Disinfection system design must be site-specific
Portacel's contracts manager Neil Savage describes the important factors to be taken into account when designing advanced disinfection systems for WTWs.
In addition, adequate process control must be used to protect employees and the public from accidental exposure to dosing chemicals at toxic levels.
The type of control system suitable for a particular plant is dependent on the complexity of the installed equipment and the site's operational requirements. Disinfection can consist of a number of treatment stages - pre-chlorination, primary chlorination, secondary chlorination, dechlorination and chloramination. The number and type of monitoring systems in operation will depend on the treatment stages and will measure flow rates, disinfection residuals and, toxic gas leaks using appropriate detectors.
The choice of control system will also be affected by whether the plant is manned or unmanned and, because modern WTWs are generally highly automated, the equipment needs to be manufactured for ease of integration into the works' whole control system.
Among the solutions currently provided by Portacel is the supply of equipment for low technology operations such as a simple 'chlorination-dechlorination' system, with stand-alone local control panels and discrete controllers for each part of the operation. Other than for alarm monitoring, this type of system is not linked to a central control system and is therefore most suitable for manned plants.
A more advanced alternative is to use a programmable logic controller (PLC), which is ideal for the optimisation of process performance.
The equipment is fully integrated with the overall treatment process and provides automatic reporting back to a central control station.
Process operation is displayed on a mounted screen and parameters such as set points and dose rate can be changed remotely. The visual display uses PC graphics in a picture-on-picture format, able to show either the whole disinfection process or individual equipment in detail.
The way in which Portacel designs systems to meet individual requirements is shown here, by a comparison of three different installations in south-east England.
Hornsey WTW, on the outskirts of London, is a 50 Ml/d plant which supplies drinking water to 250,000 people.
Disinfection is by sodium hypochlorite, with three triple validated sample monitoring systems. The water quality is seasonally variable with occasional high chlorine demand of up to 4 mgCl/l.
The plant status and operation parameters are monitored via a colour screen, with picture-in-picture graphics, within the control panel.
A link is maintained via a network to a remote station that can receive and act upon alarms and fault conditions.
The plant is unmanned and thus the custom-made control system is fully PLC controlled.
By comparison, Shalford WTW in Surrey has a capacity of 32 Ml/d and provides drinking water for approximately 150,000 people.
Disinfection is again by sodium hypochlorite; the process being pre-chlorination followed by dechlorination, using granulated activated carbon (GAC), then primary chlorination ahead of dechlorination by sodium bisulphite.
The site is manned and each treatment stage has a local control panel which reports back to a centralised alarm panel with an illuminated diagram of the plant depicting status.
The plant is also linked via the alarm panel to a supervisory control and data acquisition (SCADA) to enable remote alarm monitoring.
Shortlands WTW, a smaller 20Ml/d facility located near Bromley in Kent, has disinfection control which is best described as a hybrid of the Hornsey and Shalford plants.
It is similar to Shalford in that there is diagrammatic control panel depicting plant status. However, the plant control is centralised and incorporated into the alarm panel, offering both automatic and manual control functions.
All alarms and process variables are also available via SCADA links for remote monitoring. Although this particular plant is unmanned, there are daily visits by operators.
Portacel is also in the process of developing another system based on stand-alone local control panels, which could further increase design flexibility.
This could be networked and used in conjunction with a centralised control system, resulting in a high degree of integration with minimal cabling costs.