The removal of phosphate from the effluent of a sewage works is an essential part of the urban waste water directive: Partech has trialled closed loop feed forward control of an inlet dosing system.

The removal of phosphate from the effluent of a sewage works is an essential part of the urban waste water directive. Many treatment works have phosphorous discharge consent, but with the increasing use of detergents containing phosphate, the problem of staying within consent is growing. Phosphate removal is carried out by dosing chemicals, normally iron salts or occasionally aluminium salts, at a rate determined by a preset diurnal profile derived by analysing a series of samples over a period of time.

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.

The natural removal both of nitrate and phosphate where applicable is the method of choice.

However, in most cases it is not possible and it becomes necessary for the works operator to resort to chemicals, with Ferric Chloride and Ferrous Sulphate being the most commonly used.

Where the efficiency of iron is much reduced, aluminium salts has been applied successfully, and works normally in conjunction with an iron salt.

In all cases, the amount of chemical used is critical for the performance of the works, cost control and meeting the metal discharge consent.

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.

The Partech MicroMac sampling system is based around analysing ortho-phosphate, as opposed to total phosphorous.

This is because the ortho-phosphate chemistry is simple and fast, unlike the total phosphorous chemistry, which is complex and can be very slow.

Iron reacts directly with the soluble phosphate, which is almost totally in the ortho-phosphate form and the total phosphorous is largely in the bound form with the solids and is therefore flocculated by the iron with the solids.

It is also to be noted that in order to analyse for total phosphorous, it would be necessary to analyse an unfiltered sample, which on the inlet would be impossible.

The ortho-phosphate chemistry used is the standard ascorbic acid method, with the analyser calibrated using a 10mg/l as P standard and capable of detecting up to 25mg/l as P.

The analytical cycle time for this analysis is approximately five minutes.

Throughout the 18-month 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 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 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 dosing or under dosing both of which have financial consequences.

Three water companies provided sites for the Partech evaluation programme, these being Thames Water, Southern Water and Anglian Water.

The Thames Water site, with a Population Equivalent (PE) of 39,000, had a pumped inlet and a Gee and Co system dosing ferric sulphate controlled by a predetermined diurnal pattern.

The inward flow varied between 40 and 200 litres per second and upstream of the measuring point was a weir below which were two 'sandbanks' which release significant amounts of grit during high flow.

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.

The flow signal was obtained from the existing Warren Jones flow meter.

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 against the diurnal dosing regime.

This was later improved by a further 8% by adjusting the 'P to 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 litres per second to 300 litres per second at storm flow and the existing dosing system operated using a predetermined and optimised diurnal pattern was a Chemfeed unit with full dosing control computer.

The works also had tanker waste delivered at irregular intervals, which had some effect on the incoming phosphate levels.

The trials unit was installed above the inlet channel about 10 metres 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 8 weeks.

The third trial was at an Anglian Water site with a PE of 45,000 and contained a balance tank on the pumped flow inlet.

The dosing system on site was a Gee and Co unit with a Chemtrol computer unit for the dosing control and the unit was set up to run on a dosing profile.

The flow into the works was highly variable between 20 - 180 litres per second with a common discharge channel to 3 PSTs.

The dosing point was in a chamber prior to the channel to the PSTs and the trials unit was mounted above the common channel prior to the ferric sulphate dosing point.

This site had not been optimised prior to the trial starting and the profile being dosed was a flat dosing rate with little variation.

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 this 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 it was found that the filter did not require cleaning for a period of three months.

With the cost of the control system being in the region of GBP12k - 17k, depending on site conditions, a saving of 18% per annum at a site with a dosing rate of approximately 400 litres per day and at a cost of GBP50 per ton, the payback period is less than 12 months.

The use of an 'active' control system at the inlet to a works can have the following effect.

* Tighter and more effective control of the chemical dosing, with response to routine and non-routine events.

* Saving of dosed chemical expected to be in the range 10 to 40% depending on the status of the works as far as optimisation is concerned.

* Less likelihood of consent failure for both residual chemical - iron or aluminium - and phosphate.

* Reduced sludge production.

* Reduced potential for corrosion of the works due to over dosing of iron.

* The control system will have a payback period relative to the size of the works and volume of iron being used and is not expected to be viable on a small works.

* A reliable, robust dosing control system now exists.