Could simply adding more air holes turn a venturi aerator into a much stronger groundwater treatment system? In this experiment, three holes pushed oxygen up and removed nearly all the iron, while manganese remained the harder target. Groundwater quality is vulnerable to contamination from both natural and anthropogenic sources, and mineral dissolution, industrial wastewater discharge, and excessive groundwater extraction contribute to iron and manganese.

Iron and manganese are undesirable in clean water because they cause aesthetic issues, damage water distribution systems, and pose health risks. Their oxidation can form reddish-brown precipitates that accumulate in pipelines, causing scaling and clogging, reducing water pressure, and potentially damaging pipes.

Venturi aeration systems are extensively used in industrial wastewater treatment because they adapt to a wide range of wastewater properties and operational conditions. Air bubbles aid oxygen transfer by enlarging the surface area for contact during oxygen mass transfer.

As water passes through the converging cone, a pressure difference increases velocity and creates a vacuum effect at the port, allowing air contact and producing a two-phase mixture that enhances oxygen transfer. Previous studies have focused mainly on airflow, geometry, and discharge rate, with limited investigation of the integrated relationship between venturi air-hole configuration, oxygen transfer dynamics, and iron–manganese oxidation in groundwater.

The integration of venturi aeration with real-time monitoring of dissolved oxygen, air velocity, water flow rate, and metal oxidation behavior remains insufficiently explored. The mechanistic relationship between air-hole configuration, oxygen transfer dynamics, and iron–manganese oxidation kinetics in groundwater remains insufficiently understood.

The study’s novelty is a systematic evaluation of how venturi air-hole configurations influence oxygen transfer characteristics and iron–manganese oxidation kinetics through integrated real-time monitoring. The study aims to clarify the relationship between hydrodynamic conditions, oxygen transfer performance, and metal oxidation mechanisms to support low-cost and energy-efficient groundwater treatment systems.

Groundwater samples were collected from Rungkut Megah Raya in Surabaya, Indonesia, and stored in a twenty-five-liter polyethylene container; the bottles were cleaned and rinsed with deionized water to minimize contamination. The bottles were tightly capped after collection to minimize atmospheric exposure and potential changes in the oxidation states of iron and manganese.

The samples were transported to the laboratory, stored at four degrees Celsius, and kept without chemical preservatives to preserve the original groundwater characteristics. The samples were gently mixed before aeration, and all experiments were performed within forty-eight hours of sampling to minimize physicochemical changes.

The reactor system used a venturi aerator operating at a flow rate of zero point one liters per second, with a pump, an overflow tank, and an aeration tank for sample collection. The venturi aerator used twelve-millimeter-diameter air inlet holes, with configurations of one, two, and three air holes.

The bubbles generated by the venturi aerator were classified as microbubbles, typically ranging from ten to three hundred micrometers in diameter. The figure’s lower row makes the design comparison explicit: the venturi body is shown first with a single air intake, then with two, and finally with three, while the main schematic traces the same flow path into the aeration tank and onward to final collection.

The dashed overflow line visibly returns from the upper system to stabilize the influent tank level. The self-developed venturi aeration system in Surabaya integrated real-time monitoring and data acquisition. The monitoring system included a DN15 water-flow sensor along the venturi pipeline, an F1031V mass-air-flow sensor at the venturi air inlet, and a dissolved oxygen sensor inside the aeration tank.

The DN15 sensor operated over a flow range of one to thirty liters per minute with an accuracy of plus or minus three percent, while the F1031V sensor measured air flow with an accuracy of plus or minus two percent. The dissolved oxygen sensor had a measurement range of zero to twenty milligrams per liter, a resolution of zero point zero one milligrams per liter, and an accuracy of plus or minus zero point two milligrams per liter.

The figure’s central wiring diagram shows each sensor feeding the Arduino Pro Micro through distinct power, ground, and signal lines. The lower section then traces USB keyboard emulation from the controller to a computer and Notepad, where CSV data appears every one second, making the acquisition path explicit.

Sensor data were automatically recorded every second in comma-separated value format, including timestamps and synchronized readings from the three sensors. Before experimentation, the dissolved oxygen sensor was calibrated under air-saturated conditions and validated against a calibrated dissolved oxygen meter, while the air and water flow sensors were verified under steady-state conditions.

Measurement uncertainties were estimated as plus or minus zero point two milligrams per liter for dissolved oxygen, plus or minus three percent for water flow, and plus or minus two percent for air flow. The batch aeration process used contact times of twenty, forty, and sixty minutes.

Dissolved oxygen concentrations were monitored with an Arduino-based sensor and validated by manual measurements using a dissolved oxygen meter. Arduino sensors measured air and water velocities to characterize flow conditions during aeration.

Flame Atomic Absorption Spectrophotometry was used to measure iron and manganese concentrations. Reaction kinetics analysis was conducted to determine the reaction order governing the reduction of iron and manganese concentrations during venturi aeration.

The order of a reaction is defined by its rate law, which must be established experimentally and cannot be inferred directly from the chemical reaction equation. Standard Oxygen Transfer Rate and Standard Aeration Efficiency were used to observe the efficiency of the venturi aeration system.

Standard Oxygen Transfer Rate quantifies the rate at which oxygen is transferred from air to water and was calculated using the volumetric oxygen mass transfer coefficient and the difference between saturated and time-dependent dissolved oxygen concentrations. One-way Analysis of Variance general linear models evaluated the effects of air-hole configuration on five water quality metrics using samples from independent experimental runs.

Statistical significance was assessed at an alpha level of zero point zero one, corresponding to a ninety-nine percent confidence level, to minimize Type One error rates across multiple tests. After a significant omnibus ANOVA result, Tukey’s Honestly Significant Difference test and Fisher’s Least Significant Difference test were applied at an adjusted alpha of zero point zero one to assign statistical letter groups to treatment means.

Hsu’s Multiple Comparisons with the Best test identified the air-hole configuration associated with maximizing dissolved oxygen and pH while minimizing temperature, iron, and manganese concentrations. The groundwater sample had high iron and manganese concentrations that could cause reddish-brown staining on plumbing fixtures.

The initial dissolved oxygen concentration was four point nine milligrams per liter, and the source water had a neutral pH of seven point two. The water temperature was twenty-eight degrees Celsius, and turbidity slightly exceeded the water quality standard.

The initial dissolved oxygen level was four point nine milligrams per liter, and continuous five-minute sampling was chosen to map fine-grained kinetic profiles and stabilization behavior across the air-hole configurations. After sixty minutes, manual measurements reached six point five milligrams per liter with one air hole, seven point three with two air holes, and nine point two with three air holes.

The Arduino sensor measurements at sixty minutes reached six point six milligrams per liter with one air hole, seven point five with two air holes, and nine point two with three air holes. In panel (a), the three colored curves separate clearly after the early measurements, while the corresponding curves in panel (b) show the same configuration-dependent spacing.

Comparing the two panels, the Arduino traces closely follow the manual traces across the sixty-minute time series, with only small point-to-point offsets visible. After sixty minutes, iron concentrations decreased from zero point seven one eight two milligrams per liter to zero point one zero two seven, zero point zero nine four eight, and zero point zero zero eight eight milligrams per liter for the one-, two-, and three-air-hole configurations.

Manganese concentrations decreased from zero point three zero two two milligrams per liter to zero point two two eight seven, zero point one five five one, and zero point zero nine nine seven milligrams per liter for the one-, two-, and three-air-hole configurations. The three-air-hole configuration achieved the highest removal efficiencies, reaching ninety-eight point seventy-seven percent for iron and sixty-seven point zero one percent for manganese.

The consistently lower manganese removal compared with iron reflects slower manganese oxidation kinetics, indicating that manganese oxidation requires more stringent conditions and longer reaction times. In the upper panels, the three-air-hole curves fall more steeply over time, especially for iron, while the one-air-hole curves remain comparatively elevated at intermediate time points.

The lower panels show the same separation in removal efficiency: the bars diverge progressively from twenty to sixty minutes, with the manganese bars displaying a wider configuration gap than the iron bars at the earlier intervals. The oxidation-rate constant ranged from zero point zero one zero four to zero point zero one two three for iron and from zero point zero zero one two to zero point zero zero three six for manganese.

Larger air flow from more air holes increased the rate of metal oxidation and allowed more dissolved oxygen to enter the water, accelerating iron hydroxide and manganese dioxide formation. The consistently lower rate constants for manganese than for iron indicate slower manganese oxidation kinetics.

Longer aeration periods or stronger oxidizing environments may be required to achieve complete manganese removal. Figure seven plots the zero-order reaction-rate constant for iron and manganese reduction against one, two, and three venturi air holes. The authors report iron values of zero point zero one zero four, zero point zero one zero eight, and zero point zero one two three, while manganese values are zero point zero zero one two, zero point zero zero two six, and zero point zero zero three six.

The nearly consistent iron rates and changing manganese rates help characterize the reaction kinetics across aeration configurations. Increasing the number of air holes showed a positive correlation with a higher volumetric oxygen mass transfer coefficient, or kLa. Manual dissolved oxygen measurements showed kLa increasing from zero point zero one zero one inverse minutes with one hole to zero point zero five three four inverse minutes with three holes.

The dissolved oxygen sensor showed a similar increase, from zero point zero one zero five inverse minutes with one hole to zero point zero three nine four inverse minutes with three holes, confirming higher oxygen transfer efficiency with more holes. In the manual-measurement columns, the two-hole row sits between the one- and three-hole rows: zero point zero one seven six per minute and one point zero five five six per hour.

The table therefore shows a non-linear spread, with the three-hole entry roughly five times the one-hole entry in both unit columns. Figure ten presents one-way ANOVA results for how the number of air holes affects temperature, dissolved oxygen, pH, iron concentration, and manganese concentration.

At a significance level of alpha equal to zero point zero one, the authors report highly significant variation across all five water-quality metrics; for dissolved oxygen, the analysis gives an F statistic of thirty-seven point seventy-four and a p-value of zero point zero zero zero.

This matters because it shows that changing the Venturi aerator configuration measurably alters both physical and chemical water properties. Manganese concentration was significantly affected by aerator configuration, with a P-value of zero point zero zero one and the three-air-hole configuration forming a distinct low-concentration statistical group.

Iron concentration did not meet the strict alpha level of zero point zero one for significance, despite a reduction from zero point five seven one zero milligrams per liter with one hole to zero point three three six one with three holes. Hsu’s Multiple Comparisons with the Best screening confirmed that the three-air-hole configuration matched the target response direction across the statistically significant parameters.

The three-air-hole configuration was identified for maximizing dissolved oxygen and pH, while minimizing water temperature and remaining manganese concentration. Figure nine compares manual and sensor-based dissolved-oxygen calculations for one, two, and three air holes.

Across the four panels, SOTR and SAE are plotted over time, with the three-air-hole series showing pronounced negative fluctuations, including a sharp late drop, while the one- and two-air-hole series remain closer to zero. The authors note that such negative transient values can reflect hydrodynamic instability, sensor response lag, and numerical differentiation errors rather than actual oxygen loss.

Venturi aeration performance was strongly influenced by the number of air holes, which governed airflow distribution, bubble generation, oxygen transfer, and subsequent iron–manganese oxidation behavior. The three-hole configuration achieved the highest dissolved oxygen concentration, approximately nine milligrams per liter after sixty minutes, with iron and manganese removal efficiencies of ninety-eight point seventy-seven percent and sixty-seven point zero one percent.

Both iron and manganese oxidation followed zero-order behavior, with rate constants ranging from zero point zero one zero four to zero point zero one two three inverse minutes for iron and zero point zero zero one two to zero point zero zero three six inverse minutes for manganese.

Increased air-hole numbers enhanced oxygen transfer performance, reflected by higher kLa and Standard Oxygen Transfer Rate values, although larger fluctuations suggested increased hydrodynamic complexity and temporary instability at higher aeration intensities. These findings provide insights for designing low-cost and energy-efficient groundwater treatment technologies.

Alkalinity, bicarbonate concentration, oxidation–reduction potential, conductivity, and nutrient species were not evaluated and may influence oxygen transfer and metal oxidation pathways. Future investigations incorporating these variables are recommended to clarify the governing mechanisms and optimize venturi aeration systems for practical groundwater treatment.

The three-air-hole configuration produced the strongest oxygen transfer and the highest reported removal efficiencies, but manganese still oxidized more slowly than iron. That makes aerator geometry a practical design lever, not just a hardware detail.