Phosphate monitor cuts costs
United Utilities operates a number of WwTWs with high-phosphate influent that arises as a result of specific local industries. Discharge consents reflect this and the water company is obliged to remove a large proportion of the phosphate from the discharge.
One such plant is based on the outskirts of Burnley, where a sophisticated monitoring and dosing scheme has been developed in order to achieve the required levels of phosphate reduction.
Burnley’s consent requires either 80% removal of the total phosphorus or for the discharge to have less than 1mg/l phosphorus. As a result, the continuous measurement of phosphate in both influent and effluent is necessary, and indeed at other stages in the process.
Influent Ortho Phosphate is measured to inform the chemical dosing control system. However, continuous analysis of raw sewage influent represents a number of challenges.
In summary, influent phosphate levels are measured by an Evita Insitu monitor, and this data is used to manage the dosing of ferric sulphate. This precipitates the phosphate and facilitates sedimentation and removal. Accurate continuous monitoring is necessary to ensure sufficient dosing is applied to remove the phosphate and excessive dosing does not take place.
Overdosing would be undesirable on two counts. Firstly, from an environmental perspective, the objective is to minimise the amount of iron being added that could remain in the effluent. Secondly, ferric sulphate is expensive, and excessive dosing would be wasteful and costly and may have a negative impact on the overall performance of the treatment works.
Burnley WwTW is managed by District Manager Louisa Brown. The site has been in operation for well over a century, however, it is not clear when it was first built. Records indicate that treatment activity was ongoing as far back as 1887. It is believed that the works manager at one point was a Dr Watson who was part of the team that introduced activated sludge at Davyhulme WwTW. Burnley is reputed to have incorporated activated sludge soon after.
The influent source at Burnley WwTW is Burnley town and most of the Pendle boroughs. This represents a people equivalent of 102,000 domestic and 83,000 industrial. High phosphate levels have been a long-standing feature of the influent arising from local industry.
The complete monitoring and dosing control system, including tanks, pipework, lines, instrumentation, bunds, hard landscaping and so on cost around £1.5M.
In excess of 3,000 tonnes of ferric sulphate is used in one year in addition to almost 1,000 tonnes of caustic soda. Without the monitoring system, dosing levels would have to be either fixed rate or regulated by an influent flow measurement.
United Utilities estimates that, when compared with a fixed dose rate system, the monitoring/dosing system is saving approximately 37% of the ferric sulphate and 57% of caustic. This represents six figure cost savings every year.
Bacteria generally remove about 3mg/l of phosphorus during aerobic digestion, however, significantly higher levels of removal are often necessary. For example, at Burnley WwTW, the mean total phosphorus in the crude sewage is 17.0mg/L, with a 95 percentile of 31.4mg/l. The coefficient of variance is 54%.
So chemical treatment of the influent was chosen with consideration to these high mean and peak concentrations, phosphorus variability and the limited scope for biological nutrient removal within the existing asset.
This can be achieved with lime or the salts of aluminium or iron. Of these, iron is generally preferred because aluminium salts can be toxic to fish and lime produces high sludge levels. Ferric salts are preferable to ferrous because they have a lower oxygen demand. The stoichiometric dose of iron is approximately 1g Fe: 1g P but in practice the dose varies from 1:1 to 4:1 because of competitive reactions with organics, for example.
The reaction of ferric sulphate with phosphate produces insoluble iron phosphate, which precipitates and is removed as sludge in addition to sulphuric acid, which is buffered by additional dosing with caustic soda. This ensures that the bacteria within the biological stages are not stressed as a result of a low pH. Again, careful monitoring and closed loop control is necessary to ensure that optimal levels of alkalinity are maintained.
Traditionally, wastewater is monitored following the withdrawal and filtration of a representative sample, and the subsequent exposure of that sample to a series of
sensors and analysers. Development activity has focused on improved sensor technology and more reliable sampling and filtration methods. However, the Evita analysers were developed by an innovative group of engineers who took a different approach.
They chose to step back from the monitoring process and examine the process as a whole, and as a result they questioned the need to remove the sample. Their examination of the wastewater monitoring process highlighted the fact that many of the main sources of problems lay in the sampling process rather than the measurement stage. It became clear therefore, that a significant improvement could be made if the sampling issues could be resolved.
Evita deals with sampling issues by removing them from the process – instead of taking the sample to the analyser, the Evita takes the analyser to the sample. The Evita system simply floats on the wastewater. This eliminates the time and distance involved with sampling, as well as delays incurred by backflushing procedures.
However, the challenge is then to obtain a representative sample. In traditional systems, the sample is pumped through a filter and passed to an analyser, thereby incurring a risk that the sample may have altered in some way before the analysis actually takes place.
Evita deploys an ingenious antibacterial ion-exchange membrane that selectively allows target species (NO3, PO4 or NH4) to pass into the instrument through a process not dissimilar to osmosis. Importantly, this means that no biomass or fouling is able to pass into the measurement chamber, so that a normal colorimetric reaction can take place unhindered by contamination and interference.
In order to be able to accommodate a minimal quantity of reagents within the analyser it was clearly necessary to conduct the analysis using very small quantities. For this reason, peristaltic pumps would not be suitable and tiny ceramic micro pumps were developed specifically for this purpose. These micro pumps are so small that one rotation moves just 3µl.
The monitoring system provides a response period of approximately ten minutes. This is a fixed period, which is vitally important for the effectiveness of the overall monitoring and dosing control system.
The Evita performs an internal three-point calibration automatically every 72 hours. It also maintains contact with Hach Lange’s service engineers via GSM through Evita Net. This facility ensures that alarms can be raised via text or e-mail, and that the engineers are able to conduct routine fault diagnosis remotely.
On-site maintenance is comprised of two main activities. Firstly, the ion exchange and removal unit is changed every five weeks and secondly, the reagent pack is replaced every 11 weeks. Reagent packs are delivered in wooden reusable boxes a few days prior to the designated date and reagent change is a simple operation taking just a few minutes.
The complete unit is raised from its measuring position, the reagent pack is removed and a replacement pack is fitted. Once replaced, the instrument is restarted and automatically calibrates and resumes measurement.
In high-fouling applications, it is sometimes necessary to clean the membrane with a squirt of clean water. This can be as often as once per week under the worst conditions. Operators are not exposed to any of the chemicals and Hach Lange collects spent reagents for disposal and recycling.
Burnley WwTW employs four Evita INSITU phosphate monitors. The first monitors the influent, a second monitors phosphate levels following dosing, a third measures phosphate before a final polishing phase and a fourth monitors the final effluent before discharge to the River Calder.
Tel: 0161 872 1487