulanmaması atık aktif çamurun UAKM ve TOK konsantrasyonlarında başlangıçta yükselmeye yol açmıştır, ancak UAKM ve TOK giderim verimleri, ÇOK konsantrasyonu ve biyokütle aktivitesi parametreleri bakımından elde edilen çürütülmüş çamurlar, 28 gün ve daha fazla havalı stabilizasyona tabi tutulduktan sonra hemen hemen aynı kalitededir. Bu sonuçlar aerobik çürütmenin daha avantajlı olduğu küçük kapasiteli sistemlerde ön çökeltme ünitesi kullanılmayarak havalı çürütme işleminin performansında ve aktif çamur sısteminde arıtılan atıksuyun kalitesinde büyük değişim yaşanmadan sistem tasarımının basitleştirilebileceğini göstermiştir.

Raw wastewater sludge, produced as a result of wastewater treatment processes, contains high amounts of organic pollutants and has a tendency to rapidly decay. Thus, sludge must be stabilized before its final disposal. However, sludge treatment is the most complicated and costly step in wastewater treatment. Selecting a feasible stabilization method and conducting an appropriate disposal method is a challenging decision that an engineer has to make. For treatment plants with a capacity of 20,000 m3 or lower, aerobic digestion provides the most efficient and economic stabilization solution. Similar to other stabilization methods, aerobic digestion is directly affected by the types of applied wastewater treatment processes. The properties of wastewater sludges are directly affected by the properties of influent wastewater and the types of applied treatment processes. In a conventional biological wastewater treatment system design, inflowing wastewater passes through screens and grit collectors, firstly. Next, wastewater is directed to a primary clarification tank. After wastewater is settled for a certain period of time in primary clarification tank, it is pumped to aeration tank. In aeration tank, wastewater is biologically treated. Biological treatment process is most commonly operated as an activated sludge system that involves assimilation of nutrients in wastewater by living microorganism under aerobic conditions for prolonged periods of time. Finally, treated wastewater pumped to a secondary clarification tank and the suspended solids entailed with water was removed through gravitational settling. In this conventional system, two types of sludge were generated; very unstable primary sludge which contains high readily biodegradable organics and produced in primary clarification tank; more stable secondary or biological sludge which mostly composed of microorganisms and produced in the final clarification tank. Both types of sludge have very high moisture and biodegradable content and must be processed further before final disposal. In Turkey, there are 300 facilities conducting biological and/or tertiary wastewater treatment, according to the 2014 survey. The number of these type of treatment facilities considered to be over 600 in near future, in order to comply with EU (European Union) directives. Therefore, the amount of wastewater sludge that have to be handled and disposed will significantly increase, making sludge management an even more pressing matter. Furthermore, only 26% of sludges generated in Turkey are stabilized before disposal currently. In this context, great amounts of investment is needed to build new treatment plants and sludge processing units in near future. In this study, the effect of a primary clarification step during wastewater treatment on the stabilization of activated sludge via aerobic digestion was investigated. To achieve that, firstly, two batch reactors were operated to simulate activated sludge systems with and without primary clarification step. Next, two sludges produced in two reactors were digested under aerobic conditions. Data from the two experimental runs were compared, in order to determine the differences between two reactors during biological wastewater treatment and aerobic sludge digestion processes, in order to gain insight about the possibility of simplifying future treatment plant designs by discarding primary clarification unit and producing only one type of sludge. The two reactors were inoculated with activated sludge sampled from the aeration tank located in a domestic wastewater treatment plant receiving 60.000 m3/day wastewater. The volume of the reactors were 6 L. Initial sludge concentration was set to 4000 mg suspended solids (SS) per liter. The pH of the reactors were constantly monitored and kept above pH 6.5 by adding NaHCO3. The reactors were fed each day with fresh raw wastewater that passed through screens and grit collectors. Wastewater fed to reactors were supplied from the same treatment plant every 7-10 days and stored at 4ºC. One of the semi-batch reactors simulating an activated sludge system without primary clarification unit was fed with this raw wastewater as it was received from plant. The second reactor simulating a system with primary clarification tank was fed with wastewater that was settled. The wastewater was settled for two hours and then the obtained supernatant was transferred to another container. This supernatant was fed to the second reactor. For 46 days, the reactors were operated to produce two types of activated sludge. This 46-day period was referred as “acclamation period”, when both reactors were fed with wastewater daily and continuously aerated. The dissolved O2 concentration in the reactors was kept above 2 mg/L using air stones. Complete mix conditions were maintained using the air flow from these air stones. Solid retention time of the activated sludge unit in the treatment plant was 17 days. Thus, solid retention time of the two inoculated reactors were gradually adjusted to targeted value of 8 days in 2 weeks, at the beginning of the acclamation period. Hydraulic retention time of the wastewater in both reactors were always kept at 2 days. The semi-batch type operation procedure were explained below. Each day, the air flow to the reactors were shut off firstly and some of the sludge from completely mixed reactors were discarded. Discarded volume was calculated based on the solid retention time, or sludge age. Next, the suspended solids in the reactors were allowed to be settled for 2 hours (mimicking final clarification of sludges) and 3 L of the supernatant out of reactors’ total volume of 6 L was siphoned out. After siphoning, 3 L of fresh wastewater (raw or 2-h settled based on the reactor type) was added to each reactor. During the weekends, 6 L of wastewater added and 6 L siphoned at the start of the new week. Finally, the airflow to reactors were restarted. This procedure was followed for 46 days throughout the “acclamation period” and biological treatment was simulated in these reactors. The absence of primary settling caused an additional %27 suspended solids (SS) and %21 volatile suspended solids (VSS) load in the reactor fed with raw wastewater on the average throughout the acclamation period. Once the sludges in both reactors were stabilized at the end of 46 days, the airflow to the reactors were stopped and the sludge in the reactors were thickened by settling it and siphoning the 3 L of the supernatant. The air flow was started again after thickening and then the two types of sludges in the two reactors were aerobically digested on the next 61 days. Evaporation losses were made up with distilled water every day. The results of this study were as follows: (1) COD concentrations in effluents (58±40 mg COD/L in the reactor with primary settling and 42±27 mg COD/L for the reactor fed with raw wastewater) and COD removal rates (>90% in both reactors) were found to be very similar for both of the activated sludge systems; (2) Elimination of the primary settling phase caused a 55% increase in SS concentration and a 56% increase in VSS concentration of the waste activated sludge; (3) Despite the significant difference between SS and VSS concentrations of the two sludges, VSS:SS ratio for both reactors were initially ~78% and at the end of the 49-day stabilization process, it became ~58%; (4) According to OUR measurements, activity of biomass produced in the reactor without primary settling was 34% higher, initially, but biological activities in both reactors were decreased to similar levels after 28 days of aerobic digestion; (5) higher biological activity in the sludge produced in the reactor without primary settling was explained by that introduction of rich biodegradable organic solids, which can be removed in primary settling tank, into the reactor; (6) similar to changes in biomass activities, VSS and TOC removal rates for waste activated sludge produced in the reactor without primary settling were higher at the first 28 days of aerobic digestion compared to the other sludge; (7) In both reactors, VSS and TOC removal rates increased to 65-68% at 49. day and 53% at 61. day compared to initial values, respectively; (8) Although eliminating the primary settling phase resulted in increased initial VSS and TOC concentrations, TOC and VSS removal rates, DOC concentrations and activity of biomass for both types of digested sludges were very similar after >28 days of stabilization. Based on the observations made in this study, it can be concluded that in small domestic wastewater treatment facilities, where aerobic digestion is more economical, the design of biologic treatment systems can be simplified without a significant deterioration in the quality of discharged wastewater and stabilized waste activated sludge by eleminating primary clarification unit. Additionally, it is clear that removal of primary clarification unit causes a significant increase in the concentration of waste activated sludge (approximately 50%). Volume of the aerobic digestor and/or the aeration rate of the digestor have to be adjusted, accordingly, in order to compansate for the increased sludge concentration, when the primary clarification step was skipped. The changes in the digestor may increase both capital and operational costs. On the other hand, the simplification of the system by discarding primary clarification unit and not producing a less stable and more problematic primary sludge loaded with readily available organic materials will result in reduction in capital and operational costs. Therefore, the optimum plant size and capacity should be determined, where these increase and decrease in costs cancel each other, in future studies, before making any decisions on new plant designs.