Compass advanced automatic coagulation control system.

Achieving reliable automatic coagulation control is something many water companies struggle to maintain in highly variable (flashy) catchment areas. These areas can be at one time or another; typically low in particulates with a moderate to high natural organic matter (NOM) content or vice versa. The water quality can change very rapidly following rainfall events. This upsets previous steady state conditions, and can often reverse the balance and alter the coagulant demand parameters. Such water quality “events” present a challenge to conventional water treatment processes and coagulation control in particular.

Recent advances in instrumentation have enabled water suppliers to dispense with single wavelength UV254 monitors, or streaming current/Zeta potential controllers, and instead, install fully submersible UV-Vis spectrophotometers for continuous full spectrum online water quality monitoring.

Modern UV-Vis instruments can be used to measure the quantity of NOM and infer changes in the characteristics of NOM during water quality events. This knowledge can be used to optimise conventional treatment plants for NOM removal in the most cost effective manner, without a deleterious effect on particulate removal by regulating coagulant dose rate. It is this capability that defines successful coagulation control.

UV-VIS, SUVA and NOM Characterisation

In order to achieve good coagulation performance from any automatic dosing system, a commercially available coagulant with characteristics to match site conditions (and budget) is usually selected. In many cases this will often be what is widely referred to as Alum or alternatively Ferric, or a combination of one or the other depending on seasonal changes. Some water treatment works switch from say Alum to Ferric during summer conditions as the water matrix changes requiring different performance characteristics from the coagulant. Coagulation control is, therefore, a matter of controlling the optimal dose rate of the coagulant such that plant compliance is maintained in a robust, reliable and cost-effective manner for different types of coagulant across all seasonal conditions.

To ensure this happens from a UV-Vis measurement perspective, we are required to compensate the raw water matrix for suspended solids, and then divide the analysis into “dissolved compounds” and particulate matter. These impact on coagulant demand, often in a diametrically opposed manner. This is not as easy as it sounds. For example; “true” colour is the dissolved colour of a liquid with solids removed (filtered), “apparent” colour (that which the human eye sees) is the combined colour resulting from both coloured dissolved, particulate & colloidal matter. Any reliable system must be able to distinguish and characterise these variations such that the correct dose rate is delivered and optimal dissolved organic carbon (DOC)removal is achieved. s::can’s submersible spectrometer: the Spectro::lyser achieves this online within seconds. The Spectro::lyser combined with the Com::pass algorithms stored on the s::can analyser system are the key to reliable automatic coagulation control. Streaming current or Zeta potential systems are unable to characterise raw water in this way and therefore suffer poor control when sudden flashy environmental conditions present themselves at the water treatment works inlet, ironically, just when a reactive and responsive control system is needed!

SUVA = 254nm absorbance / DOC concentration

s::can Submersible Spectrometer & Compass automatic coagulant control software

The s::can UV-VIS spectrophotometer can be used to measure the quantity of NOM and infer changes in the characteristics of NOM during water quality events. This data, along with standard turbidity measurement, is used by the Compass software system to predict the coagulant dose required to achieve treatment aims. This form of feed-forward coagulant dose control has been successfully applied on-line at numerous plants worldwide.

Automatic Coagulation Control

Measurement across multiple wavelengths provides an absorption fingerprint profile or “spectra” from which other parameters can be derived, such as a DOC equivalent (DOCeq). The technology in the s::can spectro::lyser enables users in the field to access real time UV-Vis spectral data. This in turn has resulted in predictive, online, real time and reliable coagulation control becoming a very real possibility even under challenging and changeable environmental conditions.

Com::pass automatic coagulation control software

DOC removal tends to follow the course predicted by the graph below. Extreme ends of the scale result in treatment failure, or excessive coagulant dosing fails to provide any additional DOC removal despite increased monetary spend. It can be seen in the example below that ideally the coagulant dose rate would fall between approximately 1.1–1.7mg/l. 1.1 mg/l being the most economical dose rate of coagulant that could be used as a set point in order to maintain plant compliance with a degree of safety built in. Higher dose rates approaching 1.7mg/l provide maximum DOC removal and reduced THM production.

The Com:pass automated coagulant control system enables a utility not only to ensure they are minimising the possibility of THM formation, but optimise their coagulant dosing to minimise their chemical costs. Dosing extra coagulant “just in case” is no longer necessary.

Historically, the measurement of UV absorbance at a single wavelength of UV 254 nm has been a widely accepted surrogate for natural organic matter (NOM). However, in order to characterise NOM, additional wavelengths need to be measured.

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Fowzie Smith-Tahir

Fowzie Smith-Tahir

Marketing and Design at PMA Ltd

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