pH Measurement of Wastewater Treatment at Pulp and Paper Plants

Introduction

Pulp and paper plants consume large volumes of water, and the treatment of wastewater from these plants is a serious environmental concern. To treat this wastewater, suspended solids (SS) are separated and precipitated in a precipitation tank using flocculation agents. During this process, the pH value needs to be maintained at the proper level, the status of which is controlled by referring to a pH analyzer. In the past, pH analyzers needed to be cleaned frequently as fibers and coating agents in the wastewater adhered to the electrodes.

We tested a submersion-type holder with an ultrasonic+airjet cleaner to determine its effectiveness in the continuous cleaning of pH analyzers. Manual maintenance of pH analyzers used to be conducted on a daily basis, but our test results have demonstrated that this submersion type holder can reduce the manual cleaning frequency to just once every one or two months.

Management and control of the precipitation tank

Expected Benefits

• Continuously and stably measures pH
• Eliminates the need for frequent manual cleaning
• Enables efficient use of flocculating agents

Process Overview

Wastewater at pulp and paper plants contains effluents from the pulping, bleaching, papermaking, and recycling processes. The treatment of this wastewater involves pH level control, solid-liquid separation, oxidization, and other processes, and depends on such factors as the levels of organic, reducible and suspended matter in the wastewater as well as its chromaticity. For example, most SS are fine substances that do not precipitate easily, so they are removed by applying flocculating agents, which produce a chemical reaction. To detect the end points of these chemical reactions and to manage and control the precipitation tank, the pH of the wastewater tank is measured.

Solution Details

Measurement system

4-wire pH measurement system (with ultrasonic+airjet cleaner)
pH sensor:	KCl filling type pH sensor	PH8EFP-☐-☐-TN-TT1-N-G*A
Holder:	Submersion holder with an ultrasonic+air-jet cleaner(customized product)	PH8HS-PP-☐-T-S3-C☐*A/Z (Z: option for air bubble nozzle)
Terminal box (when needed):	WTB10-PH3
Converter:	FLXA402-A-B-☐-P1-NN-A2-WR-☐-☐-☐-NN
Ultrasonic oscillator:	PUS400G-NN-NN-☐-E

Utilities

FLXA402

• Power supply: 90 to 264 V AC, 50/60 Hz
• Power consumption: approx. 35 VA

PUS400G Ultrasonic Oscillator

• Power supply: 100/110/115/200/220/240 V AC ± 10%, 50/60 Hz
• Power consumption: approximately 15 VA

PH8HS/Z

• Air for bubbling: 10 to 50 Nl/min.

Conditions

1. Effectiveness of previous cleaning methods
   Cleaning methods using a standard jet cleaner and a brush were tested. Fibers in the effluents that had attached to the electrode could not be removed using either of these cleaning methods. (Both types of cleaning operations also had to be suspended once a day to conduct manual cleaning of the electrode.)
2. Effects of continuous cleaning using an ultrasonic+air-jet cleaner
   The amount of the contaminants on the electrode decreased significantly. Manual cleaning and calibration were found to be required just once every one or two months.

Test Data from Continuous Cleaning Using an Ultrasonic+Air-jet Cleaner

Operation period	Drift value
	Standard pH7 solution measurement	Standard pH4 solution measurement
1 month	-0.19 pH	-0.25 pH
2 months	-0.20 pH	-0.20 pH

Note: During the test, no manual electrode cleaning and calibration were performed.

Notes

Submersion holder with an ultrasonic+air-jet cleaner (customized product)

Testing conducted on the effects of pH analyzer cleaning only addressed effluents from the papermaking process. However, this submersion holder can also be used to keep pH analyzers clean that measure effluents from the recycling process, which has fibers, printing ink substances, and coating agents.