Instrumentation and control
Chapter 9: Instrumentation and Control
This chapter provides information on instrumentation that is generally utilized in sewage treatment plants. However, instrumentation is evolving all the time and enhancements are ongoing - therefore information in this chapter should be used as a general guideline for instrumentation selection and use. Controls for sewage treatment plants are also presented in terms of the general function and considerations for use.
9.1 General
The requirements for instrumentation and control will depend on the size of the sewage works, design standards and philosophy regarding instrumentation and control (I&C) and the type of processes employed. In general, instrumentation and control should allow for safe and efficient manual and automatic operation of all parts of the plant, with minimum operator effort. All automatic controls should be provided with manual back up systems.
Where some parts of the plant may be operated or controlled from a remote location, local control stations should be provided and should include the provision for preventing operation of the equipment from the remote location. Consideration should be given to providing communication via intercom between remote stations and the local stations. In some cases, the use of television equipment may be justified to provide scanning functions of local instrumentation control centers as well as process equipment.
Decisions will have to be made by the designer in conjunction with the owner and operations staff as to which equipment will be controlled locally and which will be controlled from a remote location and whether control will be automatic or manual. For instance, at a small sewage treatment plant, scum pumping may be controlled locally and manually, whereas raw sewage pumping should be automatically controlled, regardless of plant size. In addition, the points of control and the type of primary device should be selected. Decisions need to be made as to whether the instrumentation is to totalize, indicate and/or record and whether alarm functions are to be incorporated.
In making decisions relating to instrumentation and control, the following factors should be considered:
- Plant size;
- Effluent requirements;
- Plant process complexity;
- Hours in the day that the plant will be staffed;
- Potential chemical and energy savings with automation;
- Reliability of primary devices for parameter measurement;
- Preferred location for primary device;
- Parameters with useful significance to process;
- Equipment which should be controlled automatically;
- Equipment which should be controlled manually;
- Equipment which should be controlled remotely;
- Equipment which should be locally controlled;
- Owner’s design standards and philosophy for I&C and automation;
- Data requiring display at the control centre; and
- Indication, totalizing and recording functions necessary to the overall process.
For proper operation of sewage treatment plants, the following parameters should be measured:
- Sewage flow rates, including raw sewage, bypassed flows and flows through plant subsections (flow trains);
- Chlorine dosage;
- Sludge pumping rate, including raw, digested sludges and activated sludge return;
- Digester supernatant flows;
- Sludge dewatering return stream flows, where applicable;
- Chemical dosage for phosphorus removal processes;
- Anaerobic digester gas production and utilization;
- Anaerobic digester temperature;
- Hazardous gas levels; and
- Chlorine residual (if chlorine used for disinfection).
Auxiliary instrumentation may be considered to measure the following parameters:
- Air flows;
- Mixed liquor dissolved oxygen concentrations;
- Mixed liquor and return activated sludge suspended solids concentrations;
- Sludge blanket levels;
- Sludge concentrations; and
- UV intensity.
9.2 Type of Instruments
The type of instruments that will be required to measure the parameters mentioned above are classified as prime or primary element devices which transform a signal from the physical process to a suitable signal for transmission via a transmitter. These devices are broken down into various measurement parameters and each of the parameters is further broken down into features and considerations with a brief description of particular process applications.
Proper design of the instrumentation installation needs to be taken to ensure the instrument accuracy, including avoiding interference from other equipment or aspects of the plant. Flow instrumentation generally requires upstream and downstream straight lengths to avoid non-uniform flow in the vicinity of the instrument. Pipe flow instruments need to be located to avoid interference from valves and bends. Considerations for all instrumentation need to ensure the instrument accuracy by following good design practices. The instrument manufacturer’s specifications, requirements and expertise should be followed.
9.2.1 Flow Elements
Flow meter accuracy needs to account for the accuracy of the primary and secondary devices. Generally, the primary device is the physical structure in which the measurement is being made; an example is a flume in an open channel. The secondary device is generally the measuring or electronic device, such as the level sensor, for example an ultrasonic level device, on a flume. The combined accuracy of the entire flow measuring structure is often referred to as the system accuracy. A main flow measuring device such as the influent or effluent flow meter at a sewage treatment plant (STP) should have a minimum 10% system accuracy.
For the main flow device at an STP, consideration should be given to a secondary means to confirm the accuracy of the instrument, as these flow instruments are critical to determining plant capacity, upgrade or expansion requirements and for proper operations. These can include redundant flow instruments and/or the ability to conduct draw-and-fill tests or secondary checks of the physical primary flow devices. Physical primary flow devices that can be measured independently of the secondary device include flumes (e.g. Parshall flumes), weirs (e.g. V-notch or sharp crested weirs) or Venturi tubes. For these primary devices, a separate physical measurement can be made to confirm the accuracy of the flow instrument. Physical checks of this type need to take into account the proper installation of the primary structure.
Flow measurements should be conducted throughout the STP. Some areas are critical for gauging plant performance and capacity, others are key for plant operations, and some are useful for plant optimization and minimizing operating costs (e.g. electrical and chemical usage). Critical flow measurement areas include STP influent, effluent, overflows and bypasses.
Key flow measurement areas related to plant operations include:
- Unit process or plant influent and effluent streams;
- Biological return streams, e.g. return activated sludge;
- Raw and biological sludge wasting; and
- Recycle streams such as centrate and filtrates.
Useful plant optimization flow measurements include:
- Chemical and polymer usage;
- Septage and/or leachate additions, where applicable;
- Mechanical thickening and dewatering flows; and
- Gas flows, including aeration and biogas.
9.2.1.1 Magnetic Flow Meter
The magnetic flow meter operates on the principle that a conductor passing through a magnetic field will produce a DC voltage directly proportional to the speed of the conductor. The following points generally apply to magnetic flow meters:
- Preferably installed with slight incline with upward flow;
- The meter needs to be full for proper operation;
- Should have a minimum velocity of approximately 2 to 4 m/s (6.6 to 13 ft/s);
- Piping arrangement error is minimal and is not a substantial factor;
- Avoid use where entrained gases are present; and
- Accuracy approximately 1% of rate.
9.2.1.2 Parshall Flume
A Parshall flume operates on the principle of a known relationship between the upstream liquid depth and flow, under a range of operating conditions. A Parshall flume is the most common flume used in sewage treatment plants. The following points generally apply to Parshall flumes.
- Flume should be level;
- Accuracy is affected by upstream channel arrangement (should have at least ten channel widths); and
- Accurate to within ±5 % of the measured rate.
9.2.1.3 Ultrasonic Flow Meter
Ultrasonic flow meters are used in full-flowing pipes, often to measure sludge flows. The following points generally apply to ultrasonic flow meters:
- No contact with medium being measured;
- If doppler type, then the presence of entrained gases or solids are required for proper operation;
- If grease is present in the medium, then heat or ultrasonic cleaning should be used;
- Operating velocity 1 to 10 m/s (3.3 to 33 ft/s); and
- Accurate to within ±1 to 2% of the measured rate.
9.2.1.4 Venturi Tube
The Venturi tube operates on the principle that the pressure differential between the inlet and the throat is proportional to the square of the flow. The following points generally apply to Venturi tubes:
- Meter should be full for proper operation;
- Accuracy is affected by upstream piping arrangement;
- If used on sludge of any kind, then water purge arrangement should be used;
- Accuracy to within ±2% of the measured rate; however, accuracy will be much lower at low flows, depending on which type of transmitter is used;
- If time differential type, then the presence of entrained gases and/or solids will affect the accuracy severely;
- Accuracy affected by upstream and downstream piping arrangement; and
- Accurate to within ±2% of the measured rate.
9.2.1.5 Volumetric Meter
The volumetric flow meter can measure flows in pumping stations controlled by low and high level contacts.
It does not require cleaning or recalibration as no parts are in contact with sewage and can be installed remotely from the station. The measuring principle is flow rate equals volume divided by time.
- Programmed with on-site wet well volumes;
- Digital display of inflow and outflow rates, total flow and pumping time; and
- Accurate to within ±0.1% of the measured rate.
Caution should be used where variable speed pumping is employed.
9.2.1.6 Rotameter
The Rotameter is a tapered tube that has a ball float permitting rough visual readings. Mostly used in chlorinators and/or ammoniators where it is a standard fixture. Since it is installed in the flow line, it needs to be line sized and has limitations when transmission of a signal is required.
9.2.1.7 Gas Meters
Most common gas flows at a sewage treatment plant are for aeration tank air flows and digester gas flows. The following instruments are commonly used:
- Mass flow;
- Orifice plate;
- Rotary positive displacement; and
- Turbine.
9.2.2 Level Elements
The following points generally apply to the level elements presented.
9.2.2.1 Ultrasonic
- No contact with medium being measured;
- Accuracy to within ±2% of actual reading;
- Typically the sensor should be mounted a minimum distance above the high liquid level and should be located away from tank walls or other obstructions that may cause false echoes; and
- Not recommended in locations where foam is dense and persistent.
9.2.2.2 Float Type
- Range of 0 to 11 m (0 to 36 ft);
- Accuracy to within ±1% of actual reading;
- Normally located in a stilling well where turbulence is expected; and
- Commonly used for high and low level alarms and for controlling pump starts and stops.
9.2.2.3 Capacitance Probe
May be used in applications that require continuous level measurement and also as switches for alarms or start/stop control.
9.2.2.4 Bubbler
- Not common in new installations;
- Range 0 to 56 m (0 to 184 ft); and
- Accuracy to within ±0.1% of actual reading; however, the accuracy will be further affected by the type of transmitter selected.
9.2.3 Other Analytical Instruments
9.2.3.1 Density Elements
The following points generally apply to density elements:
Radiation Type
- Accuracy 0 to 15%;
- Can be difficult to keep in calibration and requires frequent maintenance; and
- Requires certified personnel to handle radioactive material.
Ultrasonic
- Range 1 to 10%;
- Accuracy repeatability is ±0.5%; and
- Avoid use where entrained gases are present.
9.2.3.2 Dissolved Oxygen Measurement (Galvanic)
- Installation details are generally related to the choice of placement of the analyzer in the process fluid; and
- Analyzers generally require frequent maintenance.
9.2.3.3 Suspended Solids Measurement
- Installation details for these instruments are unique to each manufacturer;
- Two main types are turbidity and optical;
- Turbidity analyzers are recommended for applications involving suspended solids; and
- Optical analyzers are recommended for applications involving solids concentrations from 20 mg/L to 8% solids.
9.2.3.4 Pressure Elements
The following points generally apply to pressure elements:
Bellow (Lower Pressures)
- Pressure range 0 to 2,000 kPa (0 to 290 psi);
- Accuracy to within ±1% of actual reading; and
- Installation should include the use of block and bleed valves.
Bourdon Tube (Higher Pressures)
- Pressure range 0 to 35,000 kPa (0 to 5080 psi);
- Accuracy ±1% of full scale; and
- Installation should include the use of block and bleed valves.
Liquid-to-Air Diaphragm (Commonly used in sensing pressures involving corrosive chemicals)
- Pressure to approximately 20 m water column or 0 to 3500 kPa (0 to 510 psi);
- Accuracy ±1% of scale;
- Installation should include the use of block and bleed valves; and
- Temperature extremes should be avoided and location should be as close as possible to the process measurement site.
Liquid-to-Liquid Diaphragm
- Pressures to approximately 20 m (65 ft) water column or 0 to 3500 kPa (0 to 510 psi);
- Accuracy ±1% of scale;
- Installation should include the use of block and bleed valves; and
- Temperature extremes should be avoided and location should be as close as possible to the process measurement site.
Strain Gauge (Commonly used in conjunction with a bellow)
- Should have temperature compensation;
- Accuracy ±1% of reading;
- Not sensitive enough for low pressure ranges.
9.2.3.5 Sludge Blanket Detector
There are basically two types of sludge blanket level detectors available; one is the photocell type and the other is the ultrasonic type. If the application requires an on/off type of control, then the photocell type may be suitable. However, if an analog type of control or monitoring is required, then the ultrasonic type will be required.
9.2.3.6 Temperature Elements
All temperature elements should be selected with care to assure that the appropriate device is chosen for a given temperature range. The following points generally apply to temperature elements:
Gas Filled System
- Most common in sewage treatment plant application;
- Temperature range 0 to 100 °C (32 to 212 °F); and
- Accuracy ±1% full scale.
Resistance Temperature Detector
- Use of thermowell advised;
- Temperature range 0 to 300 °C (32 to 572 °F); and
- Accuracy to within ±0.5% of actual reading.
Thermocouple
- Use of thermowell advised;
- Temperature range approximately 0 to 1000 °C (32 to 1832 °F); and
- Accuracy ±1% full scale.
Thermistor
- Use of thermowell advised; and
- Temperature range approximately 0 to 300 °C (32 to 572 °F).
9.3 Process Controls and Instrumentation
9.3.1 Pumping Stations
Pumping stations require dependable and simple instrumentation and controls. The parameters that require monitoring and control in a pumping station are: level, flow, pumps, motors and alarms.
9.3.1.1 Level Control
The purpose of a level control is to regulate the pumping rate of sewage to the treatment plant. If the pumps are variable speed an analog monitoring and control system should be used. If the pumps are constant or multiple speeds, a stepping type of control system should be used.
9.3.1.2 Flow Monitoring
The flow metering device should be selected very carefully to ensure that there are no obstructions where clogging could potentially occur. Routine preventative maintenance should also be considered in the selection of the flow meter.
9.3.1.3 Pumps and Motors
The following parameters should be monitored:
Pump
- Bearing temperature;
- Casing temperature;
- Vibration;
- Speed; and
- Suction and discharge pressures.
Motors
- Voltage;
- Current;
- Hours of operation;
- Bearing temperature; and
- Windings temperature.
9.3.1.4 Alarms
Alarms are the final warning to the operator that the system is malfunctioning and unless corrective action is taken, damage (e.g. flooding, bypassing and/or equipment damage) may occur. Audio/visual alarms are recommended in order to focus the operator’s attention on the actual fault condition.
A pumping station should have at least the high and low liquid level alarm and other pump alarms, including motor winding temperature, pump and motor bearing temperature and motor overload.
For further protection of motors in heavy-duty applications, a shock-loading relay can be installed to protect against unexpected motor overloads.
9.3.2 Mechanical Bar Screens
There are two methods to control mechanical bar screens:
- Simple manual start or stop switch requires the presence of an operator at the screen; and
- Automatic start or stop switch operates when activated by a differential pressure switch. The screen should run for at least one complete screen revolution. To achieve the minimum running time, a timer or limit switch may be used. In addition a timer should be provided to ensure
periodic cleaning of the screen, regardless of actual headloss. A means to initiate a cycle manually should be provided to permit the operator to cycle the unit as required if the automatic systems fail.
9.3.2.1 Alarms
There should be a differential pressure switch for alarm signal with a head loss setting of approximately 100 mm (4 in) higher than the setting for automatic start up of the mechanical bar screen.
9.3.3 Primary Treatment
9.3.3.1 Raw Sludge Pumping
The raw sludge pumps should be set up in such a way that the following features are incorporated:
- Selectable pump duty (manual selection of duty pump);
- Manual override of automatic controls;
- Individually selected hopper pumping times;
- Adjustable density control;
- Sludge flow; and
- Sludge density.
9.3.3.2 Scum Pumping
The scum pumps should be set up in such a way that the following features are incorporated:
- Selectable pump duty;
- Manual override of automatic controls;
- Automatic control system consisting of high and low scum level switches;
- Scum temperature indicator; and
- The duty pump should start at scum high level and stop at scum low level in the scum tank.
9.3.4 Secondary Treatment
9.3.4.1 Aeration Tank Dissolved Oxygen
Automatic dissolved oxygen (DO) control systems can be used to control the rate of air supply to the aeration tanks. The use of DO control can result in energy and operating cost savings. There are several different methods of automatic DO control. The most commonly used are:
- Flow Ratio - This consists of a fixed ratio of air volume to plant influent flow; it should be noted that this control strategy does not account for load variations that are not associated with flow changes.
- Closed Loop Control - This consists of DO probes and controllers where the actual DO reading is compared with a set point on the controller and the resultant deviation signal is used to increase or decrease the oxygen supply to the aeration tanks.
There are other methods, such as food-to-microorganism (F/M) ratio or solids retention time control. These methods require the use of a computer for calculation, forecasting and modelling.
9.3.4.2 Chemical Control Systems
Chemical addition with the exception of the chlorination system is a feed forward control system. This consists of a feeder or chemical metering pump that will dose at a fixed ratio to the influent or effluent flow of the plant, with no analyzer or feedback control.
Chlorine addition is a compound-loop control system that consists of adjustable ratio of chlorine to influent or effluent flow with trim based on chlorine residual as measured by a chlorine analyzer.
The chlorine addition for larger treatment plants should consist of at least three chlorinators and two analyzers:
- One chlorinator for pre-chlorination;
- One chlorinator for post-chlorination;
- One chlorinator standby that can be used as either pre- or post-chlorination;
- One analyzer for post-chlorination; and
- One analyzer for pre-chlorination.
Each analyzer should be capable of being switched to the standby chlorinator.
Dechlorination chemical addition will have similar requirement for control. Dechlorination control is generally based on chlorine residual after dechlorination, with a set point and alarm level identified.
9.3.5 Tertiary Treatment
Control and monitoring for tertiary treatment processes will depend on the process being used. This could include, for example, tertiary clarifiers, filters or membranes. Monitoring and control instrumentation could include:
- Water level;
- Influent and effluent turbidity or solids; and
- Pressure.
9.4 Process Narrative and Basis Of Control
The designer should prepare a report which provides a process narrative for the STP and that briefly describes each component of the plant, including the raw influent quality, pretreatment processes, biological treatment processes, pumping equipment, solids handling processes, instrumentation, monitoring and sampling equipment, as applicable. The report should also identify and explain the basis of control for the system.
Process and instrumentation diagrams (P&ID) should be developed for all sewage treatment facilities and should include all major and minor processes along with all ancillary process equipment.
Control systems should be designed with a user-friendly, human-machine interface (HMI) system to facilitate plant operation and on-line monitoring. Equipment status, flow rates, water levels, pressures and chemical feed rates should all be displayed via an HMI. All automated systems should be designed with a manual override some other form of redundancy to allow safe operation in the event of a hardware or communication failure.
9.5 Control and Monitoring Systems
Two control and monitoring systems are available. One is the conventional system with recorders, indicators, switches, push buttons, indicating lights and control panels and the other is the computerized system.
The conventional system is a passive system with limited automatic control, where the operator is responsible for decisions and actions. The computerized system is a multi-purpose system with limited scope for modification or a dedicated-purpose system with standard hardware and customized software.
Both computerized systems have two basic configurations:
- A centralized configuration where all intelligence is resident in the computers in the Central Control Station; and
- A distributed control configuration where the intelligence is distributed throughout the system.
The distributed system hardware costs will be higher than the centralized configuration; however, wiring and installation costs will be less. There are several important advantages to the distributed system.One of the advantages is that with intelligence distributed throughout the system, the software required for the computers at Central Control becomes less complicated and requires less maintenance. The intelligence contained in the other components of the system will be on firmware which requires no maintenance. Another advantage is that in the event of a communication failure, each intelligent component in the system can operate on its own and maintain some preprogrammed condition based on its own sensors. Therefore, when a lower intelligence component loses communication with a higher intelligence component, it will still function with the pre-determined fail safe program to maintain system operation.
The computerized systems can be arranged so that all operating decisions can be made by the computer based on instructions given at an earlier stage of the formal programming. Alternatively, the terminal equipment can be used for information display and manual initiation of control commands, that is as a remote manual control station.
A programmable logic controller (PLC) based system is a multi-purpose system with extensive scope for modification. The plant status, alarms, motor starters, meters and analyzers are all wired into input/output (I/O) cards located in what are called racks. The racks may be mounted separately or placed in specific plant areas to reduce wiring costs. The I/O racks are associated with controllers that are programmed to perform the required process control functions. Adjustments can generally be made easily by modification of or addition to the PLC programs.
Plant personnel require process information in real-time or in near real-time. The PLC systems accomplish this by means of an HMI. The HMI may be dedicated hardware and software or may come in the form of personal computers utilizing HMI software and connected to the PLC communications system. These systems vary widely in their capabilities and performance. The selection of hardware and software should be done carefully to assure proper current performance and future supportability and expendability.
9.5.1 Maintainability
Sewage treatment plants are becoming more dependent on control systems of all types and complexities. STP are becoming increasingly dependent on the one common feature of control systems, this being the software. Without proper documentation and maintenance of the software, proper operation of the plant is at risk. Plant operation relies on proper application programs, which could be lost without adequate system documentation.
System backup programs may also be at risk if system activities such as changes to program logic, the tuning parameters and instrument installation are not properly documented. Maintenance of the control system is difficult if not impossible to accomplish without proper documentation.
9.5.2 Identifying the Required Documents
The operation and maintenance of an STP that uses any type of programmable device for process control requires the following types of documents:
- System description in narrative format;
- System block-diagram drawing that identifies location and node names of the connected PLCs, PCs, operator interfaces, servers and modems;
- Software used for system configuration is always updated and ready to load;
- Drawings showing I/O wiring connections and address assignments;
- Address assignments identifying all of the variables within the control system, such as register and address assignments, variables and I/O tables (if required);
- Control system programs for each PLC or programmable process control device in a state that is updated and ready to load, as well as a printout of the program; and
- Narrative description of each part of the program and the software used to enter the description.
9.5.3 Smart Instrumentation
Instruments that provide the control system with both process measurements and diagnostic information about the instrument are referred to as “smart instruments.” Both pieces of information are critical in today’s control systems due to the way data is moved and used. It is common to move analytical data from the control system to a server where many people can view the data and use it in reports. If the instrument is malfunctioning, the data may be in error, but it will be used in reports generated from the server. Smart instruments can provide an indication of the quality of the data in question and, therefore, whether reports are accurate.
9.5.4 PLC Documentation Software
Specifications for STP using PLCs should include comprehensive requirements for PLC documentation software. Documentation systems, either from the PLC manufacturer or third-party software vendors, should provide functions important to maintaining a plant such as uploading, verifying and storing the application programs.
9.5.5 Reliability and Maintenance Considerations
An instrumentation and control system should be designed with both To assist in review of this vital requirement, the following list of frequent design oversights, errors and omissions has been compiled. The list does not contain any solution to these problems. It is intended only as a reminder to designers or checkers. Solutions depend on conditions or factors unique to specific projects. operational reliability and maintainability if it is to properly serve its purpose.
- Millivolt-level signals inadequately separated or shielded from parallel runs of heavy power circuits;
- Millivolt-level signals not in twisted shielded pair or triad construction;
- Electric and pneumatic signal conductors not in conduit or otherwise protected from physical/mechanical damage;
- 120 VAC control circuits too long, allowing distributed capacitance to keep the circuit energized after the primary control element is opened;
- Hazardous area (refer to U.S. National Electric Code (NEC) section 500);
- Failure to use oil-free air in pneumatic control systems;
- Failure to indicate when single-point grounding is required;
- Failure to indicate or specify required voltage regulation or over-voltage protection;
- Failure to specify adequate equipment enclosures for adverse, hostile or hazardous environments;
- Failure to consider possible or probable clogging of sensor lines by grease or solids in the process stream;
- Failure to specify or provide isolation valves on instruments connected to process piping;
- Failure to specify snubbers on pressure switches;
- Failure to provide needle valves for control of operating air or hydraulics to control valves;
- Float switches in highly turbulent areas;
- Flow meters too close to bends in process pipes;
- Installation of equipment in areas difficult or impossible to reach for maintenance;
- Failure to consider operator convenience in layout or design of control system; and
- Failure to provide operator with sufficient process data.
9.6 Security
Site security has historically been to protect property, protect staff and prevent endangering the public. Recent events have shown that in addition to these issues, sewage treatment works are also potential targets of malevolent acts of destruction and disruption from domestic and international terrorists. Purposeful contamination of sewage as well as damage to or destruction of treatment or conveyance systems, can lead to widespread and long-term environmental damage and serious public health impacts. For additional details on security issues refer to ASCE, AWWA and WEF (2004), Interim Voluntary Security Guidance for Wastewater/Stormwater Utilities.
The security systems covered in this section refer specifically to electronic type surveillance and intruder alert systems. Fencing and other safety aspects are covered elsewhere.
In larger sewage treatment plants, the main gate should have at least one of the following access control systems:
- Punched or magnetic cards with a card reader at a central control station;
- Closed circuit TV (CCTV) system where the operator has to operate the gate/door from a remote location; and
- An intercom system where the operator has to operate the gate/door from a remote location.
Legal/illegal entry alarm systems should be provided in remote pumping stations and if required in plant buildings. These systems should include door and window switches and tapes that will provide indicator signals to a central location that an entry has been made.