Monoclor^®RCS  Residual Control System

The “breakpoint curve” in the chlorination of drinking water depicts the relationship between the amount of chlorine added to the water (chlorine dosage) and the concentration of total chlorine in the water. Key concepts include the following:

• Chlorine Species: When chlorine is added to water for disinfection, it can exist in various chemical forms, primarily as hypochlorous acid (HOCl), hypochlorite ion (OCl⁻), and chloramine (NH₂Cl). The proportions of these species depend on factors such as pH, temperature, and the presence of other chemicals.
• Chlorine Demand: Chlorine demand refers to the amount of chlorine required to oxidize or disinfect other reactants or substances present in water. This demand varies depending on factors such as the concentration and type of those substances, pH, and temperature.
• Breakpoint Chlorination: Breakpoint chlorination point is the stage at which the chlorine demand of the water has been satisfied, and further addition of chlorine leads to the formation of free available chlorine (FAC) residual. This means that chlorine is no longer consumed by reactions in the water but instead exists in its free form.
• Total Chlorine Residual: Total chlorine residual is the concentration of chlorine remaining in the water after the chlorine demand has been met. It consists of both free chlorine and other combined chlorine species, such as chloramines.

While chlorination is the process of adding chlorine to establish a residual concentration of free chlorine, chloramination is the process of dosing chlorine and ammonia in a controlled reaction to generate a residual concentration of monochloramine (NH2Cl).

In the process of chloramination, “Zone I” (Mono-) on the left side of the breakpoint curve shows dosing conditions where monochloramine is predominately present in the measurement of total chlorine. Under these conditions, free ammonia is present in excess, available to react with chlorine as it is added. At the edge of this zone, total chlorine residual reaches a peak, representing the ideal state of chloramine disinfection. This typically occurs at a 5:1 ratio of chlorine to ammonia.

Past this point on the curve (to the right), free ammonia is no longer available, and chlorine added starts to react with the monochloramine species, forming dichloramines (NHCl2) and trichloramines. This causes the total chlorine residual to decrease in “Zone II” (Di-) of the chlorination curve, even though chlorine is being added. Dichloramines and trichloramines cause taste and odor issues and are less effective at disinfection than monochloramine or free chlorine.

Eventually, as more chlorine is added, breakpoint chlorination is reached. Beyond the breakpoint, in “Zone III” (Free-) of the curve, only free chlorine is present in the measurement of total chlorine residual. Under these conditions, free chlorine disinfection is being applied, rather than chloramination.  

The main differences in the use free chlorine water and chloramine water as a disinfectant are stability, reactivity, and the potential formation of disinfection by-products (DBPs). Free chlorine is more reactive and effective for immediate disinfection, but it requires careful management due to the rate of degradation and the potential formation of DBPs. Chloramine is a more stable disinfectant compared to free chlorine. It has a reduced potential for DBP formation and persists longer in the water distribution system.

A position on the breakpoint curve can be determined by sampling and measuring the concentration of chlorine and ammonia species in the water at a given time:

• In Zone I, total chlorine, monochloramine, and free ammonia are present. To distinguish Zone I and Zone II, it is necessary to measure total chlorine and free ammonia, at a minimum.
• In Zone II, total chlorine and monochloramine are present, but free ammonia is zero.
• In Zone III, no ammonia and monochloramine are present, only total chlorine and free chlorine. Total chlorine and free chlorine residual will be very similar, or equal.

The Monoclor® Residual Control System (RCS) is a product designed and manufactured by PSI Water Technologies, a Cleanwater1 Company, located in Milpitas, California.

Monoclor® RCS is designed to control chloramine/chlorine residual in a drinking water storage tank within a distribution system. The patented control algorithm used by Monoclor® RCS enables fully automated residual control to maintain the desired disinfection concentration and composition in response to variations in water chemistry over time due to seasonal changes or other factors.

Equipment configuration may vary to address site constraints or client preferences, but each system typically includes the following components:

• Active mixer to rapidly mix added chemical and to homogenize water composition in the tank, providing consistent water quality for monitoring and distribution.
• Water Quality Station (PAX WQS™) to sample water and analyze parameters in real time, including total chlorine residual at a minimum.
• Smart Control Center (SCC) with a patented control algorithm used to interpret water quality information and send commands to the chemical feed systems.
• Chemical storage and feed systems for chlorine and ammonia, integrated directly with the SCC.

The Monoclor® RCS supports the use of liquid bleach (produced on-site or delivered) at any concentration, as well as chlorine gas. While not preferred, the use of solid tablets is also feasible.

Currently, bulk deliveries of liquid bleach (sodium hypochlorite at 12.5%) is the most common choice. However, there is a growing trend toward the use of on-site generation to produce a safer sodium hypochlorite solution with a concentration of less than 1.0%. The use of onsite generation has become more common due to increases in the price of bulk hypochlorite and supply chain disruptions.

The Monoclor® RCS supports the use of two ammonia sources. Aqueous ammonia is traditionally used for chloramination, delivered at a standard concentration of 19%. Due to safety concerns, especially regarding the highly unpleasant odor linked with ammonia vapor, many utilities have opted to use liquid ammonium sulfate (LAS) as an alternative ammonia source. LAS is odorless and does not necessitate the same safety precautions as aqueous ammonia.

The transition from using aqueous ammonia to LAS as an ammonia source is a common practice; and often, the same chemical storage and feed equipment can be repurposed for use.

The chemical injection points should always be in the zone of high energy mixing in the tank, close to the mixer. Injectors can be attached directly to the mixer assembly, or a hanging nozzle assembly can be installed to discharge chemical a few feet above the mixer.

The Smart Control Center (SCC) is the main controller of the Monoclor® RCS. It interprets water quality parameters from the Water Quality Station (WQS) and sends commands to the chemical feed systems. Operators can adjust the dosing setpoint and dosing feed rate directly from the local HMI screen.

The SCC has an extensive built-in alarm management system. It automatically shuts down the dosing pump(s) when critical alarms occur. Operators can configure alarm thresholds and acknowledge alarms directly via the HMI screen on the SCC.

The Smart Control Center (SCC) is capable of integration into a SCADA system. The default method involves Ethernet communication using Modbus TCP, although other protocols can be accommodated upon request.

Hardwiring signals to the SCADA Remote Terminal Unit (RTU) is also supported. By default, there are 2 available analog outputs, but the capacity can be expanded by utilizing additional IO cards to meet the required number of analog/digital outputs.

Monoclor® RCS uses set-point control to maintain the total chlorine residual on target. The operator chooses minimum and maximum residual set-points. The chemical pump(s) start when total chlorine residual measured by the chlorine analyzer falls below the minimum setpoint. The chemical pump(s) stop when the total chlorine residual is equal to or greater than the maximum setpoint.

Other setpoints include the following:

• Disinfection mode (chloramine, free chlorine)
• Chlorine feed rates (fast and regular)
• Chlorine-to-Ammonia Ratios (5:1)

The Monoclor® RCS algorithm is implemented with the Smart Control Center (SCC). It analyzes the water quality information received from the Water Quality Station (WQS) and automatically determines the position on the breakpoint curve to trigger a preset dosing decision.

When the Monoclor® RCS is in AUTO and the total chlorine residual falls below the chlorine minimum setpoint, the algorithm starts by dosing chlorine-only first and continuously monitors the response over time.

The chlorine reacts with the excess of free ammonia present in the water (Zone I on the left side of breakpoint curve also referred as Mono-) to make chloramine and raise the total chlorine residual until the setpoint is reached, and the chlorine feed pump is stopped.

During that process, if the chlorine residual start to decrease while dosing chlorine (Zone II on the breakpoint curve also referred as Di-), the algorithm engages the ammonia feed pumps to supply both chlorine and ammonia at a 5:1 ratio and raise the chloramine concentration. When the total chlorine residual reaches the maximum setpoint, the algorithm stops the chlorine and ammonia feed pumps. 

The smart controller software continuously tracks the total chlorine residual and calculates the rate of change (slope) of total chlorine residual over time (ppm/hr). In practice, the PLC stores a certain number of points (i.e. 60) representing the average total chlorine measurement over a period (i.e. 1 minute).

The patented Monoclor® RCS algorithm determines the position on the breakpoint curve based on the rate of change of the total chlorine residual (ppm/hr) in response to chlorine dose. If the rate of change of chlorine residual is above a preset threshold, then the algorithm determines that the process is reacting on the left-side of breakpoint curve (Zone I, Mono-). On the other hand, if the rate of change of chlorine residual falls below a preset threshold, then the algorithm detects that the process is reacting on the right-side of breakpoint curve (Zone II, Di-).

The default threshold is 0.00 ppm/hr. A value slightly lower, such as -0.05 ppm/hr, is a common setpoint.  

The Monoclor® RCS Water Quality Station does not measure for free ammonia directly. The Monoclor® RCS algorithm uses the rate of change of total chlorine residual as a proxy to determine if free ammonia is present in the water.

The operator selects the total chlorine residual setpoint directly from the HMI screen. Typically, a residual setpoint between 3.0 and 3.5 ppm is chosen to uphold a high disinfectant level and compensate for the degradation of chloramine as the water ages downstream of the treatment point.

The chlorine feed rate (gph) is chosen by the operator on the HMI screen. There are two chlorine feed rates to configure:

• “Fast feed rate” used when chlorine residual is below the minimum setpoint.
• “Regular feed rate” used when chlorine residual is between the minimum setpoint and the maximum setpoint.

The feed rates are generally determined by the chemical storage tank volume and anticipated turnover. It is essential for the feed rates to be sufficiently high to meet demand and achieve the chlorine residual setpoint within a reasonable timeframe, but the feed rates should not too high to avoid overshooting and to minimize impact during a dosing shutdown.

The Monoclor® RCS is fully automated, eliminating the need for daily adjustments based on grab samples.

It is good practice to visit the site at regular intervals for routine checks.

The Water Quality Station (WQS) employs two chlorine analyzers to ensure redundancy. The primary analyzer is used for dosing decisions, while the secondary one serves as a fail-safe. During regular operation, it is expected that readings from both analyzers closely correlate, demonstrating an excellent match.

If readings between the two analyzers differ too much, the controller has the capability to initiate an alarm, prompting the shutdown of dosing operations and alerting the operator. Resolving such discrepancies may necessitate calibration and/or maintenance to rectify the situation.

Yes, amperometric chlorine analyzers require continuous exposure to chlorine levels to stay active. If a chlorine analyzer is not exposed to chlorine residual for an extended time, the probe may deactivate or become less reactive, potentially requiring a very long time to stabilize its reading.

It is recommended to store dry the chlorine probe if the WQS is not in operation for an extended period. Although it is possible to reactivate a probe by exposing it to high chlorine levels, that process may require time, and it is not guaranteed to succeed.

Routine grab samples taken by the operator for compliance can be used to compare with the total chlorine readings from the analyzers. If reading differs too much, calibration of the analyzer is done directly from the WQS screen by providing the total sample value and pressing “calibration.”

Video: Calibration Video (YouTube)

An amperometric chlorine analyzer is prone to drifting over time due to various factors:

• Fluctuations in water temperature, particularly significant daily variations.

• Alterations in sample flow since the last calibration.
• Substantial shifts in chlorine residual levels since the last calibration.
• Last calibration completed while readings were not stabilized.
• Probe maintenance required, such as electrolyte and cap replacement.
• Potential issues due to a defective or aged probe (3 years or older).
• Probe is partially deactivated and responds very slow.

Small drifts are normal and are generally addressed by calibration. If drift persists, maintenance is recommended.

ORP stands for Oxidation-Reduction Potential. It is a measurement that indicates the ability of a solution to either oxidize or reduce substances. The ORP scale is measured in millivolts (mV) as a measure of the electron activity in a solution, typically ranging from negative values (indicating a reducing environment) to positive values (indicating an oxidizing environment).

In water treatment processes, ORP is commonly used to monitor and control the effectiveness of disinfection. An ORP value for chloraminated drinking water is typically around 300-500 mV, while free chlorine water has an ORP value of 700-900 mV. Therefore, ORP can help differentiate between chloramine and free chlorine water in the operation of disinfection systems such as Monoclor® RCS.

In practice, ORP is used as a fail-safe to detect the presence of undesired free chlorine when the process goal is to control chloramine (and vice-versa). For instance, rise in ORP value suggests that the water is shifting toward the free chlorine. This can be programmed as one of the options for a critical alarm to trigger a dosing shutdown. 

While pH and ORP calibration is not required for the operation of the Monoclor® RCS, this can be done to verify readings and determine if the probes need to be replaced.

The calibration process for the pH and ORP analyzers consists of submerging the probes into a standard calibration solution and following the instruction on the WQS screen. For pH, solutions of pH 4, pH 7, and pH 10 are required. For ORP, various standard solutions are available, and a low to high range of typically 200mV to 600 mV can be used.

The Monoclor® RCS requires the chemical dosing pumps to be accurately calibrated to work with the control algorithm. The pump calibration check is a simple and quick procedure that can be done by the operator on a regular basis to ensure proper delivery of chemicals.

The frequency of calibration depends on the pump type and model used. For instance, peristaltic pumps often wear over time causing the calibration to drift, while diaphragm pumps are usually more stable and less prone to calibration drift.

Video: Calibration Pump (YouTube) 

A “dosing shutdown” alarm occurs when another critical alarm is triggered and calls for a dosing shutdown. For example, if the sodium hypochlorite level in storage tank is too low (or if tank is empty), a low hypochlorite alarm is triggered followed by a dosing shutdown.

To clear a dosing shutdown alarm, the operator must first clear the critical alarm causing the dosing. In the case of a shutdown due to a low hypochlorite alarm, filling the sodium hypochlorite tank or changing the low-level alarm setpoint would clear the alarm. Once the critical alarm conditions are mitigated, the operator can “reset” the dosing shutdown alarm on the HMI screen, and residual control will resume.

A few scenarios are possible to explain why the chlorine residual is not being maintained within the setpoints for an unusual period while pumps are running nonstop.

• Grab a free ammonia sample:
  • If the free ammonia sample is at a normal level, the chlorine feed rate may be too low, so the chlorine demand is greater than the supply. Consider increasing the chlorine feed rate.
  • If the free ammonia sample is above a normal level and the chlorine residual is slowly decreasing over time, check the chlorine feed skids and check the chlorine dosing line for leaks.
  • If free ammonia is zero, the chlorine residual decreases quickly over time, and the ORP is quickly rising, then chlorine feed may be too high or ammonia feed may be too low. The process reacts on the right side of the breakpoint curve (Zone II or III). Check the ammonia feed skids and check the chemical dosing lines for leaks.

• The chlorine analyzer(s) may be providing bad readings. The system may need sensor maintenance and calibration. 

A digital tool is available for operators to use to fill out a site visit report during routine visits to the Monoclor® RCS. By scanning a QR code on-site, operators can initiate the report, answer straightforward questions about the system's status, capture images, and log grab sample results. Upon submission, a PDF report is automatically generated and conveniently emailed to the operator.

Operators can also use the form to log maintenance and calibration tasks performed during the site visit.

Link here: Site Visit Report App