Feed Forward Coagulation Control
9 April 2009, News release from Process Measurement & Analysis Ltd
Feed Forward Coagulation Control by Chris Laidlow, Water Supply Manager - Greater Wellington Water
For the operators of a typical surface water treatment plant one of the more important challenges is that of achieving the optimum coagulant dose. This is particularly so where the source water quality is variable.
Over the years many attempts have been made at automating the process of coagulant control, arguably the most successful of these has been the Streaming Current Meter (SCM). SCM's operate in a feed back control loop, taking measurements downstream of the coagulant dosing point and feeding information back to the plant control system which then adjusts the coagulant dose.
This article is a summary of the operational impacts that we have experienced with a feed forward coagulant control system which was put into service in 2006 at the Wainuiomata water treatment plant (wtp) in Wellington, New Zealand.
The feed forward system continuously predicts the coagulant dose required to achieve treatment aims based on measurement of the UV-VIS spectra (200-750nm) and turbidity in the plant source water.
Figure 1 shows the raw water UV-VIS spectra from the Wainuiomata plant inlet over a 3 day period. The profile shows a short-term rain event towards the end of day 1. It can be seen that there is very little change in absorption in the colour range (375nm - 435nm) but a significant increase in the organics range (250nm - 350nm).
The Wainuiomata wtp is a 'run of river' plant which treats water from two catchment areas to the North East of Wellington. The catchments are bush clad and reserved solely for water supply purposes. Raw water quality can change rapidly from very good (turbidity 0.5 NTU, colour 5 deg Hazen and DOC 1.0 mg/l) to very poor (turbidity > 500, colour > 100 and DOC > 15 mg/l). Raw water alkalinity is low at 16 mg/L (average) as CaCO3.
The treatment process can, to a certain extent, handle poor raw water conditions but the plant would normally be shutdown if the cost of treatment exceeds that of other treatment plants that supply the bulk distribution system.
The plant treats a maximum continuous flow of 50 ML/d and the treatment processes are; alkalinity addition and pH control (lime & CO2), coagulation (PACl), polymer addition, flocculation, separation (DAF), rapid sand filtration, final pH adjustment and chlorination.
In 2005 we replaced our old raw and treated water colour meters with s::can spectro::lysersTM. The spectro::lyserTM measures absorption in the UV-Vis spectrum between 200 and 750 nanometres and mathematically compensates for suspended solids as illustrated in Figure 2. A range of parameters can be derived from the spectra such as DOC, TOC, SACUV254, colour and turbidity.
Streaming Current Meter Coagulant Control
The plant coagulant dose used to be controlled automatically using a streaming current meter. Whilst the SCM could be relied upon to cope with minor/moderate raw water quality fluctuations it had its limitations. Variations in pH affected the SCM output and, as the SCM control is a feedback system, it often failed to respond quickly enough or completely enough to rapid changes in raw water quality, thus leading to filter turbidity problems due to under dosing.
During moderate to heavy rainfall the plant operators needed to closely monitor filtered water quality for signs of deterioration, and, as the plant is only manned during the day, in the evening the operators would adjust the SCM set point to over dose in order to avoid being called out through the night as under dosing may result in having to wash all the filters to get the plant back on line.
Other limitations of the SCM are that it was slow to react to improving water conditions and also that the resulting dose would often be over or under the optimum dose. This is clearly illustrated in Figure 3 which shows actual trends of the dose controlled by the SCM compared with the dose predicted by the feed forward system.
Com::pass Feed Forward Coagulant Control System
In 2006 we installed the com::pass feed forward coagulant control system. This patented system operates within the s::can con::stat processor. It uses data from the solids compensated spectra and the plant inlet turbidity meter (connected to the con::stat via an analogue input) to calculate a coagulant dose set point which, in conjunction with the plant inlet flow signal, is used to control the speed and stroke of the coagulant dose pump.
The com::pass system can operate in two modes; conventional mode and enhanced mode. In conventional mode the coagulant dose is optimised for economy, which means that it will calculate the minimum dose required to achieve acceptable filter outlet turbidity and run times.
In enhanced mode the coagulant dose is optimised for organics removal, which means that it will calculate the minimum dose required to achieve maximum DOC removal.
For a few days after installing the feed forward system we stayed on SCM control and compared the output of both systems. The first thing that struck us was the instantaneous reaction of the feed forward system to raw water quality variations, it was also noticeable how the feed forward control tracks the variations in a much tighter fashion than the SCM output, see figure 3.
The feed forward system at Wainuiomata wtp has now been in service for over three years and in that time we have seen a significant reduction in plant outages due to 'front end' dosing problems. We operate in the conventional mode since we have no significant issues with disinfection by-products.
The spectro::lyserTM probe connects to the con::stat processor which is housed in an IP 65 enclosure mounted adjacent to the probe. This unit has a touch screen from which the operator can select various displays, including the current value of each parameter, historic trending, and viewing of the raw spectral data which shows a fingerprint of the water at that moment in time.
The unit can be connected to the plant PLC by either 4-20 mA output or RS485 Modbus signal. We use the 4-20mA outputs as well as connecting to our control system network (WAN).
As the spectro::lyser measures turbidity, we had it in mind that we could dispense with the existing raw water turbidity meter as well as the colour meter and the SCM. However we have yet to resolve discrepancies between the existing turbidity meter and the spectro::lyserTM turbidity measurement. We suspected that the cause of the discrepancy is due to air bubbles in the spectro::lyserTM sample chamber and have recently installed a modified instrument mounting which we hope will rectify the issue.
We had some initial issues with making sure that the instrument was properly 'zeroed' and discovered that it is important to use water that is free of organics. We use distilled water that has been passed through a mixed bed ion exchange resin.
The probe operates in a by-pass mode and is mounted in a sleeve-like fitting to which a raw water sample is connected. The only connections to the probe are a small diameter air purge connection for automatic cleaning, and a cable to connect it to the con::stat processor.
Unlike our old colour and streaming current meters, the spectro::lyserTM is a very robust piece of equipment. It has no moving parts and pre-filtration of the sample flow is not needed as the system mathematically compensates for solids. From a maintenance point of view it has been trouble free and requires little attention. The units are visually inspected weekly and we carry out a monthly zero check.
Filter run times have increased by up to 24%, giving some energy savings, and by operating in conventional mode we have significantly reduced our coagulant consumption, this, in turn, has reduced the Lime and CO2 usage amounting to a total chemical cost saving of around 20%.
In financial terms the investment in new instrumentation and process control technology netted immediate savings in the order of NZ$50,000 per year from reduced chemical use, maintenance and unscheduled plant shutdowns. In addition there will be a favourable long term impact on the capital replacement, operations and maintenance budgets for instrumentation as we will ultimately have the spectro::lyserTM replacing two or three instruments; colour, turbidity and streaming current.
The requirements of the drinking water quality standards in New Zealand (DWSNZ2008) are extremely onerous and when combined with challenging raw waters it can be hard to achieve compliance. The Wainuiomata wtp was the first plant in the country to be awarded the highest grading possible for water quality; an achievement that has been repeated every year since the installation of the feed forward system.
The combination of new instrumentation and process control has been very successful and the bottom line results speak for themselves. In mid 2008 we installed the com::pass coagulant control system at another (140MLd-1) surface water treatment plant and are seeing very similar results to those at the Wainuiomata wtp.
For further information please email Process Measurement & Analysis Ltd