Turbidity Measurement in Water Treatment: Display Instruments and Grounding Guide 2026

Accurate turbidity measurement in water treatment is essential for ensuring water quality compliance, optimizing filter performance, and protecting public health. Turbidity meters, display instruments for data visualization, and proper grounding and lightning protection systems are three critical components of a reliable water treatment monitoring system. This guide covers the principles and equipment choices for turbidity measurement in water treatment, along with practical guidance on display instruments and electrical protection. CHIWATEC provides comprehensive water treatment instrumentation solutions for industrial and municipal applications.

Turbidity Measurement in Water Treatment: Principles and Instruments

Turbidity is a measure of water clarity — specifically, the degree to which finely dispersed suspended particles reduce the transparency of water. Turbidity measurement in water treatment is one of the most important water quality indicators, directly impacting the health and safety of drinking water as well as production quality in industries such as food, brewing, pharmaceuticals, textiles, printing, and power generation.

Turbidity meters fall into two main categories: visual turbidity meters and photoelectric turbidity meters. Photoelectric types are further divided by application (process monitoring/continuous measurement vs. laboratory/portable) and by design principle (transmitted light vs. scattered light). The scattered light turbidity meter is preferred because of its high sensitivity at low turbidity levels, high accuracy, small relative error, good repeatability, and linear relationship between scattered light intensity and turbidity.

HACH 1720D and SS6 Series: Turbidity Meter Selection

In water purification plant design, the HACH 1720D (formerly 1720C) series turbidity meter and the SS6 series surface scattering turbidity meter are the most commonly used instruments.

The 1720D series is used for measuring filtered water and plant effluent. Water samples flow continuously through a deaerator to remove bubbles, then rise into the measurement chamber and overflow into the discharge port. A focused beam is projected downward into the water sample, and a photoelectric tube immersed in the sample measures scattered light at 90°. Key specifications of the 1720D include:

The SS6 series surface scattering turbidity meter is designed for measuring source water and pre-filtration water. It projects a beam onto the liquid surface and measures scattered light from the surface, avoiding direct contact between the optical system and the water sample. This eliminates the need to clean the flow cell and prevents signal loss from cell fouling. The SS6 series has a measurement range of 0–9999 NTU, with accuracy of ±5% in the 0–2000 NTU range and ±10% in the 2000–9999 NTU range.

Sampling Point Selection for Turbidity Measurement

Proper sampling point selection is critical for accurate turbidity measurement in water treatment. The following guidelines should be followed when designing turbidity monitoring systems:

• Select sampling points that are most representative of the process — coordinate closely with the process engineering team
• Avoid opening sample holes on the top of sampled pipes to prevent drawing bubbles into the sampling line
• Use a small sampling pump to ensure consistent flow and prevent scaling on the inner wall of the sampling pipe
• Determine sampling pipe diameter based on the total water demand from all instruments drawing from the same point
• Turbidity meters for filtered water are typically installed in the filter station pipe gallery (wall-mounted or cabinet-mounted)
• Plant effluent turbidity meters are usually placed in the water quality meter room of the pump station, with signals transmitted to the monitoring station

The 1720D measurement range is 0–100 NTU, but it is recommended to avoid measuring pre-filtered water with this instrument — although optically capable of 100 NTU, practical production use becomes inconvenient at higher turbidity levels.

Display Instruments for Water Treatment Monitoring

Most water purification plant projects now use intelligent display instruments with comprehensive functionality. These instruments provide digital signal processing, control functions, liquid crystal displays, data storage capabilities, and self-diagnostic features. Although some of these advanced functions overlap with the computer control system, intelligent display instruments serve as essential auxiliary tools during the commissioning stage of the computer system and as backup instruments during system malfunctions.

When both local display and remote transmission are required, a signal distributor (one input, two outputs) should be used rather than signal serial mode. One output feeds the local display instrument while the other transmits to the PLC or SCADA system, ensuring independent operation of both channels.

Instrument Grounding and Lightning Protection

Proper grounding is essential for instrument system safety and reliability. Grounding is divided into protective grounding (to prevent electric shock when equipment insulation is damaged) and working grounding (to ensure stable instrument operation). Water purification plants typically use the TN-S system: three phase wires (A, B, C), one neutral wire (N), and one protection wire (PE).

The principle of working grounding is single-point grounding. Because potential differences to ground exist, multiple grounding points create ground loops that introduce interference into the instrument system. Therefore, each signal loop and shielding layer can have only one grounding point. Instrument working grounding can be set separately or share the same grounding body with protective grounding. Based on engineering practice, grounding resistance should generally not exceed 1Ω.

Water purification plants often have scattered facilities, low structures, and flat, open terrain — conditions that increase lightning strike risk for instrumentation equipment. Flowmeter wells located outside the plant area are particularly vulnerable. Installing high-quality lightning arresters with reliable action is an indispensable protective measure. The ESP series lightning arrester from Pepperl+Fuchs (Germany) has demonstrated excellent results for protecting flowmeter signal and power supply circuits.

Frequently Asked Questions

Why is turbidity measurement important in water treatment?

Turbidity measurement in water treatment is critical because turbidity directly indicates water quality — it affects disinfection effectiveness, filter performance, and the aesthetic quality of drinking water. The World Health Organization guidelines and the 2000 Technical Progress Development Plan for the Water Supply Industry both specify a target of 1 NTU for treated water.

What is the difference between 1720D and SS6 turbidity meters?

The HACH 1720D is a scattered light turbidity meter designed for filtered water and plant effluent (0–100 NTU), using a submersed photoelectric tube. The SS6 is a surface scattering turbidity meter for source water and pre-filtration water (0–9999 NTU), projecting a beam onto the liquid surface to avoid optical system contact with the sample.

What grounding system is recommended for water treatment instrumentation?

The TN-S system (three phases + neutral + separate protection earth) is recommended for water purification plant instrument grounding. Working grounding should follow the single-point principle with grounding resistance not exceeding 1Ω.

Why should single-point grounding be used for instruments?

Multiple grounding points create ground loops due to potential differences between grounding points, introducing electrical interference into the instrument system. Single-point grounding eliminates these loops and ensures stable, interference-free operation.

How can lightning damage to field instruments be prevented?

Install high-quality lightning arresters on signal and power supply lines of field instruments, particularly flowmeters located in remote wells or exposed areas. Products like the ESP series from Pepperl+Fuchs provide reliable surge protection for water treatment instrumentation.

Conclusion & Call to Action

Effective turbidity measurement in water treatment — combined with proper display instrument selection and robust grounding and lightning protection — forms the foundation of a reliable water quality monitoring system. From selecting the right turbidity meter (1720D for filtered water, SS6 for source water) to implementing single-point grounding and surge protection, every engineering decision contributes to system accuracy, safety, and longevity.

For professional guidance on turbidity measurement instrumentation and complete water treatment monitoring solutions, contact CHIWATEC today. Email us at [email protected] or [email protected] for customized support.

Related Resources and Further Reading

• Water Treatment Instrument Selection (1): Key Requirements and Level Measurement
• Water Treatment Instrument Selection (2): Flow Measurement Guide
• Water Treatment Instrumentation Types: Physical vs Analytical Instruments
• Frequently Asked Questions (FAQ) on Water Treatment Instrumentation
• Water Treatment Accessories and Instrumentation Products