Improve dosing control
27 Oct 2005
Where applicable, the natural removal of both nitrate and phosphate from the effluent of a sewage works would be the method of choice. However, in most cases this is not possible and the works operator has to resort to chemical treatment.
Ferric chloride and ferrous sulphate are most commonly used - although aluminium salts have also been used successfully - but in all cases the amount of chemical used is critical for the performance of the works, cost control and meeting the metal discharge consent levels.
Dosing iron salts at the front end of a works requires a level of control to ensure that the pH of the influent is not made too acidic as this has a detrimental effect on the nitrifying process. Historically, the iron dosing rate was calculated by taking a series of samples throughout the day and having them analysed to derive a diurnal profile. This profile is entered into the dosing system such that a specific volume of iron is dosed at the time intervals used and this is then employed to ‘control’ the dose.
Over the past 18 months, Partech Instruments has trialled closed-loop feed- forward control of an inlet dosing system for the removal of phosphate at three sites. The results of the trials demonstrate that this approach is reliable and cost-effective, and can assist in making considerable savings in chemicals.
Throughout the trials period, Partech gave great attention to designing a sampling system that would be low maintenance and ensure reliable crude sewage sampling. The sampling system is a critical part of the control system and requires an optically clear sample to determine accurately the ortho-phosphate level. The resulting MicroMac sampling system is based around analysing ortho-phosphate, rather than total phosphorous, because the chemistry involved is simple and fast, with an analytical cycle time of approximately five minutes.
The finished design copes with low flow and low sample levels, grit, ragging and turbulent flow. In addition to low maintenance, the simple design helps with operator confidence and keeps the cost to a minimum.
The advantage of determining the phosphate level on the inlet is that the dose rate of the iron or aluminium salt can be calculated and the dosing system controlled using the combined flow and phosphate concentration. The combined output generated allows the operator to adjust the dose for site-specific conditions that allow the ‘P to Fe’ ratio to be adjusted until it is optimised. This optimisation requires the monitoring of the final effluent to ensure that the phosphate levels are within the Environment Agency consent. Once the optimisation has been done, however, there is no need to monitor the final effluent.
By using feed-forward control the dose responds to actual changes in phosphate levels and provides an active dosing regime, whereas the commonly used ‘diurnal profile’ cannot respond to changes in the inlet and can be either over- or under-dosing, both of which have financial consequences.
The three water companies involved in the trials programme were Thames Water, Southern Water and Anglian Water. The
The trial unit was positioned above the inlet channel some 20m upstream of the dosing point and the sample pump was located directly below the trial unit. After four months the system was reliably analysing crude sewage for ortho-phosphate and, when combined with the inlet flow, was successfully controlling the dosing system. During the initial stages, Partech was able to demonstrate a saving of some 25% of the iron dosed when compared with the diurnal dosing regime. This was later improved by a further 8% by adjusting the P:Fe ratio.
The second trial, with Southern Water, took place at a site with a PE of 100 000 and a standard inlet channel equipped with well performing screens and de-gritting. The flow rate ranged from 100 to 300 litre/s at storm flow and the existing dosing system ran an optimised diurnal pattern. Here, the trials unit was installed above the inlet channel about 10m upstream of the dosing point.
After just a few weeks the Partech system had provided several daily profiles and was shown to agree with the laboratory results. The positive results of this trial was the confirmation of the results and with a redesigned filter coping well with the sample, the maintenance interval was increased to greater than eight weeks.
The third trial was at an Anglian Water site with a 45,000 PE. This site contained a balance tank on the pumped flow inlet and the flow into the works was highly variable, between 20 and 180 litre/s with a common discharge channel to three PSTs (primary settling tanks). The dosing point was in a chamber prior to the PSTs and the trials unit was mounted above the common channel prior to the ferric sulphate dosing point.
After an initial period of running baselines to look at the diurnal pattern, the Partech analyser system was attached to the dosing system and a controlled trial was carried out. During the trial the amount of iron dosed was reduced by some 47% against the flat dose rate that was initially controlling the system. The sampling system worked very well and the filter did not need cleaning for three months.
To sum up the benefits of this feed-forward approach, we can say that, with the cost of a control system being in the region of £12,000 to £17,000 depending on site conditions, a saving of 18% per annum at a site with a dosing rate of approximately 400 litre/d and a cost of £50 per ton should result in a payback period of less than 12 months.
Andrew Wallace is field sales engineer with Partech Instruments.
