An Experimental Investigation on Removal and Recovery of Phosphorus Using Sludge Conditioned with Skeleton Material

Abstract

The production of excess sludge in wastewater treatment processes has been a serious issue for the operation of wastewater treatment plants (WWTPs) on both economic and environmental perspective. The disposal of sludge is a challenging task as it increases the handling and transportation cost. Therefore, sludge dewatering is the prominent to overcome the mentioned limitation. Dewatering using low-cost skeleton materials is a promising technique due to its efficiency, economic and environmental point of view. Further, it is a resource of nutrients like phosphorus and nitrates in not only in sludge but as well as in wastewater. However, phosphorus in the waste water leads to eutrophication of water bodies and result in algal blooms. As effluent discharge limits become more stringent, there is continued interest in removing phosphorus from wastewater. To prevent the receiving waters from eutrophication, the Enhanced Biological Phosphorus Removal (EBPR) process is a cost-effective and environmentally-friendly process for phosphorus removal in wastewater treatment systems. Phosphorus recovery from wastewater can help alleviate reliance on imported phosphate and reduce vulnerability to fluctuating prices. Hence in the present study, objectives have been framed in three phases. First phase deals with dewatering of sludge using the skeleton materials and its characterization. Second phase investigated on phosphorus removal using dewatered sludge by EBPR process and Finally recovery was carried out by crystallization. Reducing the moisture content in secondary sludge is a key factor in reducing the capital costs, operational costs, and transportation costs in wastewater management. In the present study, an attempt has been made to utilize granulated blast furnace slag and modified coconut shell biochar for sludge dewatering. In first phase, experimental work includes the initial characterization of the sludge and granulated blast furnace slag and evaluation of the dewatering ability of the treated sludge (capillary suction time, moisture content, turbidity, zeta potential, and heavy metal and biopolymer contents). Optimization using the Box-Behnken design was carried out with various operational parameters, and the best performance was found to be at a pH of 10, a dose of 0.34 g/g dry solids, and a contact time of 14 min. Characterization study was carried out by scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy, X-ray diffractometry, and Fourier transform infrared spectroscopy to confirm the structural features (dense), elemental composition, and the presence of different functional groups. Coconut shell is a bio waste and its availability is high as a waste in the coastal region of Karnataka, India. It is modified with ferric chloride to enhance the sludge dewaterability and it is evaluated experimentally (Capillary suction time, moisture content, settleability, zeta potential, heavy metals, and phosphate). Further, scanning electron microscopy, Fourier transformation infrared spectroscopy, and X-ray diffraction characterization were carried out to identify the structure change. A significant reduction in capillary suction time(56sec) and the moisture content (96.5%) of the dewatered sludge cake was obtained. Sludge dewatering using coconut shell biochar modified with ferric chloride was optimized by a Box Behnken method with three main factors including dosage, rapid mixing time, and slow mixing time. Optimum capillary suction time (55.8 sec) was achieved at coconut shell biochar modified with ferric chloride dosage (41% dry solids), rapid mixing time (10min), and slow mixing time (19min). The significant structural change in sludge particles was confirmed through characterization studies. During the dewatering process, the removal of heavy metal (cadmium, chromium, lead, and nickel) and phosphate (50.6%) was evident.anaerobicFor the second phase, an attempt was made to propose by EBPR induced with crystallization for the removal and recovery of phosphorus using dewatered sludge. An experimental investigation was carried out with the setup (anaerobi (anaerobi (anaerobi(anaerobi c-aerobic process) and was stably operated for 125 days. A sequential batch reactor of 4 liters was set up for the alternative anaerobic - anaerobic operation fitted with rectangular peddle mechanical stirrer and a air diffuser. Effect of pH on EBPR was studies for one complete cycle in a batch mode. Metabolism of PAOs takes place in anaerobicanaerobic anaerobic-aerobic process and that is in anaerobic process PAOs releases phosphorus and at is in anaerobic process PAOs releases phosphorus and in aerobic process phosphorus uptake takes place. The phosphorus removal efficiency was achieved in this process as 84%.