Standard Oxygen Transfer Test (SOTR & SOTE)

Essential Metrics for Wastewater Treatment Efficiency

To assess the efficiency of Mapal’s stainless-steel fine bubble diffuser, we conducted tests following the ASCE/EWRI 2-06 standard for “Measurement of Oxygen Transfer in Clean Water,” set by the American Society of Civil Engineers. Minor modifications were made to adapt the protocol to our specific test facility.

The ASCE/EWRI 2-06 standard provides a framework to evaluate the oxygenation capacity of various aeration devices, allowing for effective comparison. This method measures the time required to saturate a volume of clean tap water, initially at zero dissolved oxygen, through aeration. The resulting oxygen transfer rate is referred to as the Standard Oxygen Transfer Rate (SOTR) of the equipment. To determine the Standard Oxygen Transfer Efficiency (SOTE), the SOTR is divided by the oxygen supply rate, which in our setup is represented by the blower’s airflow rate.

Test Tank

The test was conducted in a 10 m³ tank with a rectangular floor measuring 1 m x 2.5 m and a height of 4.3 m. The water level was maintained at 4m.

Fine Bubble Diffusers Tested

Two different diffusers were evaluated in this test:

1. A polypropylene (PP) diffuser from a reputable manufacturer, used as the reference diffuser.
2. Mapal’s stainless-steel fine bubble diffuser (referred to as the Mapal diffuser).

Each diffuser was tested with both EPDM and silicone fine bubble sleeves to assess performance across materials.

Dissolved Oxygen Meter

We used three Hanna HI98193 Dissolved Oxygen BOD/OUR/SOUR Meters, each equipped with a built-in HI 92000 data logger to record the results. The meters were positioned at three specific points in the tank:

– 0.4 meters above the tank floor, in the center of the tank.

– 2 meters above the tank floor, in the center of the tank.

– 3.4 meters above the tank floor, in the center of the tank.

Flow Meter

To measure airflow, we used a CS Instruments VA 525 Compact Inline Flow Meter, which was installed on the air pipe connecting the air blower to the diffuser.

Air Blower

We used a Greenco 4RB 4 kW positive displacement blower.

Methodology

During testing, we conducted dozens of experiments following a rigorous methodology developed in alignment with the ASCE/EWRI 2-06 standard to ensure high reliability.

Each day, we began by operating the air blower and fine bubble diffuser at a high airflow rate for at least one hour to fully open the diffuser sleeve slits under maximum pressure in a filled water tank. Without shutting down the blower, we then measured the dissolved oxygen (DO) level, added deoxygenation chemicals to the tank, and checked total dissolved solids (TDS) before each test.

The deoxygenation chemicals were dissolved in a small container before being added to the tank. Quantities were calculated as follows:

– Sodium Sulfite (Na₂SO₃)

  To reduce 1 mg/L DO, 7.88 mg/L of sodium sulfite is required. For a 10 m³ tank, 78.8 g is needed. To ensure adequate levels, sodium sulfite was added at 20%–250% of stoichiometric amounts. We added between 100–150 g per 1 mg/L DO (26%–90%).

– Cobalt Catalyst

  Cobalt was added once at the beginning of each test day and maintained throughout. The required concentration was between 0.1–0.5 mg/L, so 1–5 g was added to the 10 m³ tank.

DO levels were continuously monitored until reaching saturation (stable readings for at least 2 minutes), marking the end of each testing session.

Calculation and Data Analysis

The collected raw data were analyzed using Visual Basic and Excel spreadsheet programs, downloaded from [www.seas.ucla.edu/stenstro]. We applied the Nonlinear Regression Method as outlined in section 7.2.1 of the ASCE/EWRI 2-06 standard.

Each interaction was analyzed with the following initial parameters:

– C∞ = 9 mg/L

– C₀ = 0.1 mg/L

– KLa = 0.216 per minute

– Truncation Level = 10%

System Calibration

Due to the sensitivity of the test system to variables such as air and water temperature, tank volume, and dimensions, we calibrated the system using a reference diffuser with verified SOTE results. The diffuser was tested at two different airflow levels, allowing us to compare our measurements against a diffuser with known performance metrics.

Test Results

The tests were designed to evaluate how the SOTE of Mapal’s diffusers varies under different conditions:

– Use of EPDM vs. silicone fine bubble membrane sleeves

– Various airflow rates

– Comparison of 1-meter and 2-meter diffusers

– Mapal diffuser Type S vs. Mapal diffuser Type B

Fine Bubble Diffuser with Silicone Membrane

In this scenario, three different types of one-meter-long diffusers were tested at two different airflow rates:

– One-meter Mapal Type S at 8 m³/hr (low airflow)

– One-meter Mapal Type S at 16 m³/hr (high airflow)

– One-meter Mapal Type B at 8 m³/hr (low airflow)

– One-meter Mapal Type B at 16 m³/hr (high airflow)

– One-meter calibration diffuser (Cal) at 8 m³/hr (low airflow)

– One-meter calibration diffuser (Cal) at 16 m³/hr (high airflow)