Performance of Recirculation Flow Roof in Wastewater Treatment Systems for Temperature Control Modeling in Floating Melon Greenhouses

Abstract

Background and Objectives : This study on hybrid mathematical modeling aims to enhance the sustainability of agricultural practices in the Chi River basin by developing a wastewater treatment system for high-yield greenhouse cultivation. This system is expected to reduce financial risks associated with flooding and mitigate the worsening degradation of the Chi River. The researcher has designed a rooftop wastewater treatment system using mathematical modeling to regulate the temperature in floating melon greenhouses, addressing both flood-related challenges and wastewater issues. The system utilizes the sloped greenhouse roof (tilted at 20 degrees) to remove biochemical oxygen demand (BOD), ammonia (NH3), and nitrate (NO₃) from wastewater. Furthermore, mathematical models were developed to regulate greenhouse temperature, including the Plug-Flow Volume Temperature Reactor (PFVTR) and the Completely-Mixed Stirred Volume Temperature Reactor (CSVTR). These models, based on first-order and second-order reactions (n = 1 and n = 2), control temperature (T) by adjusting the recirculation ratio (QR/Qin = R) and hydraulic retention time (HRT), utilizing the ratio of cross-sectional area to longitudinal area (A_C: A_L)to determine background concentration temperature (T*) and the Coefficient of Temperature (kT) for model validation.

Methodology : The CTFMG system (Control Temperature in Floating Melon Greenhouses) measures 5.0 meters in width, 3.0 meters in height, and 6.0 meters in length with a floating melon density factor (f) of 0.5 and a total volume of 90 cubic meters. The cross-sectional to longitudinal area ratio (A_C: A_L) is less than 1:4. The system's 18-square-meter plastic-sheet rooftop is inclined at 20 degrees. Water recirculation rates were set at 3, 4, 5, 6, and 7 liters per minute, while wastewater inflow (Qin) was maintained at 1 liter per minute. The treated wastewater was analyzed for BOD, NH3, and NO3 levels. The air exchange system operated with an airflow rate (QAin & QAout) of 576 cubic meters per day, utilizing laminar flow with hydraulic retention times (HRT) of 22.33 and 44.66 days. Temperature measurements were taken during April, the hottest month, to assess the impact on melon production. The input temperature (Tin) was controlled at 36 ± 2.5°C, and the output temperature (Tout) was recorded to determine the effect of different recirculation ratios (QR/Qin = R). Data were processed using Microsoft Excel to calculate background concentration temperature (T*), first- and second-order reaction rates (n = 1 and n = 2), and the Coefficient of Temperature  (kT) for both PFVTR and CSVTR models. The accuracy of the mathematical models was verified, and the feasibility of floating melon greenhouses was assessed based on Net Present Value (NPV) and Benefit-Cost Ratio (B/C Ratio), with a one-year project lifecycle.

Main Results : Results from the study on the Control Temperature in Floating Melon Greenhouses (CTFMG) utilize a roof area of approximately 18 square meters, inclined at 20 degrees from the horizontal, for wastewater treatment using plastic sheet materials. The recirculation ratio (Recirculation ratio: QR/Qin = R) tested at values of 3, 4, 5, 6, and 7 achieved removal efficiencies for BOD, NH3, NO3 (Removal BOD NH3 NO3 Efficiency) as follows: (5.36 ± 2.59%, 2.83 ± 2.85%, and 10.00 ± 0.031%), (25.07 ± 2.56%, 20.49 ± 2.83%, and 10.75 ± 0.0295%), (30.14 ± 2.52%, 22.07 ± 2.81%, and 14.25 ± 0.0275%), (59.72 ± 2.49%, 31.29 ± 2.8%, and 23.50 ± 0.027%) and (72 ± 2.45%, 34 ± 2.78%, and 32.50 ± 0.0265%). The efficiency of temperature reduction decreased by 6.81 ± 2%, 8.36 ± 2%, 9.86 ± 2%, 11.32 ± 2%, and 12.74 ± 2%, respectively. The Control Temperature in Floating Melon Greenhouses (CTFMG) system, with a cross-sectional area to longitudinal-section area ratio (A_C: A_L) (< 1:4), achieved the highest treatment efficiency at R = 7, with removal rates of 72 ± 2.45% BOD, 34 ± 2.78% NH3, and 32.50 ± 0.0265% NO3, and a maximum temperature reduction efficiency of 12.74 ± 2%. The system controlled the temperature from an inlet temperature (Temperaturein: Tin) of 36 ± 2.5°C to an outlet temperature (Temperatureout: Tout) of 31.00 ± 1.5°C. From the Plug-Flow Volume Temperature Reactor (PFVTR) and Completely-Mixed Stirred Volume Temperature Reactor (CSVTR) models, first-order and second-order reactions (n = 1 and n = 2) were observed. background concentration temperature (T*) was 15.55 ± 2.0°C. The Coefficient of Temperature (kT) values for both models were as follows: 0.008, 0.00004 (1/day) and 0.0009, 0.00006 (1/day). The coefficient of determination (R²) values were 0.8398, 0.8497, 0.9306, and 0.9526. From the economic feasibility analysis of the floating melon greenhouse for agriculture, based on Net Present Value (NPV), the system yielded 1,680 THB per square meter, with a Benefit – Cost Ratio (B/C Ratio) of 1.68 and a payback period of 1 year.

Conclusions : The Control Temperature in Floating Melon Greenhouses (CTFMG) system exhibited a second-order reaction (n = 2) in the Completely-Mixed Stirred Volume Temperature Reactor (CSVTR) model. The Coefficient of Temperature (kT) was determined to be 0.00006 1/day. In terms of economic feasibility, the floating melon greenhouse demonstrated a Net Present Value (NPV) and a Benefit-Cost Ratio (B/C Ratio) that supported its financial viability, with a payback period of 1 year.