A closer look at dosing systems

Washing powders may be a boon for the housewife,
but for WwTWs it means one more element
that has to be controlled – phosphorus (P). In recent years, due to the Urban Wastewater Treatment Directive a number
of WwTWs have been given phosphorus consents. With discharge consents becoming ever more stringent, treatment to lower P levels
has become increasingly important.

Les Stokes, Process Engineer at Severn Trent Water (STW), estimates nationally more than 600 sites will require P removal in the next five years. In the STW area alone, 38 sites have already had to meet new standards and another 71 will require attention. Phosphorus is not the easiest of elements to remove. Ideally, biological treatment is the optimum choice, but this is not always a viable option. To be effective it requires an influent with certain characteristics, which are not present at every site. The alternative is some form of chemical dosing.

Chemical treatment, however, has other knock-on effects on the process. If you add a metal at one point, you may need tertiary treatment to remove it later in the system, in order to meet metal consent levels in the final effluent. Since the chemicals used are either highly acidic or highly alkaline, they can affect pH values, which again may then need adjustment. There may also potential problems with ensuring continuing supply if the system relies on a specific chemical. STW’s
first choice for P removal is an iron-based coagulant such as ferric sulphate or ferrous chloride, which are generally cheaper and have a
higher percentage of metal content per unit volume. However, these chemicals are derived from waste materials, and with demand expected to rise some 250% over the next five years, relying on these alone could lead to future problems.
The existing treatment process and the characteristics of the wastewater can vary widely from site to site. A policy of ‘one size fits all’ simply does not work and, as a result, STW has spent considerable effort in devising dosing systems that will not only work in present conditions but are sufficiently flexible to meet any future changes in regime.

Much of the initial cost is taken up by the ‘hardware’ of a system – mixing tanks, pumping equipment, tertiary sand filtration, etc. But the STW team has devised a system that can also permit changes in the treatment chemical, adjustments to dosing points and provision of additional dosing so as to allow for future variations in the nature of the inflow, changes in the discharge consents or other alterations.

Site conditions are difficult to reproduce in the laboratory. “Every site is different, and the results of jar tests are not always a reliable reflection of treatment effectiveness, especially with the time lag between feasibility studies and actual installation,” commented Rosie Wilson of STW’s process design team. As a result, extensive optimisation is required at commissioning to achieve the most cost effective results. Good mixing and flocculation are essential.

At Knighton WwTW, for example, inadequate mixing proved to be a problem. The works is designed for a 3,357 population equivalent flow, treated by percolating filters and humus tanks, with an effluent total P consent of 2mg/l. Initially the flume to the primary settlement tanks at a point just upstream of a turbulent zone was identified as the best place for dosing. During commissioning, the mixing and contact time were not enough to allow formation of the metal phosphate compounds required and it became clear the works would struggle to meet P discharge consent levels.

Fortunately, to retain as much flexibility as possible, it had been decided at the design stage to add a secondary dosing and mixing point after the percolating filters, allowing P removal to be targeted at the final settlement stage. While this ‘back-end’ dosing system is more reliant on the tertiary filtration to remove the high metal carryover, the designed-in flexibility paid dividends, especially when problems were experienced with the ‘front-end’ dosing.

The switch from front to back-end dosing allowed commissioning to be completed on time and the works is now meeting its obligations for P removal. While iron-based coagulants are STW’s first choice, with some types of sewage these are not always effective. At some bacteria bed sites two chemicals, aluminium into the crude sewage and iron into the bacteria bed effluent, have had to be installed. Sometimes additional pH correction has also been required to allow sufficient alkalinity for the biomass in the beds to nitrify.

At the medium-sized Ludlow works (12,000PE) the dosing system has been designed to allow for the use of both chemicals, with aluminium injected at the front end and iron at the back end. This presented a considerable challenge during commissioning. “If not enough chemical is injected at either point, flocculation and coagulation will not occur, the P will not be removed, and most of the metal will be carried over. Conversely, if you use too much, the feed to the rest of the sewage treatment process can be starved and metal will also be carried over. This can cause problems with the biological treatment stage and the final effluent consent,” said Nick Madely, a member of the STW team.

At Ludlow, dosing was started at 50% of the assumed target amount and gradually increased until P removal reached requisite levels with no excess metal carryover across the primary tanks. If additional dosing is required to ensure effluent compliance, then the back end iron dosing system can be switched on at a low level.

STW believes designing and building-in flexibility in dosing systems will prove a wise policy in future. By building inflexibility from the earliest stages it will enable the company to meet P consent standards despite any volatility in the supply or cost of essential materials, changes in the characteristics of wastewater entering a works or lowering of consent levels.