Evaluation and Kinetics of Tofu Wastewater Bioreactor with Addition of Water Hyacinth

Abstract

Tofu wastewater is a nitrogen-rich wastewater type with a high concentration of organic contents. To enhance the digestion rate of tofu wastewater in an anaerobic bioreactor, water hyacinth was tested as an additional substrate. Water hyacinth is a dangerous weed with a high carbon-nitrogen ratio (C/N) of about 30-35. Substrate combination was intentionally used in this study to raise the C/N of wastewater. This study aims to evaluate the digestion rate of a bioreactor qualitatively based on experimental data to determine the biokinetic constants of the anaerobic bioreactor quantitatively based on microbial growth data, substrate degradation, and methane (CH₄) production. MATLAB was used as software to run mathematical modeling. The anaerobic bioreactor was designed and equipped with a circulation pump to maintain the homogeneity of the substrate and was completed with a biogas collector. Seeding and acclimatization were carried out before the main experiment started. Using a combination substrate of tofu wastewater and water hyacinth with a 5:3 volume ratio, the bioreactor was run at 20 days of hydraulic retention time (HRT) at room temperature. The bioreactor was able to remove 92.8% of chemical oxygen demand (COD) concentration and produced biogas with the highest CH₄ concentration of 56.9%. The obtained kinetic constants indicate that, in comparison with similar studies of anaerobic digestion of the low C/N wastewater, the addition of water hyacinth resulted in better performance of the bioreactor with the correlation of microbial growth rate (?_m), substrate degradation (Y_X/CCOD) and CH₄ production (Y_CCH4/X) with the consecutive values of 0.65/day, 0.64 mg cells/mg COD, and 0.62 mg CH₄/mg cells. An appropriate ratio of water hyacinth as the high carbon source and nitrogen-rich tofu wastewater is recommended to obtain the optimum ratio of carbon to nitrogen and result in a higher percentage of methane formation.

ABSTRAK: Air sisa tauhu ialah air sisa yang kaya dengan nitrogen dengan kepekatan kandungan organik yang tinggi. Bagi meningkatkan kadar penghadaman air sisa tauhu dalam bioreaktor anaerobik, keladi bunting diuji sebagai substrat tambahan. Keladi bunting merupakan rumpai berbahaya dengan nisbah karbon nitrogen (C/N) yang tinggi iaitu kira-kira 30-35. Gabungan substrat sengaja digunakan dalam kajian ini bagi menaikkan C/N air sisa. Kajian ini bertujuan bagi menilai kadar pencernaan bioreaktor secara kualitatif berdasarkan data eksperimen dan menentukan pemalar biokinetik bioreaktor anaerobik secara kuantitatif berdasarkan data pertumbuhan mikrob, degradasi substrat, dan pengeluaran metana (CH₄). MATLAB digunakan sebagai perisian pemodelan matematik. Bioreaktor anaerobik direka bentuk dan dilengkapi dengan pam edaran bagi mengekalkan kehomogenan substrat dan dilengkapkan dengan pengumpul biogas. Penyemaian dan penyesuaiikliman telah dijalankan sebelum eksperimen utama bermula. Substrat gabungan air sisa tauhu dan keladi bunting digunakan dengan nisbah isipadu 5:3. Bioreaktor dijalankan selama 20 hari iaitu masa pengekalan hidraulik (HRT) pada suhu bilik. Bioreaktor tersebut mampu mengasingkan 92.8% kepekatan permintaan oksigen kimia (COD) dan menghasilkan biogas dengan kepekatan CH₄ tertinggi sebanyak 56.9%. Pemalar kinetik yang diperoleh menunjukkan bahawa, berbanding dengan kajian serupa tentang pencernaan anaerobik air sisa C/N rendah, penambahan keladi bunting menghasilkan prestasi bioreaktor yang lebih baik dengan korelasi kadar pertumbuhan mikrob (?_m), degradasi substrat (Y_X/CCOD) dan penghasilan CH₄ (Y_CCH4/X) dengan nilai masing-masing 0.65/hari, 0.64 mg sel/mg COD, dan 0.62 mg CH₄/mg sel. Nisbah keladi bunting yang sesuai sebagai sumber karbon tinggi dan air sisa tauhu yang kaya dengan nitrogen disyorkan agar mendapatkan nisbah optimum karbon kepada nitrogen dan ini menghasilkan peratusan pembentukan metana yang lebih tinggi.