محلول سازي لجن مازاد بيولوژيكي بوسيله پيش تصفيه با حمام اولتراسونيك

فرآيند لجن فعال يكي از فرآيندهاي اصلي موجود در تصفيه خانه‌هاي فاضلاب شهري است. لجن مازاد بيولوژيكي محصول جانبي تصفيه فاضلاب با استفاده از اين فرآيند است و هزينه تصفيه و دفع آن تقريبا 60% كل مخارج تصفيه خانه مي‌باشد. يكي از مراحل تصفيه لجن مازاد بيولوژيكي، هضم بيهوازي مي‌باشد ولي به دليل ساختار لجن مازاد بيولوژيكي، ناكارآمد است. روش‌هاي مختلفي براي بهبود بهره‌وري هضم بيهوازي لجن مازاد بيولوژيكي به كار رفته است. يكي از اين روش‌ها پيش تصفيه با استفاده از امواج اولتراسونيك مي‌باشد. در اين پژوهش براي نخستين بار لجن مازاد بيولوژيكي با استفاده از حمام اولتراسونيك با فركانس kHz40، توان الكتريكي 265 وات و سطح مقطع مبدل صوت برابر با 137×240 ميليمتر مربع مورد پيش تصفيه قرار گرفت و اثر دو فاكتور چگالي صوت و زمان تابش امواج بر محلول سازي لجن بررسي شد. افزايش غلظت COD محلول و پلي ساكاريد محلول و همچنين كاهش ميزان جامدات فرار معلق نشان داد پيش تصفيه باعث محلول سازي لجن شده است. پيش تصفيه با چگالي W/mL 53/0 به مدت بيست دقيقه موجب افزايش 20 درصدي توليد بيوگاز نسبت به نمونه شاهد شد. با پيش تصفيه با كمترين ميزان انرژي در محدوده آزمايشها، شاخصDSVI در حدود 10 درصد نسبت به نمونه شاهد كاهش يافت و با افزايش ميزان انرژي، اين كاهش ادامه پيدا كرد. علاوه بر آن براي نخستين بار اثر پيش تصفيه بر شاخص هدايت الكتريكي نيز بررسي شد. نتايج نشان داد كه از شاخص هدايت الكتريكي مي‌توان براي ارزيابي ميزان محلول سازي لجن استفاده نمود.

Activated sludge process is commonly utilized for the treatment of wastewater with the benefits of high efficiency and easy operation. However, during the biological treatment of wastewater, huge amounts of waste biomass (called as “waste activated sludge (WAS)”) are inevitably generated in the process. The WAS should be treated in order to reduce the water content of raw WAS, transform the highly putrescible organic matter into stable or inert organic and inorganic residue, and finally condition the residue to meet disposal acceptance regulations. But, WAS treatment and disposal, representing 50–60% of the total operating costs of the wastewater treatment. WAS is produced in massive volumes; specifically, more than 25,000 tons of WAS is produced in Iran per year. Anaerobic digestion is one the most applicable methods in WAS stabilization due to its ability to reduce WAS volume and produce biogases. A mixture of primary and secondary sludge (WAS) passes through anaerobic digestion, but this process is more difficult for WAS than primary sludge. However, the hydrolysis stage limits anaerobic WAS digestion. To optimize the general process of WAS anaerobic digestion and increase hydrolysis performance, it would be possible to pre-treat WAS by various mechanical, thermal, chemical, and biological methods. In this research the influence of ultrasonic bath pre-treatment was studied to observe the effects of ultrasonic density and sonication time on WAS solubilisation. The charactristics of ultrasonic wave producer was surface area of 240×137 mm2, frequency of 40 kHz, power of 265W. Increases in soluble chemical oxygen demand and soluble polysaccharide concentration, as well as the decrease in volatile suspended solids, indicate that pre-treatment could cause WAS solubilisation. The cavitation produced by ultrasound waves radiation breaked down the bacterial cell wall and released the intracellular substances into an aqueous phase. Since polysaccharide is one of the main parts of extracellular polymeric substances (EPS), a polysaccharide concentration increase in the solution indicated that ultrasonication disintegrates WAS floc and the EPS value reduced in biological flocculation. Increases in ultrasonic density and sonication time caused more solubilisation, stronger cavitation arised with an increase in ultrasonic density and with increased ultrasonic density; more floc structure disintegration was achieved in less time. The best Pre-treatment efficiency was achieved in ultrasonic density 0.53 W/mL and 20 min. sonication time and it caused 20% increase in biogas production and 24% volatile suspended solids solubilisation compared to the control sample. The volatile solids after anaerobic digestion of pre-treated sludge also decreased. Increament in ultrasonic density and snication time had direct relation with volatile solids reduction. The best volatile solids efficiency was achieved in ultrasonic density 0.53 W/mL and 20 min. sonication time and it caused 48% decrease in volatile solids Given the change in WAS, electrical conductivity with ultrasonic bath pre-treatment, in addition to other tests like chemical oxygen demand and volatile suspended solids, electrical conductivity could also effectively assess WAS solubilisation.