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Ever increasingly stringent phosphorus consents is one of the hot topics in the water industry.

There are two general approaches to phosphorus removal in wastewater treatment works. One is to modify activated sludge to maximise phosphorus removal, the second is to use a coagulant such as iron or aluminium, or in some plants, a bit of both might be used.

Although this may enable phosphorus consents to be met at the moment, if phosphorus consents are reduced, it increases the likelihood that wastewater treatment works will just have to up their coagulant dosing. Without a new technology, this will lead to increased chemical costs and the potential for the breach of consents for metals such as iron.

So what should a treatment works do without a significant investment?

Software combined with instrumentation could be the answer to ensuring coagulant dosing is optimised.  In today’s world of instrumentation, providers can no longer just offer an instrument that outputs a concentration and leaves the operators to interpret what to do with it. An integrated system using software enables automation from the vast amount of data that can be produced from today’s instrumentation.

Compass CPR is the latest version of the highly successful Compass system which for several years has been used by utilities on drinking water treatment works.  Utilising the full UV-Vis capabilities of the s::can spectro::lyser combined with a site-specific calibration generated by the Compass software system, coagulant dosing can be optimised to manage consents whilst keeping chemical dosing down to a minimum.

How does it work?

Coagulant dosing is usually set manually by measuring the influent and effluent phosphorous concentrations and carrying out jar testing. Compass CPR provides an alternative which continuously adjusts the dose to the optimum level, accounting for all components of the coagulant demand.

When dosing coagulant for phosphorous removal, a large proportion of the demand is actually made up from the suspended solids, colour or other constituents in the waste water. All of these constituents are constantly varying therefore to optimise the coagulant dose, all components need to be measured continuously. Compass CPR is an online feed forward coagulant dose controller which utilises the UV-Vis spectro::lyser to continuously determine the coagulant demand.

In the example below, the alum dose required to meet the discharge limit of 0.25mg/L was 6.8mg/L as Al3+– a ratio of 2.95. 68% of the coagulant demand was caused by other constituents in the influent. So the actual stoichiometric ratio was 0.94.

The spectral data from the spectro::lyser can be used to determine the phosphorus component of the demand through the use of a surrogate absorbance or, if an online phosphorus instrument is present at the plant, this can be input to the controller instead. The phosphorous component of the demand is added to the demand calculated for the other constituents and the output from the controller is the coagulant dose.

Optimisation of the coagulant dose using Compass CPR allows you to consistently achieve ever increasingly stringent effluent quality targets, significantly reduce chemical costs and has potential to reduce sludge volumes as a result of reduced overdosing.

Compass CPR has been operating at various sites across the globe since 2008. In some wastewater plants, they have seen approximately 20% saving in coagulant in the first year of operation with no breaches of the discharge limits.

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Process Measurement and Analysis Ltd, Brook Mills House, Carr Lane, Slaithwaite, Huddersfield, HD7 5BG United Kingdom