The effect of hydraulic retention time on stabilisation unit in anaerobic – contact stabilisation (A-CST) for treating palm oil mill effluent

The increased use of palm oil in Indonesia has triggered an increase in the quantity of Palm Oil Mill Effluent (POME). The contents of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Suspended Solid (TSS), as well as oils and grease in palm oil mill effluent, need to be reprocessed, in accordance with the provisions of the Ministry of Environment Regulation No. 5 of 2014, concerning Wastewater Quality Standards, before being discharged into water bodies. In this study, a modification of the treatment unit involving the integration of suspended growth and attached growth in anaerobic and aerobic processes, known as Anaerobic – Contact Stabilisation (A-CST) unit, was conducted to reduce the high concentration of organic matter in POME. This modification can shorten the detention time and reduce the land usage for treatment. The objective of this research was to analyse the performance of the modified A-CST system, based on variations in the detention time and modelling the estimation of biokinetics under unsteady-state conditions, through numerical analysis, statistical tests, and model validation. The research focused on the stabilisation unit, with variations in Hydraulic Retention Time (HRT) of 6 hours, 5 hours, and 4 hours. The results of this study revealed that, the highest percentage of removal was obtained at an HRT variation of 6 hours, with a COD 88%.


Introduction
Palm Oil is one of the most important plantation vegetation in producing vegetable oil needed by various industries, producing cooking oil, margarine, oleochemicals, and soap.According to Statistics Indonesia (BPS), Indonesia is the largest producer of palm oil in the world, with a production of 45.1 million tonnes in 2021.The use of crude palm oil (CPO) leads to an increase in (POME), which poses a threat to the surrounding environment, if it is left untreated, due to its organic pollutant content.POME needs to be treated as it contains high levels of organic content, with dissolved solids, suspended solids, oil and grease, and high mineral content [1].POME from the palm oil industry in Aceh contains a BOD (Biochemical Oxygen Demand) of 5,713 mg/L, COD (Chemical Oxygen Demand) of 38,883 mg/L, and TSS (Total Suspended Solids) of 16,800 mg/L [2].Based on the Ministry of Environment Regulation No.5 of 2014 regarding the quality standards for wastewater, before being discharged into water bodies, organic pollutants needs to be treated.For the removal of contaminants in POME, biological treatment is one of the best processing alternatives, with several advantages, including effective reduction of organic matter, environmentally friendly, and cost saving in operational expenses.POME in Indonesia is treated by using open pond systems, which require large land areas and take a considerable amount of time.To overcome the limitations of the conventional systems, several innovative technologies for POME treatment are continuously being developed to process palm oil mill effluent [3].
A modification of the treatment unit was done, integrating anaerobic and aerobic digestion, that consists of a hybrid suspended growth system and attached growth, to enhance the effectiveness of reducing COD and ammonia nitrogen concentrations [4].This treatment combines the UASB unit and contact stabilisation, because the UASB unit is capable of degrading high organic concentrations, with a high removal percentage [5], while contact stabilisation functions to optimise the biodegradation process, so that organic compounds can be degraded optimally [6].
The contact units in the contact stabilisation system are also modified by combining attached growth and suspended growth types, with the addition of biological balls (bioballs), enabling them to achieve more than 90% removal of ammonia and combined chemical oxygen demand [7].The advantage of using this configuration is the ability to enhance overall degradation efficiency, with short detention times and limited land requirements [8].This research was focused on evaluating the performance of the stabilisation unit in the modified A-CST configuration.In the A-CST configuration, the stabilisation unit was designed to create starvation conditions, for microorganisms, and is directed to the contact unit to reduce the concentration of organic matter [9].
One of the crucial operational aspects in HRT is the amount of time spent in the biological reactor, expressed as the ratio between the reactor volume and the influent flow rate [10].By knowing the value of HRT, the capacity of the unit can be designed, according to the planned flow rate requirements.Based on the design criteria, the stabilisation unit is operated within the range of 2 hours to 6 hours [11].Based on the above description, the specifications of this research include the evaluation of the stabilisation unit in the modified A-CST configuration, through a performance analysis, based on the variations in the hydraulic retention time.This research was expected to provide alternative technological solutions for the POME management in Indonesia, with performance results that can be directly implemented, for the design, planning, and monitoring of the field scale treatment.

Methods
The sampling of POME was conducted at PT.PN VIII Cigudeg,Bogor Regency,West Java Province.Biomass samples, as the initial seeding and acclimatisation media, were obtained from the effluent of the biological treatment unit, at the Wastewater Treatment Plant II in Jababeka Industrial Estate, Bekasi.This research was carried out, from March 2023 to June 2023 in the Water Quality Laboratory, Department of Civil and Environmental Engineering, IPB University.

Tools and materials
This research focused on the stabilisation unit, which is one of the treatment units in A-CST process.The A-CST process consists of a 80 L UASB Unit, a 30 L contact unit, a 14 L stabilisation unit, and a 18 L clarifier unit.The equipment and chemicals used for water quality analysis, to evaluate the reactors performance, based on the parameters to be analysed, include COD using the SNI 6989:02:2019 method for the analysis of COD with closed reflux spectrophotometry, ammonia using the SNI 6989:30:2005 method for the analysis of ammonia levels with phenate spectrophotometry, pH using the SNI 6989:11:2019 method for the acidity level, Mixed Liquor Suspended Solids (MLSS) using the SNI 6989:03:2019 method for the TSS analysis with gravimetry, and Total Dissolved Solids (TDS).

Research procedures
The research activities were divided into 5 stages; reactor preparation stage, reactor configuration stage, seeding and acclimatisation stage, wastewater quality testing stage, and wastewater quality analysis stage.A-CST reactor was designed, based on the volume and variations in HRT.The HRT for the anaerobic unit consists of 192, 96, and 12 hours, while the HRT for the contact unit consists of 16, 12, and 8 hours, and the HRT for the stabilisation unit consists of 4, 5, and 6 hours.For each HRT variation, samples were taken daily, for a period of 10 days.The configurations of the biological nutrient removal unit reactor were determined by the operational variables, including flow and mass balance, HRT, and wastewater volume, under non-steady state conditions.The flow balance was affected by the unit volume and variations in HRT.The seeding process was conducted to propagate microorganisms.It involved the addition of an easily degradable substrate, such as glucose, into the stabilisation unit.The amount of substrate provided to the microorganisms was determined using the F/M (Food/Microorganism) ratio, which helps determine the amount of food required, based on the number of microorganisms obtained from the MLSS measurement.In addition to glucose, microorganisms also required other nutrient sources, such as nitrogen and phosphorus.The microorganisms used for substrate degradation were obtained from the sludge of the Jababeka Wastewater Treatment Plant.After a successful seeding process, the subsequent step was the acclimatisation process, which involved the adaptation of bacteria to the substrate to be treated, which is POME.The introduction of the POME for the treatment was done gradually, by providing a ratio of POME to glucose.The ratio of POME to glucose was gradually increased as follows: 25% POME : 75% Glucose, 50% POME : 50% Glucose, 75% POME : 25% Glucose.The increase in POME input was done, if the microorganisms were found to be capable of reducing COD.This gradual approach was implemented to prevent shock loading, where a sudden introduction of POME could lead to microbial die-off [12].By gradually increasing the POME input, the microorganisms could adapt to the substrate being treated.

Analysis of palm oil mill effluent
The results of analysis of the concentrations of COD, TSS, TDS, pH, temperature, total Nitrogen, oil and grease, and ammonia in the LCKS were 23,152.93±

Seeding and acclimatisation
In this research, the seeding and acclimatisation processes are necessary to adapt microorganisms for degrading the substrate, namely POME.In the seeding process, a substrate easily utilised by microorganisms in the form of 100% glucose was introduced into the reactor.The seeding process was carried out, for 13 days, until the MLSS concentration reached 14,000 mg/L.The transition from seeding to acclimatisation was performed, when the MLSS concentration in the stabilisation unit reached the design criteria of 4,000 -10,000 mg/L.The total reduction in COD concentration during the seeding process had its highest value of 95.23%, from the initial COD concentration of 6,098.24mg/L to 291 mg/L.During the acclimatisation phase, the percentage of COD removal often experiences fluctuations.This is caused by an increase in the ratio of glucose and POME provided to the microorganisms.Initially, when glucose and POME are introduced, the percentage of COD removal tends to be low, because the microorganisms are still adjusting to the given substrate.Changes in the substrate feeding ratio are made when the percentage of COD removal efficiency increases, indicating that, the microorganisms are becoming ready to degrade POME.With each increase in the ratio of glucose and POME provided, the percentage of COD removal continues to fluctuate due to the microorganisms' adaptation process.It can be observed that, the percentage of COD removal fluctuates with each change in the substrate ratio, but in the later stages of the acclimatisation process, the percentage of COD removal reaches 95.56%.This indicates that, the microorganisms are now prepared to handle POME.

Stabilisation unit performance's evaluation results
The performance evaluation of the stabilisation unit focused on variation 1 -variation 3, that is, on the detention time of the stabilisation unit of 6, 5, and 4 hours, to determine the performance of the stabilisation unit.The stabilisation unit functions are creating microorganisms in conditions of substrate deficiency, so that, microorganisms can utilise organic matter more optimally, since there are many substrates available in the contact unit [9].The process that occurs in the stabilisation unit involves further aeration of the biological sludge, after settling in the sedimentation unit, with the aim of oxidising organic matter and promoting microbial growth to yield biological sludge, with microorganisms ready to utilise substrates, when recirculated to the contact unit [15].The performance evaluation of the stabilisation unit focused on variation 1 -variation 3, on the detention time of the stabilisation unit of 6, 5, and 4 hours, to determine the performance of the stabilisation unit.The performance of the stabilisation unit can be seen from the removal percentage of COD and MLSS concentration.In the stabilisation unit, variation 1, namely when the detention time was 6 hours, the influent COD concentration of the stabilisation unit was found to be in the range of 102.98 -336.78 mg/L, with the stabilisation unit effluent COD concentration ranging from 86.67-256.67 mg/L.In variation 2, namely when the detention time was 5 hours, the stabilisation unit influent COD concentration ranged from 96.44 -3,351.59mg/L with stabilisation unit effluent COD concentrations ranging from 67.01 -3,188.09mg/L, and in variation 3, namely when the detention time was 4 hours, the stabilisation unit influent COD concentration ranged from 32.68 -1,133.01mg/L, with stabilisation unit effluent COD concentrations of 26.14 -946.63 mg/L.The COD removal efficiency for 4, 5, and 6 hours was 28%, 22%, and 16%, respectively.Previous research on leachate wastewater using a contact stabilisation reactor, with a research focus on the stabilisation unit, resulted in COD concentration removal efficiencies of 38%, 34%, and 25% at 6, 5, and 4 hours, respectively.From the results of the previous research, it can be observed that, treating palm oil mill wastewater is more challenging, compared to leachate wastewater treatment [16].The largest removal percentage in the stabilisation unit occurred at HRT 6 hours, due to a decrease in HRT, resulting in a decrease in removal efficiency in the reactor, due to the contact time of wastewater with the reactor decreasing [17].The concentration of MLSS in the stabilisation unit depicts the amount of solids and microorganisms contained in the reactor [18].The MLSS concentration in the stabilisation unit is conditioned in the 4,000 -10,000 mg/L [11].The MLSS concentration from the stabilisation unit will be recirculated to the contact unit to degrade the substrate.The MLSS concentration circulated to the contact unit in variation 1 ranged from 4600-9340 mg/L; variation 2 ranged from 3160-19980 mg/L, and variation 3 ranged from 3,100-8,460 mg/L.Based on the test results, the MLSS concentration fluctuates, influenced by the growth rate of microorganisms, the volume of the stabilisation unit, and HRT variations, so that the MLSS concentration can be below or above the range of 4,000 -10,000 mg/L.

Conclusion
The stabilisation unit plays a crucial role in the ACST reactor to prepare microorganisms, so that they are capable of utilising substrates and creating effluent quality, in accordance with the standards.The variation in HRT significantly affects the efficiency of organic pollutant removal, in the stabilisation unit.The COD removal percentages for each HRT of 4, 5, and 6 hours are 16%, 22%, and 28%, respectively, for COD.The highest removal percentage occured at the 6-hour HRT variation, where microorganisms have more time to degrade the organic pollutant.The removal efficiency of the stabilisation unit is quite low, since the stabilisation unit is not a unit that focuses on processing, but rather on conditioning microorganisms, in a state of substrate deficiency, so that they can utilise the substrate, when it is recirculated to the contact unit.The MLSS concentration fluctuates, influenced by the growth rate of microorganisms, the volume of the stabilisation unit, and HRT variations.

Figure 1 .
Figure 1.A-CST modified reactor physical condition of the unit, during operation.

Table 1 .
Operational design parameters of the biological nutrient removal unit.

Table 2 .
The quality of palm oil mill effluent.
4,053.22 mg/L respectively; 65,950 ± 34,367.35mg/L ; 1,605.93 ± 374.15 mg/L ; 3.97 ± 0.56 ; 27.43 ± 1.86 °C ; 289 mg/L ; 4,217.2mg/L ; 24.60 ± 19.80 mg/L.Based on Ministry of Environment and Forestry Regulation No. 5 of 2014 concerning Industrial Wastewater Quality Standards, POME needs to be reprocessed to meet quality standards.The analysis of POME processing on a laboratory scale was carried out through a biological nutrient removal unit.

Table 3 .
Reduction of organic pollutant concentration based on HRT variations.

Table 3 .
Reduction of organic pollutant concentration based on HRT variations (cont.).