The Water Treatment Process

Published Date: 02 Nov 2017

Before moving on with the study of PLCs, SCADAs and the design of the system to be automated, an overview of the treatment process is necessary. This will allow identification of the specific areas where automation is required and how it will be beneficial to the operators and consumers.

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Figure 2. Typical water treatment process

Pre-treatment: pH adjustment

It may be needed to adjust pH of raw water to improve coagulation and flocculation and reduce corrosivity. When alum is added to the water pH may drop out of range. In this case, water may be considered to be unstable. Lime /soda ash is added to increase alkalinity in the rapid mixing chamber.

Coagulation and Flocculation

In the coagulation process, coagulant chemicals are added to the water as it passes through the rapid mixer. Turbulence created by the mixer is used to mix the chemicals into the water quickly. Dissolution occurs in 10 to 30 seconds. Primary coagulants are chemicals that are responsible for the main coagulation reactions, being the formation of flocs. Of all coagulants, aluminium sulfate (alum) is most commonly used.

Coagulant aids (also called flocculant aids) are used to increase the density of slow settling floc particles, or to strengthen them so they do not break up during settling and filtration. Polymers are popular coagulant aids.

Following the rapid mixer, flocculation is the slow stirring process that causes the flocs to grow and to come into contact with particles of turbidity to form larger particles that will readily settle. The purpose is to produce a floc of proper size, density and toughness for effective removal by sedimentation and filtration.

In conventional systems, using separate tanks for flocculation and sedimentation, detention time is about 30 minutes. Detention times shorter than 20 minutes may result in incomplete floc formation especially during cold water conditions which lower floc formation rates. Longer detention times may break up large flocs and result in wasted capacity. It takes very excessive mixing intensity to cause floc break up. A variable speed motor on the flocculator is a must.

Operational problem

The most common operational problem is a sudden change in raw water quality, usually a result of changing conditions, or quantity, examples being droughts or cyclonic conditions.

A significant change in raw water quality means adjustments may be needed including:

Adjusting coagulant dosages.

Adjusting flash mixer and/or flocculator mixing intensity (speed).

Adjusting alkalinity or pH.

Sedimentation process

Sedimentation is the separation of suspended material from the water by gravity. In the water treatment the main purpose is to reduce solids loading on the filters. Sedimentation is carried out in the sedimentation tank, a settling tank or clarifier-different words for the same process. Clarification of the water is the direct result of the sedimentation of particles. The rationale of having the coagulation/flocculation process is to increase the size of the particles hence reducing the amount of time to settle the particles during sedimentation.

Sludge removal

Water treatment plant sludges usually consist of aluminium hydroxide floc and the settled particles, which were originally present in the water. The concentration, composition, and quantity of sludge depend on:

The coagulant dose.

The characteristics of raw water.

The type of sludge removal system.

The concentrations of solids can vary from less than 0.25% to more than 10%. In general sludges with concentrations of less than 3% can be discharged by gravity, denser sludges require pumping. In sludge removal from the clarifier, an important consideration is called "coning". Coning can cause a dilute sludge to be wastedwhile leaving significant amounts of sludge in the clarifier. A simple solution to this problem is to remove less sludge more often.

Filtration

The primary purpose of filtration is to remove suspended materials such as small particles and micro-organisms from the raw water stream. There are two types of rapid filters, gravity filter or pressure filter. The water is usually pre-treated by coagulation and settling to remove majority of the suspended matter before the actual filtration process. Because very little bacterial purification occurs, chlorination is practiced after filtration to achieve disinfection.

Wash water troughs

The wash water troughs are located above the surface wash equipment. They are installed, half the bed depth above the bed to provide a free space between the underside of the trough and the top of the bed. This space is normally provided for when the filter is backwashed to allow for the filter media to expand when cleaning without losing filter media. The depth of the wash trough to the top of the bed will vary widely.

2.1.2.2 Necessary accessories

Other necessary components include:

The influent wash water valve;

The effluent wash water valve;

A valve to control the flow of water to the surface wash equipment; and

Pumps.

All of these valves are controlled from a console in front of and facing the filter. Built into the control console are gauges showing loss of head, rate of flow through the filter, backwash rate of flow, and effluent turbidity.

2.1.2.3 Backwashing

The rapid sand filter can be cleaned of accumulated turbidity by reversing the direction of the flow of water. This process is called backwashing. In backwashing, the flow of water expands the sand, scours the bed and carries the accumulated solids to the sewer or waste treatment facility.

The normal method for backwashing a filter involves draining the water level above the filter to a point six inches above the filter media. The air scour is then turned on and allowed to operate for several minutes to break up the crust on the filter. After that, the backwash valve is opened allowing backwash water to start flowing into the filter and start carrying suspended material away from the filter.

The time elapsed from when the filter is started until full flow is applied to the filter should be greater than one minute. After a few minutes, the filter backwash valve should be fully opened to allow full expansion of the filter media. Generally, this expansion will be from 20 to 40 percent over the normal filter bed volume. The filter will be washed for 10 to 15 minutes, depending on the amount of solids that must be removed. The best way to determine how long the filter should be washed is to measure the turbidity of the backwash water leaving the filter. In most cases, a filter is washed too long. This could be costly as too much backwash water is used and must be treated after use.

Backwash valves must be opened slowly. Opening the valves too quickly can cause serious damage to the filter underdrain, filter gravel and filter media. Filters should be backwashed when the head loss reaches about 69 kpa or everyday, but no less than every 2 days to prevent cracking.

2.1.2.4 Air scour wash

A method used to assist in cleaning the filter is accomplished by introducing compressed air into the backwash stream before it reaches the filter. Air scour systems blast the filter media with jets of air from the bottom of the filter and are activated prior to backwashing and remains on until the wash water troughs begin to fill with wash water. A common problem with air scour systems is that they inadvertently remove filter media into the wash trough damaging the filter. This can usually be remedied by reducing the backwash velocity, by properly guarding the filter media and by ensuring air scour is turned off before the backwash reaches the wash water troughs.

Programmable Logic Controllers

2.2.1 Basics of a PLC

A programmable logic controller is a digital computer optimized for control tasks used in the automation of complex industrial processes, aiming at providing cost-effective, more flexible and reliable systems. The PLC uses a programmable memory where the instructions and implementing functions such as logic, sequencing, timing, counting and arithmetic are stored. These instructions and functions control electrical and mechanical input elements as required by the processes being automated and send the appropriate signals to the output elements so as to achieve the desired control action.

The PLC system

The PLC consists of the Central Processing Unit (CPU) where input signals are interpreted resulting in the execution of control actions according to the program stored in its memory, and communication of the decisions as action signals to the output elements.

The power supply unit converts the mains ac voltage of 230 Volts to low dc voltage of 5 Volts as per the requirements of the processor and the circuits of the input/output (I/O) interface modules.

Figure 2.2.1 Block diagram of a PLC (Bolton, 2006)

Programming devices are used to load the program onto the memory of the processor. Examples of such devices are hand-held programming devices, desktop consoles, personal computers.

The memory unit stores the program.

The processor sends out and receives information from external devices through the I/O sections. There are two types of signals, either discrete, where the signals are sent as on or off or analogous, where the signals are sent in proportion to the variable parameters being monitored.

The PLC also consists of a communications interface which is used to receive and transmit data on communication networks from or to other remote PLCs (see figure 2). It is concerned with actions such a device verification, data acquisition, synchronisation between user applications and connection management. (Bolton, 2006)

Figure .2.2 Basic communications model

2.2.3 The PLC hardware configuration

The typical PLC hardware consists of a backplane to serve as communications bus. This connects the PLC processor with an array of the individual I/O devices. These devices are in fact several modules which are plugged into racks comprising of several slots. Each module has several I/O points and is coupled to the processor by an I/O bus. The slots are coupled together by a main bus thus coupling the modules to the CPU. The backplane of the PLC also contains a specific slot in which the CPU card can be plugged. This modular construction of a PLC allows easy reconfiguration to meet the demands of the process being controlled.

Figure 2.2.3 PLC hardware

2.2.4 Why PLC?

The "old way" type of control is problematic as it is based on mechanical relays (weakest link in systems). Mechanical devices have moving part that can wear out. In the event of failure of one of the relays, it may be required to troubleshoot the whole system which implies that the latter will be shut down until the problem is located and corrected thus having a negative impact on industrial processes. Another major problem with hardwired logic is that if a change (even a minor change) in the sequence of operation of the system is made, it again requires complete system shut down and rewiring of the panels, leading to major expenses and loss of production time.

The advent of PLCs caters for all the short-comings of the "old way". The price of a PLC is competitive with the relay system it replaces. It is a solid-state device, operated electronically rather than mechanically. Its I/O devices are easily replaceable and it has the flexibility of a computer. One of its most significant pluses is its ability to function in an industrial environment as it is robust and designed to withstand vibrations, humidity, heat, dirt, noise, etc. its modular construction allows easy removal of subassemblies for repair and replacement. Furthermore PLCs make troubleshooting an easy task as it consists of an LCD display and I/O status LEDs on the front of the PLC or modules. Another advantage is the ease and simplicity of programming using ladder logic (most common programming language used to program PLCs). New ladder can be written off-line while the systems are still in operation and can be downloaded to the PLC in a very short time, thus no rewiring is required.

PLC Ladder Programming

A very common way of programming PLCs is based on the use of ladder logic diagrams. Writing a program is equivalent to drawing a switching circuit. The ladder diagram consists of two vertical lines representing the power rails. Circuits are connected as horizontal a line, that is, the rungs of a ladder, between the power rails. In drawing a ladder diagram, certain conventions are adopted as follows:

The vertical lines of the diagram represent the power rails between which circuits are connected. The power flow is taken to be from the left-hand vertical across a rung.

Each rung on the ladder defines one operation in the control process.

A ladder diagram is read from left to right and from top to bottom. Figure 4 shows the scanning motion employed by the PLC. When the PLC is in run mode, it goes through the entire ladder program to the end, the end rung of the program being clearly denoted, and then promptly resumes at the start. This procedure of going through all the rungs of the program is termed a cycle. The end rung might be indicated by a block with the word END or RET (return).

Figure 2.2.4 Scanning the ladder diagram (Bolton, 2006)

Each rung must start with an input or inputs and must end with at least one output. The term input is used for a control action, such as closing the contacts of a switch, used as an input to the PLC. The term output is used for a device connected to the output of a PLC.

Electrical devices are shown in their normal condition. Thus a switch which is normally open until some object closes it, is shown as open on the ladder diagram. A switch that is normally closed is shown closed.

A particular device can appear in more than one rung of a ladder. For example, we might have a relay which switches on one or more devices. The same letters and/or numbers are used to label the device in each situation.

The inputs and outputs are all identified by their addresses, the notation used depending on the PLC manufacturer. This is the address of the input or output in the memory of the PLC.

Figure 5 shows standard IEC 1131-3 symbols used for input and output devices. Some slight variations occur between the symbols when used in semi-graphic form and when in full graphic. Note that inputs are represented by different symbols showing normally open or normally closed contacts. The action of the input is equal to opening or closing of a switch. Output coils are represented by just one form of symbol.

Figure 2.2.5 Basic Symbols (Bolton, 2006)

To illustrate the drawing of the rung of a ladder diagram, consider a situation where the energizing of an output device, for example a motor, depends on a normally open start switch being activated by being closed. The input is thus the switch and the output the motor. Figure 6 shows the ladder diagram.

Figure 2.2.6 A ladder rung (Bolton, 2006)

Starting with the input, we have normally open symbol || for the input contacts. There are no other input devices and the line ends with the output, denoted by the symbol ( ). When the switch is closed, representing the presence of an input signal, the output of the motor is activated. In the case of a normally closed switch |/| with the output, then there is an output signal until the switch is opened. Only while there is no input to the contacts is there an output. (Bolton, 2006)

2.3 Supervisory Control And Data Acquisition

Supervisory control and data acquisition (SCADA) is a type of industrial control system which is used to accomplish functions such as acquisition and processing of data and remote control of processes and devices by the operator. The use of such a system allows significant reduction in labour costs and at the same time improves plant performance and reliability.

The SCADA consists of the following components:

Human Machine Interface (HMI) which consists of a graphical representation of the plant to the operator.

A computer system which gathers data and sends instructions to remote equipment.

Remote Terminal Units (RTUs) which convert analog signals from the sensors to digital signals which are sent to the computer system for further processing.

PLCs

Communication links relating the computer system to the RTUs. These can be wires, fibre optics, radio waves, telephone line (telemetry system), microwave, and satellite.

2.3.1 Human Machine Machine

The HMI is a graphical presentation of the processed data of the processed data, providing a means through which an operator can monitor the process or even make alterations to the system by modifying control variables. The HMI also provides a means of output that allows the system to respond according to any alteration made. It is implemented on a PC-based system or panel viewer using line graphics and schematic symbols. This greatly simplifies troubleshooting and alarm-handling (the process whereby the SCADA system monitors that alarm conditions are being met and determines the time of occurrence of an alarm). Once an alarm is detected it may lead to the activation of other alarm indicators and/or send text messages or email to management or remote SCADA operators.

The SCADA system uses the concept of distributed database or tag database containing data elements called tags or points. A tag or point represents a single value or output value at a particular address in the PLC or even from a remote sensor which is being monitored and controlled by the HMI. There are two types of points, namely hard points and soft points. A hard point is an actual input or output within the system whereas a soft point is produced by logic and mathematical operation performed on other points. Each point consists of the value and information about the time at which the value was retrieved.

Advantages of the SCADA

There are a number of advantages to having a SCADA installed such as:

It facilitates troubleshooting.

It provides safety of operators and equipment due to alarm handling.

It allows real time data acquisition.

Large amount of data can be processed and stores by the computer.

It allows remote monitoring of large number of sensors over a wide area.

It provides automatic generation of reports.

The lifespan of equipment is increased.