4-Klorofenolün aktif çamurda kometabolik ayrışması üzerine biyosurfaktan etkisi

Effect of biosurfactant on cometabolic degradation of 4-chlorophenol in activated sludge

Özet

Bazı kimyasal endüstri atıksularında bulunan klorlu fenolik bileşikler organizmalar üzerinde önemli toksik etkilere neden olurlar ve sıklıkla biyolojik ayrışmaya karşı direnç göstermektedirler. Kometabolizma son zamanlarda klorlu solventler gibi ayrışmaya direnç gösteren bileşiklerin biyolojik arıtımında önemli bir teknik olarak belirmiştir. Kometabolizma ile biyolojik ayrışmada hücre büyümesi için uygun bir büyüme maddesine ihtiyaç duyulmaktadır. Biyosurfaktanlar su ve toprak ortamında ayrışmaya karşı direnç gösteren kirleticilerin giderimini artırabilmektedir. Bu çalışmada büyüme maddesi olarak glikoz kullanan aklime edilmiş karışık kültür ile 4-Klorofenol’ün (4-KF) ayrışması üzerine biyosurfaktanın etkisi, çamur yaşı 10 gün ve hidrolik bekletme süresi 17 saat olarak sabit tutulması ile aktif çamur reaktörü kullanılarak incelenmiştir. Biyosurfaktan olarak JBR 425 rhamnolipid, kritik misel konsantrasyonunda (15 mg/l) kullanılmıştır. Biyosurfaktanın eklendiği reaktör (test reaktörü) ile, eklenmediği kontrol reaktörü aynı 4-KF ve KOİ yükleme hızlarında paralel olarak çalıştırılmıştır. 4-KF konsantrasyonu 40-250 mg/l aralığında uygulandığı zaman, kontrol ve test reaktöründe 4-KF giderim verimleri %97.1-91.1 ve %98-96.5 aralığında olmuştur. Bu 4-KF konsantrasyon aralığında, 4-KF giderim verimindeki azalma biyokütlenin 4-KF’e adaptasyonundan dolayı önemsizdir. 4-KF konsantrasyonu 350-450 mg/l aralığında uygulandığı zaman, kontrol reaktöründe arıtma verimi %80-76.64 aralığında iken, test reaktöründe %86.5-84.6 aralığında olmuştur. Test reaktöründe biyosurfaktan glikoza ilave olarak biyokütle üretiminde kullanıldığından dolayı biyokütle konsantrasyonu kontrol reaktörüne göre daha yüksek olmuştur. Biyosurfaktan mevcudiyeti 4-KF’ün biyokütle üzerine olan toksisitesini azalttığından 4-KF’ün ayrışma hızı artmıştır.

Abstract

Chlorophenols are introduced to the environment through man-made activities, such as waste incineration, uncontrolled use of wood preservatives, pesticides, fungicides and herbicides as well as bleaching of pulp with chlorine. Chlorophenols discharge into the environment is of great concern because of their toxicity and suspected carcinogenicity. Hence, the removal of phenol and chlorinated organic compounds from wastewater is necessary task to conserve the water quality of natural water recourses. Different physical, chemical and biological methods such as activated carbon adsorption, chemical oxidation and aerobic/anaerobic biological degradation were used for removal of chlorophenols from wastewater. Adsorption and ion exchange methods were usually used to concentrate the chlorophenols on the solid phase, which require further treatment by chemical or biological oxidation for complete mineralization. Chemical oxidation methods are fast, but expensive and also may result in formation of undesirable by products. Despite the recalcitrant nature of chlorophenols, there are still some efforts toward their biological treatment with specialized culture conditions, because of economical reasons and a low possibility of byproduct formation. Aerobes are more efficient at degrading toxic compounds because they grow faster than anaerobes and usually achieve complete mineralization of toxic organic compounds, rather than transformation, as in the case of anaerobic treatment. However, it has been reported that chlorinated solvents generally cannot serve as a single carbon and energy source for microbial growth, but rather must be biodegraded by cometabolism. The nongrowth substrate, then, can only be transformed in the presence of a growth substrate, a phenomenon called cometabolism. For biological degradation of toxic compounds degraded through cometabolic pathways, a suitable growth substrate, which serves as sources of carbon and energy to support cell growth, is required. It is quite common that an organic compound is chosen as a growth substrate because it can support cell growth of the cometabolizing bacterium naturally. Numerous studies have focused on the biodegradation of 4-CP under aerobic conditions in fed-batch reactors, in sequencing batch reactor and in special culture. Limited number of studies was reported on biological treatment of 4- CP using activated sludge by continuous operations. When practical application of engineering systems are considered, however, the fate and effect of 4-CP in continuously operated systems with a mixed culture gains importance. Surfactants can either be chemically synthesized (synthetic) or microbially produced (biosurfactants). Biosurfactants are usually classified based on their biochemical nature and the microbial species producing them. For specific applications, biological surfactants have advantages over synthetic surfactants due to their structural diversity, biodegradability, and effectiveness at extreme temperatures, pH and salinity. Biosurfactant applications in the environmental industries are promising due to their biodegradability, low toxicity and effectiveness in enhancing biodegradation and solubilization of low solubility compounds. A number of researchers indicated surfactant enhancement in microbial degradation of organic contaminants. However, there are no studies in literature on enhanced biodegradation of 4-chlorophenol using a biosurfactant in an activated sludge bioreactor. In this study, the effect of biosurfactant on degradation of 4-Chlorophenol (4-CP) by acclimated mixed culture using glucose as a growth substrate was investigated by an activated sludge reactor. JBR 425 rhamnolipid was used as biosurfactant. Test reactor with added biosurfactant and control reactor (without biosurfactant) were used in parallel tests. The results of this study show that 4-CP degradation can be enhanced in the presence of biosurfactant by cells grown on glucose as the growth substrate. When the 4-CP concentration was applied between 40-250 mg/l, 4-CP removal efficiencies ranged between 97.1-91.1% and 98-96.5% in the control and test reactor. In this range of 4-CP concentrations, the decrease in 4-CP removal efficiency was not significant in both of the reactors because of the biomass adaptation to 4-CP. When the 4-CP concentration was applied between 350-450 mg/l, while 4-CP removal efficiency ranged between 80-76.64% in the control reactor, it ranged between 86.5-84.6% in the test reactor, respectively. Addition of biosurfactant in the test reactor would increase the COD removal capacity in the presence of 4-CP. The presence of biosurfactant may have attenuated the toxicity of 4- CP on biomass, and consequently enhanced the biodegradation rate of 4-CP and COD. As a result of using glucose as the growth substrate, competitive inhibition with 4-CP can be avoided. Moreover, the use of glucose would not result in additional environmental pollution as opposed to using phenol.