Application of hybrid ultrasonic cavitation/adsorption and coagulation for treatment of palm oil mill effluent

Abstract

Ultrasound cavitation (US), coagulation treatment (natural coagulant chitosan and synthetic coagulant ferric chloride (FeCl3)) and activated carbon as adsorbent were applied for treatment of raw palm oil mill effluent (POME). The findings showed that for US alone, increasing pH >11 COD removal increased due to more hydroxyl radical (OH·) present in alkali solution phase. The COD, colour and TSS removal at pH 11 were 26.3%, 52.7% and 58.2%, respectively after 60 min. Application of coagulants (Chitosan and FeCl3) required acidic medium for coagulation (i.e. between pH 4.5 and 5) to be effective. A dosage of 100 mg/L chitosan at pH 4.5 removed 15.4% COD, 85.8% colour and 97% TSS from POME. A dosage of 450 mg/L FeCl3 at pH 5 removed 38.54% COD, 88.6% colour and 91.5% TSS. It was observed that FeCl3 removed COD better than chitosan. Adsorption studies indicated that 800 mg/L of activated carbon removed 64.3% COD, 99.16% colour and 99.5% TSS. The dosage needed for adsorption was much higher compared to chitosan and FeCl3 coagulants required. However, activated carbon could be recycled and reused. The hybrid treatment of ultrasound cavitation and coagulation (US-FeCl3) removed 56.3% COD, 92.4% colour and 96% TSS. The US-Chitosan removed 35.1% COD, 86.8% colour and TSS 89.2%. The eventual hybrid treatment of ultrasound cavitation, FeCl3 coagulation and activated carbon adsorption in series removed BOD5 89.7%, COD 88.1%, colour 99.9% and TSS 99.5% cumulatively. The final effluent concentration of the treated POME was in the accepted range set by Department of Environment (DOE) Malaysia. The study showed that a combination of ultrasound cavitation, adsorption and coagulation (Chitosan and FeCl3) treatments were effective for removal of BOD5, COD, colour and TSS in POME wastewater. In addition, the efficiency of the treatment will further improve when these treatment technologies are combined. INTRODUCTION Malaysia contributes about 39% of the world palm oil production and this translates to 44% of palm oil world export [1]. Therefore, palm oil is a very important sector and significantly affects the gross domestic product (GDP) of Malaysia. Due to the importance of palm oil industry, large area of land has been converted into oil palm plantation estate, at the same time, many more palm oil mills have been built to process the increasing amount of oil palm fresh fruit bunch (FFB) into crude palm oil. Palm oil mill effluent (POME) generated by processing 1 ton of FFB, contains about 29-30 kg at 30C, 3-day biochemical oxygen demand (BOD3) [2]. From the data of POME produced in year 2014, if the raw POME is discharged into the environment without any further treatment, the BOD discharged would be equal to the waste generated by 75 million people, which is the 2.5 times of the current Malaysia population [3]. The most popular method to treat the POME in Malaysia is using the ponding system as it has a low equipment cost and it is easy to operate. There are more than 85% of palm oil mills that have adopted this method to reduce the BOD of POME to reach an acceptable limit, which is less than 100 mg/L (West Malaysia) and 50 mg/L (East Malaysia). Due to the pollution potential of the POME, the Department of Environment, Malaysia has proposed more stringent regulation on the discharge limit of POME. For example, decrease of the BOD discharge limit from 100 to 20 mg/L. 6th International Conference on Environment (ICENV2018) AIP Conf. Proc. 2124, 020008-1–020008-12; https://doi.org/10.1063/1.5117068 Published by AIP Publishing. 978-0-7354-1864-6/$30.00 020008-1 This will be challenging for all the palm oil mills in Malaysia. Therefore, an effective polishing technology is needed to degrade the POME before discharged [1]. In the ponding system, the POME undergoes biological treatments which includes anaerobic digestion process followed by aerobic ponding with hydraulic retention time of 40 d or more. However, ponding system has some drawbacks which are long hydraulic retention time (HRT), huge land needed and the release of greenhouse gases (methane). There are also many palm oil mills which are unable to achieve the discharge limit by using ponding system [3]. Ultrasonication (US) is an irradiation of ultrasound with frequency beyond the normal hearing range of humans (>15–20 kHz) or it is simply mechanical waves at a frequency above the threshold of human hearing. It can be generated at a broad range of frequencies and acoustic intensities. It has been widely used as a green technology to treat various wastewaters with higher degradation rates and shorter reaction times compared to conventional methods [4, 5]. Cavitation is the formation, growth and subsequent collapse of bubbles over a small time period which results in the generation of large magnitudes of energy over specific location [3]. There are four types of cavitation which are acoustic (ultrasound), hydrodynamic, optic and particle cavitation. Among these four types of cavitation, acoustic cavitation and hydrodynamic cavitation is the most common and have been investigated. US cavitation occurs when a passage of very high frequency sound wave of 16-100 kHz transmit through wastewater. Hydrodynamic cavitation occurs when a liquid passes through a constriction. Ultrasound cavitation which degrades pollutants can occur either through pyrolysis of pollutants or through the production of OH· [5]. Pollutants which are volatile, non-polar and hydrophobic can easily enter into the cavitation bubbles and exposed to the collapsing conditions of bubbles [3, 6]. The advantages of using ultrasound technology as an environmentally friendly, compact and low-cost wastewater treatment option is notable. Ultrasonication is expected to decompose complex organic pollutants in the effluent due to the formation and collapse of high-energy cavitation bubbles [5]. This method has been reported to be successful in several processes such as bioprocesses, biohydrogen production, aerobic and anaerobic treatment processes [7-11] etc. Chitosan is a kind of biopolymer coagulants which is non-toxic, biodegradable, renewable and environmental friendly [12]. Chitosan is a type of marine polymer which has been widely used in practical fields such as wastewater management, pharmacology, bio-chemistry and biomedical. Chitosan is a cellulose-like polyelectrolyte biopolymer which derived from de-acetylation of chitin. Chitin can be easily found in marine nature, it is occurring in the insects, yeast, fungi and exoskeletons of crustaceans [13]. Chitosan contains high amount of amino functions that provide novel binding properties for heavy metals in waste water [14]. Chitosan can coagulate effectively at pH less than 4.5 as strong acidic condition exaggerates POME to form unstable flocs [15]. There are no reported literatures on the hybrid ultrasonic cavitation/adsorption and coagulation treatment of POME as an alternative to ponding system. The main focus of the research was the treatment of POME through the combination of ultrasound cavitation, adsorption using activated carbon along with pretreatment or post-treatment using natural and synthetic coagulants. The objective of the study was to investigate the polishing and reduction of organics in order to decrease the biochemical oxygen demand (BOD), total suspended solids (TSS), colour and chemical oxygen demand (COD) of the POME. To achieve this, the optimization of the ultrasound cavitation in reducing pollutants in POME, determination of the optimum dosage of adsorbent (activated carbon), chitosan and ferric chloride (FeCl3) and the performance of the hybrid POME treatment by combining ultrasound cavitation/adsorption and coagulation for treatment of the POME so as to meet the effluent discharge standards was investigated. MATERIALS AND METHODS Materials Basically, the POME and chemicals used were the materials required for the study. Wastewater Sample Collection POME was collected from a nearby local production mill in Sibu, Sarawak, Malaysia. It was collected from the initial discharge of the raw POME. The POME were freshly collected when needed.