Chemical oxygen demand (COD) refers to the consumption of oxidizing agent in the treatment of water sample with strong oxidizing agent expressed in milligram of oxygen per liter. COD reflects the pollution degree of water by reducible substances including organism, nitrite, perferrite and sulfide, etc. It is common that water will be polluted by organism. Therefore, COD serves as one of the indicators of relative content of organism.

1.      Applications

On-line COD monitoring instrument has been widely used in rivers and lakes to monitor water quality. It can also be used to monitor wastewater discharged from industrial and mining enterprises and urban wastewater treatment factories. It finds wide applications in the fields such as petroleum, chemical industry, power plant, pharmaceutics, food, electronics and water works.

2.      Characteristics

•     The instrument uses free hydroxy group produced by electrodes in electrolysis to measure COD value in water solution. The oxidation-reduction potential of hydroxyl (OH) is 2.85V, ozone is 2.07V and dichromate is 1.36V. It has a high sensitivity and good repeatability.   

•     The measurement will not be affected by the color and turbidity of water solution.

•     The instrument uses glucose and sodium sulfate (NaSO₄) solution, so that no harmful and hazardous chemical substances such as potassium dichromate (K₂Cr₂O₇) and concentrated sulfuric acid (H₂SO₄ 98.3%) are needed and no secondary pollution will be caused.  

•    A new sampling system is adopted, so that there is no need to maintain and clean the instrument frequently.

•    Touch-screen control is adopted with a Chinese operating interface, simple to operate. 

•    Fast analysis, 6 minutes/time.

•     Automatic calibration, measurement, data saving and remote control.

3.      Main Technical Parameters and Specifications

3.1 Measurement range:   10.0mg/L～200.0mg/L,

200.0mg/L～500.0mg/L,

500.0mg/L～1000mg/L.

3.2  Accuracy: ±5%(FS)

3.3  Repeatability：3% (FS)

3.4 Zero Draft：±5mg/L/24h

3.5  Range Draft：±10%/24h

3.6 Normal Operating Conditions:

a) Ambient Temperature: (0～40)℃；

b) Relative Humidity: No great than 90%;

c) Power Supply: AC 220 V±22V，50 Hz±0.5 Hz；

d) No corrosive gases in air nearby;

e) No performance-affected vibrations nearby.

f) No performance-affected interference from magnetic fields nearby except from the geomagentic field. 

4.      Main Functions

4.1  A large single color touch-screen of 5.7 inches with Chinese menu and touch-operation.

4.2  Parameters Setting

4.3  COD value correction: building regression straight lines or broken lines related to various pollution sources.

4.4  Calibration: manual and automatic

4.5  Measuring Data Processing: test data displaying, storage, viewing, printing (extension), remote transmission and alarming.

4.6  Output Bus Extension: using RS-232 port to connect with RS-485, Modem and internet.

4.7  Remote Communication and Control: using computer software to realize remote communication and control over the test data from instruments operating in the field.

4.8  Input: collecting test data from other instruments such as pH meter and flow meter.

5.      Construction

5.1  Meters

The meter consists of a touch-LCD display, sensors, a measuring pond, a stirring apparatus, a computer board, a control board, a power board and a transformer, etc (refer to Fig.1)  

Figure 1   Construction of Meter

5.2  Liquid Path

The liquid path consists of solenoid valves, a creep pump and an electrolyte heater, etc (refer to Fig.2).

Figure 2. Construction of Liquid Path

5.3 Sampler

The sampler consists of PVC pipes, a pressure regulator valve, a tap for manual sampling and a pressure meter, etc (refer to Figure 3).  

Figure 3. Schematic Diagram of Sampler

6.      Main Inputs and Outputs

6. 1 Main Inputs

6.1.1 LCD touch-screen input

6.1.2 pH meter current input  (4~20mA)

6.1.3 Flow meter RS232 communication input

6. 2  Main Outputs

6.2.1  LCD displaying COD, pH and flow data outputs.

6.2.2  COD, pH and flow RS232 data communication outputing pressure.

6.2.3  Alarm switch control signal output (AC 220V/2A).

7.      Preparation of Common Solutions

7.1  Calibration Solution (200mg/L, 500mg/L and 1000mg/L)

7.11  200mg/L: Weighing out 0.25795 grams of glucose monohydrate powder to be solved in water. The solution shall be totally transferred to a 1000ml-measurement-bottle and water shall be added to dilute the solution to the graduate line.

7.1.2 500mg/L: Weighing out 0.5195 grams of glucose monohydrate powder to be solved in water. The solution shall be totally transferred to a 1000ml-measurement-bottle and water shall be added to dilute the solution to the graduate line.

7.1.3 1000mg/L: Weighing out 1.0318 grams of glucose monohydrate powder to be solved in water. The solution shall be totally transferred to a 1000ml-measurement-bottle and water shall be added to dilute the solution to the graduate line.

7.2 Regenerated Solution

Weighing out 355 grams of anhydrous sodium sulfate to be poured in 2L of deion water while stirring until being completely solved in the deion water. Then pour this 2L solution and 3L of deion water into a clean container.  

7.3 Electrolyte

Weighing out 71 grams of anhydrous sodium sulfate to be poured in 2L of deion water while stirring until being completely solved in the deion water. Then pour this 2L solution and 3L of deion water into a clean container.  

8.      Routine Maintenance

8.1  Agent Change and Addition

The consumption of distilled water, electrolyte and regenerated solution varies with automatic measuring periods. As soon as you find there is not enough agents left, agent shall be added in time, otherwise it will affect measurements even damage electrodes. Once a week, the standard solution shall be changed for a fresh one.

8.2  Pipe Checking

Check whether flexible pipes (tubes) are properly fixed to each solenoid valve and the creep pump. Flexible pipes will become ageing having been used for a long time and shall be replaced in time.

8.3   Adding Solution in Reference Electrodes

Check frequently the liquid level of reference electrodes. In case the liquid level is 2cm lower than the feed (charge) inlet, reference solution (saturated K2SO4) shall be added in time.

9.      A Complete Set of Instrument

9.1  One set of On-line COD Monitoring Instrument, Model 580

9.2  One Measuring Electrode, Model COD-1

9.3  One Counter Electrode, Model COD-2

9.4  One Reference Electrode Model COD-3

Series of Laboratory pH Meter

General

Laboratory pH meters are suitable to be used in the labs of universities, colleges, research institutes, mines and industrial enterprises to measure pH and the potential mV value of water solution. It can measure the temperature of water solution with a temperature sensor and measure the potential of the electrode with an ion selective electrode as well as measure the oxidization reduction potential with an ORP electrode.