Keeping an eye on oxygen
Around 60-70% of a WwTW's energy costs can stem from the aeration of activated sludge. Graham Meller reports on Hach Lange's LDO - a dissolved oxygen sensor that helps eliminate excessive aerationWater companies are constantly searching for technologies that can both improve processes and create operational savings, and one such area of improvement is in the measurement of dissolved oxygen (DO).
Many water treatment technologies involve aeration in order to maximise the effectiveness of bacteriological degradation. It has been estimated that 60-70% of a plant's energy costs come from the aeration of activated sludge. But, in addition to the need for ideal bacteria growth, it is essential that wastewater is not aerated excessively because this wastes energy. So, accurate DO measurement represents a vital component of successful plant management. Three years ago, Hach Lange made the claim that most of the problems associated with DO measurement had been eliminated as it launched the Luminescent Dissolved Oxygen (LDO) at Iwex 2003.
User feedback now confirms the initial claims for the sensor and it is now the most common aeration DO probe used in replacement in UK water companies.
For more than 50 years, galvanic and polarographic sensors have been used to measure DO. These sensors employ membranes, anodes, cathodes, and electrolyte solutions that generally require a high degree of maintenance. The sensors also suffer from drift, and as a result have to be recalibrated frequently.
Historically, there have been a number of problems associated with galvanic and polarographic sensors. The membranes are relatively delicate, and can become contaminated or damaged, in which case it would be necessary to replace the internal electrolyte. The sensor's anode is consumed over a period of time and will require replacement if it, or the electrolyte, becomes poisoned by gases such as hydrogen sulphide.
There are other factors that can affect the accuracy of these traditional sensors, including variations in pH or the presence of chemicals that induce voltage, such as iron and aluminium salts, and polymers.
Hach Lange claims that the LDO has overcome these problems because it does not consume oxygen as part of the measurement process, and it does not require frequent recalibration because it does not suffer from drift (gradual loss of accuracy).
How does it work?
The sensor is coated with a luminescent material, called luminophore, which is excited by blue light from an internal LED. As the luminescent material relaxes, it emits red light. And this luminescence is proportional to the dissolved oxygen present. The luminescence is measured both in terms of its maximum intensity and its decay time. An internal red LED provides a reference measurement before every reading to ensure that the sensor's accuracy is maintained.
Peter Packham is process co-ordinator for the South-east provinces of Thames Water, which includes 49 WwTWs, 15 of which employ aeration lanes. He has around 24 LDOs installed at WwTWs such as Aldershot, Bordon, Camberley, Horley and Lightwater, with more units being deployed as part of the ongoing replacement strategy.
Peter reports that the older LDOs have been in place for around 18 months and have provided trouble-free operation. He says: "We chose the LDO because of the reduced lifetime costs that it offered. This new technology is easy to install and requires a lot less maintenance than traditional, membrane-based probes, so labour costs are lower.
We have found that an occasional visual check is all that has been necessary.
"The cost of consumables is also considerably lower, largely because we no longer have to fit expensive replacement electrodes. We have also experienced improved accuracy. The membrane probe tended to drift over time, but we have found this not to be the case with the LDO. Naturally, we regularly check the readings but we generally find the values to be spot-on."
Mark Gibson is a technical adviser at Northumbrian Water. He first began using the LDO at Bran Sands WwTW about three years ago where the sensor has been a success and is now being used at some of the company's municipal WwTWs. But, while the reduced requirement for maintenance is a great benefit, there are a couple of sites that have suffered from rapid fouling. And as a result, it has been necessary to install air cleaners. This reduces the cost of ownership benefits that the LDO offers.
"Our work has indicated that fouling can be detected by monitoring the rotor speeds of the aerators," he says. "Consequently, manual cleaning of the LDO probe has been necessary when higher rotor speeds are detected. The cleaning process only takes a few seconds but it is vitally important if we are to avoid excessive use of energy."
Dave Manley, ICA coordinator for Southern Water in Sussex, reports that, as a result of early trials at Newhaven and East Grinstead during 2003/4, the LDO has been installed more widely across the region. He says: "Results at Budds Farm WwTW, where all of the DO probes have been upgraded to the LDO, were particularly encouraging. Previously, we would have to carry out regular calibration, spending around half an hour changing membranes, handling chemicals etc. And the staff would need specialist training. However, we have found that no recalibration is necessary with the LDO. We simply change the caps once every two years.
"Another benefit has been the accuracy of the readings. The readings from the membrane-based probes used to drift. But the LDO remains accurate unless the probe becomes completely fouled, in which case there is a sudden and clear drop in the DO reading. And it is then necessary to wipe the tip of the probe. We therefore wipe the probes every 2 to 3 weeks as a precautionary measure," said Manley.
Manley employs the SC100 controllers with the LDOs, and says: "The software that comes with the SC100 makes it very easy to collect data, change probes etc. So the whole process of measuring DO is now much easier, less costly and more accurate."
Steve Tough is ICA team co-ordinator for Hampshire and the Isle of Wight, and currently has more than 50 LDO sensors installed in aeration lanes at WwTWs across the region. He says: "The main benefit of the LDO has been the amount of time/labour that they have saved in comparison with the older DO probes."
Anglian Water originally tested the LDO nearly three years ago at its Innovation Centre and as a result of the positive trial results it was passed to the Energy Innovation Team and is now being rolled out, among other probes, to Anglian Water's operational teams.
The Energy Innovation Team was created to find ways of improving energy efficiency and therefore aeration naturally warranted particular attention. Adam Brookes was a member of the team and is now an energy auditor. He says: "Effective DO control is key to energy efficiency in aeration and an accurate, reliable DO sensor is therefore essential. Membrane-based probes are capable of providing the level of reliable accuracy required. But their high maintenance requirement has meant that we now use approved probes such as the LDO in most replacement circumstances."
Dave Biss, an Area Energy Engineer for Anglian Water, has had experience with the LDO at plants such as Clacton, Jaywick, Morden, Colchester, Witham and Burnham. Dave says: "One of the key advantages that we have experienced has been that, following installation, local staff have taken greater ownership of process control. In the past, specialist contractors were necessary because of the chemicals and training necessary for calibration and maintenance procedures. But these are no longer necessary and this has resulted in an improvement in process management. The price of the LDO is also very competitive and the SC100 Controller is dual channel, which means that we can control two sensors with one controller."