Deri endüstrisi atıksuyu için biyolojik arıtma sırasında ozonlamanın optimizasyonu

Özet

Bu çalışmada, biyolojik arıtma sırasında ozon ile kimyasal oksidasyon prosesi için optimum ozonlama noktasının belirlenmesi araştırılmıştır. OTH profillerinin elde edilmesine dayalı respirometrik ölçümler yardımıyla – çalışmaya konu edilen fiziko-kimyasal ön arıtmaya tabi tutulmuş – deri endüstrisi atıksuyu numunesindeki (A Numunesi’ndeki) KOİ bileşenleri belirlenmiştir. Respirometrik analizler aracılığıyla biyolojik arıtma sırasındaki reaksiyon sürelerine bağlı olarak B, C ve D numuneleri tanımlanmıştır. Ozonlama deneyleri başlangıç anı (A Numunesi), kolay ayrışabilen KOİ bileşeninin tümüyle giderildiği an (B Numunesi), başlangıçtaki yavaş ayrışan çözünmüş KOİ bileşeninin yarı yarıya giderildiği an (C Numunesi), geriye sadece çözünmüş inert KOİ bileşeninin kaldığı an (D Numunesi) için yürütülmüştür. Ozonlama deneyleri sonucunda, farklı oksidasyon ürünlerinin oluşumuna bağlı olarak değişik arıtma verimleri elde edilmiştir. Kolay ayrışabilir nitelikli organik maddenin biyolojik arıtmada giderilmesinin ardından ozonlama prosesinin (ara ozonlama prosesinin) uygulanması, tüm numuneler içerisinde mg/l cinsinden KOİ giderimi bazında en iyi sonuçları vermiştir. Ön ve son ozonlama prosesleriyle karşılaştırıldığında; kolay ayrışabilir nitelikli organik maddenin biyolojik arıtmada giderilmesinin ardından 40 mg/dak’lık optimum ozon akısında uygulanan ozonlama prosesi, biyolojik arıtmaya söz konusu prosesin entegrasyonu açısından en uygun seçenek olarak belirmiştir. KOİ fraksiyonları arasındaki dönüşüm mekanizmalarından hangisinin baskın olduğunu daha iyi algılayabilmek için ozon ile kimyasal oksidasyon prosesine tabi tutulmuş numunelerde respirometrik analizlerin de yapılması önerilmektedir.

Abstract

Industrial wastewaters contain various organic compounds each in a different oxidation state. Due to this varying organic content, every industrial effluent has a unique fingerprint in terms of COD fractions. Biological processes are usually prescribed for treating the industrial effluents with considerably high COD content, mainly composed of soluble biodegradable fraction, as they have economic advantages over chemical oxidation. Some industrial effluents, i.e. tannery wastewaters on the other hand, may contain considerable amounts of biorefractory organics; so that applying biological treatment alone may not yield adequate COD removal efficiencies to meet the discharge standards. Such cases necessitate the usage of an advanced chemical oxidation method i.e. ozonation along with biological treatment. In a combined biological and ozone treatment, preozonation is executed to enhance the biodegradability by producing more oxidized and soluble organic compounds. Post-ozonation, alternatively, is used as a complementary treatment step to achieve additional reductions in terms of organic matter and to provide a polishing effect on the biological treatment effluent. Ozone can also be applied within biotreatment as an in-mid treatment step where easily biodegradable COD can be removed by the first biological treatment and the inert COD is converted to biodegradable forms by the following ozonation process to ease the further biological treatment. In a combined treatment scheme comprising biological treatment and chemical oxidation with ozone, the assessment of the optimum location of ozonation depends on the treatment efficiency, appropriateness and economical feasibility analysis of the integrated system. Thus, the objective of this study is to investigate the suitability of ozone application before / within / after biological treatment. In this framework, a chemically settled tannery effluent characterized by a large amount of organic matter with different biodegradability is selected as a strong wastewater that requires an additional treatment step before /within / after biological oxidation. The investigated sample taken from the tannery wastewater treatment plant located in Rehau, Germany, is subjected to ozonation experiments in order to choose the optimal treatment scheme for ozonation within biological treatment. Conventional characterization performed on the chemical settling effluent reflects a strong wastewater character with a total COD content of 2020 mg/l, almost entirely soluble in nature. Respirometric evaluation of chemically pre-treated wastewater sample indicates that 46% of the total COD is rapidly hydrolysable COD. The slowly hydrolysable COD component amounts only to 3% of the total COD since the chemical treatment almost completely removes the particulate organic matter. Consequently, the total COD includes a total biodegradable COD fraction of 81%, while the remaining 19% is classified as initial inert COD portion which only composes of soluble compounds. The required reaction periods to obtain different COD fractions are also determined from OUR profiles. After respirometric measurements, the chemical settling effluent (Sample A) and the biologically treated wastewater (Sample D, containing only soluble inert COD portion as the remaining soluble fraction) are subjected to ozonation experiments. Ozonation is also conducted on two different phases in biological treatment; namely in a phase where the readily biodegradable COD is completely depleted (Sample B), and in another point where the rapidly hydrolysable COD is at half of its initial concentration (Sample C). In order to obtain the samples of B, C and D, a lab-scale fill and draw reactor operated at an F/M ratio of 0.2 g COD / g VSS, fed with Sample A, is used. During the course of ozonation studies, the sequence of biological treatment and ozonation depicts different COD removal efficiencies, as the performance of ozonation step is strongly dependent on the extent of biological oxidation. A value of 40 mg O3/min is determined as the optimum ozone flowrate because the total COD reduction of biologically pre-treated samples cannot be significantly improved beyond that level. Total COD abatement rate constants of biologically pre-treated samples are by far bigger than those obtained for pre-ozonation process. Compared to pre- and post-ozonation processes, the best integrated process is the application of ozone at a stage of biological treatment where readily biodegradable substrate is entirely consumed; as this treatment scheme leads to the highest decreases in COD concentrations among other integration alternatives. Respirometric evaluation of the ozonated wastewater samples is recommended to highlight the conversions between COD components.