An integrated UMAS for POME treatment

The direct discharge of palm oil mill effluent (POME) wastewater causes serious environmental hazards due to its high chemical oxygen demand (COD) and biochemical oxygen demand. This paper proposes a new approach for integrated technology of ultrasonic membrane for a POME treatment. The paper evaluated the economic viability based on the changes of the new design of ultrasonic membrane anaerobic system (UMAS) when a POME introduces this approach. Six steady states were attained as a part of a kinetic study that considered concentration ranges of 13,800–22,600 mg/L for mixed liquor suspended solids and 10,400–17,350 mg/L for mixed liquor volatile suspended solids. Kinetic equations from Monod, Contois and Chen and Hashimoto were employed to describe the kinetics of POME treatment at organic loading rates ranging from 1 to 15 kg COD/m/d. throughout the experiment, the removal efficiency of COD was from 92.8 to 98.3% with hydraulic retention time from 500.8 to 8.6 days. The growth yield coefficient, Y, was found to be 0.73 gVSS/g COD, the specificmicroorganism decay rate was 0.28 day and the methane gas yield production rate was between 0.27 and 0.62 L/g COD/d. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wrd.2016.124 N. H. Abdurahman (corresponding author) Faculty of Chemical and Natural Resources Engineering, University Malaysia Pahang-UMP, Lebuhraya Tun Razak, 26300 Gambang, Kuantan, Malaysia E-mail: nour2000_99@yahoo.com N. H. Azhari Faculty of Industrial Sciences and Technology, University of Malaysia Pahang-UMP, Lebuhraya Tun Razak, 26300 Gambang, Kuantan, Malaysia


INTRODUCTION
The palm oil industry has grown tremendously in recent years and accounts for the largest percentage of oil and fat production in the world ( Jundika et al. ).Over the last few decades, the palm oil industry has been growing rapidly.
Palm oil has risen to become the most produced and consumed vegetable oil in the world, widely used in food, cosmetic and hygienic products due to its affordable price, efficient production and high oxidative stability ( Jundika et al. ).Palm oil is the most produced vegetable oil in the world with a global production of almost 60 million tonnes and a global vegetable oil market share of more than 35% by weight in 2015 (Hansen et al. ; Malaysian Palm Oil Board MPOB ).The industry continues to generate huge revenues for the producing countries.Accordingly, it is not surprising that the oil palm industry is expected to grow further in the coming years, as shown in Figure 1.
The conventional methods for palm oil mill effluent (POME) treatment include aerobic, membrane, evaporation, fluidized bed, anaerobic filtration and continuous stirrer tank reactor.The main drawbacks of these methods are either large volume of digestion, long retention time, lower methane emission, clogging at high organic loading rates (OLR), high power requirement, high cost of carrier media, not suitable for high suspended solid wastewaters, and less efficient gas production at high treatment volume.
This study introduces ultrasonic membrane anaerobic system (UMAS) to overcome the above-mentioned drawbacks of the conventional methods because UMAS has the following advantages: • treats high organic load wastewater efficiently and effectively; • reduces the anaerobic treatment time; • reduces the plant floor area; • produces renewable energy efficiently; • reduces biomass sludge discharge.Thus, it is not surprising that the highest yields have been obtained from palms grown in this region, which is far from its natural habitat.Moreover, the Malaysian

Wastewater preparation
Raw POME was collected from a local palm oil mill in Lebah Hillier, Kuantan, Malaysia.In the first stage, raw POME was pre-settled using an ordinary sedimentation tank.In the second part of this study, raw POME was chemically pretreated to remove suspended solids and residual oil.
The samples were then stored in a cold room at 4 W C. POME stored under such conditions has no observable effects on its composition.Table 1 shows mathematical expressions for specific substrate utilization rates (SSUR) for three kinetic models (Monod, Contois, and Chen and Hashimoto).

UMAS bioreactor operation and experimental set-up
Raw POME wastewater was treated by UMAS in a laboratory digester with an effective 200-L volume.

Bioreactor operation
The UMAS performance was evaluated under six steadystates (Table 2) with influent COD concentrations ranging

RESULTS AND DISCUSSION
The performance of UMAS The performance of the UMAS was evaluated and is summarized in       growth of VFA.The COD removal efficiency did not differ significantly between HRTs of 500.8days (98.3%) and 14.7 days (96.0%).At HRT of 8.6 days, COD was reduced to 92.8%.As shown in Table 2, this was largely a result of the washout phase of the reactor because the biomass concentration increased in the system.This may be attributed to the fact that at low HRT with high OLR, the organic matter was degraded to VFA.The HRTs were mainly influenced by the ultra-filtration, UF membrane influx-rates which directly determined the volume of influent (POME) that can be fed to the reactor.

Determination of bio-kinetic coefficients
Experimental data for the six steady-state conditions in    on methanogens bacteria that might contribute to biogas production.Because methanogenesis is also strongly by pH, methanogenic activity will decrease when the pH in the digester deviates from the optimum value.
Mixing provides good contact between microbes and substrates, reduces the resistance to mass transfer, minimizes the build-up of inhibitory intermediates and stabilizes environmental conditions.This study adopted the mechanical mixing and biogas recirculation.The integrated technology of UMAS is a more attractive solution compared to the case when the POME was treated individually using either ultrasonic or membrane technology.Moreover, the integrated technology, UMAS, showed improved economic viability, which is the most profitable approach compared to installing each technology alone.
Over the long term, global palm oil demand shows an increasing trend as an expanding global population gives rise to increased consumption of palm-oil based products (Meryana et al. ).Sayer et al. () stated that since POME is a major contributor to the economies of several developing countries, the global production and demand for palm oil is increasing rapidly and the plantations are spreading across Asia, Africa and Latin America.The five leading palm oil producing countries are Indonesia, Malaysia, Thailand, Colombia and Nigeria (Mba et al. ), as shown in Figure 2. The development of palm oil industry in Malaysia has turned into a phenomenon in which the area of plantation expanded from year to year.The country is experiencing a robust development in new oil palm plantations and palm oil mills.This commodity plays a significant role in the Malaysian economic growth (Awalludin et al. ).Throughout the year, Malaysia is blessed with favorable weather conditions which are advantageous for palm oil cultivation (Yusoff ).
palm oil industry has grown to become a very important agriculture-based industry, and the country is today one of the world's leading producers and exporters of palm oil.According to Yacob et al. (), 381 palm oil mills in Malaysia generated about 26.7 million tonnes of solid biomass and about 30 million tonnes of palm oil mill effluent (POME) in 2004.Discharging the effluents or by-products on the land may lead to pollution and might deteriorate the surrounding environment.There is a need for a sound and efficient management system in the treatment of these by-products in a way that will help to conserve the environment and check the deterioration of air and river water quality.The main objective of this study was to evaluate the performance and kinetics of the new designed UMAS in treating POME based on three models: Monod (), Contois () and Chen & Hashimoto ().
Figure 3 presents a schematic representation of the UMAS which consists of a cross-flow ultra-filtration membrane apparatus, a centrifugal pump and an anaerobic reactor.Six multi-frequency ultrasonic transducers, operated at 25 KHz, are bonded to twosides of the tank chamber and connected to a Crest Genesis Generator (250 W, 25 KHz; Crest Ultrasonic, Trenton, NJ, USA).The principle of ultrasonic treatment relies on cavitation to disintegrate cell walls.High density intensity ultrasound enhances the disintegration of particulate matter, as shown by a reduction in particle size and increase of the soluble matter fraction.The ultra-filtration membrane module had a molecular weight cut-off of 200,000, a tube diameter of 1.25 cm and an average pore size of 0.1 μm.There were four tubes, each 30 cm long, and the total effective area of the four membranes was 0.048 m 2 , and the pH ranged from 2 to 12.The reactor was composed of a heavy duty reactor with an inner diameter of 25 cm and height of 250 cm.The operating pressure in the UMAS was maintained between 2 and 4 bars by manipulating the gate valve in the retentate line after the cross-flow ultra-filtration membrane unit.Analytical methods Biogas volume was daily measured with water displacement, using a 20-L water displacement bottle, the methane (CH 4 ) and carbon dioxide (CO 2 ) content were analyzed by a J-Tube analyzer and a gas chromatograph (GC 2011, Shimadzu) equipped with a thermal conductivity detector and a 2 m × 3 mm stainless-steel column packed with Porapak Q (80/100 mesh).Total suspended solids (TSS), volatile suspended solids (VSS), volatile fatty acids (VFA) and alkalinity were determined according to Standard Methods (APHA ).The chemical oxygen demand (COD) was measured using a Hach colorimetric digestion method (Method #8000, Hach Company, and Loveland, CO, USA).
Figures 4-6.The Monod and Chen and Hashimoto

Figure 7
Figure 7 shows the percentages of COD removed by UMAS at various HRTs.COD removal efficiency increased as HRT increased from 8.6 to 500.8 days and was in the range of 92.8-98.3%.This result was higher than the 85% COD removal observed for POME wastewater treatment using anaerobic fluidized bed reactors (Idris & Al-Mamun ) and the 91.7-94.2%removal observed for POME wastewater treatment using MAS (Fakhru'l-Razi et al. ), and the 93.6-97.5% removal observed for POME treatment using MAS (Abdurahman et al. ).It is observed that there are no COD values for the period between 100 and 500 days, this could be attributed to the

Figure 6 |
Figure 6 | The Chen and Hashimoto model.

Figure 7 |
Figure 7 | COD removal efficiency of UMAS under steady-state conditions with various HRTs.

Figure 8
Figure 8 shows the SSUR values for COD at steady-state conditions HRTs between 8.6 and 500.8days.SSURs for COD generally increased with HRT decline, which indicated that the bacterial population in the UMAS multiplied (Abdullah et al. ).It observed that there are no SSUR values for period between 100 and 500 days, this may be attributed to the growth of VFA.The bio-kinetic coefficients of growth yield (Y) and specific micro-organic decay rate (b) and the K values were calculated from the slope and intercept as shown in Figures 9 and 10.Maximum specific biomass growth rates (μ max ) were in the range of 0.248-0.474d -1 .All of the kinetic coefficients that were calculated from the three models are summarized in Table3.

Figure 9 |
Figure 9 | Determination of the growth yield, Y and the specific biomass decay rate, b.

Figure 10 |
Figure 10 | Determination of the maximum specific substrate utilization and the satur- ation constant, K.
Figure11shows the gas production rate and the methane content of the biogas.The methane content generally declined with increasing OLRs.Methane gas content ranged from 64.6 to 81% and the methane yield ranged from 0.39 to 0.70 CH 4 /g COD/d.Biogas production increased with increasing OLRs from 0.48 L/g COD/d at 1.0 kg COD/m 3 /d to 0.81 L/g COD/d at 15 kg COD/m 3 /d.The decline in methane gas content may be attributed to the higher OLR, which favors the growth of acid forming bacteria over methanogenic bacteria.Thus the methane conversion process was adversely affected with reducing methane content and this led to the formation of carbon dioxide (CO 2 ) at a higher rate.The gas production showed an increase from 290 to 540 L per day during the study.In this scenario, the higher rate of carbon dioxide (CO 2 ) formation reduces the methane content of the biogas.CONCLUSIONSPOME is always regarded as a highly polluting wastewater generated from palm oil mills; however, reutilization of POME to generate renewable energies has great potential, especially when coupled with wastewater treatment technologies.This study proposed treating POME by-products through the integrated technology of ultrasonic and membrane production, UMAS at University Malaysia Pahang, UMP.This study evaluated the economic viability based on the changes of the new design of UMAS when a POME introduces this approach.

Table 1 |
Mathematical expressions of SSUR for known kinetic models

Table 2
. The UMAS performance at six steady-states was established at different hydraulic retention times (HRTs) and influent COD concentrations.The kinetic coefficients of the selected models were derived from Equation (2) in Table1by using a linear relationship; the coefficients are summarized in Table3.At steady-state conditions with influent COD concentrations of 70,400-90,200 mg/L, UMAS performed well and the pH in the reactor remained within the optimal working range for

Table 2 |
Summary of results

Table 3 |
Results of the application of three known substrate utilization models

Table 2
were analyzed; kinetic coefficients were evaluated and are summarized in Table3.Substrate utilization rates (SUR) and SSUR were plotted against OLRs and HRTs.