Redesigning the Wastewater Treatment Plant for the Tofu Industry in Ledok Kulon Village, Bojonegoro Regency

Currently, the wastewater treatment plant (WWTP) for the tofu industry in Ledok Kulon Village, Bojonegoro Regency uses an anaerobic-aerobic-anaerobic system. However, treated wastewater quality is still over maximum standard. To rectify this, the objective of the study was to technically improve the WWTP so that the treated wastewater could comply with the required quality standards for disposal into the environment. This was achieved without increasing the capacity of the WWTP by changing the function of each unit and modifying the flow patterns of the system. Additionally, the treatment system was altered from an anaerobic-aerobic-anaerobic to an anaerobic-aerobic system. Consequently, the WWTP redesigning calculations showed that the levels of BOD, COD, and TSS produced were 9.65 mg/L, 61.23 mg/L, and 2.16 mg/L, respectively, which met the required quality standards.


Introduction
According to Mufarida and Abidin (2021), tofu industrial waste is the result of the tofumaking process, especially when the soybeans are washed.The waste consists of solid, gas and liquid waste [5].Rahmiati (2021) stated that the tofu waste contains 23.25% protein, 5.87% fat, 26.92% carbohydrates, 17.03% ash, 16.53% crude fiber, and 10.43% water [8].According to Herlambang (2001), tofu liquid waste contains gases such as oxygen (O2), hydrogen sulfide (H2S), ammonia (NH3), carbon dioxide (CO2), and methane (CH4), which originate from the decomposition of organic matter in the waste [2].Muliyadi and Safrudin (2020) stated that the composition of tofu liquid waste consists of 99.9% water and the remainder consists of dissolved solids and suspended solids particles of 0.1% [6].According to Rajagukguk (2020), solid particles consist of about 70% organic matter and 30% inorganic matter.Organic matter consists of about 0.1-0.8%protein, 1% carbohydrates (mainly stachyose and sucrose), 0.4-1.0%fat, and 0.4% minerals [9].Pohan (2008) stated that the high content of organic compounds causes tofu liquid waste to contain high BOD, COD, and TSS.Therefore, if the waste is discharged into the waters without prior treatment, it can cause pollution [7].Kaswinarni (2007) stated that the impact of organic pollution from tofu industry waste is the disruption of biotic life and the decrease in water quality due to the increase of organic matter content.The liquid waste from the tofu-making process contains dissolved and suspended solid particles which will undergo physical, chemical and biological changes.This can produce toxic substances and become a growing medium for germs or other disease germs that can endanger tofu products and human health.If waste water is not handled properly, the color will turn blackish brown and smell bad.This unpleasant odor can cause problems in the respiratory system.If this liquid waste seeps into the ground near the well, then the well can no longer be used.If the waste is dumped into the river, the river will be polluted and cause health problems such as itching, diarrhea, cholera, colitis, and other diseases if it is still used [3].
According to Faisal et al, (2016), to deal with the impact of waste pollution from the tofu industry, it is necessary to find ways to treat liquid waste that can be done effectively, efficiently, and are easy to operate [1].Generally, there are three types of processing methods developed, namely physical, chemical, and biological methods.The physical method involves separating part of the pollutant load, especially suspended or colloidal solids from the effluent, by using physical forces.Several processes that can be used in the physical treatment of tofu industrial wastewater include filtration and sedimentation.In filtration, filter media is used to separate coarse particles and suspended solids from liquid waste.Whereas in sedimentation, solid flocs are separated from the flow using gravity (Metcalf & Eddy, 2003) [4].
By using biological methods, dissolved organic matter levels can be reduced using microorganisms or aquatic plants.In principle, biological methods involve breaking down complex molecules into simpler molecules.However, this process is very sensitive to various factors such as temperature, pH, dissolved oxygen (DO) content, and inhibiting substances, especially toxic substances (Samer, 2015) [11].Microorganisms such as bacteria, algae, or protozoa can be used in waste treatment (Rittmann & McCarty, 2001) [10], while water weeds (Vidyawati, 2019) [12] can be used as plants.Biological methods consist of aerobic and anaerobic processes.In aerobic processes, organic compounds are decomposed using free oxygen as the final electron acceptor, whereas in anaerobic processes, organic compounds are decomposed without the presence of free oxygen and these organic compounds are used as the final electron acceptors (Rittmann & McCarty, 2001) [10].
Currently, the WWTP in Ledok Kulon Village has used physical and biological treatment to treat tofu industrial wastewater.Physical treatment is carried out using settling tanks, while biological treatment is carried out through anaerobic and aerobic reactors to decompose organic compounds.The treatment stage starts from the inlet, then to settling tank 1, anaerobic reactor 1, aerobic reactor, settling tank 2, anaerobic reactor 2, settling tank 3, and finally to the outlet.Even though the two processing methods have been used, the WWTP processing results still do not meet the set quality standards.Based on laboratory tests, the parameter values of BOD, COD, and TSS at the existing WWTP outlets were 352 mg/L, 768 mg/L, and 80 mg/L respectively.BOD and COD parameter values still do not meet the expected quality standards.
This paper aims to provide suggestions for technical improvements to existing WWTP, with the hope that wastewater treatment can reach the set quality standards and problems arising from tofu industrial wastewater can be overcome.

Methods
The first stage of this study was an evaluation of the existing industry, followed by a site survey which was conducted by observing the location of the tofu industrial WWTP in Ledok Kulon Village which was experiencing problems related to wastewater treatment.
The data collected in this study consisted of primary data obtained through direct observation in the field such as the design of the existing WWTP, dimensions of the WWTP unit, and samples of wastewater for laboratory testing, as well as secondary data obtained from published references and East Java Governor Regulation Number 72 Year 2013 concerning Wastewater Quality Standards for Industry and Other Activities/Businesses.
The laboratory test phase is carried out to analyze and test the wastewater discharged from the existing WWTP as primary data to be used in the calculation of the evaluation and redesign of the WWTP.The data processing and analysis stage is carried out to process and analyze data from laboratory test results and secondary data that has been collected, then used to evaluate and redesign the WWTP.

Discharge and quality of WWTP influent
The quantification of wastewater discharge is done through direct monitoring using a flow meter installed at the WWTP outlet.The results of the monitoring have been presented in Table 1.  1 shows that the average wastewater discharge is 157.8 m 3 /day, with the highest discharge being 166.8 m 3 /day.The maximum discharge will serve as the reference for the redesign of the WWTP.
Table 2 presents the properties of the wastewater that enters the WWTP, as revealed by the laboratory tests.

Existing WWTP condition
The wastewater treatment process utilized at the current WWTP involves an anaerobic-aerobic-anaerobic system that comprises of a settling tank, an anaerobic reactor, and an aerobic reactor, as illustrated in Figure 1.The wastewater initially enters settling tank 1 and is then transferred to anaerobic reactor 1. Next, the wastewater flows to the aerobic reactor and then is directed towards settling tank 2. Following this, the wastewater is sent to Anaerobic Reactor 2 before flowing to settling tank 3, and ultimately discharged into the Bengawan Solo River.

Settling tank 1
Settling tank 1 has a partition that divides the tank into 7 compartments.The effective depth of settling tank 1 is 0.67 m.The effective size of settling tank 1 can be seen in Table 3. Anaerobic reactor 1 Anaerobic reactor 1 has 7 tanks which are all filled with biofilter media.The effective volume of tanks 1, 4 and 5 is 42 m 3 , there are 4 places for biofilter media.Tanks 2, 3, 6 and 7 have an effective volume of 32 m 3 , there are 2 places for biofilter media.Biofilter media is made of plastic and is shaped like a honeycomb.The effective volume of each place of biofilter media is 1.27 m 3 .

Aerobic reactor
The aerobic reactor has 8 tanks with an effective volume of 42 m 3 each, namely tanks 1, 2, 3, 4, 5, 6, 7, and 8, all of which are filled with biofilter media.Each tank consists of 4 pieces of biofilter media.Biofilter media material made of plastic and shaped like a honeycomb.The effective volume of each place of biofilter media is 1.27 m 3 .The aerator used in each tank is a blower type LW-200.

Settling tank 2
Settling tank 2 has length, width, total depth and effective depth of 5.46 m, 3.30 m, 3.51 m, and 3.16 m, respectively.The total volume is 63.24 m 3 and the effective volume is 56.94 m 3 .

Anaerobic reactor 2
Anaerobic reactor 2 has 7, namely tanks 8, 9, 10, 11, 12, 13, and 14, with an effective volume of 32 m 3 each.All tanks are filled with 2 biofilter media containers made of plastic and shaped like a honeycomb.The effective volume of each place of biofilter media is 1.27 m 3 .

Settling tank 3
The dimensions of settling tank 3 are as follows: length of 4.41 m, width of 3.72 m, total depth of 3.56 m, and effective depth of 3.10 m.The total volume and effective volume are 58.40 m 3 and 50.86 m 3 , respectively.WWTP effluent quality Laboratory tests have been carried out on WWTP effluent with the results shown in Table 4. Table 4 shows that there are still 2 parameters that still do not meet the quality standards, i.e.BOD and COD, so it is still necessary to redesign the WWTP so that the treatment results can meet the quality standards that have been set.set.

WWTP redesigning
WWTP redesigning is carried out by changing the function of each WWTP unit and changing the flow direction of the existing WWTP system without having to increase WWTP capacity.The result of the WWTP redesign is a combined anaerobic-aerobic system consisting of an equalization tank, a pre-settling tank, an anaerobic reactor, an aerobic reactor, and a final settling tank as shown in Figure 2. Changes from the existing WWTP to the proposed modification WWTP can be seen in Table 5.To ensure that the redesigned WWTP can treat tofu wastewater with better results and that the effluent meets wastewater quality standards, a mass balance calculation for each WWTP unit is performed which is presented in Figure 3.According to the mass balance, the final result of wastewater treatment with modified WWTP meets the effluent quality standards of the tofu industry, TSS = 100 mg/L, BOD = 150 mg/L, and COD = 300 mg/L.

Conclusion
The technical enhancement of WWTP can be achieved by modifying the operation of each unit and adjusting the flow sequence from the existing condition, which is inlet, settlement tank 1, anaerobic reactor 1, aerobic reactor, settling tank 2, anaerobic reactor 2, settling tank 3, and outlet; to a new WWTP layout, which Is inlet, equalization tank, pre-

Figure 2
Figure 2 Flow diagram of the proposed modification WWTP

Figure 3
Figure 3 Results of modified WWTP mass balance calculation

Table 5 Change of existing WWTP to the proposed design Proposed design Change of existing WWTP Reason for change
To get the volume of the aerobic reactor according to the design criteria Final settling tank Settling tank 3 changed to final settling tank The design and function are the same, only the name has changed to make it easier to identify existing WWTP with redesign WWTP.