For many wastewater treatment companies, natural removal both of nitrate and phosphate is, where applicable, the preferred method. However, in a great many cases this is not possible and therefore the dosing of chemicals is used to assist the natural process.
In the case of activated sludge process (ASP) plants, natural processes are capable of achieving a high percentage removal and so the use of chemicals is not so prevalent. However, this is not the case for operators of trickling filter works where it is necessary to resort to dosing chemicals, most commonly in the form of iron or aluminium salts. Where the influent properties, such as low alkalinity, make the use of iron salts unsuitable, aluminium salts can be used. In all cases, the amount of chemical used is critical for the performance of the works, cost control and meeting the residual metal discharge consent.
Ensuring compliance with consent level in many water treatment plants across the UK is achieved using iron salt dosing to control phosphate. However, with the price of iron salts increasing substantially in recent years, companies are setting out to find ways and means of reducing dosing chemical costs whilst maintaining discharge consent levels.
The removal of phosphate from the effluent of a sewage works is an essential part of the urban wastewater directive. Many treatment works have phosphorous discharge consents and there is anecdotal evidence that suggests that effluent containing phosphate makes staying within consent levels more of a problem particularly where the level has been reduced to 1 mg/l total phosphorus.
The most widely-used methods of dosing control are fixed diurnal pattern and flow proportional dosing, both of which have limitations. A fixed diurnal has to be programmed into the dosing control system and as the name indicates, the dosing pattern is fixed until the diurnal pattern is altered. This diurnal should be altered for seasonal and influent changes, but in reality very few are. Flow proportional dosing controls the chemical dosing in line with the inlet flow and assumes that the concentration of phosphate will remain a constant within that flow range. Normally the dosing will be capped for high flows (storm flow conditions).
These established approaches are now being challenged by UK-based water instrumentation manufacturer Partech, which has developed an alternative approach for controlling the dosing of chemicals. Termed Feed Forward Control, this new approach is now being actively taken up by UK water companies.
With Partech’s Feed Forward dosing control (see image), the system measures the concentration of reactive phosphate in the crude sewage directly after the screens and de-gritter, but before the dosing point and the influent flow. By calculating the phosphate load and applying an algorithm which includes Fe (Al) to P ratio, an accurate dose can be calculated and applied to the influent.
The sampling system
The Partech feed forward system comprises a sample pump, filter unit, filtrate pump and sample pot, dosing control unit and MicroMac C phosphate monitor. The sample pump contains an impeller designed for use in raw sewage and wastewater containing solids typically up to 7 mm. The pump is suspended in the intake channel and linked to the main sampling unit in the kiosk.
The self cleaning filter unit comes with 20 micron stainless steel wedge wire filter element in a specially designed PVC body incorporating compressed air inlet, sample inlet/outlet, filtrate outlet and cleaning port. The self-cleaning filter is supplied with a control system linked to the MicroMac C, which instigates a “sample on demand” cycle that controls the sample pump and the filtrate pump in an operator/site programmable manner. On the filtrate side of the filter unit a small peristaltic pump is installed to supply the sample to the sample pot, which has been designed to hold a minimum amount of sample. The analyser draws its sample from this sample pot and the excess sample flows to waste via an overflow built into the sample pot.
The dosing control unit employs an electronic control system for signal input and output for the control of the dosing system. This involves the input of a flow signal and a phosphate concentration signal. Including a fail-safe operator settable defaults and alarm set points, this module links to the dosing system via an analogue input to the pump via an existing cable and transmits a combined flow /phosphate mA output.
The Partech MicroMac sampling system is based around analysing reactive phosphate, as opposed to total phosphorous. This is because the reactive phosphate chemistry is simple and fast and actually measures the phosphate that will react directly with the chemical. Iron/aluminium reacts directly with the soluble reactive phosphate, whilst the total phosphorus, which is largely in the bound form with the solids, is flocculated with the solids. This competitive reaction accounts for the variability of the iron/aluminium to P ratio which is very dependant on the ‘strength’ of the incoming sewage. 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 MicroMac C single chemistry colorimetric analyser determines ortho-phosphate in the range 0–20 mg/l as P. The analyser uses the standard molybdenum blue chemistry as per methods for the examination of waters and associated materials. The analyser uses a patented loop flow analysis (LFA) system which gives it the flexibility to match the performance of a laboratory analyser with very much less attention. LFA is also very flexible in programming and it has been possible to incorporate much of the routine maintenance of the colorimeter in the operating cycle.
Feed forward versus feed back
The feed forward control responds to ‘actual influent’ conditions, the system calculating the phosphate load to the works and then calculating the required amount of chemical, with the ratio being altered to achieve fine control of the dose for different seasons. With the feed back approach, the system relies on a predetermined profile, which is either flow proportional or fixed. As a result, the system doses the chemical to the pre-set level and only corrects ‘after’ the event.
With feed forward, the risk of over or under dosing the chemical is reduced, whereas with feed back, over and under dosing remain a high risk. Although the feed forward sample is more difficult to analyse and has a potentially wider range (0-70mg/l as P), the potential benefits are greater whereas the sample for the feed back is much easier to analyse and has a much smaller dynamic range (0-5mg/l as P).
The savings on this site during the initial set up period of one month indicated that savings in the region of 30% were achievable, this being influenced by the amount of high flow/storm flow that happened during this period. The direct chemical savings over the two year period since the installation has been shown to be 20% to 25% and the system has controlled the effluent such that the works has been in consent for both phosphate and iron.
Summary
Analysers can be used to control dosing systems, but it is important that they are simple to operate and maintain. By keeping analytical solutions simple, operators will have confidence in the system which they are using. Field experience has demonstrated that feed forward control can increase the efficiency of the dosing system and gives a direct saving on chemicals. Fixed diurnal or flow proportional control of a dosing system is better than nothing, but active control is a better solution. Feed back control can be effective, but it gives less control than feed forward.